Building an electric racing car

A small team of school kids and myself have built and raced an electric racing car. This is how we did it.

Step 1: The Chassis

We built this car to enter a series of national races here in the Uk - Details can be found at http://www.greenpower.co.uk. To some degree this controlled how we built our car and what we used as the race series has rules that must be followed.

The chassis was a simple "Ladder" design made from 19mm aluminium square section tubing. This is easy to work and light as well as moderately low cost.

The various sections were joined with aluminium plates and pop rivets. We followed a Greenpower design as the basis for our car but made numerous changes along the way.

Comments

[tezzachs]

nice to see a greenpower team using instructables, we were going to, but got engrosed in building and forgot the pictures! might do one for a goblin kit car at some point though

[rickharris]

Sadly an ex greenpower team, I retired and no one else would take it on. They gave the cars to another school that- as far as I know has yet to do anything with them.

[TheMadScientist]

these motors can by aquired at your local flea market/ resale shop for sure! old and new air compressors alike love these things, and they tend to be pretty massive... also, backup generators use these in reverse to generate electricity, which means that they can be taken out and used as an electric motor aswell. a third source would be the obvious washing machine/drier but they're a bit more difficult to get loose...

[rickharris]

Mmm these are purpose made for the job and are 24 volts.

You can get DC motors second hand but they are surprisingly rare these days. About the best source is Ebay and ex golf cart motors or up to 300 watts (claimed) electric scooters.

[rickharris]

Take care you are getting a DC motor of the required voltage and power rating there are a lot of toys (electric scooters for example) out there that use relativly small motors. Our motors are designed to operate on electric wheel chairs although there they have a step down gear box fitted. You need something in the order of 250 watts to be useful. BUT take care about using a generator to act as a motor. It will not be as effectent as a purpose built motor and could be and AC alternator which is useless. Car / lorry starter motors provide plenty of power BUT eat electricity - there is a drag car contest that uses them but after 200 yards the batteries are cooked. If you can find one perhaps a car DYNAMO will do at a push bit I wouldn't expect the range or power we get.

[The_Tom]

I got a motor from an old treadmill is says its a
2.5 hp at 130 VDC
17.2 AMPS
7090 RPM
25C
Class B
Thermal switch: 120 VAC MAX. 1800 VA MAX

Im kind of unsure as to what most of these mean lol and would it be a good candidate for a small go kart like yours?

[rickharris]

1. You need to get 130 volts to drive it - That's a LOT of battery!

2. Other than that it looks very useful.

Power is a function of Voltage and current so As you increase the voltage you can reduce the required current for a given power. This is why the F1 KERS systems use 300 volts + to power their system.

[omnibot]

I'd say so. RPM looks a little high but gearing it down should take care of that. 2.5hp is a good amount of power for a small buggy.

[The_Tom]

im guessing it meant 2.5hp at 130 dc volts?

[FrozenIce]

great instructable :) really liked it :) another thing about the breaks, wouldnt it be cool if u also put in some sort of wind break, like in the bugatti veyron?? it would look soo epic! just saying +5*

[rickharris]

Cool yes, but more weight to carry. When your racing the aim is to be as light as possible to make the battery go further.

thanks for the comment.

[tim127]

im trying to build an electric bike and the battary i want to use says its 10 ah and 24 volts how many amps is it and how well will it work with a 100 watt motor?

[rickharris]

A 10 Amper hour battery will in principle supply 10 amps for 1 hour - or 20 amps for 1/2 an hour or 5 amps for 2 hours - get the idea?

In actual fact it will fall somewhat short of this if you expect it to recharge and have much of a life.

Assuming the motor is 24 volt you may get 3/4 to 1 hour real running out of it. In principle the 100 watt motor will draw 4.amps to produce 100 watts at 24 volts.

100 watts is a little small for powering much other than a very light weight scooter. 200 - 250 would be better and not a lot more expensive.

in this car a 70 Ahr battery gave us about 2 hours running flat out - 30 Mph.

[tim127]

ok thank you, ill get a 250 watt moter and a better battary

[rickharris]

The steering mechanism is chosen for it' simplicity to manufacture and repair during the race. It has never let us down and works well under race conditions .

You have to understand the rules. The drive must be able to exit the car in under 6 seconds - a wheel gets in the way and to make it removable as in F1 is just too complicated. This isn't intended to be a raod car but has been built for a precise purpose.

As to having poor drivers in the UK I see in the current Formula 1 line up drivers from the UK - Australia, Germany Scandinavia in fact all over Europe and south America but No North American drivers have driven since 1993 and ever won the drivers championship.

Perhaps your driving experience is more suited for driving round simple oval circuits?
it's easy to be critical when you don't have the full picture!

[bmêttź]

Re: quick disconnect steering wheel.

A easy way to make a quick disconnect steering wheel is to use a socket wrench extension and a socket with a nut welded into it that is bolted to a steering wheel.

Use air socket wrench parts as they are better built for this purpose.

You can also use impact wrench extensions and the universal joints on them with small gas and electric motors if you have to mount a motor offset.

[benrice33]

Plus Britain has the greatest car show, or TV show in general for that matter, Top Gear

[fatboy106]

Great instructable, very helpful for when my team enters nxt year.

[rickharris]

If your entering next year a few tips:

Ensure you FULLY understand and comply with the rules.
Brakes are a BIG failure point - make sure the exceed the minimum demand.
When you race take spare tires and inner tube (You will get punctures)
Make a tool/parts list make sure you take them all.
Get to the race early so you can unpack, set up and get scrutinised.
Make sure every member of the team has a job to do and does it -
Practice packing and unpacking, changing batteries and drivers a LOT.Try to have more team members than the minimum or you will find some days not everyone turns up.
try to involve parents.

[fatboy106]

Thanks, it's nice to hear pointers from someone with hands on experience (the greenpower lot didn't help much).

[rickharris]

We got to the final twice - call it coincidence but we got punctures both times from drawing pins? How freaky is that!!!

Our down fall was that it took so long to change the tire that put us out of the running.

Build simple and reliable, you will outlast may of the fancy, even corporate entries who have spent BIG money on their vehicle.

Find somewhere to let your drivers practice driving. This can be really difficult as wide open spaced are few and far between.

Remember every year you need to bring in new members. We used to keep the car in the school foyer as an advert - It not only impressed pupils but Ofsted as well.

try hard to get a range of staff interested so your not short of adults when you need to race. i was lucky that out groundsman was interested and almost always available.

[Billgun]

Over in the UK you people keep the kids so uninformed about cars that they don't even know they are supposed to have steering wheels? wow thats so sad. I was driving MY one-ton around my house and down a old road by the age of 9 years old Let kids drive cars around thats what teaches them Because when you put a kid in the driver seat for the first time at the age of 16 they are going to be REAL terrible drivers thats probably why the UK has a reputation of bad drivers

[soccer man]

Cool car I'll try to build it. :) P.S how much money did you spend on that?

[rickharris]

The basic car cost around £250 as it is with the aluminium body - Then on top of that the motor was also £250 the battery set (4 x 12 volt 75 Ahr) cost £120. A lot depends on what you have and can scrounge for nothing. If I were to do this again and not for racing where the aim was as light as possible I would probably use plywood as a building material.

[teslafan100]

cool :)

[iminthebathroom]

The wheel chair motors you have used can be found SOMETIMES cheaply at medical supply centers. Most suppliers of powered wheelchairs that also do repairs have very odd contracts with suppliers. Most of the time if there is an electrical/mechanical erros with the chair, the standard rule is to A. either give the new person a new chair, or B. replace a very large portion of the wheelchair completely. Now the odd part is, that the parts that have been removed or even the whole chair are ordered to be scrapped. This is for insurance reasons on there behalf. Now most of these places will actually strip the motors and sometimes the speed and controller units, all though the latter is not often kept. The motors and the like are then used for people who's warranties have expired, or for people just wanting the motors for there kids to make go-karts or the what not. Remember to be kind when talking to these people, explain what you are doing and if they can't help you right now, ask them to keep your number.

[oscarthompson]

Heres The pictures of the electric Go-Kart I am making.
I have decided to split the current through 2 relays as I couldnt find any 40A relays. I am using 2, 350w 24v 19A 2550rpm motors geared down so they have suffient torque.

I am using light wieght alluminuim for the frame

Thanks

Oscar

[rickharris]

Looks good. You might find you will need to move your switch closer to your thumb but you will find out when you drive it.

How about brakes? Stopping is more important than going!!!

Have you modified an existing kart or built from scratch. Getting steering right is very tricky.

[robert]

i cant do that at all

[rickharris]

Can't do what exactly?

[oscarthompson]

I was thinking of that when I made the whole although my thumb just reaches it the push button. The brakes are not disk brakes but the same as the ones on bikes, just modified. I have build this from scratch from an old swing set

The steering is still needing to be perfected.. The spindles and the steering column work fine its just i need to find sutiable push rods and joiners. I used m6 bolts (the long ones you can cut to size) but it keeped locking up.

Thanks

Oscar

[rickharris]

I get several queries a month on this project asking for more details, often asking how to make it go 60 - 100 MPH on 2 car batteries and how suitable is it for a college project.

Well There are some serious issues here.

Please people who are interested read the whole instructable AND follow the links for further information/

Please research what others are doing. IF we could get 100 MPH from 2 car batteries and go 500 miles then Ford etc would be already doing that!

Lastly please attend to the points below.

1. This is a BIG task - I also made an electric car fro my collage degree and it took me almost a year of very long hours and hard work  to do  - make sure you can finish the task.

2. You will not get 50 - 60 MPH and a reasonable distance on a couple of  car batteries. THINK - if this were possible then commercial manufacturers would be doing it.

3. You must do some in depth research there is a LOT of information on the web, this is only a starting place. You must look to see what others have done and how they did it or you are bound to fail. As a teacher of design  I am worried that you don't have enough grasp of the essential issues to complete the task.

4. At best you will probably get about 30 or 40 MPH on the flat for about 2 hours from 2 car batteries - so a range of about 60 miles.

5. The most energy dense electrical power supply at present that is easily available is Lithium ion batteries like in mobile phones BUT these are going to cost you a LOT. commercial cars combine a HUGE power pack with a higher voltage motor to get a better power to current rate.

6. Our motor was 250 watts and if you rad all of the instructable and follow the links you will see runs at about 2000 RPM and was geared to give 35 MPH at the wheels - I leave it to you to work out what the gearing has to be as it depends on the wheels you use.

7. Our car cost us about £1500 to build say $3000 so it isn't a cheap project to do well.

8. Your better bet would be to develop a part of such a system and show how it is integrate into the whole vehicle.

9. For a commercial day to day car you will need to look at Pulse width modulation speed control. This will give good control without wasting energy, Energy recovery on braking (KERS system). A suitable material and construction for a VERY light weight body/chassis. A suitable source of mechanical running gear. Some way to steer and some way to stop the vehicle.

10. As I said at the start It is a long and complex engineering problem, it can be done - we did it, but you MUST ensure you have the time and skills to complete the task.

Good luck

[knuckel]

 This is a amazing instructional, I was just about to make my own electric vehicle!

[rickharris]

Your welcome - good luck. It's a big undertaking but lots of fun.

[oscarthompson]

hello.

I would like to know what gear ratio you are using? And does gearing the motor down increase the torque. eg. a 300w motor geared down so 1 revalution of the wheel is 5 revalutions of the motor, would mean the torque is equivelant to 1.5kw motor?

thanks
oscar

[rickharris]

14:1 geared down. The motor runs most efficiently at 2000RPM and draws about 20 amps. yes gearing down increases torque.

[oscarthompson]

Do you have any diagrams/schematics for you elctrical system?

Thanks

[rickharris]

The attached image shows the wiring. this is very simple. The 2 batteries are wired in series to give 24 volts. A key lock isolating switch prevents accidental operation of the car when not wanted and a way to isolate the batteries in case of emergency (they do happen)

The breaker protects against excess current - initially we set it to 75 amps but later changed it for a 100 amp breaker - That is a LOT of amps. -

The motor is controlled by a simple on off push button - this quite enough for the type of racing was were doing anything more complicated adds weight and gives a higher failure chance.

It is IMPORTANT you use correctly rated automotive wire for this as the motor can and does draw around 75 to 90 amps on occasion. A car battery is quite capable of melting insulation and wire and starting a fire so safety MUST be a first concern.

[oscarthompson]

Thank you Very much.
I have two motors and each one draws about 19amps. I am also using a automotive relay to switch the power on and off. I know the the gauge of wire for the motor connections have to be thick but for switching it on, does it have to be thick?

My gauge of wire is that type out UK (united kingdom) extension leads, I dont know how to measure it ?

Oscar

[rickharris]

If your 2 motors are wired in series they will need 2 x the voltage.  If wired in parallel  then 2 x the current.

Are they driving the same wheel? if not will they run at the same speed?

[oscarthompson]

I am wiring the motors in parrelel and they will run at the same speed. Hopefully!  The are driving 1 wheel each eg. 1 motor to one wheel

[rickharris]

No in general the relay will not draw very much current. In a later circuit we also went to using a relay as the high current through the switch rapidly caused problems with burning contacts.

Your 1.5 mm mains cable should be OK cable is measured by it's cross sectional area.

[oscarthompson]

Thanks, I'll upload some pictures of  my go kart I am building later.
I am wiring two, motors wired in parrelel that draw about 19 amps each, I guess i should be looking at a 40A relay?

[b2gills]

I have been involved in the Iowa Elecrathon, with the Real World Education team, although it has been a while.

You really should should use a Pulse Width Modulating (PWM) controller, because it lets you control how much amperage is running through the motor, which is really important when starting from a dead stop.

You may want to try to charge the batteries in parallel if you have a strong enough charger, which will reduce how long the batteries sit in a drained state. We have actually gotten our batteries to read 30 volts total, when completely charged.

For the motor I would go for a Lynch/Etek motor, the Lynch is hand assembled, while the Etek was machine assembled (no longer made). Another alternative is to get a controller and a motor from http://www.marselectricllc.com/

You could visit Cedar Rapids Kennedy to find some information on batteries, motors, motor control, and chargers. They have a very competitive team, I think they were the first to get over 35 mph.

[rickharris]

We race for 4 hours (used to be 6) and best distance in the time is the winner. Although we are allowed to use 4 car batteries we can not charge them. Typically the winning cars are doing 110 - 140 miles in 4 hours This includes the time to pit and change drivers at least 5 times (compulsory) Top speeds of 40 to 50 MPH are getting common so rules have been introduced to slow the cars down - Not fully covered cars for example. Good to see others getting into the area though it's LOTS of fun and not roo costly if the investement is controlled - I like the fact we all have the same motor/battery set so only the car/drivers make the difference.

[oniman7]

I have an Idea that you might try to lengthen the battery life. This is fairly complicated to someone with no mechanical experience ( me ), but I assume you could find a way to do it. Put an alternator from a car on the bottom and attach one to both the front and rear axles. Then hook it up to your batteries, and it should charge them as it drives. May not make it unlimited range, but I imagine you could get at least another 20-30 miles off of it.

[rickharris]

It is not possible to get something from nothing. driving the alternator would take more energy than it produces and so make matters worse.

If this was easy we would all be riding electric cars now.

[hbjngf]

haha "If this was easy we would all be riding electric cars now."

there's more to cars than regular transportation and ease

[rickharris]

As you say I invested a lot of time and energy - not to say money to develop a racing car even though I knew we would be 2 or 3 years before we stood any chance of winning.

The Greenpower contest and web site www.greenpower.co.uk/gallery/index.php has many good examples of how to do it. BUT easy it aint!

http://www.greenpower.co.uk/gallery/index.php

[TheIrishApe]

You could easily just add some solar panels or wind turbines such as some VAWT's, but add them kind of in the back to add less resistance. The turbines could lengthen the battery and the solar panels could probably just make the battery life unlimited take it that you will ride in the sun.

[rickharris]

Oh how I wish this would work. You could solve the worlds energy problems by connecting an electric motor to a generator, tap off enough electricity to run the motor and use the rest. It doesn't work - The energy used ot generate even small amounts of electricity would be more than you produce - It would actually use more power. We could (in principle) use a generator and pedle power it but again the human energy demands would be considerable for little gain. Light weight (including the driver) - low wind resistance, good smooth engineering/running, correct gearing are the main fators.

[rickharris]

Unfortunately - or perhaps not - we are strictly controlled by the rules of the contest.

We are allowed a push start - PWM has not yet proven in 10 years of this contest to be a sure winner at all. There is much thoguht that free wheel is good to allow you to boost and coast to reduce battery usage.

You will be able to get off load voltages of around 14 to 14.5 ish volts although it quickly falls back once under load.

Had a look at the sites - We wouldn't be able to run those cars as they are fully covered. Our rear wheels also have to be at least 500 mm apart. Otherwise our new car has many of the same body features.

Long and low.

2500mm (9 feet) long and only 700 mm (2' 3) high - that's about knee hight.

[rickharris]

Just had another quick look at the electrathon contest: In our race the drivers hardly ever use the brakes - coasting into the corners gets to be an art as braking just uses up power far better to free wheel to drop speed. We are looking for endurance so I guess it makes a difference and I guess regenerative braking could be worthwhile if the gear didn't weigh too much. after 20 MPH DRAG is the issue - An open cockpit creates serious aerodynamic problems that are difficult to evaluate and /or solve without a wind tunnel - HOWEVER Some schools have begged access to wind tunnels or build one - Strangely their results seem to be similar to those who manage without. Light weight is a common winning factor - A good aerodynamic shape - Well practised drivers who understand what they are trying to do and drive intelligently not just to get past the guy in front (which isn't the point at all). All of the top 10 cars have a very low static rolling resistance - give them a push and they go for ages with no power. They are all also very quiet when running - Good engineering. Good battery husbandry - Not letting them get too discharged - keeping them charged during the off season.

[crazyhalofreak]

i am building a car for a race- something like 1 mile or so. for the power, i was thinking of using a normal 9v battery and using a wall step-down transformer to bump it up to 120v. would this be viable for an electric vehicle, or would it drain the battery much faster?

[hbjngf]

you could use and inverter but it would still drain the battery ridiculously

[rickharris]

In principle yes - BUT as you suggest the inverter will have losses i.e. it needs energy to drive it - There isn't any simple way to do this.

If you study what the commercial systems do they tend to use special low loss motors coupled with BIG expensive banks of Lipo batteries or similar to reduce weight whilst giving a high energy density.

As a home build you are far better off with low voltage motors (and therefore high current -) and good old readily available lead acid batteries. around 24 to 36 volts seems to be about optimum power for weight.

Reduce the weight of the vehicle by using aluminium and a light weight body 9 Carbon Fibre is good or better no body at all. (see the aerial Atom - www.arielmotor.co.uk/full_screen_v2.html) accept that speed and range do not go together and go from there. It involves complicated engineering, a lot of time and development as well as spending some real cash. Sorry no easy route everyone would like it otherwise.

[rickharris]

No I am sorry. it's all about power. If you increase the voltage using a transformer then you decrase the current (Amps) available because the overall power available must remain the same, if it didn't you would be getting power for nothing!.

As an aside most wall warts do not use transformers any more they use a type of power supply called a switch mode power supply. and so wouldn't work anyway.

You don't say how big your car is intended to be - person carrying or a model??

[crazyhalofreak]

person carrying

[rickharris]

Sorry to say it won't work for you.

Assuming the car is not a small model.

A 9 volt battery has very little current capacity. they are intended for powering transistor radios and the like.

taking a transformer to increase the voltage will automatically reduce the current available to almost nothing. There is nothing for free!

You will need at least a 12 volt car battery - perhsps 2 depenfing on how big / heavy your car is and how fast you want it to go.

Even if it is a model you can expect to be using a battery pack similar to the radio control car batteries.

[crazyhalofreak]

would a tesla coil work instead of a switch mode?

[rickharris]

No sorry and neither will your next idea to get around the simple fact that moving any amount of weight required power.

Assuming you want to achieve a reasonable speed (say 20 to 30 MPH then you are looking at an electric motor of >250 watts. Typically a wheelchair type motor without the low speed gear box or one made for an electric vehicle.

To produce 250 watts for a fe mins you need either a lead acid battery (car type) OR a bank of other batteries capable of giving around 20 Amps of current at 24 volts or 40 amps of current at 12 volts  or for at least 5 mins.

This is going to be a considerable banks of rechargable batteries 9and quite costly) even the car battery here in the UK costs about $80 and lasts a year under race conditions.

There is NO way around this - if there was we would all be driving electric cars now.

[crazyhalofreak]

what if i took  6 d-cell batteries and hooked them up to the transformer, or would that not work very well either?

[rickharris]

correct.

[wee_man]

Around how much did this cost and can you tell me what currency its in and also how much did the gokart weight with motor adn everything in execpt the driver cheers

[rickharris]

IN GBP about £500 - this includes safety items like the 4 point harness, electrical cutouts, tip tilt batteries required by the rules of the contest. Weight around 80 -100 pounds 40 to 50 kg two of us can lift it easily - in fact it is often carried, turned on it's side to exit a narrow door by 14 year old students. I think with a little thought you could build a similar running car for under £100 without the motor cost if you try. - Tubular steel is cheaper than aluminium, Ply wood is even cheaper - look at how simple boats are made - look up stitch and tape boat building. Look at the greenpower site there are many hints there . The motor is the difficult part to get cheaply and there is no easy substitute for the wheelchair type motor.

[wee_man]

thanks i was thinking of making something similar to the one you made 24v motor etc etc. but more in the style of a gokart. but weight will be a problem alwell and i was thinking of putting a lawn mower motor in the back with an generater running off it just incase i was a long away from home and ran out of battery

[rickharris]

Mmm - why not just drive from the lawn mower engine? It would take hours to charge the battery once discharged.

Alternative? - and I haven't tried this - you could use a mains generator - there are plenty about and drive a mains motor again plentiful BUT the law will regard this as a petrol powered vehicle.

[wee_man]

but with a lawn mower motor the spinning bit is horizontal whick would be a pain to use. You could use a chainsaw motor but then would it have enough power and speed required?

[rickharris]

A standard type lawnmower puts out 3 to 4 Hp - Not a lot anyway - It would move you about but at no great speed. The drive direction depends on the mower - often vertical but there are ways to overcome that - e.g. Gears - belts. Chainsaw motor - No - Not powerful enough. If you have the mower engine and can get hold of a chainsaw engine and have lots of space to use the vehicle go for a hover craft - Lots of information on the web and great fun. Otherwise have a look at recumbent bikes. -

[wee_man]

i would rather a go-kart feel but i would rather use a eletric motor because petrol costs so much now. but its cool how greenpower is interesting people in the use of eletricty but i reacon i more countrys their needs to be a contest like this

[rickharris]

Of course I agree with you - Organisation costs and take a lot of effort so not many people are prepared to do it. Australia has a similar contest..

Perhaps if low cost - different - transport is what you need have a look at ground hugger An electric assist drive could be added.

You must remember that our car is designed for a race series and wouldn't do very well in normal road / traffic conditions. - if it were that easy every car company in the world would be making them. -

For the 4 hours we run and the 100 miles + we can cover it serves its purpose. In traffic it would be much less distance and would need to be more complex even as a basic go kart.

[DFree]

What about using a car alternator on the wheel in the back that isn't attached to the motor. It would charge the battery while you drive. I don't know if it would put out enough power to keep the battery charged since its always running on electricity but it would certainly increase the amount of time you have.

[wee_man]

(addingto joeofloath's comment)
The reason why no alternator is 100% efficient is because there is wasted energy.  In the motor (for say) 50w of energy is applied.  Some of that will be turned into heat and sound and finally the rest into kinetic energy.  Then theirs also friction with the tires on the ground. heat sound in the alternator too.

[quote]
When energy is transferred,
some of the energy turns into forms we don't want.
This energy is called wasted energy.
Wasted energy takes the form of heat and sometimes sound or light.
[/quote source http://www.gcsescience.com/pen20-energy-efficiency.htm]

[joeofloath]

It won't. No alternator is 100% efficient, so it will take (say) 50w to turn over, but only generate 10w. You can't generate power from no where anyway. You Use the battery power to move the car, slowing it down with an alternator is pointless.

[snipe8]

you can make your go cart be faster by. putting the bigger gear on the motor and a smaller one on the wheel

[rickharris]

true in theory - In reality the car wouldn't go at all because the motor wouldn't have enough power to drive the wheels with such a high gearing. IF it were possible the car would be capable of exceeding 680 miles an hour!

[snipe8]

lol. thats true

[kjjohn]

is there an equation or something for figuring out how fast it will go with a certain gear ratio? I am designing an electric motorcycle and I would like to know.

[rickharris]

Measure the wheel diameter - Work out the gear ration by counting the teeth on the gears - Gear teeth A divided by gear teeth B = gear ratio.

Once you know how fast the wheel is going in Revs per min and how far it is round you can work out how far the wheel travels in a min and so multiply by 60 for MPH.

HOWEVER your motor will have an ideal speed to run at for maximum power. For this you need a data sheet for the motor. e.g. mine was 2000 RPM at this speed it draws 20 amps. and produces maximum torque. So the gear ratios were chosen as a compromise between acceleration and top speed verses battery life.

A spread sheet would help you out.

[bmlbytes]

For a different type of speed control, you could add a potentiometer in series with the motor's positive line. This would give the motor a little less power when turned down, and will be better on the motor than "pulsing" the power switch.

[rickharris]

Sorry to disagree - The Potentiometer will absorb energy and release it as heat - this means that at the 70Amps our motor may draw the pot would need to be HUGE. to get rid of the heat. The push switch is simple and easy to use. We may like to a pulse width modulated system in the future to give proportional control.

[annodomini2]

Yup, the switching is necessary, the pot would have to be huge and expensive to handle the current. Also the system would permanently be drawing some power so the battery goes flat faster A Higher frequency electronic PWM system would be an improvement, but obviously more expensive.

[rickharris]

Some, in fact quite a few cars do use Pulse Width Modulation (PWM) to control speed, Mainly because this allows them to have a boost and coast system that is computer controlled so as the speed drops off only enough power is feed in to keep the speed up - thus using less battery overall. It certainly works - BUT so does an intelligent driver and an on off butten is the watch the speed and use it correctly.

[annodomini2]

A driver can't operate at 5-30Khz though

[al9595]

only one wheel is moterized doesn't that mean it will go in circles?

[rickharris]

All of the cars only drive 1 wheel so I guess the answer is NO it makes little difference. Thi prevents one wheel trying to go faster than the other round corners. Look up car differential to see the principle. We avoid the weight of a copmlex gear system by driving 1 wheel.

[al9595]

ok i get it

[jivard]

What's your recommendation for a good strong electric motor. To power, say, a Dodge Sport Truck? I'm looking to convert it to battery from gasoline

[Asiwa]

I really wouldn't go forklift. As much torque as their capable of, they really aren't the most efficient thing for electric vehicle conversions. I would check out a site like cloudelectric.com for vehicle-specific parts. They even sell kits for certain vehicles. I would use one of their motors.

[Blenderizer]

I'm working on that instructable right now :) The most common motor would be something out of a forklift, generally hooked right into the transmission. My instructable is a big one, but should be done in the next few months. We're converting a Chevy S10, but pretty close.

[Xamu]

How's the instructable coming?

[Blenderizer]

Slowly! We have the truck all ready, and we have the puchase order lined up, we just need money for batteries and a control board.

[rickharris]

I am sorry I don't really have one - A full sized car would be far outside my experience or skill. BUT I can tell you this: It will be horrendously expensive. Most commercial electric vehicles use 240 volt motors and LOTS of batteries, often Lipo which need careful handling. Look at it this way - Take the Horse power of the dodge truck - There are approx 760 Watts in a horsepower - To get the same power multiply your current horse power by 760 to give you a figure in Thousands of watts - our motor is only 250 watts. For such a heavy car this isn't practical.

[skunkbait]

Excellent instructable!!! I'll poiunt this out to my kids. We've got a couple of spare electric motors, and most of the other pieces lying about.

[Resident Expolsives Expert]

(removed by author or community request)

[JZ Price]

another compettion is FRC (first robotics competion) this is a international compettion. it hasnt' helped me spell, but it has taught me a lto about cadd, data systems. mechanims. electronics, etc. usfirst.org this competion is a big deal. if your team is able to win a competition you are almost garentied a full ride for a good college.

[rickharris]

Again we are Uk based - some years ago we entered - with a different team - a UK TV robotics contest - TECHNOGAMES - a sort of robotic Olympics with many different contests. We can third and second in the 2 contests we entered and got on TV. Sadly the contest didn't run after the 3rd year.

[rickharris]

Thanks for the information. That looks very interesting. Sadly though we are in the UK so it wouldn't apply to us. Our organisers are having a lot of trouble with finance this year as sponsors are dropping out due to the credit crisis. Hopefully it will get through somehow. although this year only about 1/2 as many races will be held.

[JZ Price]

FIRST is ""INTERNATIONAL"" we have faced a few UK teams during our competition. Those teams could use more people. Even if you can't make it to a competition. Seeing the Robot work in a garage/gym is amazing

http://en.wikipedia.org/wiki/FIRST_Robotics

Here are the links to the UK teams that i know of:
https://my.usfirst.org/myarea/index.lasso?page=teaminfo&team=1933&event_type=FRC
https://my.usfirst.org/myarea/index.lasso?page=teaminfo&team=1884&event_type=FRC

Most teams would be glad to have a few more sets of hands to work on the robots. This years build season is over, but there is next year. And it would be best to ask now, to be sure you can be apart of the other teams, that is if you can't come up with the overly large sum of money required.

[Resident Expolsives Expert]

(removed by author or community request)

[rickharris]

Perhaps some day when the financial world improves. A transatlantic electric car contest would be interesting !!!

[carbon12 based]

Hi, Did you do any calculations for the frame or just did a qualitative analysis. By looking at the pictures, you have the motor driving "one wheel" am I right? How does this impact the steering and handling? Would it steer to one side? If so, maybe getting some camber and toe in on the rear probably would fix that. You would loose some power due to the increased friction however that maybe worth it if you are constantly having to steer it back in a straight line. I know for simplicity reasons a Diff is not used and not enough power for a fixed 2 wheel (Drift action!). Great design wish i had this when I was in school, you must be like god to the students.

[rickharris]

The basic chassis came as a kit - It was developed by experimentation and is far stronger than it needs to be although it is very light compared to the weight of the motor and batteries. Driving one wheel makes little difference to handling and removes tyre wear. Driving two wheels is also more complicated - in general more complicated meanes heaver something we try to avoid.

[David Loll]

You could use a trick to gain aerodynamics the way wheelchair raceing pilots do, they use 2"wide packaging tape. Make some formers out of the same foam you are now using and tape in two or three layers to cover the body.pull it tight and keep it smooth, you can get some complex shapes very easily. Another consideration would be to use three wheels instead of four. with your CG that close to the ground tipping shouldn't be a problem,in that conifguration you would loose 1 wheel 2 bearings 1 brake and put the drive wheel in the middle of the car, and contouring the tail would be easier. Those are thoughts for consideration looks as though you and the kids are having fun.Keep up the good work with the kids.

[jesse blaha]

how fast can it get up to?

[rickharris]

Our car last year peaked at about 26 MPH - The fastest cars could hit 50 MPH This year we hope to be able to complete the races with an average of 25MPH which will call for 40 MPH peak. Higher spees generally means shorter range so there is always a trade off. If you want to drag race over a short distance - a few 100 yards for example - then you can go very fast. 80 MPH +

[stasys]

could you post details on the steering!?? PLEASE or do you race on a straight track?

[rickharris]

"do you race on a straight track" :-) - NO - The drawing below show the steering mechanism in a little more detail. If you study that and the photographs in the instructable you should get the idea of what we did. The steering is done with 2 levers one on each side - the drivers quickly get the hang of it and it is MUCH simpler and safer than a steering wheel. It also makes for more room to get in and out of the car. The materials are 3 mm steel with a 12 mm bolt through the steering as a king pin. A search for gokart steering will throw up some other ideas as well.

[balisticjoe]

It's not very eco-frendly but I hear that fiberglass resin and nylon make a strong plastic like body that is fairly cheap.

[rickharris]

It is indeed, very strong and light. Often used in larger model aeroplanes. For the most part using standard fibre glass would be enough - The only drawback is you need then to makes a complicated mould for the car - This may well be beyond most people.

[jopey12]

Very interested in your racing formula. We also have a eV challenge - see http://evchallenge.swantafe.wa.edu.au/
Read some informative comments & answers.
What are most teams using to make the body kits? Also what material for the screen?

[rickharris]

Most cars use fibreglass, a few carbon fibre. One winner even used canvas and bamboo to make a very elegant body. Ours used aluminium 1.5 MM it's fairly cheap - The whole silver body cost about £30

The aluminium body is very light and was hand formed over the chassis with little drawing but a lot of head scratching. Cut out with a jig saw and lots of pop rivets.

Greenpower will give you lots more ideas for bodies

The screen is perspex (clear acrylic_ easily available at DIY outlets here in the UK. Bent with a hot air gun over the edge of a table.

[Jalakahops]

Is aerodynamics really a concern at 20mph? I would think overall weight would be the biggest factor in a vehicle of this type. I guess without a wind tunnel or some intensive math you wouldn't even be able to tell how much the wind would slow you down to tell me how wrong I may be about that. Great instructable though, makes me want to build one for my kids.

[rickharris]

Weight makes a difference to acceleration. As we are doing an endurance race this is less of a problem. The subject of drag/aerodynamics is a very active one. We do have bricks that do fairly well but never in the top 20 - lightweight undoubtedly helps to reduce batter usage and so increases range. The designs of the Shell eco marathon cars are ALL highly aerodynamic and they rarly go over 15 MPH - Many have been designed by universities and commercial organisations who have wind tunnel access and they seem to find advantages in low drag designs. i agree that the conventional wisdom is under 15/20 MPH no need for aerodynamics over a great advantage - BUT we shall see with our new car in September. I have to confess much is just Bling to make a car that looks good.!!

[rickharris]

We do have a fairly simple way to measure drag and improvements - by measuring the motor current - less drag = less current.

There are other ways to measure drag easily without a wind tunnel detailed on the web. Much can be done empirically rather than using complex maths etc.

Build one - its good fun.

[jexter]

Very nice project! Thanks for posting the Instructable.

Also, your link to Greenpower has a "." after "uk", rendering it invalid Here's the good link: http://www.greenpower.co.uk/about/

[rickharris]

Thanks for the correction

[wee_man]

im thinking of making one of these im currently designing mine and its going to have 48v bike wheels speedo other things kind of like the blue one above but a fully inclosed cockpit what do you think

[rickharris]

Wheels over 20 inch dia are liable to fall apart with the stresses. Bike wheels are not intended to stand sideways stress and work best on 2 wheels. 20 inch and below seem to survive. The rest of your ideas are fine. Personally I would go for an aluminium body like our car in the finals.

[wee_man]

i was making mine offroad/onroad mostly offroad

[vanbo]

Add some form of suspension and this would make the single most AWESOMEST death trap EVER.

I must try this. How would you go about putting basic (not off road, but not racetrack) suspension in this? Feel free to contact me.

And just out of interest, did you fiugre out the range? Thanx for a great i'ble

[rickharris]

Suspension is tricky - Look at bike suspension parts as they are simple and fairly cheap. Oceall it has no advantage if the road is smooth and if the road is rough the electric car isn't powerful enough.

[rickharris]

on 2 batteries - about 30 to 50 miles depending on how smooth - how flat - how fast.

[screemrhed9]

ah, that would do it, we're racing on pretty mucha nything thats smooth enough to have a track run on it, even if its dirt, and awesome about the new welded frame, you do it yourself or get an outside source to do it?

[screemrhed9]

youre not allowed to cover the cockpit? i woulda thought that you could sice its a race, or is it a distance race, not a circuit race?

[rickharris]

Rules say no body over the drivers head so they can get out quickly (6 seconds) in an accident - bear in mind they do once in a while break out into flames, generally wireing faults.

[screemrhed9]

i could imagine wiring faults.

[rickharris]

We have the facilities to weld the sub chassis at school although the main body/chassis has been built from plywood using fibreglass stitch and tape techniques as if building a plywood boat. The body you will see will be carved from polystyrene foam and we intend to use the photo below as a model. Although this car isn't a greenpower car we know it is highly aerodynamic and effective. although we have to be a little higher and are not allowed a covered in cockpit.

[panzer]

where did u get the wheels from??? i'm havin truble findin some thnks panzer

[rickharris]

Try Northern tools Wheels

Although my wheels are designed for a wheel chair. Our new car uses 4 x 20 inch standard BMX bike wheels. Also uses a 3 speed gear box to better use the motor power.

[screemrhed9]

the pop rivets didnt snap or wobble a lot? 'cuz i do green kart racing here in the states, and i pop rivetsed my frame the first time around, and my frame nearly shookitself apart on the first lap.

[rickharris]

Seems OK - remember we are racing on tarmac rack tracks so fairly smooth although without suspension the drivers say different. New car is welded!

[maxpower49]

how big is the moter not horse power but the dimensions

[rickharris]

About 173 mm long by 100 mm diameter

[maxpower49]

can you get a picture of the fins on your motor or if possible a picture of the watter cooling system

[rickharris]

Of all the pictures I have none show the motor heat sink clearly - I will take some tomorrow. We do not use water cooling although many cars do. In general a mist of water frequently is better than soaking it once in a while. The cooling effect is by evaporation not by sitting in a pool of water. A regular squeeze spray gun can easily be modified to put the spray head at the end of a pipe to spray onto the motor. IF you choose to wrap some absorbant material round the motor to hold the water remember IF you run out of water this becomes a good insulation blanket.

[robonut123]

Cool car this looks like it would be a lot of fun to do but what kind of battery is use and how can you recharge it.

[rickharris]

We have to follow the rules of the competition so are limited to using standard car batteries. They are charged with a standard battery charger as for a car.

[rickharris]

The second version was better and the third will be WOW. It uses a 24 volt motor so 2 regular car batteries - it can do about 30 to 50 miles on the 2 batteries at average speeds of 30 to 50 MPH. The race is 4 hours long so we have to change batteries half way through. We use a regular commercial car battery charger - takes 14 hours per battery.

[cool hand]

yeah, but to power a 24v motor requires a 24v battery and usually cars have 12v batteries. with a motor that is DC compatible it is impossible to do so because even with 2 or even with 100 batteries in DC the power output would still be 12v. so how did you power it?

[rickharris]

As qdog says the car carries 2 x 12 volt car batteries. They are wired in series i.e. pos on one to the neg on the other then the 2 free terminals to the motor. This gives you 24 volts (really it does) The batteries are charged from a standard car battery charger one at a time - It takes 56 hours to charge them all (14 hours each) from scratch. The batteries will last several years IF you take care not to discharge them too low - Below 11 volts and you are at the limits - below 10 volts and you may say goodbye to the battery without special charging equipment. Charging from a car alternator or dynamo would call for some thing to drive the alternator or dynamo - this isn't going to be very simple - in addition the car system isn't intended to charge a battery that has been deep discharged but rather to provide a constant small charging current to replace the losses from running the car.

[cool hand]

Well, yes, you 're right about the battery wiring. It's a pity that you cannot adjust the speed smoothly. Anyway I have already started building the chassis on my own same project. Thanks for the info. It's a great Instructable that I plan to work and experiment on the whole summer. Thanks again!

[skaltura]

Look at brushless systems and LiPo. However the amps you are pushing are very far from even near the limits of NiMh. I haven't worked on this scale of systems, infact 1/10 scale cars, but on these, more power is being used than this on the top end. Older technology is limited on them to under 300watts however. But with modern technology, on sane bucks, you can already achieve 4kW and driving time of hours (as you very very rarely use even nearly the max). Under 1,000USD you can buy an 1/10 RC Electric Car with all you need, which easily outruns a nitro car, top speeds of over 100km/h, or around 65mph. And that's seriously scaled down stuff, weighting under 1kg total for the electrics, imagine 2-2.5kg RC car cruising at 65mph ;) You need scaled up for this, or either seriously short gearing (like 1:13 final drive), and preferrably multiple small motors, if you use what's found for RC cars. Look at Castle Creations, they are releasing new ESC, which can pull through continuously multiple kWs, and motors to go with it :) Also, those speaking of generators to use as motor --> Change the timing, it's all in the timing with brushed motors, however, no matter what kind of brushed motor, it's simply inefficient. Unfortunately, it seems you are restricted on the batteries & motor :( Fortunately, there's A LOT of things you can do besides these to increase overall efficiency and speed :)

[qdogg]

To those wondering about how to get 24V out of 2 batteries: EASY-- connect them in series. Plus on one to minus on the other, then the remaining 2 cables go to the motor. How do you think a 3 volt flashlight works using only 1.5 V batteries? The same way.

[cool hand]

rickharris i wanted to ask you a few things about the project because i'm working on my own these days. i've had a little trouble with the battery/motor system. did you use car (12v) batteries to power the car and if you did how could you power a 24v motor with two 12v batteries in DC? i just don't get it. apart from these i just wanted to ask if you have any ideas for a recharging system with a dynamo or even directly with household electricity because here in greece batteries are not cheap and i don't want to spend money every, like, 2 or 3 weeks for new ones. thanks a lot

[jakesllama]

well i soppose it would be good for turning right... sweet video by the way did you enter the contest? i would love to build an electric car but i only have wood scraps... tah

[rickharris]

everything is possible - the car cost about $100 to build. It could have been cheaper and a LOT more expensive.

[TheMadScientist]

rick, may I ask if you know how to convert a belt drive type electric motor to a chain? the belt mechanism is seemingly friction welded to the shaft, but I have a strong dislike of belt drives...

[rickharris]

Your best bet is to drill the Pulley and a suitable gear and bolt them together. A lot of people do this to make the input gear on a standard 3 speed gear box bigger. At the power ranges we use this works well 4 or 6 1/2 - 3/8 bolts depending on the size of the gear.

[jakesllama]

sorry i ment what contest did you enter?

[rickharris]

The contest is run for British Schools. The cars are built by and raced by 11 to 16 year old students (12 of them) over a 4 hour race. The aim is to go the furthest in 4 hours on the power in 4 car batteries. This is a fair challenge as all the cars use the same batteries and motors. Everything else is up to the designers. Top cars do 40 to 50 MPH and will travel around 110 to 130 miles in 4 hours. It was started as a way to get young people interested in engineering We were number 48 in the video the all aluminium car. We had some problems - a puncture which lost us a lot of time and finished 64th - Still for the first year this was very good. Next year top 10.

[maroadrunner]

There is a similar contest in the United States. It is called Electrathon. People build electric cars and race them on 2x12v batteries over one hour. The goal is like above to go the farthest in that amount of time. As far as I can tell it is very similar as the best cars go 40 to 50 mph too.

[rickharris]

We race for 4 hours on 4 batteries - the top cars get over 100 miles. Similar contest great fun get a team together and take part - it's not that expensive or difficult.

[jakesllama]

Wow 50 mph not bad. Im british too but im in america and home schooled so im just building one for the fun of it. I have a few more questons how did you get a motor that big? Do you think i could do 4wd? (i know it wont be good on fuel but i live in a forest and only one wheel would be a problem...) and finally is it dangrose because volts that high can't be safe thanks jake

[rickharris]

Framco make the motors to go onto electric wheelchairs. but a lot of companies make 24 volt electric motors around 250 to 350 watts. You need to do some local research.

4wd is theoretically possible but would be complicated to do easily, also expensive because of all the gears.

2 Car batteries give 24 volts - this is not dangerous IF you are sensible and understand how to deal with the batteries and care for them.

In your case you would more than likely be better off thinking about using a lawn mower engine to drive a small buggy - there are plans on the web. to build things like This

[jakesllama]

Perfect you have been great help to me thanks a bunch i actully had an idea for you're contest if you carry 4 battery paks then when two of them are dead you could charge them with a generator (or dymo) that can collect some spare electric from motor that way you could possibly use only three battery paks (if you didnt alrady think of it) good luck with it any way thanks again Jake

[rickharris]

Not allowed in the contest Greenpower will give you a better idea of what is required and possible.

[nerys]

Holy crap man I need some information from you !! 67mph on TWO BATTERIES !! Here is what I need. I need 65 miles drop dead range (IE can not be shorter than this) and here is the kicker I need 50mph. Acceleration is not an issue if it takes me 30seconds to get to 50mph US thats FINE as long as it has to guts to get there and stay there (almost flat level drive to work 54 miles each way charging at work NO problem) 24volt rocks!! thats only 20 D cells I could make a 50amp hour pack from 100 cells for under $500 !!! Where did you get the motor what does it cost? Do they have a 36v version? (I am 400 pounds so think two person car but for one person) All I need is to go 65miles (75miles would be safer) at 50mph - thats it !!! Please if you can provide some sources prices etc.. for your components that would be so helpful!!

[nerys]

67miles not 67mph my typo

[keebmn82]

I might have missed it but what size gears did you use and where did you get them? I have a Jazzy power wheel chair that I'm going to strip down for the motor, speed controls and batteries. The way gas prices are going I'd drive this to town to pick up grain and what nots, I only live a mile out. I have a elec scooter but they don't come w/ a lot of room to carry very much...Appreicate any help. Here's a photo from the Rhoades website

[Yerboogieman]

looks like a truck from a LONG time ago

[rickharris]

The motor runs at 2000 RPM at the shaft. the motor gear has 11 teeth and the driven gear on the wheel 44 teeth. The wheels however are only 12 inches in diameter.

The choice was made from calculations to acheive a top speed at 2000 of about 30 MPH - Actually we seem to ba about 28 MPH peak speed from road tests so the gearing needs to be raised slighthly - 1 or 2 teeth.

Many engineering suppliers will provide a range of gears which they sell to fit to electric motors. have a search in your area because any addresses I give you will be UK based and not a lot of use.

The greenpower site has a number of links to UK suppliers where you can get a feel for the type of product you need to be looking for.

Remember we are running very light! and get about 40 miles from our 2 batteries - If you load it up the range will drop dramatically (that's why ther e are, as yet, few dedicated electric cars on the road)

[keebmn82]

I plan on adding a couple more batteries and solar panels, but all that will have to wait til the snow melts. The bike is actually buried under a snow bank. Thanks so much for your help. Keith

[markiscool]

where did you get the motor from

[rickharris]

In our case the Motor comes from a Uk firm called FRAMCO - They make motors for electric wheelchairs so look around maybe you can pick up a spare from somewhere local. Low voltage motors are getting more common with electric bikes getting popular etc.

[mitchell12]

Just wondering if you could use a motor to drive each rear wheel. That way you'd get the turning radius(i think) and still have power to both wheels for straight driving.

[rickharris]

We are not allowed to use 2 motors. The rules of the race series we have built the cars for are very strict.

Imagine you have a car built by 11 to 15 year old students that is capable of 40 to 50 MPH. You go to a race track and put 11 year old drivers in the car!!!

It has to be controlled and safe or we couldn't do it so very strict rules - See [http://www.greenpower.co.uk. Greenpower]

[mitchell12]

Oh... Makes sense then. Great job and best of luck in future projects.

[crisl001]

super snail will win

[turbohacker]

Why not use a belt instead of a chain as it uses less energy? Would the torque of the motor be to substantial for for the belt and cause slippage? Just a thought, excellent vehicle by the way!

[rickharris]

Belts have been tried by other cars but slippage is a real problem - if not at first the belts stretch during the race and keeping them tight is a nightmare for little gain other than easy to use pullys and quieter running. the chain works fine especially if you use an1/2 inch industrial chain rather than a bike chain.

[Johenix]

Just out of curiosity, have you considered using a two wheels forward steering,one wheel behind driving design? It was used in the "Panasonic Oxyride Battery Car" powered by 2 AA size "Oxyride" batteries for over a mile. The car was driven by a 90 lb. secratary. Since then they have a new battery and a new car. Are your batteries standardised as to type? I would recomend rolled foil battery like those made by Gates?

[mitchell12]

Plus in turns the inside wheel would have less draw so more power available to the outside wheel? I dunno just guessing at this stuff but this is really kool.

[robonut123]

Where did you find your 24 volt engine at and how much did it cost, I also noticed that on the back wheel for your design the motor was only hooked up to one wheel wouldn't this cause it to go in circles.

[rickharris]

The motors come from a Uk firm called Framco - They are a 250 watt 24 volt wheelchair motor without the gear box. Driving 1 wheel reduced weight and complexity and provided a sort of differential. No probs with turning right all the time! Also no gear box - This adds weight mechanical complexity with little gain as experimentation has shown. A choice of final drive sprockets for different tracks could be valuable though.

[jakesllama]

if you connect the wheel to only one wheel wont it go side ways? (nice car)

[rickharris]

Not at all. This arrangement is essential if you want to corner efficiently, otherwise one wheel would fight the other and waste energy.

As you can see from the Youtube video at video

The car works fine - any minor self turning effects are automatically corrected by the drivers who do not report the car pulling to one side at all.

Wait for next years model! now that will be good. in the build at present. Sleeker, Faster and that cool I want one.

[KevinSaw]

This might be a dumb question, but how does this work out if the motor/chain is only attached to 1 of the back wheels? Won't it just make the thing go in circles?

[rickharris]

Evidently it is OK! - Actually it is necessary - if you drive both wheels when you corner one wheel will scrub on the ground slowing you down and using more energy from the battery unless you have a differential (that is extra weight that we don't want) - there is a slight tendency to turn the car - the drivers compensate automatically for this.

[incorrigible packrat]

Nifty car. A fairly easy speed control could be realized by rigging up some sort of multiple contact switching dingus, that will switch your battery configuration between parallel (12V high amps) and series (24v lower amps). I'm not in linear thought mode at the moment, or I'd draw a circuit. A bonus: the low speed configuration provides higher amperage for more toque... torque, rather (POS keyboard has malfunctioning R key) -mind you, toque is a nice word too... A caveat: the switch contacts need to be pretty substantial,for the current involved. I'm thinking maybe repurposed electrical panel switches, or flattened copper pipe or something.

[rickharris]

Weight issues here - also reliability and the loss of battery power as heat in the resistor pack - see above for PWM solution

[incorrigible packrat]

Were you replying to my post? I made no mention of resistors.

[rickharris]

Sorry I read into your comment more than was there - but the essentials remain the same - simple on off seems to do the trick

[truk]

A large potentiometer (variable resister) would be a heck of a lot easier though and allow for almost infinite options within the cars capacity, rather than two. It really needs a gearbox to add some extra acceleration though (I know I know, minimal weight)

[mondaymonkey]

Im not familiar with electric cars very much, but doesn't the rule that a DC electric motor will deliever maxiumum torque as you have less rpms but same power in? I interpret this to mean that adding gears won't do much unless your overreving your motor which i find hard to do.

[rickharris]

The rules forbid revs over 2000 the motors rated output. BUT hard to police! However the idea behind gears is that: In an endurance race acceleration isn't much of an issue but consistent lap time are. In order to get to 100 miles + in 4 hours we need to be lapping at about 30 - 35 MPH to account for stops etc. On a flat track you could optomise the motor speed with a suitable gear and just go with that but in the real world of up and down you find yourself in many cases with either a motor that is being loaded and so revs and speed drop off or in a downhill situation where you could pickup speed if you were able because the motor is unloaded. A simple cycle gear system gives you that possibility. In addition you need to be as aerodynamic as possible to reduce motor load as well - making sure you use all the battery power available is important as is not running out of power on the track - Ooops!. Of course gears are no good without the instrumentation and skill to use them effectively. An addition that comes with cycle gears is a free wheel allowing boost and coast to conserve batter power.

[rickharris]

Would use power in the form of heat and so reduce endurance so if a speed control is required the Pulse width Modulation (PWM) is the way to go. electronically switching the motor on and off very quickly.

[thrudd]

PWM motor speed controller is definitely the way to go. You may be able to coax a simple module from someone in the industry since the basic ones are nor very expensive and fairly easy to set up. Start / stop / speed control. A clutch and a belt drive may be something to look into. A lot of industrial equipment use belts instead of chain drives. If you look at an older design lath you may see a practicable and inexpensive variable speed drive system.

[esrun]

It is www.greenpower.CO.UK not .com :) This is a really inspiring write up and I'm glad I know about greenpower now but it's a shame you didn't go into more detail.

[rickharris]

I have changed the web address - thanks. It is difficult to know how much detail to put in - a lot can be obtained from looking at the greenpower site - BUT what else would you like to know in terms of details? I will try to oblige.

[bop96]

nice use of duct tape.

[rickharris]

Ok I should justify the duct tape - We built the basic chassis between easter and June 2007 - My workforce then broke off to do their exams and left school bringing the project to a virtual halt. Intending to do some further body work - fiber glass at home in the summer holidays I put off doing any body design work. The holidays got taken up with bad weather in the UK and holidays so no body on return to school in September - Trials set up for 11 Sep and only 2 weeks notice so the body had to be fast and easy hence the paper, plastic sheet and duct tape. To be honest I didn't think we would do so well in the race and qualify for the final as this was the first year - in fact just finishing would be a good result. New all aluminium body, much better streamlined and aerodynamic is now in construction ready for the final on 14 Oct and not a strip of duct tape in sight (i rather miss it somehow) Pictures to come.

[alienwear]

what about a differential?

[rickharris]

Valid but not necessary - With a relatively low power electric motor laying rubber down isn't an option so we just drive a single wheel thus doing away with the need for a diff and saving weight! Lots of gokarts use the same idea.

[alienwear]

yes but gokarts have both rear wheels driven and skid steer

[rickharris]

Not as far as I know - when I say gokart I mean a racing machine designed to enter recognized races. They all use correct car type steering as does ours minus the steering wheel and don't use a diff.

[alienwear]

See here Kartbuilding

[alienwear]

I meant the rear axle is solid and the rear wheels skid round the turn

[rickharris]

Whilst I am in here, we qualified for the finals 47th out of about158 - not bad for the first attempt. Final 14th october.

[T3h_Muffinator]

This is awesome! Btw... where's your school? I wanna transfer, STAT! Amazing work, Keep it up!

[rickharris]

In the UK :)

[T3h_Muffinator]

BLAM!

[Easy Button]

Any chance you could put off road tires and go off road with it? With out breaking it.

[rickharris]

It isn't practical you wouldn't get any reasonable battery life out of it. This is intended for fairly smooth racing tracks.

[Easy Button]

Ok,thanks.

[brainiac]

yha you would half to add shoocks and universil hinges

[garageprinter]

You may want to try using Superkote or something similar for your "skin". It's used for model airplanes and gets heat shrunk over your frame so it's nice and smooth. The UK has a rich heritage of skin on frame aircraft, from the Spitfire to the Mosquito. Reach back into history and use it for the future.

[rickharris]

thanks for your comment and suggestion. interestingly we had a meeting of the build team to discuss what we had learned from our first real race and the suggestion was made to re-body the car using such a film, other perhaps less costly possibilities were ripstop nylon over a bamboo frame and or perhaps doped cotton such as on fabric covered aircraft. A little research shows these to be covered in dacron - have to find out the price. Could well be a possibility - if so I will add to this instructable. Thanks

[thrudd]

You might want to look at what was used for the various long haul and human powered vehicles and aircraft. (makes a great research project for the students - historical strength weight compromises, etc) Or to get rid of the framing all together and go pure monocoque. This is the model car builders favourite by vacuum forming the body shells in one piece, though something the size you are working with it may be best to make it up from discrete sub panels / sections.

[rickharris]

They tend to use fairly exotic materials that cast the earth. Whereas this isn't necessarily cheap we don't have an endless budget.

[nicholast]

If you wanted to be able to run more current through the system you could use an eTek Brushed Motor. They can take175 Amps at 48 Volts without over heating. I'm using one in my Electric dirt bike (80cc)

[rickharris]

Like it or not we are limited by the rules of the contest. We all use the same motor and the same battery type.. This brings differences down to vehicle design and driver skill.

[Xamu]

You say, "The motor runs at about 80 deg C in normal use but if stressed, e.g. pulling away from a slow spped too often, ..." But I also notice you are not using a simple clutch in place of your small drive gear. Granted, that would not give you great torque off the starting line but it might reduce the strain on your motor. Whaddya think?

[rickharris]

The slow speed tends to come from going into corners to slowly and loosing speed. We try to get the drivers to carry speed through the corners by taking the racing line and whilst the wheels point straight ahead keep the power on. The start can be pushed off. The motor draws about 35 amps in normal use but can draw up to 70 amps if the car slows down. I agree gears would resolve this but there would also be a lot of gear changes and weight issue - Bike gears tend to be unreliable when they are heavily stressed.

[Xamu]

Hi Rick,
I think you may have read more into my observation than I thought was there. I wasn't suggesting a gearing system.

I was thinking of replacing your drive sprocket with a centrifugal go kart clutch. When used with gasoline motors, at idle the wheels do not receive power. As the power increases the clutch engages. Currently, your motor is direct drive to the wheel. That must be a bit jerky as the power slams on and off. A centrifugal clutch like this would probably put much less stress on your motor. It would also make it much more "youth friendly" as it would apply the drive power more smoothly. Not a thrilling plant-you-in-the-seat feeling but you would be much less prone to damaging your motor.

If you want gearing without a gearbox you could replace your chain and sprockets with a torque converter and have the effect of a clutch and variable gearing without a gearbox, shifter, clutch pedal, etc. Not the cheapest way to go but sounds like it would achieve many of your stated goals.

[rickharris]

Thanks for the links the TC looks a lot like a DAF vairomatic gear box. Weight is always an issue. Even in a tight race you only get to use 3 gears 1:1 + perhaps 30% and - perhaps 25%. The power to weight is very tight. Not much more than a normal bike.

[rickharris]

Thanks for the suggestions guys - The racing situation is rather complicated as outlined below: The race series we have entered is an endurance race over 4 hours. We use 2 sets of 2 batteries (4 x 12 volt batteries in total) Because the target is endurance (70+ miles mid field 100+ miles if you want to win) you have to build very light. Somethings can not be controlled the motor and the battery type are fixed by the rules and all the same. The variables are the car and driver. Some, in fact many cars use bicycle gears to try to better spread their motor energy but so much depends on the track, is it flat for example and the weather wind etc. there is a weight penalty to using gears as well as a reliability issue. The start allows a push start so a clutch or gear box isn't totally necessary and direct drive cars have indeed won in the past. we chose this for simplicity in our initial efforts but ar looking at perhaps a Rohloff gear box - although expensive it will give 8 gears allowing a dead start and the ability to match the motor current with speed and track conditions. To use the gearbox effectively we will have to design and fit instrumentation to tell the drivers the state of the motor - current - temperature - speed - road speed -battery capacity, all of these effect the way you use the gears. All of this is more weight and complexity. Some cars have sophisticated telemetry fitted to radio data back to the pit crew to allow them to manage their car better - again complex and costly. There are many things we could do - However we always have to think - Will the "improvement" use more power than it saves. Will it be cost effective - Will 12 year old drivers be able to use it in an effective way, Will it make us go faster - further - longer. If the answer anywhere is no than we don't do that as the disadvantages outweigh the advantage. For my current thoughts we are a little slow and I would like to raise the gear ratio by 1 or 2 teeth and see how it goes. Other wise our drivers are instructed not to keep the on switch pressed to the "floor" but to try to use the battery power in an economic way by pulsing the motor manually. In the long run some of the above improvements will get incorporated (watch this space) One thing many point out are the wide wheels - high drag so go to bike tires - What they dont realise is the tires are very hard and the running patterns show no more tread touched the road than on a normal bike tire - Without going to ultra narrow racing slicks we would gain only a little and have puncture and unreliability problems we don't presently have.

[Paulcet]

Love the project. I hope my son's school will do something similar when he gets to that age. The only suggestion I might propose would be an ammeter. The driver can monitor the meter and adjust his/her driving accordingly. In fact, your driver is the PWM controller!

[njzammit]

have you thought about a centrifugal clutch like the ones in chainsaws etc?? a suggestion.. why dont you make the throttle and brake as 2 small simple foot pedals, should make driving on the whole easier as hands would be left only for steering.. oh yes another thing.. do you have any air ducts for the motor?? nice rose joint works on the steering as well i liked it.. anyways great little car best of luck in further races.. wish i was a driver!!

[monkeyplate]

holy crap. your car is actually being held together with duct tape. thats amazing.

[njzammit]

well i know a guy with a racing car with a suzuki 4cyl bike engine doing about 150 mph.. still duct tape and cable ties hold the body down.. very strong actually..

[rickharris]

well the bodywork is - We had to remove much of it after a practice day to realigne the rear drive system and in the time available duct tape was the only alternative. We appear to have qualified for the finals so we need to put together a new body over the next 4 weeks - aluminium or minimal weight rip stop nylon on a bamboo frame a decision yet to be made.

[raytruant]

Nice project, and a really nice activity for kids. But if this is to fly here in North America, we'll have to figure out how to make it burn petrol and make smoke. Electric Karting is gaining momentum in Europe, and current lithium polymer powered karts are capable of 160km/hr. Strange to see a race that is as quiet as a swim meet. Looks like the battery can be charged with a solar plate (or we can use a diesel generator here with the muffler cut off).

[jongscx]

I'm assuming you also had a TIG welder in the shop somewheres as I see some joints in the back that would be hard to explain with just rivets and JB weld... Also, if you don't mind, I'd really like to see some more detailed pics of the steering system... I'm probably just missing something really simple, but I don't quite understand how you tied the vertical steering sticks to the wheel bearing...

[rickharris]

Another picture of the linkage assembled out of the car is on the steering page. if you still want closer detail I will have to take some more pictures tomorrow.

[rickharris]

Can do on the pictures the rear was professionally welded as the roll bar is a critical safety feature. all of the rest is pop rivets.

[whatsisface]

This is awesome! Great project.

[omnibot]

This is great!!! I'm totally gonna steal this design for my VeloCar-project.

[bowmaster]

DUCT TAPE RULES!!!!!!!!!!!

[gyromild]

Gotta love those duct tapes.. : - )

[Patrick Pending]

Great project and a good instructable. Pat. Pending

[Grimling]

I like your project very much , it's nice for kids to do such things , and make them familiar with technologie meanwhile ! The only thing i don't really see it = Why didn't you use a slide pot meter to control the motor speed? Isn't hard to use , isn't hard to install , but it does save battery if you use it well , and you can drive slow if needed without having to break

[Tool Using Animal]

Pots work by turning the excess electricity into heat, very inefficient. You can get high current PWM speed controls fairly reasonably now from the hobby industry, but, full on/full off is a cheap way to got. Brilliant project sir, nice to see kids doing something this cool.

[rickharris]

Thanks have a look at the greenpower.com site there are lots of better examples there

[Tool Using Animal]

Typing in greenpower.com takes me to a juicer site

[rickharris]

Aha try

http://www.greenpower.co.uk/about/

sorry

[rickharris]

Actually in practical terms a variable motor speed isn't necessary - The motor draws 20 amps normally and up to 70+ amps under stress so a direct pot in line would need to be a very high wattage. Electronic motor control is possible but not really necessary (it's also relatively expensive)