1HP Electric Car

This instructable gives an overview of building two small cars from the ground up, both powered by electric bike motors.  The build includes  chassis, fibreglass body, steering and drive train.  The cars are designed for efficiency to extract the maximum performance from a 1hp motor and were used to compete in an electrathon competition.  It can be built for around $1500 including motor, controller and the latest lithium batteries and will go flat out around 50km/h for 1 hour before recharging.
Similar in size to a gokart, Roadbot 3 and Roadbot 4 have streamlined bodies, minimal frontal area and low rolling resistance tyres.  They are powered by brushless DC electric motors and a lithium battery packs.
They were built to compete in the Perth EV Challenge in Western Australia by the River Raiders team. 

Step 1: Planning and Models

After doing some drawings and measuring up the drivers, a scale foam body model is made from polystyrene foam and a full scale ergonomic study is made from plywood.  The shape of the sides shows how the vehicle profile will look and identifies a couple of places we can shave a bit off the cross sectional area.

This year we are using 20” wheels and are building 3 and 4 wheel cars.   The 20” rear wheel, down from 26” in previous years allows a more reclined driving position which reduces the frontal area.  The 20" wheels are also stronger for side loads than large bike wheels (note a bike does not apply side loads to a wheel when cornering) but have a slightly higher rolling resistance however at speeds over about 30km/h, reducing wind drag becomes more important than rolling resistance.

Step 2: Building the Body

The body is made in the following steps
   1.  Drawing up plans to determine the smallest aerodynamic package
   2.  Make a foam model 
 3.Make a full size plug
   4.  Make a fibreglass mould from the plug
   5.  Make a fibreglass body in the mould

The Plug
It started off looking more like a flintstone’s car!  I found an old pine log in the paddock that was big enough to carve a plug for the body from and cut two large slabs, one each for the top and bottom car body.
Reasons for using a log are twofold, firstly cost - had an old log in the paddock so doesn’t cost anything other than the time and secondly the timber will give a stronger more rigid surface than foam to mould onto.  Timber can be formed easily with woodwork tools and can be finished easily to a good quality surface.  After the plug was completed it was sprayed with gloss varnish and given several coats of mould release wax and sprayed with a pva mould release film

Fibreglass Mould
The plug was cut longitudinally into 4 quarters.  See photos to clarify how the plug was cut.  This enabled the plug to sit flat on a melamine particle board which provides and smooth, flat surface for mould joins.  For the first quarter mould, a second smaller melamine board was used at right angles to the first board to create a second mould joint plane.
I won't go into any detail on fibreglass layup as there are other good instructables on fibreglass.  We used three layers of chopped strand mat with polyester resin for the mould.  In the corners the glass matting needs a gentle radius so we mixed wood fibre (sawdust from sanding) with some resin to smooth out any tight bends before laying up the glass matting.

After the mould has set (allow a few days to harden) bolting holes were drilled along the joints before removing from the plug.  The holes will be used to align and assemble the mould after the plug is removed.

Body
The mould was polished and wax, reassembled in two halves and the internal surfaces coated with mould release agent.  Don't rush or skimp on this step or the body will stick to the mould and be very hard to remove and may damage the mould.
After waxing and applying pva mould release, one layer of woven mat and one layer of chopped strand matt were layed up in the mould by hand.  A team of three people made it easier to complete including handling cloth, working quickly with resin before setting and wetting out the cloth and eliminating air bubbles in the fibreglass. Its a smelly, messy job but there are very few alternatives for a cheap, light, strong, curved body skin. 
After release, trim the flanges and cut a hole for the driver.  

Step 3: Steering

The steering follows basic ackerman principals, that is the steering pivot points are in a line between the steering pivot (king pin) and the centre of the rear wheel.  This results in a different steering angle for each wheel - the outer wheel having a slightly large radius and smaller angle when turning than the inner wheel.  If there was no allowance for the different steering angles between the inner and outer wheels the car would still work OK but there would be tyre scrub when turning and would not be as efficient of fast through bends.

A model was made using ply and nails to represent steering joints to test the steering geometry before fabricating.

The steering was made by welding a stub axel turned from 14mm rod onto a piece of 14mm steel tubing.  The inside diameter of the tubing was 10mm, neatly pivoting on a 10mm (3/8") bolt.  The stub axel pivoted in a U shaped bracket bent from 4mm steel bar and the back of the U bracket was welded to a 25mm (1") square hollow tube.  

Looking at the steering from the front, the steering pivot (king pin) is not vertical, an angle of about 10 degrees (king pin angle)  reduces steering scrub and makes the steering self centre, improving stability.  From the side the king pin is angled at about 5 degrees (caster angle).  Also the wheels are not completely vertical, they have a slight camber.  Camber is usually provided on road cars so the tyre sits flat and has more grip allowing for some roll when cornering.  In this case the reason for camber is very different.  We are not looking for grip, but a little bit of camber helps offset some of the side loading on the outer wheel when cornering, a bit like a bike leaning into a bend.  



There is no suspension here - its designed for a smooth track.  Creature comforts are not important, efficiency is the main goal.

Step 4: Chassis

Here keeping it simple is best.  The weak points are probably the welds, so we keep these to a minimum.  This also help make it quicker to build.  Essentially the chassis comprises two 25mm (1") thin wall square hollow steel tubes, one each side of the driver with another for the axle at the front.  The three wheel car has a couple of tubes welded on like a horizontal rear fork.

The four wheel car is more complex.  It uses a small chain drive diff from a tricycle supplier.  It also has swing arm axles with rubber stoppers for suspension.  The suspension is not for comfort, but is required to keep all four wheels on the road.

Step 5: Drive Train

The obvious source of motors for a small electric car are the bike kits that are cheaply and widely available.  They range from as low as 200w to over 1500w.  The motors usually come matched to a controller and a battery pack is used to suit the voltage and range required.

The  first choice to make is whether to use a hub motor or a chain drive.  A hub motor is simpler and more reliable, but cannot adjust the gearing and tends to be limited to about 40km/h maximum speed.  With a chain drive motor, the gearing can be adjusted and speeds can be alot higher for an aerodynamic body.

We used an 800w brushless dc motor and the lithium battery pack voltage and size was determined to suit the rules of the competition which limited the battery pack to 36volt 5kg maximum.

The use of standard bike components meant that chain sprockets and brake discs could be bolted direct to the wheels.

Step 6: Bringing It Together

The bottom of the body rivets directly to the main chassis tubes and the top half of the body joins by pivoting at the front on a repurposed door hinge.  

Add paint, graphics, a windscreen to complete a simple electric car.

Step 7: Testing the Drive Train

More Testing Footage

Step 8: Race Day

Roadbot 3 (three wheeler) was noticeably faster than Roadbot 4 despite identical motor, controller and battery combinations.  The extra cornering grip and stability of an additional wheel could not make up for the additional drivetrain and aerodynamic losses of a 4 wheel vehicle.

Roadbot 3 was fastest on track setting a lap record and still had battery capacity at the end of the race.  Roadbot 4 was about third fastest on track and the battery pack and could not use its full speed if it was to make the full race distance because of the extra current draw from the motor.

After a botched pit stop (rules require two mandatory pit stops) where the next driver was missing, Roadbot 3 recovered ground to finish second overall.

Picture (l-r): Wayne, Rod and Jamie, the River Raiders Team

Acknowledgement:  EV Power supplied batteries and battery management for the cars.

2012 ITC Global Perth EV Challenge Official Event Video
http://youtu.be/i2-fg-HefL8

River Raiders Race Footage

Comments

[charliebike58]

Hello Ganhaar,
my compliments for you ecar, could I know please where can I buy that kind of wheels? Thank you.

[Ganhaar]

Samagaga from China produces the wheels and the diff for 4 wheeler

[Ganhaar]

Hi yes the wheels are great, not only look good but the aluminium spoked design gives reasonable side loading strength that is generated with 3 and 4 wheel bikes. I ordered them online from samagaga. It was the same supplier who provided the rear diff for the 4 wheel car. If you do order some i'd appreciate if you let him know where you came across it as maybe he might want to link to the instructable. thanks Wayne

[sjors_02]

Impressive instructable!

For a school project I need to build a similar vehicle, and I saw this one and it is just how i imagined it, my question is: can you guys send me the schematics and detailed build instructions for this project? I know your instructable was made 2 years ago, but i thought let's give it a try, You can contact me on [e-mail], I also made a render in Fusion 360 showing what I'm trying to make. Sorry for my english, I'm dutch

[sjors_02]

I meant 4 years ago

[T0BY]

Very impressive!

[BryceK4]

Hey Ganhaar,

Firstly, thank you so much for this instructable, it does a REALLY good job. So, I'm starting an electrathon team this year, and I just have a couple questions.

I'd like some more info on the race you competed in, specifically, length of time that the cars had to drive for, were there any hills on the track.

On the subject of batteries, I have to use SLA batteries, but I would like to know the specs of your batteries, and the voltage of your system. I would also like to know amperage draw of the motor in your system, if you know that off the top of your head.

Finally, I would like to know your thought process when you determined your angles for steering (camber, caster, and kingpin). I would also like to know how exactly you built your steering to align with your Ackermann angle. I understand the principle, but I want to know how you determined your measurements.

Thank you in advance!

-Bryce Karlins

Peachtree Ridge HS Electrathon

[mnelson14]

Thanks so much for putting this on instructables. We are just attempting to put our first electric race car together and need all the help we can get. Cheers!

[Ganhaar]

Thats great, good luck and let me know how you go.

[carscarscars]

What is the name of the motor you used?

Thanks in advance!

[Ganhaar]

The motors came from Cylone Motor in Taiwan

[leo.gomez.568847]

Congrats! nice job you did with this vehicles.
what diff are you using for the 4 wheeler? can you let me know where can i find one? been trying so hard to get one but i only find atv ones.
Greetings from Mexico
Leo G.

[Ganhaar]

Got the diff from Jeffrey Samagaga at www.samagaga.com
It was about $100 new including axels.

[Ganhaar]

36V lithium (lithium ferrous phosphate chemistry). Batteries limited in the rules to 432Wh and 5 kg. We have tried a few different types of batteries such as pouch cells, but for this event we used 11 headway cells.

Wayne

[rajaram23495]

Kindly give the battery spec please..

[vipinbatheriyil]

please I need more informations about this car

[Ganhaar]

Happy to answer specific questions,

[Ganhaar]

Happy to answer specific questions,

[vipinbatheriyil]

please I need more informations about this car

[vipinbatheriyil]

please I need more informations about this car