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Electric Vehicle - A simple lightweight EV platform

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I've been looking forward to the arrival of electric car technology. Not just for the smooth quiet power, wide torque range, cheap running costs and minimal maintenance, but to design and build cars to take advantage of the simplicity and flexible packaging offered by electric technology. Some new knowledge and skills of battery and electric drive systems are required, but once you have an understanding of this, putting it all together is much simpler and quicker than using a petrol or diesel drivetrain. Why? The motors and controllers come ready to bolt on and plug together and all the sensors required are usually built into the motor.  Cooling systems, fuel storage and pump, gearbox, differential, exhaust system and complex wiring harness are not required. There are a few more components required to deal with high voltage electrical power, but other than this, it is really not much more complex than building a radio controlled electric car. With less complex mechanical systems to worry about, building your own car has become more achievable and more fun.
Electric car components have been available for a while now.  A friend converted his first electric car 10 years ago then changed it to lithium batteries 6 years ago. The technology is now becoming more readily available, costs are coming down and performance is increasing and this trend is set to continue.
Why build your own? Because you can and it is great fun to build a very light, simple car reasonably inexpensively.  It is a heap of fun to drive and has excellent performance because of the light weight.  Buggies, gokarts, grass roots racers and kit cars such as Lotus 7 clubman style car that is still going strong since the 60's have spurned their own industries.  Electric cars bring new opportunities for a fresh look at homegrown performance.
 
Concept and Design
This Instructable provides a summary of a basic layout for an EV platform that suits a wide range of applications and can be easily tuned with different size motors, batteries, gearing and size. It demonstrates a simple and compact system with a low centre of gravity that is strong, stiff and straightforward to build. The Instructable does not go into the design and fabrication of bodywork, I will leave this to others and your imagination. It is pretty easy to see that this rolling chassis is very flexible in the bodywork it could accommodate, but keeping the body light will maximise performance and range.
Key Design Parameters
When designing a new car platform from scratch, there are a lot of choices. A lot of thought and design effort has gone into keeping the design as simple, light weight and very easy to build - simpler than a Locost or clubman style car.
I will get straight to the point here and outline some of the key design features and why.
Drive - Rear wheel drive, one electric motor powers each rear wheel. Eliminates the need for a differential and CV joints.
Motors - AC Induction. Have good torque over wide speed range. Simple and robust with a motor controller for each motor. Mounted inboard.
Batteries - Large lithium cells. I used 45 Lithium cells for a total of 148V and 100Ah. This needs to be matched to the motors and controllers. This is a relatively small pack compared to production EV's vehicles but is ample for a car that is light and is not used for long range driving. Keeping the battery pack size down helps keep down the vehicle weight and cost. My large lithium cells are good for a peak current draw 3 to 5 times the rated hourly figure above (3C to 5C). Lithium polymer cells are available that have a higher energy density and will do much higher peak currents than this and they are commonly used in model cars and planes, but at present the large lithium cells are a lot more economical for larger packs and the 3C peak current is not a major limitation unless you need a high peak demand such as for drag racing.
Chassis - Folded aluminium box. The batteries are contained in the box which also handles all the vehicle loads. This is the key to a simple, light and very easy to construct car. It provides a high level of strength and stiffness from a very simple and light structure.
Suspension, Steering and Brakes - Double wishbones were used and they are the best choice for a number of reasons including lower height for maximum flexibility in body design, height adjustable again for flexibility in body styles and optimum handling performance. There are numerous vehicles that can be used to source suspension and steering components. I used parts from a Mazda MX5 (Miata) which has front and rear wishbone suspension and rear wheel drive so all the parts could be obtained one source. It also has 4 wheel disc brakes and a straightforward steering rack. Using mass produced parts helps streamline the project, keeps costs down and ensures that these important items are robust and reliable.
Gearbox - Nil. The electric motors have such a wide torque range that they will operate effectively with one fixed gear. I use a toothed drive belt at a ratio between 1:3 and 1:5 for smooth quiet and maintenance free transmission. A chain drive would also be ideal and would be lighter and cheaper but a little noisier.
Weight - The weight of the EV platform including motors and batteries is approx 500kg. Major components of the weight come from the batteries (150kg), wheels and suspension (140kg) and motors (118kg).
Vehicle Platform - A vehicle platform is basically rolling chassis with drivetrain installed. It is drivable and just needs some bodywork to complete the package. I avoid any body styling discussion in this Instructable and rather present a very flexible platform that will suit a range of body styles.
Driving the Car  -  With one gear and heaps of torque (300Nm from the twin motors) Driving is simple and effortless and the car rapidly gains speed and without a body the sensation of speed is greatly exaggerated.
 

 
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MJDQLD16 days ago

Thank you so much for taking the time to be a pioneer in Australia and step outside of the box for a new direction. I came across your project on the ev-power website. I am new in wanting to create my first EV and I'm excited to be able to read this. I won't talk tech as I'm just happy to look at the theory of your design choices so i can understand more later.

As a starting Man Arts/Graphics teacher this is a project I'd like to be able explore with students and put some money behind it to make something very similar to your platform with an intent to make it road legal in AU. Your earlier comments are very valuable and I'd be interested in following the progress of this design if you ever did want to put a body on.

I agree with your choice to make sure that there are 2 design projects and making the choice to document the design of the chassis first. This is effect may lead you down the path of early car design. From memory (I am probably wrong) Holden was a coach builder before they built cars. This probably means that early examples of their "cars" were them building coachwork over someone else's chassis. 4WD vehicles still use a body on frame approach and from memory so do existing urban vehicles (Busses and trucks).

Without investigation of the rules and further investigation and nailing down of my own design requirements I am just wanting to look to your platform as a basis of ideas. I have the idea buzzing in my head to look to creating a EV platform that can be used to drive me to school, be a technology demonstrator to my classes and peers and be used as a rolling home battery pack to be plugged into the house. I cant help but think that my "ideal" battery pack may need to be bigger but I may just be greedy (or wanting more range).

Great outcome that I can't help but feel came from the use of the ever popular KIS design philosophy. If you are the same author as a school project EV challenge in WA then top marks for both.

Ganhaar (author)  MJDQLD13 days ago
Yes my school EV Challenge car instructable is called 1HP Electric Car. I also have an instructable on redesigning the gokart as an electric microkart with the batteries housed in a composite timber deck that doubles as the kart chassis.

It is all about keeping it simple and using a separate body and structure allows for one simple platform to be used for a wide variety of purposes. The big auto companies incorporate the vehicle body into the structure, but it is not practical to build a one off or small volume like this. Most small volume cars use a chassis of some type, mostly different to the ladder chassis that utes and some 4wd's use such as space frame or composite tub.
In my design, I have chosen to further develop an idea that as far as I am aware first emerged with the first Lotus Elan. The Elan had a torsion tube down the centre of the car to which the engine and suspension was attached and a fibreglass body was fitted. Fifty odd years on, it is now practical to make a larger torsion tube from pressed aluminium that contains the batteries for an electric car and electric motor/s simplify the fitting of the power source.
Yes, one of my future projects is to build a coupe of bodies for the EV platform and I have a stack of foam in my shed waiting for me to find the inspiration and the time.
Ganhaar (author) 18 days ago

Have had a query about sizing motors. The size of the motors depends on the size of your vehicle and the performance you want to achieve. There are three key factors. Acceleration, top speed and climbing ability.

Acceleration. This can typically be calculated using the short term or peak power output of the motor and while it is possible to calculate power required from first principals using newtons laws of motions plus drag forces, I would suggest working from a graph that shows empirical data for power to weight ratio versus acceleration. A graph that I have put together from data published in car magazines is included in the previous comment.

Top Speed and climbing ability both are longer term power requirements and are typically limited by the maximum temperature or the motor. They are calculated using the long term or rated power output of the motor. Top speed can be calculated from aerodynamic drag formula i.e. Power = Cd x V^3 A / 2g (S.I. units) where Cd = coefficient of drag, ranging from about 1.5 for a motorbike, to 0.3 for the most streamlined production cars to about 0.15 for a highly streamlined solar challenge car. V, velocity m/s. A, cross sectional or frontal area and 2g is a constant 2 x gravitational acceleration = 19.6m/s^2 at sea level

Climbing ability can be estimated from first principles using basic newtonian physics i.e. potential energy = weight x height and power = energy / time.

Real life will be a combination of all three of the above plus losses including electrical losses, drivetrain losses and rolling drag of the tyres so probably add on at least additional 15% to the power required.

Ganhaar (author) 2 months ago

Stuart,

The size of the motor depends on the performance you want. I have attached a graph below that you could use as a starting point, but there is also more to the performance equation e.g. top speed which is influenced by gearing and aerodynamics. Examples are for a 12 second 0-100kmh time you need a weight to power ratio of about 15.5 kg/kW and for a 6 second 0-100kmh time a weight to power ratio of 7 kg/kW. For a 500kg vehicle this equates to 32kW for a 12 sec 0-100 and 70kW for a 6 sec time. I should be able to do 12 sec with one motor or 6 sec with two motors. I am currently a bit slower than this but my gearing is too high. With lower gearing and bodywork with an average aerodynamic efficiency, I should achieve these times.

I would recommend sealed motors to prevent ingress of dust and dirt. Many of the bigger motors come with water jackets that give better cooling and allow you to run the same motor at a higher output. Air cooled motors may deliver a similar burst power, but yes cooling is important for sustained high power outputs. A more efficient motor will also heat up less and running a motor at higher voltage will also generate less heat for a given power rating because the heat generated is proportional to the current draw and a higher voltage will give a lower current draw for the same power output, however as voltage increases, higher performance insulation, controllers, switches, fuses etc are required and greater safety precautions are required for dangerous voltages.

regards

Wayne

Car Performance Graph weight power ratio to 0 100 time.tiff

This is a great post indeed. You probably want to try this book which shows you how to make your car run forever without recharging the battery. It really works:

http://www.ivantic.net/Energija/fuelless_engine_50-350hp.pdf

I'm pretty sure that if this worked, it would be breaking one of the fundamental laws of nature, the Law of Conservation of Energy.

Suffice to say, I'm skeptical.

Stuart212 months ago

Hi Ganhaar, would 1 motor (this size) be enough? i.e. have you tested the output you are using, compared to the capacity of the motor?

If you were to go to one motor, what hp / kw would you use? (Anticipating load plus bodywork, in the future)

I think you said the motors are sealed, which is great for noise reduction, but any temp rise issues?

TIA, Stuart.

The only problem i see with this is the motors are not protected against dust and dirt. it could effect the motors.

Ganhaar (author)  Dashing Rainbow Dash3 months ago
The motors are sealed. Putting additional protection around the motors will keep them cleaner but it would reduce cooling and thermal performance. Heat is the biggest enemy of electric motors.
What about water cooling?
CJSchecter968 months ago

very cool Itd be cool to make one of these someday. would it be possible to have energy recovery where like, you have a motor on the front wheels but not being driven, so when you accelerate and the axle spins the motor produces electricity and puts it back into the battery? or a wind turbine-esque turbo?

Ganhaar (author)  CJSchecter968 months ago
A generator creates drag when generating power. Try shorting out the three thick wires on a brushless motor and you will see how much drag that can be created. The extra drag created will be slightly more than the power generated as these things are not 100% efficient. Similar result with a wind turbine although it will generate energy from the wind. Sorry no free energy. Better idea is to connect your car to solar panels or a wind turbine when it is parked in the garage to recharge the batteries.
regards
Wayne

Use solar panels or a wind turbine on the front of it. That way, it doesnt add drag to the wheels.

henryjimenez9 months ago

NIce design.. this are a big solution, im working in a small software for a electronic diferential over raspberry pi. Congratulations from Colombia.

Henry

Ganhaar (author) 11 months ago

Update on energy efficiency.

Measured energy consumption over normal driving on gravel roads and am achieving 0.74Ah/km or 106Wh/km

Squidyman11 months ago

so how much did this all cost?

Ganhaar (author)  Squidyman11 months ago

about $15k. Major costs are batteries $150 x 45 100Ah cells = $6750 and motors (with controllers) are about $3000 each.

Wheels, hubs, brakes, steering was only $500 because I bought a damaged car for $1500 and sold off engine, gearbox and computer for $1000.

Chassis was about $400 in materials including pressing the aluminium box.

wrsexton1 year ago

In researching further, I have a question about the bearings/mounts for the drive half shafts. Realize this car is together, but a photo would be useful. Also, type/brand of bearing you used and how its mounted might help me visualize this.

Ganhaar (author)  wrsexton1 year ago

I have added a sketch to step 2 showing a section at the rear pulleys and halfshafts. I don't have a photo and don't have any plans to pull it apart in the immediate future.

The inner bearings are mounted in a steel bearing holder with a flange and four bolts. A standard bearing face mount block would be ideal, but I had a lighter steel one that I recycled from a previous project. For the outer bearing block I machined this from recycled plastic to be larger without getting too heavy for spreading the loads into the thinner wall of the chassis box.

I think they were skf metric bearings, but any standard bearings are fine. The loads on these are not high, no side loading like the wheel bearings, they are just there to hold the shaft and tension the belt.

I see. A pair of bearings for each half shaft. That makes perfect sense and is, no doubt, a very trouble free solution. Thanks for your response.

dk_20131 year ago
Awesome work! Sorry for a bit stupid (newbee) question, but how do you think - is it possible to use instead of electric accumulators diving cylinders connected to electrogenerator to run these motors? I understand that direst air engine probably will provide more efficiency in converting energy of pressed air to car engine movement. But these equipment can decrease the cost of energy storage part for this kind of automobiles, provide all advantages of electric motors and can be used in something like publik transport applications (when vehicles moving on predifined routines and can be refilled with pressed air at end stops with compressing stations). And the all equipment is widely used (diving cylinders and compressors) and well tested.
Ganhaar (author)  dk_20131 year ago
Never built a compressed air car but I think you will find there are two major hurdles to overcome - volume of storage cylinder and heating / cooling when the air is compressed and released.
Yes, these are a lot of different pneumatic cars (serving pneumocars) which uses the energy of compressed air directly in pneumatic motors. Just interesting, is it possible to use hybrid scheme, but instead of internal combustion motor to use pneumatic motors like this - http://www.grainger.com/category/air-motors/pneumatic-motors/pneumatics/ecatalog/N-brn for example and standard diving cylinders. This will take more space for compressed air cylinders, but it will allow to increase speed of charging (with compressor stations) and decrease cost of energy storage devices, like batteries (by using instead of them cylinders). Ok, nevermind. Thanks for answer :) This is just my common suggestions.

Compressed air invlolves a lot of friction, and is very lossy compared to batteries.

ANDY!1 year ago
That's one heck of a EV! Just wondering what sort of master cylinder you used for the brakes, vacuum assisted or manual. I'm working on an EV myself and have to work on modifying the brakes...
Ganhaar (author)  ANDY!1 year ago
I used the Miata vacuum assisted cylinder because I had a spare one off the car that I used for wheels and uprights. It has been working fine without the vacuum assist connected, probably because the car is much lighter (around ½ the weight) of the Miata, however they are a bit heavy and I have looked at fitting a vacuum pump which is available for around $200 suited to EV conversions. I haven't gone this way as the philosophy of the car is to keep it as simple and light weight as possible, so f I decide to go ahead with a brake upgrade it will involve removing the vacuum cylinder and increasing the mechanical leverage of the brake pedal or fitting a refurbished brake cylinder from an older car with unassisted brakes which will have a smaller diameter thus a larger hydraulic ratio.
If you are converting a car and the original car had boosted brakes then you probably have little choice but to fit a vacuum pump for compliance with vehicle licensing requirements.
Ganhaar (author)  ANDY!1 year ago
I used the Miata vacuum assisted cylinder because I had a spare one off the car that I used for wheels and uprights. It has been working fine without the vacuum assist connected, probably because the car is much lighter (around ½ the weight) of the Miata, however they are a bit heavy and I have looked at fitting a vacuum pump which is available for around $200 suited to EV conversions. I haven't gone this way as the philosophy of the car is to keep it as simple and light weight as possible, so f I decide to go ahead with a brake upgrade it will involve removing the vacuum cylinder and increasing the mechanical leverage of the brake pedal or fitting a refurbished brake cylinder from an older car with unassisted brakes which will have a smaller diameter thus a larger hydraulic ratio.
If you are converting a car and the original car had boosted brakes then you probably have little choice but to fit a vacuum pump for compliance with vehicle licensing requirements.
shonoe181 year ago
I'm 13... I'm going to try and tackle this project so by the time I'm able to drive..15-16... I was wondering what lightweight body I could put on top of the model? Any thoughts
Ganhaar (author)  shonoe181 year ago
I quite like the Porsche Spyder (e.g. Chamonix) or a Sylva J15. Both small and lightweight for a road body, although I am doing something more like a cross between a go-kart and ktm crossbow myself at the moment.
Kiteman1 year ago
What sort of range, speed & recharge time are you getting so far?
Ganhaar (author)  Kiteman1 year ago
It uses around 1% of the battery's charge per kilometre on unsealed farm roads and off road, so that equates to 100 km or 60 miles per charge.
For my application and testing, the range is a lot more than required around the farm and the pack was sized more to ensure safe peak current draw than range. The advantage is the extra pack capacity gives a mobile power source around the farm.

Speed is ridiculous for farm tracks and haven't explored the upper end of the speed range, this will have to wait for track time. On the tree lined farm tracks I wouldn't go more than 100km/h or 60mph. Estimated top speed is around 160km/h or 100mph but dependent on the aerodynamics of the body as the car good low end torque geared for a top speed of 200km/h (1:3 motor to drive ratio, 6000 rpm motor speed) although it will not be able to reach speeds this high without a low drag, streamlined body.

The recharge time is about 5 minutes per 1% of charge or 8 hours for recharge from fully discharged using a 10amp 240 volt single phase battery charger. The charge rate is not linear and as the battery approaches fully charged, the charge rate drops off.
espdp2 Ganhaar1 year ago
I love this project. Over the top, and yet I'm tempted to think "I could totally do that!" Fantastic documentation as well.

What about length of driving time? Assuming you are just doing "normal driving" (I know, impossible...), how many HOURS of driving around the farm roads before it really has to be recharged? Thank you.
Ganhaar (author)  espdp21 year ago
Driving around the farm is very much short drives and stopping and starting frequently and doing jobs along the way, so a bit difficult to say. Would be guessing about 1.5 hours of continuous driving.
Awesome, thanks.
wrsexton1 year ago
Perhaps I missed this somewhere, but with two motors, I assume you use two controllers? If so, how are the linked to a single throttle? Or is your controller one controller operating both motors?
Ganhaar (author)  wrsexton1 year ago
two throttles bolted together to operate as one feeding two motor controllers is the current solution because my electronic skills are very basic. An electronic throttle splitter would be more elegant and I am hoping to find someone who can help point me to a circuit diagram or supplier.

I guess Google really is my friend. Once I read your description of what to look for, a search for two motor controller gave me this thread: http://www.diyelectriccar.com/forums/showthread.php/two-alltrax-controllers-single-throttle-84495.html. You'll probably have to cut and paste it as I don't think hyperlinks work in these comments. It references the attached diagram. Thank you for inspiring me to learn!
DWG-ONE-THROT-TWO-AXES.jpg
To put in a hyperlink use the rich editor.
lelelevi1 year ago
Roughly how much did this total project cost you in USD if you don't mind?
Ganhaar (author)  lelelevi1 year ago
About $15k AUD for a fully driveable rolling chassis, which is similar to USD but I think you would be able to source parts cheaper in the US. When considering costs of an EV keep in mind that your running costs are a lot lower but capital is higher. Also we are talking recent technology, early stage of mass production, all new drive train and battery components so costs are very reasonable.
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