Electric Vehicle - A simple lightweight EV platform

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Picture of 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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Step 1: The Chassis

Picture of The Chassis
chassis box section.tiff
The chassis is one of the few items you need to fabricate. The majority of items are sourced and attached to the chassis. We use a pressed aluminium box from 3mm thick aluminium sheet that doubles as the main structural spine and the battery box. The approach keeps fabrication very simple, maximises rigidity, keeps the weight low down and concentrated in the centre of the car and keeps the battery pack away from damage in impacts.

One of the disadvantages of using a thin walled box section is that large concentrated loads cannot be applied directly to the aluminium walls. This is easily overcome by using tubular steel subframes or bulkheads to spread loads. The tubular steel subframes are relatively small and are not difficult to fabricate.

Aluminium Box
The chassis box requires a large press to bend up. Because it's a simple box, your local metal roofing supplier should be able to supply the material cut and pressed to size.. I sourced a 3m long box, 270mm high x 300mm wide made from a single 1.2m x 3m x 3mm sheet complete with lid for $300.
The width of the box is wide enough for 4 batteries across plus a thin ply lining. The ply lining helps to protect the batteries, stop any drumming noise being transmitted through the chassis, ensures rivet heads don't rub on the sides of batteries. Attach the lining to the aluminium chassis using sikaflex automotive or marine polyurethane flexible sealant/adhesive.
Note that the lip on the aluminium box serves not only as a place to fix the lid, but also strengthens the structure when the lid is not fixed in place.

Inside the chassis box, a series of bulkheads, either aluminium or ply plates are fitted. The bulkheads have a number of functions. They support the chassis box against buckling, support batteries from acceleration and deceleration loads (including an impact) and provide reinforced mounting points for heavy components such as motors and seat frames or floor.

The box also needs a lid that can be opened to access batteries, but it needs to be securely fastened as an integral structural part of the box. While riveting would give a quick and strong attachment, it is not suited to testing and development requirements of a custom vehicle. A suitable alternative to using rivets is using rivnuts and socket screws. 6mm to 8mm rivnuts and screws are suitable. They should be spaced reasonably close and stainless rivnuts and screws are not recommended as the threads tend to bind. I have used 6mm socket screws at a spacing of 50mm. I do not recommend using stainless socket screws and rivnuts, having learned the hard way when several of the socket screw threads binding and needed to be drilled out and replaced.

A drawing showing dimensions for a chassis box is shown above along with an Autodesk Inventor rendering showing the basic chassis layout. The chassis box design is optimised 45 CALB 100Ah lithium cells fitted four wide.

Suspension Subframes
External subframes that slide over the box section are used to mount the suspension. The subframes are welded up from 25mm x 1.6mm steel box section. They are attached to the aluminium box using structural rivets from inside the box. An angle attachment for your drill or a right angle drill is essential for drilling the rivet holes this and a pneumatic riveter is needed to apply sufficient pressure to set a structural rivet as you will not fit a large manual style riveter inside the box.

Structural rivets such as Megalock Rivets should be used for attaching subframes. In Australia they will probably need to be sourced from a specialist supplier such as Profast. At the time of writing there was limited availability of structural rivets on Ebay, but suppliers such as Profast will post out supplies. A pneumatic riveter is available online starting from under $100 and is needed for the higher pressures required to set structural rivets, particularly in confined spaces.

The front and rear subframes mounted to the chassis box are shown above. The suspension mounting points are visible in the photos. The angle of the outer members matches the suspension mounts.

Step 2: Electric Drive

Picture of Electric Drive
farmbot drive x section.JPG
The car uses two electric motors mounted on a subframe that sits on the chassis, driving the rear wheels via belts. There is one fixed speed and keeping the belts (or chains) inside the chassis box keeps them clean, minimises maintenance and protects fingers.

This platform is easily adaptable to virtually any motor, either face or foot mounted.
I have used AC motors which are 58kg each. They are a good price for their size and have excellent torque over a large rev range. BLDC or permanent magnet AC motors will give a higher power to weight but are more expensive for an equivalent power output at this size due to the cost of large rare earth magnets. My AC induction motors were sourced from EV Power and came with controllers. Brushed motors are an even more economical option and an Etek / Mars style motor such as the Motenergy ME0709 are available from around $600 each.

The motors also require a motor controller. I won't go into detail about motor and controller selection here as there are other good resources available, however most motor suppliers will offer a motors either with a controller, or will recommend suitable controller options. A purpose built motor controller for a vehicle drive system is designed with forward and reverse and often regen programs. They typically also have switch inputs that will be compatible with a 12V ignition switch to switch the controllers on and off. Connecting up a motor controller is about as complex as fitting an aftermarket stereo to a car, although you need to be very particular about not making any errors or there could be smoke and tears.

The motors transmit power through a toothed belt to the rear wheels. A 30mm wide Gates GT3 will transmit up to 30kW peak power and run smoothly and silently. Chain drives can be cheaper and lighter and provide a higher torque rating. The use of a belt or chain drive eliminates the need for a gearbox as the electric motors have more than adequate torque for single speed operation, particularly with a light weight vehicle.
I am using two AC motors put out a combined 300Nm of torque and with a 1:3 gear ratio from the motor to the rear wheels and while the acceleration from a standing start is excellent, it is currently geared for a very high top speed, thus even better low range performance could be achieved with lower gearing.

Rear Axle
The rear axle comprises two half shafts bolted to the original mazda drive shafts. A large pulley is mounted directly on each half shaft and is driven from a small pulley mounted directly on the motor.  The half shafts are supported by a basic bearing each end, approx 20mm inner and 30mm outer bearing size with a pulley mounted keyed onto the shaft.  The diagram above illustrates the rear axel arrangement.  The half shafts have an outer flange welded to the shaft and machined to match the rear drive shaft flanges of the MX5 / Miata.

There is no interconnection between the two driveshafts or motors - each drives completely independently of the other. The torque characteristics of the electric motors inherently distribute torque between the driven wheels, thus there is no requirement for any differential.  This arrangement will also facilitate simple development of true control systems in the future (torque vectoring) 

HOW TO....
Order your selected motors and controllers
Order small pulley or sprocket with hole and keyway to match electric motor
Order drive belts or chains
Order rear pulleys with a ratio between 1:3 and 1:5. Ratios can easily be changed and experimented with and the best ratio needs to consider the motor selection, target vehicle speed and acceleration characteristics. The larger rear pulley will typically require a taperlock bush. This makes it easy to fit the pulley firmly onto the driveshaft and to change pulley ratios in the future.
The only specially machined items in the drivetrain are two half shafts that have a flange at one end to suit the drive shaft flanges from the suspension donor parts. A large rear pulley or sprocket is mounted on each half shaft and the half shaft is supported with a bearing each end attached to the chassis box and rear suspension subframe.

Step 3: Seating

Picture of Seating
A pressed aluminium floor box is riveted on each side of the chassis box and a 25mm x 1.6mm square hollow steel tube extends across the chassis at the front and rear of the floor box to assist in spreading loads across the chassis so the riveted joints on each side work in unison. Alternatively a frame for the floor boxes hangs over the chassis box and sits on rubber pads making fabrication more suited to interchangeable modular components and providing additional shock absorption for a smoother ride.

It is critical that the passenger cells are well attached to the chassis as considerable loads are encountered in the event of an impact. For the direct riveted floor, loads are shared across a large number of rivets, providing considerable shear strength. Where a floor box subframe is used with a couple of attachment points to the chassis box, reinforcement of the chassis box will reduce the risk of any localised buckling or tearing of the chassis box at the subframe attachment points in the event of higher speed impacts.

The photographs show a ply finish to the top of the floor box. In this example a ply and aluminium composite floor box has been used. The composite panel is constructed by riveting and glueing a ply and alumium sheet over aluminium ribs and a foam core. This increases stiffness, reduces any 'drumming' of the floor and provides a nice finish. It requires more time to fabricate but can reduce the cost of materials as ply is quite cheap and it permits a lighter gauge of the more expensive aluminium to be used.

The seats shown are Jaz Pro blow moulded poly seats. They are light weight, economical and ideal for outdoor use.


Step 4: Battery

Picture of Battery
Battery Pack.jpg
cell module.jpg
Batteries need to be selected to suit the peak motor current draw, operating voltage of the motor and controller and a capacity that will provide the required range.

The motors and controllers that I am using operate at 144 volts so I have used 45 lithium cells in series. The peak current draw is 600 amps for brief periods with a maximum rated current for longer periods of 300 amps so I have used 100Ah cells to limit current draw to between 3C to 5C. This gives a 14kWh battery pack which is a bit more than half of the capacity of Nissan Leaf's 24kWh battery pack.
Each cell is 3.3kg giving a total battery weight of 150kg.

A battery pack of this voltage is enough to do some serious damage if you drop a tool across the terminals and can give a dangerous electric shock. A qualified electrician with experience in DC power circuits should complete work on the battery pack.

An essential part of the battery pack is the battery management system. The battery management system is required to ensure individual cells are not over or under charged. A good battery management system will also provide information about the battery state of charge and the current draw in or out of the battery. The cells sourced for this project were CALB 100Ah cells from EV-Power, ordered and supplied complete with EV-Power's own battery management system and a compatible single phase battery charger. The individual cell and battery pack installed in the car is shown in the photos above.

Some background on lithium cells for electric vehicle use from the battery supplier...
LFP batteries have many advantages over Lead Acid, half the weight, higher voltage under load, double the usable capacity and ten times the cycle life! The total cost of ownership is less for LFP batteries than for lead acid.
  • Voltage: 3.2-3.4V nominal, 2.5V min, 3.9V max
  • Cycle life: 2000+ to 80% DOD, 3000+ to 70% DOD, measured, not just claimed.
  • high discharge rate
  • Consistently low internal resistance. (=longer life)p
  • Safe LFP chemistry, proven performance in EVs.

Step 5: Suspension and Steering

Picture of Suspension and Steering
A vehicle platform with double wishbone suspension on each wheel and rear drive has been developed. Wishbone suspension provides maximum flexibility both with body design and ride height as wishbones give easy height adjustment and typically require lower height above the wheels than strut type suspensions. Wishbones also provide optimum handling performance while driving the rear wheels is a little simpler, avoids the need for CV joints on the steered wheels and provides more entertaining handling. Rear wheel drive is also suited to more central placement of the electric motors for optimum placement of the heaviest components closest to the vehicles centre of mass.

The cheapest and easiest way to obtain suspension components is to source from a wrecking vehicle, however there is a limited number of vehicles with front and rear wishbones and rear wheel drive. Two fairly light weight vehicles are the Mazda MX5 / Miata and Honda S2000. I have used a wrecked Mazda MX5 as they are more commonly available in Australia. The suspension from a MX5 is conveniently attached to subframes that can be detached from the car by removing a few bolts. The subframes include all of the suspension mounting brackets so they could be reused, but the chassis box would need some work to cut and fit around the original subframes, so a simpler (and lighter) approach is to fabricate your own front and rear subframes and attached the suspension, wheels, hubs, brakes, springs etc. complete onto the new subframe.
Photographs: The front and rear wheel and suspension after removal from an MX5 is shown in the above photos along with the original front subframe and a rear subframe. Note the front suspension subframe is reasonably heavy as it also incorporates the front engine mounts. The new fabricated subframe can be easily seen on the car in the last photo.

Even though the MX5 is a light weight vehicle, there is still significant unsprung weight, with the four wheel, brake and suspension assemblies weighing a total of approx 140kg, which is nearly 1/3 the weight of the car. For lighter weight (but more expensive and more fragile) aluminium racing wheel uprights and hubs with fabricated wishbones and rod ends can be sourced eg from Formula Ford parts suppliers.

HOW TO.....

Firstly if you need to disassemble the host car to get the components out, refer to a useful guide such as how to remove an MX5 body from the makers of the exocet kit car, although suspension components can easily be removed just by removing wishbone pivot bolts and driveshaft flange bolts at the rear and wishbone pivot bolts at the front.

Secondly press up brackets for the suspension pivots from 50mm wide x 3mm thick steel plate. The bracket widths need to match the width of the suspension bushes on the inner ends of the wishbones. They may need to be slotted to cater for camber angle adjustment. The Mazda uses a simple offset washer to position the suspension mounting point in the slotted hole for camber adjustment.

Thirdly attach the brackets to the suspension arms in the middle of their adjustment range. With the suspension and wheel still assembled, clamp the brackets onto the suspension mounting subframes and check the position and alignment of the wheels. Clamp long lengths of tubing to the wheels to assist in aligning them. Once the wheels are in position and aligned, tack the mounting brackets onto the suspension subframes with a welder, then remove the suspension arms and weld into place.

The steering rack sits at the front of the chassis and two tube or angle arms are welded to the front suspension subframe to mount the rack. The angle of the steering shaft and column needs to be determined to suit the seating position in the car. Placement of a pivoting joint on the steering column mount caters for a height adjustable steering wheel.

3d CAD drawings of Mazda MX5 / Miata suspension assemblies are available online from grabcad and can be used with free 3d CAD programs such as Autodesk Inventor Fusion for development of the suspension design.

The mazda steering rack with power steering has a higher (faster) ratio than unassisted racks. With the lighter weight of this vehicle design, hydraulic assistance is not required and the assisted rack can easily be depowered. Instructions for depowering an MX5 steering rack are available online on MX5 / Miata community forums.

Step 6: Driving Experience

Picture of Driving Experience

The experience of driving and fine tuning a light weight EV designed around a central battery box has been a lot simpler than a petrol powered car. Basically it was plug everything in and check that the motors are running in the right direction, fine tune the two throttle pots, tighten the drive belts and very little else to do.

The concept of twin motors independently driving the two rear wheels has worked perfectly and the motor torque and speed characteristics automatically distributes power to the two rear wheels without the need for an "electronic differential". There has been some drifting in the adjustment of two separate throttle pots that send signals to their respective motor controller. This doesn't cause problems in normal driving and in the future will be addressed by going back to one throttle pot with an electronic splitter.

The EV is being used around a farm with a small tray and is proving convenient, smooth and quiet and with the short range trips typical around the farm, the battery pack does not get discharged below about 85%.

The large section size of the chassis box provides excellent stiffness and there is no discernible scuttle shake and the design will continue to be tested over rough farm roads to prove the strength and reliability of the concept and to expose any weaknesses.

The level of performance of the motors far exceeds that which can be explored on gravel farm roads and some track time will need to be booked in the future for further performance testing. The gravel farm tracks do provide an excellent testing ground for testing torque vectoring systems to get the best performance on slippery gravel roads and a future project to develop an Arduino based torque control system is planned, although not in time for the current Arduino challenge.

Range, Speed and Recharge Time

Update May 2014 typical energy consumption 0.74Wh/km, normal driving.
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 excessive 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. A lower gear ratio is planned and will increase acceleration to 100km/h which is faster than needed.

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.

Step 7: Resources

BotEV Chassis Platform November 2013 2D CAD file (dwg)
Chassis Box cross section CAD file (dwg)

3D CAD model information

Suppliers & Parts
Motors, Controllers, Batteries and Battery Management - EV Power (Australia)
     Smaller motors for light weight EV projects also check out Cyclone Motor, Golden Motor and Kelly Controls
Drive pullies and belts - Gates GT3, Busselton Bearings
Aluminium Pressings - Combined Metal Industries
Seats Jaz Pro - Ebay, Sydney supplier

I would like to thank Rod Dilkes from EV Power for his encouragement, support knowledge and assistance. Rod is the brains behind the electrical setup for this project.
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Hi Ganharr,
I was wondering if you had to make any modification to the steering
I would imagine the miata has a power steering pump, and you don't.
I was wondering if you had to do anything special, or if you could just basically slap it right on

Ganhaar (author)  nickydlax23 hours ago
Yes the miata did have a power steering pump. I did not want power steering on the electric car as this would require an electric drive for the power steering pump, extra weight and extra complexity. You can get power steering pumps for EV conversions, but this car is much lighter than the Miata, particularly over the front wheels where the Miata normally supports a fairly heavy engine so the power steering is not needed. The power steering rack has a faster ratio than a manual rack which is also good for a light weight performance platform.
The modification required is either to join up the power steering tubes so that the hydraulic fluid can flow from side to side when the piston in the rack moves or to cut a notch in an internal seal to let hydraulic fluid pass. The modification is a reasonably common performance mod on the Miata and is well documented on Miata forums. I chose the later method as it provides less internal friction but you may wish to use the first method if you want to keep the option to fit a power steering pump in the future.

do you have more pictures of how you set up the rear axle for the gears and the motors, i wanna know how i would weld it together, and use bearings

Ganhaar (author)  cool austin3 days ago

Did you see the diagram showing cross section of the rear axle in step 2?

There are no gears, just a single 1:3 belt drive. Unfortunately I have had some problems with the drive belts stripping and breaking due to the torque involved. 150Nm was supposed to be within the capacity of the 30mm wide belts but I think I might be getting much higher torques in regen. Also the minimum diameter pulleys for a belt drive has restricted the gear ratios I can use so I have just sourced some 5/8" chain and sprockets (530 motorbike chain and sprockets) and am going to lower the final drive ratio and should give a much more reliable, albeit more noisy transmission.

thanks for pointing that out. im looking a similar frame, but changing a few things, i might do a 4 wheel drive instead, with either 10kw or 20kw motors. my only issue is going to be, convincing my government (canada) to licence said vehicle, and getting an insurance company that will acccept the vehilce / not rip me off

MJDQLD1 month 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)  MJDQLD1 month 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) 1 month 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) 3 months ago


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.



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:

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.

Stuart213 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 Dash4 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?
CJSchecter9610 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)  CJSchecter9610 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.

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

henryjimenez10 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.


Ganhaar (author) 1 year ago

Update on energy efficiency.

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

Squidyman1 year ago

so how much did this all cost?

Ganhaar (author)  Squidyman1 year 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 - 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?
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