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EV Chassis Selection

This page goes through some of the considerations you need to address when selecting a vehicle to convert to electric.

My Criteria

There are lots of considerations that came into play when looking for a vehicle that is a good candidate for conversion to electric. However, the first and most important thing to decide is what you need in a car. Will an EV even work for you? My EV Background Page goes into some details about what modern EVs are capable of. Once you know what you need, you can go through the below criteria, and perform some EV Performance Analysis on your selected chassis, and see if it pencils out.

In my case, I want a second car that is good for running around town. It needs some cargo capacity, but not a lot. A two-seater is fine for me. I also want to get maximum range that I can. Performance needs to be adequate for freeway usage.

Note that some people convert a pickup truck because they want cargo capacity or because it is easier. Others will convert a larger passenger car because they want to be able to carry the whole family. I don't follow these priorities to their logical conclusions, since That isn't what I was going for.

Chassis Selection Criteria

There are hundreds of different car models. How did I decide on a Toyota MR2? Well, the first thing I identified was what kind of car I need. In my case, a compact sports car fits my criteria from above: two seater, some cargo space. So what then? I looked at these criteria:

  • Weight
  • Aerodynamics
  • Precedent
  • Conversion Issues
  • Parts availability
  • Cost

One could probably identify more things to worry about, but these were the big ones for me. More details follow.

Weight

Weight is a big issue with an EV, even more so if you live in a hilly area like me. An EV conversion can lose 500 to 1000 worth of internal combustion stuff and gain 1000 to 2000 pounds worth of EV components and batteries.

Why is weight important? It affects performance. Acceleration, hill climbing ability, and rolling resistance all get worse as the car gets heavier.

It makes sense to start with the lightest vehicle that you can that will meet your usage requirements. Bear in mind though that looking at curb weight (generally defined as the weight of the vehicle ready-to-drive, with a full tank, but no people or cargo) is not the only number to look at. You need to consider that different cars may have substantially heavier or lighter drivetrains. For example, two similar sized cars may weight the same, but one has a 500 pound iron block V8 in it, and the other has a 300 pound 4-cylinder. You would be able to lighten the first car more. In the case of my MR2, its starting curb weight was quite good at 2265Lbs, and I stripped out its fairly light engine (245lbs) and another 275 pounds of other stuff, making the conversion-ready chassis weigh in at about 1700lbs.

I used various consumer websites to locate Curb Weight information for cars I was considering, and I used some engine weight tables I found online to estimate the weight of the drivetrain components that I would be removing. In the case of the MR2, I estimated 400-500 pounds of take-off weight, which came out pretty close. Links to such sites are on the EV Reference Material page.

Following is a list of some of the cars I considered, and their advertised curb weights. Sometimes curb weight was given as a range, in that case I took something in the middle. Note also that many of these cars have "twins". For example, the Mitsubishi Eclipse also came badged as a Eagle Talon and Plymouth Laser. This list is not all-inclusive of course. I only bothered to research cars I figured I might find in an acceptable price range.

CarCurb Weight (Lbs)
1989-1991 Honda CRX2100
1993-1996 Mitsubishi Mirage Coupe2100
1992-1995 Honda Civic Hatchback2100
1985-1989 Toyota MR22250
1991-1996 Saturn SC12300
1989-1994 Mitsubishi Eclipse2550
1993-1997 Ford Probe2690
1984-1990 Pontiac Fiero2700
1990 Nissan 240SX2700
1992-1992 Honda Prelude SI2800
1990-1992 Mazda RX72800
1989-1994 VW Corrado2800
1993-1995 Mazda RX72826

Aerodynamics

This is probably the next most important property of the chassis to consider, unless you never intend to drive much at higher speeds. Aerodynamic drag goes up with the square of speed, so doubling your speed will quadruple the aerodynamic drag. There are a few things you can do to streamline a car, but it is still best to start with the most aerodynamic car you can to begin with if this is important to you. In my case it is important as I want to be able to cruise on the freeway. There is a fair amount of online information for many makes and models relating to aerodynamic drag (techically, CdA for Drag Coefficient multiplied by frontal surface area). The 1985-1989 Toyota MR2 has one of the best CdA (0.535m^2) of any cars I looked at. There were several other cars that were very close, including the 1989-1991 Honda CRX (0.53m^2). There were several other aerodynamic cars I considered, including the Ford Probe, Mitsubishi Eclipse, Pontiac Fiero.

Below is the same list of cars I considered as above, but this time listing CdA. There are much more complete lists of CdA by car make and model online, follow the links from the EV Reference Material Page. You can also follow the link provided there to the page that discusses aerodynamics modifications you can make. Note that all these cars are good in terms of aerodynamic drag, but there is still a 20% difference between the best and the worst in the bunch. That easily can translate to a 5 to 10 mile difference in range as an EV. Just for comparison's sake, the CdA on my 1962 Land Rover is closer to 2.0m^2, or more than three times worse than the MR2. Yikes. As you can see, the MR2 was very good in this category as well as with its weight.

CarCdA (m^2)
1993-1995 Mazda RX70.52
1989-1991 Honda CRX0.53
1985-1989 Toyota MR20.535
1989-1994 Mitsubishi Eclipse0.535
1990 Nissan 240SX0.54
1990-1992 Mazda RX70.547
1993-1996 Mitsubishi Mirage Coupe0.59
1992-1995 Honda Civic Hatchback0.59
1989-1994 VW Corrado0.59
1991-1996 Saturn SC10.603
1992-1992 Honda Prelude SI0.61
1993-1997 Ford Probe0.63
1984-1990 Pontiac Fiero0.63

Conversion Issues

Some makes and models are easier to convert to an EV than others. This issue focuses around two or three major factors. The first is the ease of converting the drivetrain to electric. The second is battery carrying capacity. The third, applicable only to newer cars, is ensuring that the complex, computerized vehicle electronics still work.

A typical EV conversion mates an electric motor to the original transmission. So, if the engine/transmission connection in the target car is anything other than a standard flat connection plate, it will require a complicated adapter to be made. Some makes of cars have engines that turn the opposite direction from others, though any modern EV motor can be tuned to work in the correct direction. In the case of the MR2, the engine turns the standard way, and transmission does not require anything unusual in an adapter plate. Note that if you are doing a direct-drive conversion and eliminating the transmission, then any adapter plate problems would go away too.

The second issue is Battery Capacity. In my case, I wanted to be able to carry enough batteries to make for a 120 volt system. With 8V batteries, that means 15 of them, or 20 with 6v batteries. The MR2 is a small car, but its mid-engine chassis has three compartments: A front compartment, the engine bay, and the rear trunk. With Chassis Modifications (acceptable extra work to me, but maybe not everyone), I was able to fit 17 batteries: the front bay can hold 7 batteries, and the engine bay can hold 10. The trunk can hold many as well, but I wanted cargo space left over. Basically, with the MR2 there is enough battery capacity. I did not look into any of the other cars I considered closely for battery carrying capacity since by the time I was considering this I had ruled out everything but the Honda CRX and Toyota MR2.

The final issue, not a problem with the MR2 but probably a significant one with many newer cars, is that the vehicle electronics are so complex and computerized in newer cars that removing a big chunk of the car (the engine) may cause the remaining systems to work incorrectly or not at all. The MR2 had an engine control computer (ECC) but it went bye-bye along with the engine.

Precedent

By Precedent, I mean to say that it makes sense to convert a car that other people have already converted. This means that advice may be available, and possibly the opportunity to see a finished product before you start. If the car in question is a very commonly converted model, kits and/or prefabricated parts may even be available. I was able to find at least two other toyota MR2 conversions that had been done. The MR2 is not a commonly converted car, but it has been done successfully, and that was good enough for me.

Parts Availability

This one is a no-brainer. If you intend to actually drive your car, you will still need to get parts for the chassis, transmission, interior, brakes, etc. So While a 1965 Corvair Coupe EV would be deliciously ironic, it would be rather difficult to find parts for. In the case of the Toyota MR2 (and anything else I was looking at) Finding parts is no big deal. They are all fairly common cars, and they all have highly standardized drivetrains.

Cost

This one is a no-brainer too. I did not want to spend a fortune on a nice running car that I was going to tear apart. Cost is why I ended up giving up on finding a Honda CRX. Good running examples with decent bodywork are still several thousand dollars. Even project cars could cost two thousand. I was able to find a solid MR2 for $400.

Note that you can leverage your conversion plans here: You don't need an engine, so if you find an otherwise good late-model car with a bad engine or high mileage, bonus! You will be in the driver's seat on price.

Conclusion

The 1985-1989 Toyota MR2 is a good fit in all these categories. It is lightweight, very aerodynamic, and it has sufficient capacity for batteries. Replacement parts are readily available and there is a large aftermarket parts supply too. I was able to find a solid MR2 chassis for a very low price. Other MR2's have been converted, so I know it is possible to do. Case Closed.