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Which option would work the best?

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Discussion Starter · #1 ·
I'm currently working on the plan for a "conversion" of an '82 Corvette to electric. I say conversion because it's mainly a thought experiment to help me learn more about electric cars. This project won't be started for at least 3 years, in which time I expect that the landscape will have changed, and some of the planning done now will become obsolete. Anyway.

My desired specs are:
A sub-2.5 second 0 to 60 time.
Top speed of more than 180 mph.
Range of more than 200 miles.
I'd prefer if it didn't leave me destitute.

I'm currently looking at the AM Racing AMR 250-90, in either it's dual or single configuration.
I've got a very nice little calculator set up here: Curves
This lets me fiddle with gear ratios, motor setups, etc. and it's lead me to a problem.

The AMR 250-90 dual has a rpm redline of about 10,600 RPM, but for practical reasons I'll be limiting it to 9,500. This means that a lot of the more aggressive EV gear reduction solutions won't work for me, because they are designed for motors with higher max RPM. This effectively limits me to a fairly conservative gearing ratio if I want to have any sort of high top speed: about 2.7:1 to 3.1:1 between motor and wheels is the range I'd be facing. This is actually the range that the stock '82 final drive falls in, so the original plan was to take out the engine and trans, replace the driveline, and swap in the motor where the transmission used to go. Unfortunately, this gearing ratio limits my acceleration, with 0 to 60 being about 5 and a half to 6 seconds. That's not bad, but it's not good enough.

My second plan was to use a transmission. I know, I know, everybody says that electric cars don't need transmissions, but this one might, for precisely the reason I outlined above. The problem with this was that there are basically two transmissions designed for electric cars that I could find: one is in the Porsche Taycan, and one is made by Inmotive, who only supplies to OEMs. Even then, the Taycan transmission is A: impossible to find on the aftermarket and B: Still too aggressively geared for the AMR, and I'm not sure that any OEMs actually use the Inmotive transmission. That said, they're both fairly recent, so it seems that 2-speed EV transmissions could become a thing pretty soon. The other option is to use a standard automotive transmission, which for a while seemed like an okay idea, because the AMR is closer to standard automotive RPMs than most electric motors. I was considering using the stock '82 transmission, which is an automatic 4-speed that had gear ratios that fit my specs nicely. I would have needed a slight (~1.5:1) reduction in between it and the motor to get it down to spec speeds, but other than that it seemed fine... until I ran into torque. The AMR dual has 854 Nm of torque, which is enough to rip my transmission out of it's housing and turn it into an automotive chassis blender. (Actually I'm not sure if it would do that, but it's an amusing image.) I looked at other transmissions, including some manual transmissions, but it seemed like more trouble than it's worth. So. Option 3!

Option 3 is, I think, the closest to success. It involves different motors: instead of one insane AMR 250-90 dual, it uses 4 half as insane AMR 250-90 motors. For those keeping track, that's twice as insane. It's also twice as expensive, but eh. The plan is, as you might have guessed, to use one motor per wheel. With a gear ratio of 3.5:1, this gives me sub 2.5 second 0-60 times and a top speed of 165 mph. That's not entirely accurate, because the Corvette has 19 inch front wheels and 20 inch rear wheels, and I'm calculating as if both wheels are 20 inches, so acceleration will be a little higher and top speed a little lower. It also doesn't quite reach my target speed, but it's something to work with. This has a fair number of engineering problems to deal with, though, so I'll go through them:

The '82 Corvette is a front-engine, RWD car. Converting it to an in-wheel motor AWD car is going to require me replacing both the front and the rear axle, and I'm not sure how that works for in wheel motors.
There are some inherent issues with in wheel motors concerning un-sprung mass. Each AMR 250-90 single weighs 150 pounds, so that will increase my unsprung mass considerably. Additionally, all 4 motors will add 600 pounds to the car, so I'll have to do some pretty heavy trimming.
I'm not sure how many systems there are that support 4 motor drive, but I do want to utilize this as much as possible.
4 motors come in at just under 40,000 dollars. Yikes.

Now, some upsides!
4 motor drive gives me a lot of control, and could have hidden performance benefits.
I did all my calculations with the constant supply figures, not the peak figures. If I should have used peak figures for acceleration instead, then I'll have a much easier time of things. For starters, I'll only need a gear ratio of 3.3:1, and I'll have a top speed of 170 mph. That doesn't seem like much of a change until you look at how the acceleration decreases over time and realize that it doesn't. Even if I hard limit to 0.9 of peak as I get towards the top, I'll have a 7.5 second 0-170 time, which is enough to make a Bugatti Chiron want its mother. That's what the calculator is set to right now, so you can check for yourself Of course, this is all theoretical and I expect heavy losses in the real world, but still. If it's anything close to that in the real world, I would gladly give up my top speed goal. I could gear the front and rear wheels differently to get comparable acceleration and a higher speed, but at the top speed, that would mean rotating the more aggressively geared wheels faster than 10,000 RPM.

Anyway, why did I post? A few reasons:

1. How can I get a 3.3:1 linear reduction?
2. Where can I find replacement front and back axles?
3. What is the best 4 motor electric vehicle controller?
4. If I do end up going with different gearings for the front and back wheels, what would be the ramifications of having two motors (not the main powerhouses at this speed) rotating faster than it's top speed. Should I include some sort of clutch to mechanically disconnect them?
5. Are there any glaring holes in my plan?

That's all for now, thanks!

TL;DR:
0-170 mph in 7.5 seconds, read the whole thing to see how.
 
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