Converting an '82 Corvette to use 4 electric motors

5631 Views 38 Replies 12 Participants Last post by BudgetDreams, Jun 14, 2021

Jomp135

Discussion Starter · Mar 23, 2021

So, I'm currently going through the design process for a high performance '82 Corvette electric car conversion. I'm looking at using AMR-250 90 single motors from EVWest, and after building a rather nifty calculator for spec analysis, I've figured out that the best way for me to meet my performance goals is to use 4 of them, one to power each wheel. I've looked at in wheel hub, and because of the associated performance issues with unsprung mass and the size of the motors, it's not practical. Because of this, I'm now looking at putting the motors in the axle assemblies: each motor will have a planetary gear reduction on the output, and that will be connected to a halfshaft connecting it to the wheels. It's essentially 2 motors pretending to be a very large differential. My problems are twofold:
First, what is the best way to convert an '82 corvette from having a nondriven front axle to having a driven front axle? I don't need most of the features associated with AWD. The way I see it, I just need an FWD axle that can go in an '82 Corvette, but what would that be? Tell me if I would be better off posting this in another forum, by the way.
Second, would my plan with the motors work?

Thanks!

1 - 20 of 39 Posts

wjbitner

· #2 · Mar 23, 2021

Jomp135 said:
So, I'm currently going through the design process for a high performance '82 Corvette electric car conversion. I'm looking at using AMR-250 90 single motors from EVWest, and after building a rather nifty calculator for spec analysis, I've figured out that the best way for me to meet my performance goals is to use 4 of them, one to power each wheel. I've looked at in wheel hub, and because of the associated performance issues with unsprung mass and the size of the motors, it's not practical. Because of this, I'm now looking at putting the motors in the axle assemblies: each motor will have a planetary gear reduction on the output, and that will be connected to a halfshaft connecting it to the wheels. It's essentially 2 motors pretending to be a very large differential. My problems are twofold:
First, what is the best way to convert an '82 corvette from having a nondriven front axle to having a driven front axle? I don't need most of the features associated with AWD. The way I see it, I just need an FWD axle that can go in an '82 Corvette, but what would that be? Tell me if I would be better off posting this in another forum, by the way.
Second, would my plan with the motors work?

Thanks!

Hi!

That certainly sounds interesting. Can you quantify the performance goals you want? As I think about this, I would be more concerned about the batteries. Specifically, when the throttle is applied assuming 4 controllers, that's a lot of current. To get a battery pack to supply that much current is a very high discharge rate, meaning not a lot of range. What type of range are you looking to achieve? Simiilar to ICE engines, the whole system has to work together, fuel supply, cams, compression ratios, suspension, tires... Some depends on how much of the car you are willing to
alter. Where can you put all those batteries. Look forward to hearing more about this project.

Bill

reiderM

· #3 · Mar 23, 2021

Jomp135 said:
So, I'm currently going through the design process for a high performance '82 Corvette electric car conversion. I'm looking at using AMR-250 90 single motors from EVWest, and after building a rather nifty calculator for spec analysis, I've figured out that the best way for me to meet my performance goals is to use 4 of them, one to power each wheel. I've looked at in wheel hub, and because of the associated performance issues with unsprung mass and the size of the motors, it's not practical. Because of this, I'm now looking at putting the motors in the axle assemblies: each motor will have a planetary gear reduction on the output, and that will be connected to a halfshaft connecting it to the wheels. It's essentially 2 motors pretending to be a very large differential. My problems are twofold:
First, what is the best way to convert an '82 corvette from having a nondriven front axle to having a driven front axle? I don't need most of the features associated with AWD. The way I see it, I just need an FWD axle that can go in an '82 Corvette, but what would that be? Tell me if I would be better off posting this in another forum, by the way.
Second, would my plan with the motors work?

Thanks!

You're better off using 2 tesla drive units, one for the front and one for the rear. All the power you could possibly dream of, while only needing to deal with 2 inverters instead of 4.

Like Bill said, you would probably have a hard time finding a battery pack that can power 2 powerful inverters, much less 4.

You're correct in that there is not a good reason to use hub motors for just about any project.

What kind of performance are you looking for? 2 large Tesla drive units will produce around 1000hp, which is in truth probably too much for an '82 corvette.

Electric Land Cruiser

· #4 · Mar 23, 2021

This guy made a 4WD C3 Corvette by using a Caddy Eldorado front end, so maybe the Cadillac FWD axles and hubs would bolt up to the C3 suspension, or close enough to make work: The World’s Only 4WD Hemi-Powered Corvette -

Personally, I think RWD suits the Vette more. EV conversions should be built to the original car's strengths. One great thing about a Vette is the rear transaxle, which means that it's already set up for a rear engine EV conversion!

brian_

· #5 · Mar 23, 2021

Jomp135 said:
... each motor will have a planetary gear reduction on the output, and that will be connected to a halfshaft connecting it to the wheels. It's essentially 2 motors pretending to be a very large differential.

That's a perfectly workable configuration, used by a few existing EVs (such as the Rimac Concept cars) and used at one axle only (three motors total) by some hybrids and proposed EVs.

But you may want to reconsider the use of planetary reduction. It's hard enough to get a high enough ratio in a single planetary stage, and two planetary stages are complex(and take more axial length) compared to a two-stage set of parallel gears. For instance, GM used single-stage planetary reduction in the Chevrolet Spark EV but it only had about 3.5:1 reduction, then for the Chevrolet Bolt they use parallel gears; both have the axle outputs concentric with the motor shaft.

brian_

· #6 · Mar 23, 2021

wjbitner said:
Can you quantify the performance goals you want? As I think about this, I would be more concerned about the batteries. Specifically, when the throttle is applied assuming 4 controllers, that's a lot of current.

It isn't any more battery current than the same power from one huge motor or two large motors - it's just a question of total power, regardless of how many ways it is split up.

But yes, performance requirements need to be understood to assess battery requirements, regardless of the number of motors.

wjbitner said:
To get a battery pack to supply that much current is a very high discharge rate, meaning not a lot of range. What type of range are you looking to achieve?

Performance only matters to range if you use that performance. If you are racing, using everything the car can do continuously, obviously range is short.

Reactions: 1

Jomp135

Discussion Starter · #7 · Mar 23, 2021

Yes, I guess I should lay out my performance goals. I want to use the AMR motors because of their power curves. I used Desmos to whack together a crude calculator to analyze performance(found here: Curves ) , and I found that this specific layout meets my goals quite nicely, with a little extra bonus. My goals going in were:
180 mph top speed
2.5 second zero to 60
At least 100 mile range.
I was looking at an 80kwh battery bank, but I haven't settled on what batteries to use yet.
However, looking at the power curves I found something which is very nice indeed: namely, acceleration stays pretty much constant throughout the full speed range of the car, at about 10 m/s^2. This is assuming 95% drivetrain efficiency, though, so I'm obviously not expecting those results in the real world. Still, that kind of acceleration means 7.5 second 0-170 times, which is fantastic. I feel that the calculator is definitely flawed in that the power draw at low speeds is really low, giving me ranges that are much too good.

4 AMR racing motors produce about 737 continuous horsepower, and 1622 peak, which may well actually be too much for an '82. What would be the limiting factor there? I'll have more of a look at battery packs, but I'm planning to supply the inverters at 700 volts to decrease the current draw. Do you have any suggestions as to potential batteries?

I thought for a while about using a larger motor (AMR 250-90 dual) on the rear axle, but I couldn't find a way to get that to meet my goals without using some sort of transmission, and those are very underdeveloped for electric cars at the moment. The reason Tesla uses AWD for their high performance EV's is because they couldn't get transmissions working reliably, so they went to a different gearing for the front and back.

I found some Tesla Drive units at StealthEV, I'll run the numbers for those.

Also at Stealth EV I found my motor controllers: Cascadia Rinehart PM250Z's. And they are ludicrously expensive, at $13,000 apiece. And I would need one per wheel. I'll have a look at the Tesla Drive units.

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brian_

· #8 · Mar 23, 2021 (Edited)

Jomp135 said:
First, what is the best way to convert an '82 corvette from having a nondriven front axle to having a driven front axle? I don't need most of the features associated with AWD. The way I see it, I just need an FWD axle that can go in an '82 Corvette, but what would that be?

All Corvettes have double A-arm (double wishbone) front suspension, and should retain that. So the two options are to change the hubs and hub carriers to ones which could be drive, or to completely swap the suspension for something that has driven double A-arm from suspension... which is more likely to be found in an AWD vehicle than anything just front wheel drive. Regardless of the suspension, the frame rails will probably be in the way of the axle shafts; pickup trucks and SUV routinely have driven front axles with double A-arms and a ladder frame, but the are designed to fit that, with the frame well above the axle line.

Electric Land Cruiser said:
This guy made a 4WD C3 Corvette by using a Caddy Eldorado front end, so maybe the Cadillac FWD axles and hubs would bolt up to the C3 suspension, or close enough to make work: The World’s Only 4WD Hemi-Powered Corvette -

This is a rare example of front wheel drive with double A-arms, and would be good if the parts work... although all of the components are antiquated. That 4WD Hemi-powered C3 uses the complete Eldorado suspension and part of the frame, not just hub components; I don't know how well they would suit the Corvette. These components are not specifically Cadillac (Eldorado); the same chassis was used first for the Oldsmobile Toronado, and later for the Buick Riviera and even the GMC Motorhome, from 1966 through 1985 1979-1985 was a lighter version).

Electric Land Cruiser said:
Personally, I think RWD suits the Vette more. EV conversions should be built to the original car's strengths. One great thing about a Vette is the rear transaxle, which means that it's already set up for a rear engine EV conversion!

A 1982 is the third-generation Corvette (C3), and doesn't have a rear transaxle; that didn't start until the C5 for 1997. Even then, the rear face of a motor attached to the front (input) of the C5 transaxle would be well ahead of the rear wheels; the front of the motor would run into the seats. Splitting the transmission section from the final drive would move the motor back, but it would still be entirely ahead of the axle (how far depending on how much length the required reduction gearbox would take).

But I agree - the proportions of the car are designed for RWD. With any rational component placement it will be neutral to tail-heavy, and during acceleration load shifts to the rear tires, so most of the power will be needed at the rear even with AWD. Some very high performance cars that would normally be RWD have gone to AWD for better control, and that makes sense, but in that case the front motors can be substantially lower-power than the rear.

Rear suspension
The discussion of front suspension brings up an issue: the rear dual-motor drive unit would replace the stock rear final drive (differential) unit, but the suspension uses it. In the C2/C3/C4 rear suspension, the axle shafts also serve as suspension components, transmitting lateral load through the diff bearings and final drive case to the vehicle structure. The new drive unit would need to handle the same forces (which are not normally handled by a final drive or transaxle), unless the suspension is converted to use a separate upper control arm. This same concern exists with any EV drive unit, including one from a Tesla.

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brian_

· #9 · Mar 23, 2021

reiderM said:
You're better off using 2 tesla drive units, one for the front and one for the rear. All the power you could possibly dream of, while only needing to deal with 2 inverters instead of 4.

Total power isn't the problem. One Tesla large drive unit is enough for the car, if total peak power is the only concern. Dealing with more inverters also means not needing differentials and having (if the controls are done properly) excellent traction control (better than any limited-slip diff can achieve).

brian_

· #10 · Mar 23, 2021

Jomp135 said:
Also at Stealth EV I found my motor controllers: Cascadia Rinehart PM250Z's. And they are ludicrously expensive, at $13,000 apiece. And I would need one per wheel. I'll have a look at the Tesla Drive units.

Yes, high-power AC motor controllers/inverters are insanely expensive. This is a major reason why the current trend is to salvage complete drive units from production EVs, complete with the controller/inverter.

brian_

· #11 · Mar 23, 2021

Jomp135 said:
I was looking at an 80kwh battery bank, but I haven't settled on what batteries to use yet.

Okay, but have you considered where all that battery would go in a C3, especially with a front drive unit taking some prime space?

Reactions: 1

brian_

· #12 · Mar 23, 2021

Jomp135 said:
I thought for a while about using a larger motor (AMR 250-90 dual) on the rear axle, but I couldn't find a way to get that to meet my goals without using some sort of transmission, and those are very underdeveloped for electric cars at the moment.

What do you mean by "transmission"? A gearbox that transmits power is a transmission - every EV has one, and you will have four (or two if you give up on individual motors per wheel and go with one per axle). If you mean a multi-ratio (typically only two-speed) transmission... yes, almost all EVs avoid that. But they work for the few that use them, and most companies which make transmissions for EVs offer a two-speed version.

The AMR 250-90 dual motor is simply two AMR 250-90 single motors in the same case. Using that dual-core motor instead of two single-core motors would make no difference to the performance or gearing requirements.

Jomp135 said:
The reason Tesla uses AWD for their high performance EV's is because they couldn't get transmissions working reliably, so they went to a different gearing for the front and back.

That's nonsense from some Tesla fan who with no technical understanding. Tesla failed with two-speed transmissions early in the Roadster era, and never built an AWD Roadster. AWD and two-speeds are unrelated.

Tesla has AWD for the same reasons as everyone else: better traction, better stability. The front and rear drive units in some Model S and X variants are only different in ratio because they have different motors (small at the front and large at the rear) due to re-using existing components (small from the original AWD with matched front and rear motors; large from the original RWD). Someone at Tesla came up with the idea of changing the power distribution between front and rear based on speed to take advantage of the different drive units' operating characteristics, but the gearing different is both small and irrelevant. Having two motors both driving all the time through unchanging ratios is very different from having a motor driving through a two-ratio transmission.

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Jomp135

Discussion Starter · #13 · Mar 23, 2021

brian_ said:
.. yes, almost all EVs avoid that. But they work for the few that use them, and most companies which make transmissions for EVs offer a two-speed version.

I didn't know that, so far all I've been able to find are:
The Taycan Transmission, which is impossible to find on the aftermarket
The Inmotive transmission, which only supplies to OEM's (and I haven't been able to find any that use it)
Borg Warner, who made both a transaxle and a linear transmission, but who I think have stopped producing those. Plus, I can only find the transaxle, which can't take the loads I want.

I'm still looking at Tesla Drive units, but I'm trying to figure out what motors they use so that I can get power curves to plug into my equations. Anyone know what motors are used by this, or this?

Isaac97

· #14 · Mar 23, 2021

Jomp135 said:
I'm still looking at Tesla Drive units, but I'm trying to figure out what motors they use so that I can get power curves to plug into my equations. Anyone know what motors are used by this, or this?

Those are Tesla's motors so it's not very easy to get detailed specifications. However, you should be able to figure some things out by looking at max torque and max horsepower; when torque * rpm = max hp, there you have the 'knee' of the torque curve. (there are of course other conversion factors necessary.)

One issue with Tesla units is that they have fairly tall gearing and thus will have issues getting up to 180mph; 9.73:1 in the large motor, with a maximum motor RPM of 18,000. And at that RPM you won't have much torque. So different gear sets might be necessary; Zero EV sells a 4.5:1 gearset which should give you excellent top speed at the expense of torque. (300mph top speed anyone?)

brian_

· #15 · Mar 23, 2021

Jomp135 said:
I didn't know that, so far all I've been able to find are:
The Taycan Transmission, which is impossible to find on the aftermarket
The Inmotive transmission, which only supplies to OEM's (and I haven't been able to find any that use it)
Borg Warner, who made both a transaxle and a linear transmission, but who I think have stopped producing those. Plus, I can only find the transaxle, which can't take the loads I want.

Yes, they're rare in production, and some of the two-speed transmissions which are available have not been selected by any manufacturer. As an individual builder you can't buy from the manufacturers, aftermarket suppliers won't have them, and salvaging used units from production EVs is of limited practicality.

The Taycan rear transmission is impossible to find because the vehicle is too new, and even in a few years it will still be expensive because the car is so expensive. The best-known example might be the Rimac Concept One... and they only made about 7 of those cars.

I'm not really suggesting the you could reasonably find a two-speed transmission to use, just that they are possible and even reasonable to make - they are rarely used because they are not needed, not because they can't be or haven't been built.

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brian_

· #16 · Mar 23, 2021

Isaac97 said:
One issue with Tesla units is that they have fairly tall gearing and thus will have issues getting up to 180mph; 9.73:1 in the large motor, with a maximum motor RPM of 18,000. And at that RPM you won't have much torque. So different gear sets might be necessary; Zero EV sells a 4.5:1 gearset which should give you excellent top speed at the expense of torque. (300mph top speed anyone?)

I think you meant that they have fairly short gearing for high speed: the terminology (which can be confusing) is that a greater reduction ratio is suited to lower speeds and is called "short"; a lower reduction ratio is suited to higher speeds and is called "tall".

The overall gearing of the Tesla drive units (and almost every EV drive unit) is short compared a typical engine-driven car in high gear, with an overall ratio of about 10:1 versus about 3:1 for a typical car. The short gearing is made possible by the high motor speed which is possible, and it limits the top speed of the car. The motor can produce full power to nearly the top motor speed, so top speed is limited by the safe motor speed, not the availability of power.

At the other speed extreme, the overall gearing of the Tesla drive units (and almost every EV drive unit) is a bit tall compared a typical engine-driven car in low gear, with an overall ratio of about 10:1 versus about 15:1 for a typical car. The tall gearing is made possible by the high motor torque at low speed, but still limits the low-speed acceleration of the car. In the highest-powered EVs it's not a problem because even with this gearing the available torque to the wheels still exceeds available tire traction and is far beyond what is useful on the road.

To reach extreme high speeds and still have extreme low-speed performance two ratios are beneficial, and that's why the Taycan has them. For sane people, that's not required; even on a race track, more than 250 km/h (155 mph) is of little use.

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MattsAwesomeStuff

· #17 · Mar 24, 2021

I've never seen an EV converted this way.

I love weird solutions and I love Corvettes, so, I'm quite excited to see what you come up with. Please stick around and keep documenting your build!

Bratitude

· #18 · Mar 24, 2021

Jomp135 said:
So, I'm currently going through the design process for a high performance '82 Corvette electric car conversion. I'm looking at using AMR-250 90 single motors from EVWest, and after building a rather nifty calculator for spec analysis, I've figured out that the best way for me to meet my performance goals is to use 4 of them, one to power each wheel. I've looked at in wheel hub, and because of the associated performance issues with unsprung mass and the size of the motors, it's not practical. Because of this, I'm now looking at putting the motors in the axle assemblies: each motor will have a planetary gear reduction on the output, and that will be connected to a halfshaft connecting it to the wheels. It's essentially 2 motors pretending to be a very large differential. My problems are twofold:
First, what is the best way to convert an '82 corvette from having a nondriven front axle to having a driven front axle? I don't need most of the features associated with AWD. The way I see it, I just need an FWD axle that can go in an '82 Corvette, but what would that be? Tell me if I would be better off posting this in another forum, by the way.
Second, would my plan with the motors work?

Thanks!

sounds very very very expensive. just get a Lexus gs450h hybrid transmission, and the evbmw controller. it’ll pull hard. And be much much simpler, while keeping the rear end stock, and giving you a engine bay for batteries

brian_

· #19 · Mar 24, 2021

Bratitude said:
sounds very very very expensive. just get a Lexus gs450h hybrid transmission, and the evbmw controller. it’ll pull hard. And be much much simpler, while keeping the rear end stock, and giving you a engine bay for batteries

That would package nicely but it wouldn't even remotely approach the builder's performance goals, and wouldn't be AWD.

Why comment if you're not even going to read the discussion so far?

Bratitude

· #20 · Mar 24, 2021

brian_ said:
That would package nicely but it wouldn't even remotely approach the builder's performance goals, and wouldn't be AWD.

Why comment if you're not even going to read the discussion so far?

he hasn’t stated numbers, and that set up could yield 350hp +
And there is a version with a awd transfer case.

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