# Miata direct drive



## santossrg (Jan 6, 2016)

I'm thinking to convert a 1998 Mazda Miata to EV 

Can a Warp 9 motor connected directly to the transmission shaft move the vehicle even in a incline grade? What should be the voltage?

Best regards.


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## dcb (Dec 5, 2009)

been seeing a few threads about miatas lately, that tunnel looks like trouble for a motor though. you might want to give it a search.


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## santossrg (Jan 6, 2016)

How can I calculate the torque/power necessary to use directly connected in the transmission shaft?

Best regards.


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## Duncan (Dec 8, 2008)

Hi
You need to know
Motor torque
Diff ratio
Vehicle weight
Wheel/tire size

Motor torque is proportional to motor current - I think a Warp9 is about 230ftlbs at 1000amps

230 ftlbs x 4:1 (diff) = 920ftlbs
Your wheel/tire is going to be about 1 ft radius 
So you will get about 920lbs "pushing" your car
a Miata weighs about 2100lbs - so just less than 1/2 G acceleration
Should give about 7 seconds zero to 60


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## santossrg (Jan 6, 2016)

Duncan said:


> Hi
> You need to know
> Motor torque
> Diff ratio
> ...


Thank you Duncan by the answer.

0 to 60 mph in 7 sec, is enough to me. 

So you mean I can go with direct shaft transmission with no problem.
Is that correct? And in a incline grade?

Best regards.


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## Yabert (Feb 7, 2010)

santossrg said:


> So you mean I can go with direct shaft transmission with no problem.
> Is that correct? And in a incline grade?


If your miata stay lightweight and you use a 1000 A or more controller, the Warp9 will move your car, even in a incline grade.
BUT.... you will maybe overheat the motor by working at low speed / high amps.
Example: few stop and go in a incline street will cause a lot of heat to be generate in the motor.
Force air cooling could be an obligation. Higher rear end ratio could help considerably, but that mean lower top speed.

Also, think about how you will go in reverse.


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## santossrg (Jan 6, 2016)

Yabert said:


> If your miata stay lightweight and you use a 1000 A or more controller, the Warp9 will move your car, even in a incline grade.
> BUT.... you will maybe overheat the motor by working at low speed / high amps.
> Example: few stop and go in a incline street will cause a lot of heat to be generate in the motor.
> Force air cooling could be an obligation. Higher rear end ratio could help considerably, but that mean lower top speed.
> ...


To the reverse movement, I thought the controller can handle this.

Any way, in your opinion should I keep the gear box?

Best regards.


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## Duncan (Dec 8, 2008)

santossrg said:


> So you mean I can go with direct shaft transmission with no problem.
> Is that correct? And in a incline grade?
> 
> Best regards.


To amplify Yarberts comment
A Warp9 will take 1000amps for a few seconds - which will get you to the speed limit when you will have to lift off

If you want to drive slowly up a long steep hill then you may start hitting the thermal limits of the motor

You will need a reversing contactor from a forklift to reverse,
There is a possible problem here, the Warp9 is set up for forwards going backwards it is not correctly timed
I don't find that a problem BUT I don't use full (or even 1/4) power in reverse


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## Moltenmetal (Mar 20, 2014)

With a gearbox, you have options for both fun acceleration, great top speed and climbing hills without overheating your motor.

Without it, you have quite a few potential problems.

Your choice, of course. As Duncan said, it can be made to work- it's just a matter of whether or not you will a) design it correctly and b) not mind the inevitable compromises on the way there.

What is your main concern about keeping the gearbox?

The clutch is optional. I kept mine for ease and comfort and I don't regret it, but many manage without one with no problems.


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## dougingraham (Jul 26, 2011)

I kept the transmission in my RX-7. Drained it weighs 72 lbs. The gear lube is probably around 5 lbs. The Miata manual transmission is going to be similar. If you don't keep the transmission you also don't need the flywheel, clutch and pressure plate. Those things are probably another 30 lbs. Lets just call it 100 lbs of weight savings if you don't keep the transmission. I've got a dyno graph that shows 277 ft-lb at 1000 amps. But it is in the range of 250 ft-lb at 1000 amps. The RX-7 differential ratio is 3.909 or 4.10 depending on motor. The torque curve on the WarP 9 is flat up to about 3200 rpm at 144 volts. You can run it pretty safely up to 170 volts and with the Helwig brushes I have been told 192 volts. The reason I mention this is that operating at a higher voltage will widen the torque band. I believe the torque band at 1000 amps extends up to around 4800 RPM if the motor sees 192 volts. With a transmission the motor is not going to see these kinds of power levels for very long. It could see these power levels frequently if you do direct drive. Third gear is 1.419 making the total reduction 5.55:1. With a 5.55:1 torque amplification and 250 ft-lbs and assuming 1 ft radius tires this gives 1388 lbs of thrust which is more than 1/2G. Redline is 6000 RPM which gives 79.99 MPH with stock size tires in 3rd gear. But I am not sure I would ever get there because torque drops off quickly above 3200 RPM. At 3200 RPM the vehicle speed is 42.7. This is too low to be the only gear. So you get good accel up to 43 mph. With higher voltage of 192 you would see this up to 64 mph. That would be really usable as a single speed around town car. 

I have the 3.909 differential. The car feels heavy if I start out in 4th gear with 1000 amps. It feels pretty good starting out in 3rd gear.

It really is all about expectations. I don't think I would be happy with the accel if I had eliminated the transmission. At one time you could get a ring and pinion set for the differential that gave a 5.3 but it was horribly expensive. The highest available ratio now is 4.778 for $525. This would be better but not quite enough. Going to a controller that could source higher current (2000 amp Zilla for example) and going to 1400 amps would give around 350 ft-lb and more than make up for the loss of torque amplification (1672 ft-lb at the tires). But at the higher current the torque band is narrower. There is no free lunch here. I expect you would be running on the raggedy edge at 192 volts and 1400 amps. And yes I know that John Metric runs 210 volts and 2000 amps but his runs are less than 9 seconds and his brush holders are not stock and he is not using brushes appropriate for street operation. Your brush and comm life are going to be diminished.

For any kind of performance oriented driving you want to keep the transmission, at least in a DIY environment when using a WarP 9 or WarP 11HV). When they get figured out for the DIY crowd the small Tesla drive train would be great for smaller cars. For larger vehicles the big Tesla drive train would be great. For the mentally ill the big Tesla drive train would be acceptable in a Miata. My computer model shows around a 2 second 0-60 time assuming you could get enough rubber on the road.


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## miscrms (Sep 25, 2013)

This is old school, but I still find it very useful for estimating the effects of different gear ratios, voltage, current limit, etc on top speed, gradability, etc.

http://www.evconvert.com/tools/evcalc/

Its got a 98 Miata in there already, and motor parameters for a warp 9, so you probably just need to come up with reasonable battery and controller specs.

Rob


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