Ben, you are right. *if* you can find a 6:1 rear end
(the highest that my vehicle can be equipped with is 4.2:1)
then at 12.65 MPH the drive shaft is doing 1000 RPM.
Just a random data point: my previous truck (S10) when doing
a constant 55 MPH on level road, no wind, consumed 15 kW to
maintain the same speed.
Scaling that down to 1/4 of the speed: 14 MPH should take way less
than 4 kW since the wind resistance is not linear (I believe it is 4th
power of speed) so 5 hp at 1000 RPM is a good continuous power rating,
as long
as the higher RPM rating is equally higher.
The saving grace for EVs is that the "continuous" trip is almost always
less than an hour (Tesla excluded) so you can typically look at 1/2 hour
rating of compnents such as motors and those are way higher than 24/7
continuous ratings.

Cor van de Water
Chief Scientist
Proxim Wireless Corporation
Email: Private:
Skype: cor_van_de_water Tel: +1 408 383 7626

-----Original Message-----
From: Ben Goren [] 
Sent: Friday, July 25, 2014 7:42 PM
To: Cor van de Water; Electric Vehicle Discussion List
Subject: Re: [EVDL] Hybrid Mustang: batteries

Thanks for the detailed explanation, Cor.

I think I'm still having a hard time warping my head 'round torque and
power requirements and all the rest. I'm thinking I need some good way
to calculate (or at least guesstimate) *actual* power requirements so I
can compare with what I'm seeing on the performance graphs.

For example, the AC-51 has torque out the wazoo from a standstill, so
I'm assuming that's nothing to worry about.

Continuous (not peak) power, on the other hand, is 5 hp @ 1000 RPM. If I
have the math right, with the 6:1 rear end recommended by HPEVS and
25.5" tires, that's at 13 MPH. But how much power does it actually take
to maintain 13 MPH in a ~3500 pound 60s-era car? If more than 10 hp
(with both motors), I've got a problem. If less, I should be okay,

That same gearing would put the motor at 8000 RPM at about 100 MPH,
which I'm sure I can live with as a do-not-exceed speed (whether the car
gets there through the gas engine or the electric motor or both). *do* I figure out how much power is needed for a given speed?
If I knew that, I'd have a much better idea of how to figure out the

I've plugged some best-guess figures into these two calculators, and
they suggest that I'm probably okay...I think...but am I really guessing

> I am guessing that the question is if you can place the electric motor
> *before* the transmission?

Not if I want to go the hybrid route with the existing engine. And if I
was going to do a pure BEV conversion, I don't think this would be the
car for that -- probably instead a Ghia or the like.

> Or find the highest reduction diff for your Mustang that you can find,
> you need to keep the Revs up to keep the electric motor alive!

I'm pretty sure that's HPEVS's idea with the 6:1 rear end...which
wouldn't be cheap...and neither would be the custom transmission needed
on the other side of the electric motor to keep the gas engine happy.
All in all, I'd much rather keep the rear end and a generic

> Another alternative would be to place the electric motor next to the
> drive shaft with a sprocket on the motor axle pointing forward (next
> the output from the transmission) and a 4x larger sprocket on the
> transmission output.

My shade-tree mechanic thought of that some time back, possibly using a
Gilmer belt. The electric motors are much too powerful for that type of
a side-load application, alas.

Thanks again for all the help!

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