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#19
11-06-2010, 10:01 AM
 lloydn Junior Member Join Date: Dec 2009 Posts: 7
Re: Smart Fortwo EV..... High power version!!

That looks like an interesting equation there Duncan.

Just wondering where you got it from, and the units you are using, because I can't seem to get it to work with numbers I've been running, and I'd be quite interested in it.

Just to illustrate, I have an 02-05 Ford Fiesta currently. Weight is about 1045kg. Max Torque from the 1.25 is 110Nm. Wheel radius is 0.292m.
 Motor Torque (Nm) Gear Ratio Wheel Torque (Nm) Wheel Force (N) 110 15.2866 1682 5760 110 8.2411 907 3106 110 5.4656 601 2058 110 4.0565 446 1527 110 3.2452 357 1223
"Vehicle weight on the driven end x tire coefficient of friction (0.8 for "inexpensive" road tires = Max force the tires can use"

and an estimate of 60% weight on the driven wheels (627kg)

627 * 0.8 = 501.6N

Much lower than the force available in any gear. And Force divided by mass gives acceleration, so that would be 0.48 m/s^2, or a 0-62 of about a minute. Pretty sure my car isn't that slow... So I think it must be a different units thing, please let me know what you're equation is using.

What I've been using to judge the maximum usable torque is:

Torque = distance (radius of wheels) x mass (kg) x acceleration (m/s^2)

Then looking at other road cars to see what a reasonable max acceleration is. The maximum seems to be about 0.5g for FWD, and 1g for RWD.

Using that equation on my Fiesta gives about 1497Nm (about 5 times the force of the previous equation). First gear makes more sense with that result.

And for Yabert:

On the Smart, 0.287 radius wheels, 750kg, RWD, gives 2112Nm maximum. I'm not sure the smart is really set up to handle that much though, so just think of it as an absolute maximum.

In order to achieve 0-50 in 5.2 seconds (0-60 in 6.3) with direct drive, you'd need about 1200Nm of Wheel Torque, or 300Nm of Motor Torque with a 4:1 ratio. That is about 56kW of power.

If 90mph is the max speed, and 4:1 is the ratio, then 60mph is 3440rpm; so you need a motor that can develop 300Nm or more Torque from 0-3440rpm. That's 0-60 in 6.3s ish.

If you did get the torque of a warp 11 at 1000 Amps (434Nm, according to evsource), you'd get better than Tesla Roadster level acceleration.

I don't know much about motors, so hope for one of the more knowledgeable guys to come along, but this is my best guess with what you have.

The 9.6kW at 48v is 200A continuous, similar to the warp motors. The commutator section looks quite big, so should overload quite well, and will probably take the 1000Amps if they Kelly really supplies it, for a short time. I don't know how much torque per amp you will get though, if there is a torque rating on the motor, please let us know.

5000-6000rpm for that size motor would seem like a reasonable limit. If the motor has an rpm rating, it would help to know that too.

My concern would be that at 150v it might be spinning too fast, or if it is spinning at a good speed, would not be producing the torque that you need.

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