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Old 04-08-2009, 04:30 PM   #9
FC
Solving problems
 
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Join Date: Oct 2003
Carmudgeonly Ride: M5 / 718 GTS / Cooper S / GTI / LR4
Location: Metro Boston
Posts: 25,215
Quote:
Originally Posted by John V View Post
WHEEL torque is what matters, and WHEEL torque is engine torque times the reduction factor of the transmission.
Aboslutely.

In machine design in general it's all about the final delivered torque. My biggest machine, for example, has an electric motor that puts out a max of just 21Nm. But in my case, I had functional requirements that led me to know I needed about 1600Nm "at the wheeels" (with some room to spare given only approximations of drivetrain losses). At that point, it's a matter of finding the most (cost-)efficient way to supply that torque. Given the available gearing I could package/supply (and its trade off wrt the speed requirements) the 21Nm motor was the best option I had. And that guy gives me the 1600Nm I need.

In my case, I have no transmission. I have one gear ratio. I rely on the characteristics of the motor to allow me to work through the speed range.

As an example, decades ago, electric motor performance was poor and required us to actually have 2 speeds. Nowadays with brushless motors you can get away without that. Again, as with cars, since I can't change gears at ALL, I need the torquiest motor I can find that can deliver the most torque as far up in the rev range as possible. And I do run out of torque anyway (that's just physics). Fortunately, the performance envelope of my machine is such that I don't need much torque as speed goes up.

EDIT: This also explains why the little extra hp/tq that the ZHP gets over regular 330i's go a long way. It's not just an extra 8lb-ft, it's 8lb-ft AND the shorter final gear, AND an extra 300rpm to use it all.
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