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Originally Posted by Blainestang
I have to disagree with a LOT of that explanation...
If you DO read that, you really should read THIS POST as a follow-up.
It's a post of mine from the last time someone pointed out that vettenet explanation. |
I have to agree more with vettenet than you. For the moment, forget torque and horsepower and recall from high-school physics a = m / f. Any force applied to the mass of your car is derived entirely from torque. Consider a car weighing 2000 lbs and is generating 100 ft/lbs of peak torque at 4000rpm. Let us also assume a 4:1 total drive ratio (let's say a 4:1 final drive ration and 1:1 transmission gear ratio). This means that without any losses we're generating 400 ft/lbs of torque to the drive wheels with a drive wheel speed of 1000 rpm.
(To keep everything else simple, let us also assume no frictional losses, no wind resistance, a perfectly flat surface, no wheel slipage etc. etc. You know, the usual though experiment assumptions.)
But to determine the force applied to the car, we also need to know the diameter of the wheel. Let's say the diameter of tire is 26", that makes the radius 13" so the force transmitted to the pavement is 400lbs * 12 / 13 = 369lbs. More specifically, the
thrust is 369 lbs.
Back to our weight of 2000 lbs, treating this as mass with an applied force of 369lbs we get an acceleration of 369 / 2000 = ~0.18G
Now let's change one thing: we have a car of the same weight but now we're generating our 100 ft/lbs of torque at 8000rpm. So to be at the same drive wheel speed of 1000 rpm, we'd have to downshift a gear of a 2:1 ratio thus doubling the torque with a total drive ratio of 8:1. It shouldn't be any surprise that our thrust has now doubled to 738 lbs as does our acceleration to ~0.36G
Hence vettenet's statement that it's better to make torque at high rpm than at low.
But what about peak horsepower? Again, it's a matter of taking advantage of gearing to maximize
torque applied to the wheels. Our car makes 100 ft lbs of torque at 8000rpm which translates to (100 ft/lbs * 8000 rpm) / 5252 = 152 hp. But let's say at 9000 rpm our horsepower peaks at 163hp which means our torque has dropped to 95 ft/lbs. This means, that for the same drive wheel speed of 1000rpm we were doing with the 8:1 total reduction ratio, we now need a 9:1 ratio. This increases our 95 ft/lbs to 855 ft/lbs and has increased our thrust to 789 lbs and our acceleration is now ~0.39G
But what if we hadn't changed our total drive ratio to 9:1 and left it at 8:1? Our drive wheel speed would have increased from 1000rpm to 1125rpm but the torque to drive wheels would have dropped from 800 ft/lbs to 760 ft/lbs, thrust would have dropped from 738 lbs to 701 lbs, and acceleration dropped from ~0.36G to ~0.35G
So in summation: the only thing that causes acceleration is force. Torque is force. Horspower is an expression of torque over time. Acceleration is what you feel and you feel the most at peak torque.
- Mike