Quote:
Originally Posted by Pete_vB
Let me explain how force at the wheels is calculated, so you understand where I'm coming from.
Engine Torque (ft lbs) x Gear Ratio x Final Drive Ratio / Tire Radius (ft) * Drivetrain Efficiency
For the 1M:
370 (ft lbs) x 4.11 (1st gear ratio) x 3.154 (Final Drive) / 1.06 ft x 85% = 3846 lbs
For the M3 DCT:
295 (ft lbs) x 4.78 (DCT 1st gear) x 3.154 (Final Drive) / 1.06 x 85% = 3566 lbs
So unless one of the numbers above is wrong or the losses for the two cars are much different, the 1M should have 8% more "Force At Wheels" in 1st gear. Your graphs show 9% less, or an 18% difference. Hence my questions.
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There is nothing wrong with this basic calculation (you should include wheel radius though as well). I'm not debating that the 1M can output more peak wheel torque than the M3. You can see evidence of the same thing in my acceleration graphs (bottom right most ones, from about 1-1.25 seconds).
It is just that in the real world your calculation is fairly meaningless.
Why? Because one needs to examine power delivery over time, not at an instant. These are peak/instantaneous values matter much less than the observation that the average acceleration values at the same time and across time are higher in the M3. Also note that having so much torque to the wheels often simply overwhelms the available grip (in 1st gear) and thus you have wheelspin. The metric that with a single number best captures the dynamics of acceleration across a given time and through multiple gears is power, to be more specific indeed power to the grounds per weight. Assuming losses are pretty roughly constant across different models and that gear ratios are pretty well optimized by the factory we can simpy short cut that longer statement to:
Power to weight is the key metric in determining a cars performance.
Another way to help realize this is simply that F=Pxv. Given two cars each at the same velocity the one producing more power (at that velocity, i.e. using the best (lowest) gear and highest rpm) will produce the most accelerative force. When you look at it this way gearing and torque disappear from the equations!
Again another way to look at this is that the M3 on WOT operates typically from 7000-8400 rpm (not counting the very begining of a launch) and thus is producing a minimum of ~350 hp (7/8.4 x 414). The 1M never even produces that much! Well if under rated, its peak might be just about that much. Again the power of high redline and broad torque curve is infinitely more important than peak engine or wheel torque.
Hope this all helps.