Originally Posted by
ericgu
Well, it's really pretty simple. Your motorcycle produced 20 NM, but it did it at 7500 RPM, while the human produced it at 75 RPM. To get 75 RPM out of the motorcycle, you'd need to gear it down 100:1, and that would multiply the torque by 100, giving you 2000 NM.
That's why higher RPMs are sought after in engine design. If you can double the RPM range of an engine without lowering the torque (you can't, but bear with me..., then you could gear the engine down 2:1 and get double the torque.
I think Eric's got it right, on an average a human produces around 30-40 N-m of torque at 80-90 rpm at the crank, if you have 46 teeth on your crank and 26 on your wheel, you have a ratio of 0.59, and therefore the torque at the wheel is 30x0.59=17.7 N-m, and 80/0.59 = 135 rpm at the wheel. And this the first gear of your mountain bike, as you shift down the ratio changes, 0.41, 0.33, 0.24 etc.
The scooter engine on the other hand produces 20 N-m at lets say 1000 rpm, therefore you'll need a different gear ratio, something like 2-3, to see an rpm of 300-500 at the wheel, the torque in which case becomes 20x2=40 N-m or 20x3=60 N-m at the wheel.