acotrel said:
Surely the fastest you can go on a long straight stretch of road is determined by getting the highest gear the motor will pull while having the revs at the point where maximum torque occurs - If that point is lower than the revs where maximum horsepower occurs, would the motor still rev further than the revs at max torque ?
Surely when the heavy crank is wound up and the motor is producing maximum torque and pulling the highest gear it can without dropping revs ... that is all the speed you will get ?
Not really.
It should be clear from simple physics that max top speed comes at max horsepower, but that doesn't seem to have worked for everyone, so I'm trying for a more intuitive explanation of why you don't gear for top speed at the engine torque max. What you are doing is gearing for max torque at the rear wheel with overall gearing that will still let you reach maximum top speed, and that doesn't turn out to be the max torque point on the engine curve. This is my best shot at explaining that in a more intuitive way.
On a given bike that is geared for maximum top speed, let's assume that it's a Commando with a torque peak at 5,000 rpm. Let's assume that we've tuned it up well enough that it will do 120 mph at 7,000 rpm for it's maximum top speed. Those are actually fairly realistic numbers. Now you think, great, I can just change the overall gearing so it's hitting 120 mph a little below the torque peak, and then it should be able to pull past that as it gets to the peak torque point. Let's say you think 4,900 rpm would be a good number to gear for. To do so, you have to change the overall gearing by a factor of 4,900/7,000, or 0.7. For example, if your overall gear reduction from crank to rear wheel was 4:1, you'd have to reduce it to 2.8, so you would be at 5,000 rpm at 120 mph. Unfortunately, you've also just reduced the torque multiplication from crankshaft to rear wheel by the same 0.7, and you no longer have enough force at the wheel contact patch to overcome drag, so you'd never get near 120 mph. If you just look at engine torque, you are leaving out the torque multiplication in the gear train. If you gear so that you get maximum speed at the horsepower peak instead of the torque peak, you are getting less torque at the crankshaft, but because of the gearing, you are still getting more torque at the rear wheel, where it counts. Because horsepower is a product of torque and rpm (and some constants), as long as the horsepower curve is still increasing, you can gain speed by gearing taller, until you reach the point where the torque curve drops off faster than rpm increases.
People get confused about the relationship between torque, horsepower, and top speed fairly often here. There's probably a much better explanation than mine somewhere in books on how to set up race car gearing, but I couldn't find one to copy. More graphs and examples would probably help, but it's more work to generate them than I'm willing to do. This is the best I could manage, so take it in that light.
Ken
