This really is an Old one... Chriso pretty much got it though, hp is a function of torque. If you want more information read the Wiki article on it here: http://en.wikipedia.org/wiki/Horsepower . To sum it up, the term horsepower originated from the days of the horse and buggy. It was used as a way for the sellers of horses and then afterwards cars to measure the output of their products. It is calculated from the torque, which is the twisting force of a rotating object.

The one interesting thing to consider with a sport bike, is due to the fact that they are so light, the bike will actually lurch forward when shifted out from redline. The engine is spinning so fast that when you let the clutch out it will bring the bikes speed up as well as lower the speed of the engine, even if you shift and immediately cut the throttle.

 

ummm, if i am supposed to shift near the torque peak what is that extra 4,000 rpms there for? making a loud ruckus and pissing off the neighboors?

hear is my rough, not mechanical engineering look at it.

if you shift based on the torque peak you are maximizing the torque out put of your engine, but since you are changing gears, you reducing the amount of torque delivered to the back wheel, because you are changing the gearing to go faster with out the engine spinning faster. but if you wait to shift until the HP peak (or after, anyways, baseing you shift on the HP peak) you don't reduce you effective torque on the back wheel until you have to (you want to go faster but you engine can't) Thats why the HP number is used.

Its like say you are riding a 10 speed road bike, you legs can put the most force on the pedals going very slowly, like 5 rpms. (lets say 100 pounds and the cranks are 1 foot long, that equals 100 ft*pounds). so to accelerate you should be in a very high gear right? no, because, because the back wheel is going to be rediculously hard to move compared to the lower gears, but you legs do have a limit to how fast they can pedal before they just start spinning uselessly, that limit is like your rev limiter, and some point inbetween that and the 5 rpm torque peak is your peak HP for your legs.

if you dont get what i am talking about take road bike turn it up side down and put it in the highest gear, have a friend put both hands on the pedals and try and turn them, while you try and turn the wheel the other way with one hand, you will win.

thats why your bike is a monster in first gear, but tames out by the time you get to 6th. because of the gearing you engine is able to deliever less torque to the back wheels, you gear box converted that torque into speed (angular velocity)

think of a wrench on a tough bolt, if you put a breaker bar on the end you an deliver more torque to the bolt because torque is a function of force and distance to the fulcrum. if you double the length of the wrench, keeping the force the same, you double the torque. or you half the force required to deliver the same torque

but to turn the bolt a full turn (keeping the resitance constant) it will take the same amount of work, because work is force x distance. and if the lever is twice as long, it takes half the force to turn it, but it has to go twice as far.


have fun thinking that through. feel free to correct me, or ignor me and shift at 11,000 rpms