Power and Torque

The power of a bike is measured in horsepower (HP). One horsepower is the energy required to move 33,000 lbs by one foot in a minute, which is the strength of some perfect horse. It measure the rate of energy use - which is the same thing which 'watts' measure, eg. for lightbulbs. A human being can produce something like a quarter of a horse power. Bike engines produce anything from 10 to 200 horsepower.

In practise, you see bikes rated in brake horsepower (BHP). Brake horsepower is named after the machine which is used to measure horsepower, which is called the prony brake. The bhp rating is the maximum attainable hp for the engine. It doesn't account for losses of energy through the gearbox, chain, wheel bearings. Even if you have a 50bhp engine, you wont be getting your full 50hp of rotation power at your back wheel if you've got a rusty chain and corroded wheel bearings.

An engine clearly delivers a different amount of power at different speeds. When it's going slowly, it provides less energy each second than if it was going quickly. If you look at the power graph for your bike, you'll see what power it produces at different engine speeds (rpm).

Torque measures ability of accelerate. Again, your engine will produce different amounts of torque at different speeds. Typically, it has low torque at low revs, maximum torque at something like 60% of it's maximum speed, and then the torque drops off as it reaches maximum speed. The more torque you have, the faster you can accelerate. That's why bikes feel most responsive at something like 60% of peak revs - you're at the point of maximum torque.

Why are bikes less responsive when the engine is cold?