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Power and Torque

Power and Torque is best explained by starting with torque and then move on to power

Torque (expressed in lb-ft) is defined as a FORCE around a given point, applied at a RADIUS from that point.

The easiest way to picture what torque is, is by loosening a wheel nut. Try to loosen the nut by pulling on the spanner at the same end as the nut (very little torque) and you'll struggle to loosen it. Pull on the spanner at the end of its shaft (maximum torque) and you'll loosen it quickly.

As car scribe Ralph Hosier explains: if you weighed 180lbs and the spanner was a foot long and you had all your weight bearing down on the end of the spanner, then the torque at the bolt is 180lb-ft.

Power (expressed as horsepower) is torque (the twisting force) multiplied by speed (how fast it's spinning). Power is therefore a measure of how much WORK can be done in a specified TIME.

Which is better: Power or torque?

The answer to this question (for bikes) is dependent on your riding style. If you're running a torquey, big volume engine, you'll find that you don't need to down-shift quite so often when accelerating from lower speeds - this is because more torque gives you more available access to power at lower revs. This makes driving (generally) more relaxed.

If your engine has little torque, you'll have to change down a gear more often to explore the rev range to unleash the engine's power. Vehicles with lower torque figures are generally cheaper to run, but the engines have to be worked harder to get up to speed.

Advanced Info

1. Horsepower = Torque x RPM/5252.

2. One horsepower is the power needed to move 550 pounds (249kg) one foot in one second.

3. A cbike's gearing can make a high-revving engine put down a lot of effective torque to the ground. If the gearbox and differential reduce crank RPM to wheel RPM by 9 times, you also get 9 times the torque at the wheels. Gearing can essentially convert low engine torque to high effective torque, but at the cost of speed (RPM).

Power is expressed as;

horsepower - hp or bhp
kiloWatt - kW
1 hp = 0.746 kW

Torque is expressed as;

pound per feet - lb-ft
kilograms force - kgf-m
Newtons - Nm

Much power, high rpm

Much torque, low rpm