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Torque and horsepower are often confused when it comes to discussions of dirt bike and ATV power. You know they are not the same, but what exactly is the difference, and how are the two related?
TorqueTorque is a measure of the amount of twisting (rotational) force acting at a distance and it is measured in foot-pounds (lb-ft) or Newton-meters (N-m). Torque is higher at low rpm while horsepower is higher at higher rpm. You generally want to have as much torque as possible at low rpm's, to get you moving. Bikes with lots of low-end torque do great wheelies and are better able to claw and scratch their way up chunky and steep trails at slow constant speeds.
Horsepower (HP)Horsepower is a measurement of power, which takes into account the rate at which force is applied, in addition to the amount of force. In radial (circular) motion, horsepower becomes the direct result of torque and rpm. The formula listed below is a constant formula, so you can figure out the horsepower of any engine if you know the torque and rpm (revolutions per minute).
Horsepower = Torque x RPM/5252Taking a look at the equation reveals, interestingly, that at exactly 5252 rpm, the horsepower and torque numbers must be equal, so if you ever see a torque vs. horsepower graph where the lines don't cross at about 5252 rpm, it's time to start asking questions. (Why the puzzling 5252 constant? One Horsepower is defined as 33,000 lb-ft / minute. The circumference of a one-foot circle is 6.2832 feet. Since these are both constants, you can divide 33,000 by 6.2832 to get 5252, reducing the equation to a very simple approximation.)
So which is more important?Both are equally important. One is a measurement of force and the other a measurement of power, which is force over duration of time. So if having high torque is like saying you can bench-press 300 pounds, having high horsepower would be like saying you can bench-press 300 pounds, per second… quite different!
Dynamometer Test and Torque CurvesPower isn't everything when it comes to dirt bikes. On the trail, often times, less is more. But, if you want to know the actual power of a bike, you'll undoubtedly be able find someone online who has dyno tested it. This form of engine analysis will give you accurate torque and horsepower numbers at any rpm. The tricky part is figuring out whether you can trust the technique and motives of whoever did the testing! Remember to look for the cross at 5252 and to pay attention who is making the claim, and why. Also don't forget to take into account where the measurement was taken; at the crank? at the wheel? All kinds of things can affect the measurement. Measurement at the engine's crankshaft is a more accurate representation of what the engine can produce, but measurement at the wheel gives a more realistic look at the power you are actually putting to the ground. The gearbox of the bike and the inertia of the rear wheel itself will all affect the power delivered to the ground.
What To Look ForTrail bikes are usually manufactured and tuned to have plenty of low-end torque, but any bike can be geared for the demands of slow and steep trail riding. If you don't have the cash to go out and get an aftermarket exhaust kit to change the torque curve on your bike, you can try the simpler, cheaper solutions. For example, motocross bikes, including smaller two-stroke bikes like 100cc and 125cc can be made into great trail bikes by simply adding a 7-12 ounce flywheel weight, and a larger rear sprocket.