Posted by hogryder on September 03, 1999 at 02:11:30:
Here's the best explanation of torque vs horsepower I've been able to find. It's from the Cycle Doctor's March 1, 1999 Q & A page. Happy reading.
I could talk about horsepower and torque for hours and we should probably do a full story on these funny concepts sometime. For now I will just answer your particular question.
Torque is strength; horsepower is speed. So to speak. Torque is a measure of how much weight you can move; horsepower is a measure of how fast you can move a specific weight. For example, human weightlifting contests are contests of torque. Each competitor must lift the same amount of weight and the guy who can lift the most is the guy who has the most torque. If weightlifting contests were contests of horsepower then the winner would be the guy who can get a specific weight over his head the fastest.
Horsepower was established exactly in that manner. A rope was harnessed to horse, then pulled straight back, over a beam and down to the ground. A known weight was attached to the bottom end of the rope and then someone spanked the horse while someone else held a stopwatch and timed how fast the weight was lifted a specific amount. The weight lifted to time ratio of that experiment was then labeled one horsepower. So, by and large yes, horses generally have about one horsepower.
One horsepower equals 550 foot-pounds per second. This is where many get confused because we all know that our "torque" wrenches read in foot-pounds too, and we've never seen anyone using "horsepower" wrenches. But you must remember that "torque" wrenches don't have any "per second" requirement. If you held your torque wrench at 40 foot-pounds all day then you would be applying 40 foot-pounds of torque all day long. But while doing that in hp terms you are applying only 40 foot-pounds per twenty-four hours, which as you can see is piles less than 550 foot-pounds per second. So, while doing that you are expending a very small fraction of one hp. If you used your torque wrench as fast as you could then the more foot-pounds per second you applied would be the result of expending more hp.
So torque is the measure of the strength it takes to pull a stump. Horsepower is the measure of how fast you can pull a stump. But as you know, stump pulling requires a great amount strength, not strength applied quickly.
Again, horsepower is a measure over time; torque is simply a measure of strength. This is why peak horsepower numbers are silly and meaningless. They don't represent a picture of a motorcycle's potential because a peak number only shows horsepower at one specific rpm, which is usually only achieved for only one specific moment in time. Since horsepower is a measure over time you need to look at what an engine is doing throughout its complete horsepower curve. How fast a bike accelerates is its measure of available hp throughout the machine's revs. You have probably noticed that, when you reach revs that produce more hp, the revs then climb quicker, and you go faster quicker. If you have 50 hp at 5,000 rpms and 150 hp at 10,000 rpms then you only have 50 hp when your bike is at that lower rev number. Period, end of story. You only get to use the 150 hp at the revs at which your bike produces that amount of hp. If you never rev your engine to that peak number, then you do not experience 150 hp. Torque, though, peaks out at a very low number so most everyone feels the full push of a bike's torque as a matter of course.
We all talk about hp in reference to how fast a motorcycle accelerates but for some reason many seem to forget that while a motorcycle is accelerating it is driving through its revs and not just spinning at redline. Bikes that accelerate quickly have lots of hp throughout a broad range of rpms or they are geared in such a way that the rider can keep the machine within the small range of rpms that produce the most power. Racers don't go below 9 or 10 grand rpms because the hp below those revs is much lower and they don't have time to sit around waiting for only a few horsies to push them forward.
Peak numbers are only for bragging between squids. The excuse for mentioning a peak number is because there is a misconceived probability that the hp numbers below that peak will be generally high, also. Unfortunately, the reality is that there are probably dips and jumps in many bikes' power curves and that they are actually nasty beasts to ride. But it's much easier to blab about a fairy tale number than to carry around a printout of a curve. That has its own foolishness, too, though.
So, in a sentence, hp is what makes you go faster quicker and torque is what gives you the ability to get going in the first place.
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