Torque Calculation for Dummies

motor torque vs speed in RPM

Want a good question? Well, here you go: what kind of answer do you expect from an English Literature student if you ask him "How to Calculate the Torque of a Motor"? Odds are that – especially if the BA type answering the question is an "eHow.com contributor" – you're going to get one that's pretty much nonsense. That's almost exactly what some poor schmuck would get if he or she depended on an answer out of the likes of Soren Bagley at ItStillRuns.com.

After some hemming and hawing about torque, rubbish like

"The torque of a motor is the amount of force that an engine generates. This force is used to rotate a lever arm attached to the engine, which in turn propels the entire vehicle"...

...which, we guess, is a description of an antique steam engine, but doesn't quite capture what most people think of when discussing a vehicle's torque. But we'll get back to that later. According to Soren, if you want to calculate torque (and he supposedly got this from the now-defunct ask.com [not really], which is a perfect example of the blind leading the blind, in our estimation), you calculate torque by, Calculate the length of the motor's lever arm... Calculate the force that the motor is capable of exerting on the lever arm... Multiply the length of the motor [sic] by the force of the motor...

That's a half-baked approximation of the physics formula for torque – T = F*r*sin(θ) – but it most certainly is not a method of calculating motor torque. Torque of a motor is sometimes estimated (not calculated) by the formula  T = 5252 * (horsepower / RPM). In other words, Soren's totally ridiculous set of instructions, which supposedly require that you measure the "length of the motor."

What is most disturbing is that DMS (Demand Media Studios: you can't spell "dumbass" without "DMS") actually paid Bagley for this tripe to replace a version of this content mostly copied from the CarsDirect article cited in the previous paragraph. While obviously written by someone who knew only slightly more than Bagley, at least it was correct(ish)... meaning that the original author would not have been named the Dumbass of the Day.

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