Torque Defined, With Math

As anyone who knows me will tell you, I like to talk cars... A LOT. So naturally I attend many car meets and events and have had a chance to talk to lots of car enthusiasts. I often hear people mention that such and such car has x horsepower and X torque.

The other day I found myself stumbling over the definition of torque. Don’t let this happen to you. Read an outstanding definition of what this “torque thing” really is.

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As anyone who knows me will tell you, I like to talk cars… A LOT. So naturally I attend many car meets and events and have had a chance to talk to lots of car enthusiasts. I often hear people mention that such and such car has x horsepower and X torque, but I should be extra impressed with the torque number because that’s what’s really important. If you consider yourself a car enthusiast than you have certainly heard some version of this, or maybe the saying: “horsepower sells cars, but torque wins races.” In reality, this misconception couldn’t be farther from the truth.

Horsepower and torque

Horsepower and torque are related in that horsepower is a function of torque and rpm. In other words, horsepower is the rate at which torque is delivered. The equation for horsepower is

hp = (torque x rpm) / 5252

To explain how this works, let me first give a basic physics lesson. A unit of torque consists of a unit of distance followed by a unit of force (or in some cases a unit of weight, which on earth can be considered a unit of force due to gravity). The unit of torque defines a situation in which the unit of force is attempting to rotate a hypothetical lever of length defined by the unit of distance.

So 1 ft-lb describes a situation in which a hypothetical lever of length 1 ft has a force of 1 lb trying to rotate it about a pivot. The force and distance values are multiplied together to give the total torque, for example a 2 lb weight sitting at the end of a 2 ft long lever would provide 4 ft-lbs.

Imagine for a second that you were to attach an 8 ft long 2×4 to the wheel of your car and you stood on the far end to attempt to turn the wheel. I weigh 150 lbs, so in my case this would provide 1200 ft-lbs (8 x 150) of torque. So if you can make 1200 ft-lbs of torque with your body weight and a 2×4, why do we need engines? Because even though you are making as much torque as a Mack truck for an instant, as soon as the wheel begins to turn you will hit the ground and have to reposition the lever in order to move the wheel again. Since horsepower is the rate at which torque is supplied, and torque cannot be continuously supplied in this situation, horsepower is essentially 0.

What is Torque?

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