Starts With A Bang

What does Torque in a Car do?

You’ve seen it a bunch of times when you’re reading the specs on a new car: the number of foot-pounds of torque that it puts out. Well, the 2009 New York Auto Show just happened, and I was reading the synopsis of a new Mercedes that just came out:

369 foot-pounds of torque, it says. (That’s 500 Newton-meters, for you mks/SI folks.) Torque is the amount of “turning power” you have, much in the same way you turn a wrench. 369 foot-pounds means that if you had a wrench that was 1 foot long, and applied a force of 369 pounds directly perpendicular to that wrench, you would get 369 foot-pounds of torque.

Well, what can this do to a car? The answer is: cause it to accelerate! The torque specification they give is the maximum torque of the internal-combustion engine, which is usually a higher value than the actual torque on the wheels. (See wikipedia for more details.)

But this torque can tell you a lot about how fast the car can accelerate. Let’s turn it into a physics problem. We’ll assume that this “500 Newton-meters” is an actual, legit value for how much torque the tires experience. We can estimate that the mass of a typical car is about 1500 kg, and that the typical distance between the center of mass of the car and the wheel’s rotational axis is about 20 cm; this gives us a moment of inertia for the car of 60 kg m^2. The car’s wheel size plus the sidewall radius of the tire is about 20″, or 51 cm.

The acceleration of this car? 4.25 m/s^2, or (more commonly), it can do 0-60 miles-per-hour in about 6.3 seconds. Want a car that can accelerate faster? Here are the things that can help:

Want to know which street-legal car hold the world’s record? This Sunbeam Tiger does 0-60 mph in 2.6 seconds!

So yes, it’s just a prototype, and in theory it can do it in 2.3 seconds, but that’s pretty good for some real-world physics, and now hopefully when someone brags to you about how much torque their car has, you’ll actually know what they’re talking about!