2.6 Torque

A wheel does not exert force directly. A wheel exerts torque. Torque is closely related to force. Torque is force applied at a distance from the center of rotation. The SI unit of torque is $\mathrm{N}\mathrm{m}$.

In our example, the diameter of a 185/60R14 wheel/tire combo is 596mm. The radius is 298mm. The amount of torque is $4445\mathrm{N}\times0.298\mathrm{m}=1325\mathrm{N}\mathrm{m}$. Because there are two wheels applying torque at the same time, each wheel only needs to exert 663Nm.

The U.S. unit to measure torque is pound-foot. Let see how we can do the unit conversion. A kg at one gravity pull is 2.2 pounds (force). One gravity is $9.8\mathrm{m}\mathrm{s}^{-2}$. This means one kg at one gravity pull is $9.8N$. Since $9.8N$ is 2.2 pounds, the conversion is that one pound is 4.45N. One foot is 0.3m. This means that one pound-foot is 1.35Nm. The required torque in our example is, then, 491 pound-foot at each wheel.

The physics symbol of torque is ``$\tau$'' (tau).