Candid Cameron

Candid Cameron

Paul Dean’s answer about the wisdom of mounting car tires on motorcycles (October, 2004) was right on track, especially regarding rubber compound, construction and the changing effective diameter when a bike is leaned into a corner. But what about the effect of the contact-patch area on traction? In engineering classes, we’re taught that the force necessary to begin to slide an object is dependent upon only two factors: coefficient of friction and the force normal to the plane. The simple formula for a tire on pavement would be: the force required to slide equals the coefficient of friction times weight. The area of contact is not part of this equation. This makes sense if you consider that as the contact area increases, the load/unit of the area decreases proportionately. Likewise, as the weight decreases, the load decreases proportionately. The net effect is zero. But now I’m wondering: Does a wider tire (greater contactpatch surface area) really give greater traction? Mac McReynolds Tampa, Florida

The classic "laws of friction ” apply to the limited class of materials in which the actual area of contact is in direct proportion to the applied load. Two surfaces in contact with each other consist of profiles that are rough when seen at some small scale. In light contact, only the tips of the surface “asperities ” touch each other; but as the load is increased, the asperities in contact are crushed somewhat, allowing lower asperities between them to come into contact. And so it goes, the area in actual contact increasing directly with applied load. At the points of contact, various kinds of short-range effects—ranging from outand-out welding of the materials to each other down to weaker forces—cause the actual surface-to-suiface friction, and their action is, again, proportional to the actual area of contact.

Rubber is different and does not obey the classic friction law. Because rubber is highly elastic but under extreme stress becomes much less so, its defection under the first unit of applied load is significantly larger than that under the second unit, and so on. Thus, if a large area of contact is desired, it is better to apply the load to a large area than to a small one. This non-linear “spring constant ” of rubber is why wide tires do indeed grip better than narrow ones. -Kevin Cameron