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Where the rubber meets the road

Tires. They're round, they're rubber and they roll. They come in a bewildering number of sizes, shapes and tread patterns. But have you ever considered what part of the tire actually does the work?

It is that small portion of the tread called the "Contact Patch" -- the shaded elliptical area in the picture at the start of this article. Everything that happens between the car and the pavement happens in this small part of the tread, an area you can probably cover with your hand.

When the car is moving and the tire rolling, the contact patch is, paradoxically, "stationary," relative to the road; or rather, each small part is, as the tire rolls. Think of it interlocking with the pavement the way one gear interlocks with another as they turn. What's more, this interlocking works in all directions (see "The Friction Circle" for a further explanation).

If you accelerate, it is the contact patch that takes up the fore-and-aft strain, same with braking. When you turn a corner, the contact patch keeps you from sliding sideways. It even twists as tire goes around a curve, allowing you to stay on course, rather than sliding off the road.

When the tire is rolling freely, the contact patch keeps changing, relative to the tread, the same way individual gear teeth only contact each other for an instant. When the tire is not rolling, that little patch of rubber takes abuse. If you lock up the brakes in a panic stop or spin the car, rubber is worn off in that one place. That is when the tire develops a "flat spot" -- the tread is no longer circular, but "C-shaped."

Depending on how much rubber was ground off, a flat spot can go unnoticed or it can be a serious problem. What you would hear and feel is a rapid thumping noise each time the flat spot hits the pavement. And if it is very badly worn, you could grind entirely through the tire, causing a hole in the tread...and a flat. Fortunately, this doesn't usually happen at legal speed.

So be careful on the roads, be nice to your tires and have some respect for those little ellipses of rubber. Do that and they'll see you safely home every time.

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