When a Tyre Stops Working

A tyre is designed to keep your vehicle in contact with the road. When it stops doing that it is no longer working and your vehicle develops a skid. The dynamics of skidding are for another website; here we briefly look at why your tyre stops working.

The micro detail of a tyre would show its micro irregularities in its tread surface interacting with similar micro irregularities in the road surface. Rather like a lizard climbing a sheet of glass, its microscopic hairs on its feet clinging to microscopic roughness on the glass.

When the car is sitting still, the tyre is sitting still and there are no dynamic changes in that interface between two surfaces. When the tyre begins to roll though those micro interfaces create a resistance that results in grip. Close examination of the tread block will show that as force is applied by accelerating (or cornering) the tread block distorts allowing the tyre casing to shift slightly in relation to the tread interface with the road. This distortion of the tread block is called "slip". The amount of slip in the tread block is determined by its size and its compound. A small block will have more slip than a large block, a deeper block more than a shallow block. And the amount of "slip" is also determined by how much the compound will allow itself to be distorted.

Whatever the compound and the tread depth, once the slip has reached its maximum level, the tyre casing then distorts, this happens both in acceleration and deceleration and also in cornering.

When the tyre casing reaches the limits of its distortion, the tyre has nothing left to give and, suspension settings accounted for, the tyre loses its grip at the point of micro interface with the road and the tyre skids across the tarmac.

It is highly unlikely that any normal driver would ever notice the tyre actually deforming. However most drivers will experience from time to time what happens when the tyre stops working: The skid.

There is one area where this is of critical importance in tyre choice. Most car drivers select tyres that suit their vehicle and its everyday use. There is one exception. The SUV/4x4 driver. There is a propensity amongst some 4x4 drivers to opt for the biggest, chunkiest tyres they can find. If they actually spend a great time of their driving off the road and in conditions that require those chunky tyres then there is no problem with that selection. However, when the vehicle is on the road for the majority of its life there are safety issues to be considered.

Those big chunky mud-plugging tyres may work well in the mud, and may even give comfort in snow, but, they offer a false sense of security on asphalt in the wet, and can add metres to the stopping distance, even in the dry. Not really important unless that extra metre or so happens to be occupied by another vehicle, or a pedestrian.

Those issues on stopping distances also apply to car and van tyres. Though the visual differences are not always so obvious. In tyre tests there can be varied results, tyres can be good in one area of measurement and not in another. So, when making your tyre choice do listen to the experts, read any tyre reports you can find. Most reputable tyre fitters have copies of test reports lying around. And remember, the most important metre in your braking distance is the very last one.