Getting a grip on tire traction


Want to know about tire traction?

 

The amount of traction a tire has depends on a large number of things:

The construction of the tire affects its effective traction: stiff-walled tires (such as run flat tires) behave differently from those with more flexible side-walls.

The type of surface that where the tire used such as a wet or dry road, concrete, asphalt, dirt, sand, and others. Engineers talk about the coefficient of friction (COF) when comparing tires. For normal road tires the COF varies between about 0.7 on dry and 0.4 wet pavement.

The kind of rubber - the composition of the tread compounds will affect tire traction. Soft rubber molds to the shape of small irregularities in the road better than harder compounds and therefore grips better. But, there is a cost: soft rubber wears out more rapidly than hard rubber so there is a difficult compromise to be made in designing the ideal tire for a particular application.

The temperature of the rubber affects how 'sticky' it is. This is why the pit crews in a Formula 1 race team will keep spare tires in special heated blankets in readiness for a pit stop. Race drivers will sometimes deliberately slide their cars from side to side to try to heat up the tires prior to the start of a race.

The tread design can also make a big difference. In terms of grip, a bald tire (or a deliberately smooth racing 'slick') has good grip on dry pavement, while the same tire on the same pavement but in wet conditions has poor grip. When the road is wet, a grooved tire allows water to be squeezed out from under the part of the tread that's in contact with the road into the grooves between the treads. This allows more of the rubber to be in contact with the road providing better traction.

Under dry conditions the traction is not as good as a smooth tire because the grooves on the surface decrease the tire's contact area.

In the end, different tread patterns offer different compromises between wet and dry handling. And, to further complicate matters there are specialized treads for winter conditions, off-road driving and other uses.

The weight of a vehicle also contributes to the amount of friction created - this is why Drift Cars are stripped down to the bare minimum weight, so that there is less friction and the initiation of the drift is relatively easy.

When braking, accelerating or cornering, the weight of a vehicle shifts from one set of wheels to another so that some tires may lose traction while others are still gripping the road adequately.

Suspension alignment angles also affect a tire's traction. The best tire traction happens when the tread surface is flat on the road. Preloading a tire through adjustment of camber and toe angles is used to great effect in establishing and adjusting handling characteristics of a vehicle.

Inflation pressures have a further effect. An under-inflated tire presents a larger surface to the road compared to one that is over-inflated. However, side-wall flex is greater in an under-inflated tire - which can be dangerous. It also greatly increases fuel consumption.

Understanding the precise behavior of tires is no simple matter: the difference between dynamic and static friction of rubber is significant and the degree of flex in the side-walls of a tire also plays a large part in how it will perform in practice.

The force required to make a tire lose grip is much greater than that required to keep the tire slipping once it's moving. In practice this means that if a vehicle is made to turn too tightly or at too great a speed, causing it to start to skid, it can very easily turn into a skid that is impossible to control. In other words, it's far easier to prevent a skid than to get out of one once it starts. Remember this: it might save your life.


tireguy

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