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Wider tires & braking
By Mark Warner* (10/2/2000)

Do wider tires improve braking distance?

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Before we go any farther, let's more precisely understand what is meant when you say that wider tires improve braking distance. I assume that we're talking about braking distances vis-a-vis threshold braking (as opposed to just stomping on the brakes and locking it up). This means that we're applying the brakes JUST to the point of braking free, then very quickly releasing them, and then very quickly reapplying them. You repeat this sequence many times until the car actually stops. (In effect, you're acting like a human ABS system.) The distance between first touching the brakes and the point where you actually stop the car is what we'll call the braking distance. Further, let me assume that we're comparing two tires that are identical in all respects (diameter, material, etc), except for their width.

Now let's look at what happens at the tire-to-road interface. In a perfect physic's laboratory world, one usually asserts that the friction force (i.e., the braking force) between two objects is a function of two things: the coefficient of friction at the interface, and the normal force. (Muddying the waters more than a little bit is the fact that this is only true for two "smooth" surfaces sliding against each other. It doesn't take into account that rubber is relatively malleable, and tends to fill the small nooks and crannies in a roadway, thereby changing the nature of the "braking" problem into one containing both frictional and shearing-type forces. This turns out to be a VERY difficult complication. Worse, the vulcanized forms of rubber used in most automotive tires has rather weird coefficient properties that don't behave nice and linearly. But I digress...)

Regardless, let's assume for a moment that we're testing the brakes in a laboratory, where we can just consider coefficients of friction and normal forces. For two identical cars, the normal force (or vehicle weight) is the same, so this is a non-factor in affecting the braking distances. That leaves only the coefficient of friction. If we assume that the two tires are constructed of the same material, then they should have the same friction coefficient, right? Well, yes, except that the wider tire has more surface area than the narrower one. I'm talking about the "circumferential" area around the outside of the tire. This is given as Pi x Diameter x Width. Pi and Diameter are constant, but the Width's are not, by definition, the same. Therefore the wider tire has more surface area.

Why is this important? Well, it turns out that the coefficient of friction of rubber on asphalt is dependent upon temperature (actually, it's usually measured as a function of sliding velocity, but it ultimately is a function of the heat generated at the sliding interface). In our case, during threshold braking, we get instantaneous, "micro" sliding action between the rubber and the road. This causes heat to build up, which drops the coefficient of friction. A wider tire, having more surface area, can more easily dissipate this heat energy than can a narrower tire. One reference I have claims that this effect can be as much as 5-20%.

But wait a minute! Why do F1 drivers want to warm their tires up? Isn't this to improve their tire's "stick?" In a word, yes. But as I alluded to earlier, rubber ain't linear. It ain't even progressively constant. Racing tires need to get up into a bounded temperature range to exhibit their best performance. Too little heat, and the coefficient of friction is lower. Higher temperatures, oddly, have the same effect-- namely lower friction coefficients.

*While Mark is no tire company engineer, he is a degreed and licensed mechanical engineer, so is probably as qualified as anyone to answer the posed question.


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