Monday, March 2, 2009

The Tire Contact Patch

Tires are a very interesting element as they have such a significant impact on the handling of your car. Humorously though, they are pretty misunderstood. How a tire keeps the car gripping the road is an extremely in depth and technical issue, so lets preface this discussion by saying we are going to only go in to the broader generalizations and some of the more easy to understand points. It gets very complex when you start trying to approximate contact patch deformation, sidewall spring constant, pressure distribution etc. There are a lot of great books out there, so definitely find them and read them, but the hope here is to get you thinking about a few simple items to begin with.

So far starters… wider is better right? There isn’t a straight answer to that. So lets discuss what happens with the tire first of all. Think of a balloon for starters. If you could put it on a piece of glass and look at the area that is “flat” where the balloon contacts the glass you’ll notice one thing quickly and simply – the more pressure you apply the bigger that area gets. This is a pretty simple concept. When you have a weight applied to an inflated item it has to respond with a force equivalent to the weight to be able to support it. This applies to your tires on your cars as well. As I mentioned before, though, this gets pretty complicated when you really consider how much support the sidewall gives etc, but for now lets ignore that. Most tires for people on high performance cars will run in the 35-45 psi range. So lets choose 40 psi as your inflation pressure. Let’s also use my BMW 318i as an example. It weighs about 2800lbs and is very close to 50/50 weight distribution. So we’ll assume the car is properly corner weighted (corner-weighting is when you adjust spring height and rate etc to distribute the car’s weight evenly across the diagonals and each side/edge of the car) so that the weight is evenly split between each of the 4 tires. This means that each tire is getting 700 lbs of weight on it. At 40 lbs per square inch (psi) that means we need 17.5 square inches of area to support that for my car. Sounds big right? Not really. My car during the summer runs a tire I can only get in a 195 width. So that means it is roughly 7.7 inches across at the point where it touches the pavement. If I’m 7.7 inches across, to get to 17.5 inches means I only have a rectangle that measures 2.3x7.7 inches. Not a lot of area to keep that car on the road is it! So now lets go to a wider tire and see what happens. If I keep the tire pressure the same but go to a 235 width tire my contact patch now measures 9.25x1.9 inches. It is still 17.5 square inches at the road, it’s just wider now. Yep, that’s right, you just went WAY up in tire size and you still have 17.5 square inches in contact with the road! What you have done is changed the shape of your contact patch. Now, this is somewhat misleading because with many of the larger tire sizes you can run a little less tire pressure which will increase your contact patch for real. However, as you reduce pressure you lose some of the structure effects in the tire and you may actually end up with strangely shaped or poorer contact during cornering as you roll on to your sidewalls or have other issues. This is where you need to work on properly balancing the pressure for your tire.

It is also very important to keep in mind tire compound. Often you can achieve much more grip with a smaller contact patch area due to the adhesion factors of certain tire compounds. Tires generate their “grip” in a few ways. One way is simple generic friction between the surfaces and the way they bond to one another. This is greatly affected by compound. A second way is mechanical grip that is generated by interference between the objects, for example, if the rubber complies to the shape of a gravely pavement, you will have some significant vertical walls the rubber is being pushed against and torn away from the tire by. This generates grip as well. The third major grip component is adhesion. Some tire compounds actually are truly “sticky” and will adhere to the surface they roll over. This allows coefficients of friction greater than 1 to be achieved! What this means is that for every 1 lb of force applied down on the tire, it can generate more than 1 lb of force perpendicular to that. This is HUGELY dependent on compound and is a significant factor in the advantage that race slicks will have over street tires (although that changes more and more every year with some of the crazier street tires).

You also need to consider what happens to the contact patch during maneuvers. A tire actually provides the MOST grip at a point when it is “sliding” a little relative to the angle of travel. This is referred to as slip angle and varied greatly depending on tire design. It is often in the range of 3 to 10 degrees though. This matters because as you make your tire contact patch wider you change the amount of difference between the arc radius the inner portion of the tire travels versus the outer portion. This can lead to those portions of the tires actually traveling at different slip angles from one another and it can lead to conditions such that one portion of the tire may be at optimal slip, while another is not. This is one relatively extreme case where wider would indeed NOT be better. Also, as tires get wider things like turn in response and their behaviors under braking or acceleration and the changes in the tread deformation introduces a lot of other variables and possibilities. Again, the topic is very complex so it is hard to argue it fully in a short essay. I’ll try to revisit the topic in the future and get into some of the more specific details for the above, but hopefully this got you thinking and drop at least a few new ideas out there for you.

Happy motoring!