All vehicles are supported by a cushion of air contained in four flexible rubber tires. If you could place a car on a glass floor and look at it from below, you would see four patches of rubber, most folks are surprised at the size of these four patches, depending on the vehicle – each patch is a little smaller than a hand. These are the only points of contact between the vehicle and the road. Each of these four small patches of rubber is known as the “contact patch”. It is these four patches that create the traction which makes the vehicle – go – stop and turn. It is these four patches that send the feedback back to the driver, and it is these four patches the driver has to manage. Consider them to be a source of information needed to control the vehicle.
How Much Rubber Do You Have?
At VDI we tend to be anal about “The Patch”. We check tire pressures often. In our opinion, you have to know the size of the contact patch your vehicle creates. Here is how we do it – jack up your vehicle, put ink on the bottom of the tire, and let the car down on a piece of paper (we use fingerprint ink). Have someone keep their foot on the brake as you let the vehicle down, it keeps the tire from rolling, and lower the tire on the paper – the tire will leave a mark on the paper that represents the tire contact patch. The paper should be outline paper divided into one-inch blocks, it makes measuring the contact patch easier.
Once done you have a “picture” of the tire contact patch, and can easily measure the size of the patch in square inches (width of the patch times the height of the patch). If you take that number and multiply it by four that is how much rubber is on the road.
In our vehicles, police package Crown Vics with P225/60R16 tires with 32 PSI, we have 36.75 square inches per tire (the drawing is not to scale) and all four tires will give us a static contact area of 147 square inches of rubber touching the road. The size of the patch is a function of the load on the tire. Using the science of Vehicle Dynamics, VDI simulates various loads on the patch for different scenarios – from the analysis, we acquire the information needed to coach the student through the scenarios.
Impressive material. ..
What would it be for your average motorcycle?
Bring your Bike into your garage, wetting the tire with a inert liquid that and lifting the tire off of the floor. Place a sheet of paper under the tire and slowly lower the tire until all of the bikes weight of off of the jack. Lift the bike again and you will see the contact patch of the tire. Do this for the front tire as well, as the front and rear tires can and will likely have different specs on width and diameter. Those two factors can cause a contact patch to change wildly.