The design of a tire is far more complicated than an inflatable piece of round rubber mounted to a wheel or rim. While many people are under the impression that tires are crucial parts of an automobile's design, few understand the true importance of tires and how much a vehicle relies on tires to achieve its intended performance and safety.
While the pneumatic (pressurized) tires used on autos today maintain a circular shape before they are mounted onto a wheel and vehicle, they change shape under the pressure of the vehicle and the road. The part of the tire that becomes flat and touches the driving surface at any given moment is known as the contact patch. It is also referred to as a tire footprint.
What is the Contact Patch?
The contact patch is a combination of tread, tread grooves, and any other tire design features such as siped rubber, which maintain contact with the road as the tire rolls. This design allows pneumatic tires to possess the most versatile traction options at both low and high speeds. The contact patch will vary based on tire design, construction materials, type of tire, tread and shoulder design, and sidewall construction. While the actual contact patch can't be seen by simply glancing at a vehicle, it is easy to see its location by viewing the profile or the outside of the wheel.
How the Contact Patch Works
As the vehicle remains in contact with the road surface, the contact patch will continually mold and morph based on the rolling of the wheel and shape of the driving surface. For example, the contact patch will slightly vary on bumpy road surfaces or uneven pavements. It will also vary while traveling over loose road surfaces such as dirt and gravel.
The tread pattern that rolls through the contact patch will offer grip on the road surface, and this grip may also vary based on any number of driving maneuvers. When a tire enters a turn or is in the process of cornering, it may slightly slide. This is known as the slip angle.
The contact patch is designed with such phenomena in mind in order to offer the safest and smoothest ride. The contact patch is also designed to aid in traction during inclement weather. Tires that are designed for wet weather travel contain special features that help the contact patch get their grip on the road.
Consider a tire that boasts rain grooves. As the tire rolls through the contact patch, the rain grooves are able to channel the water back and away from the actual tread blocks, which remain in contact with the road surface beneath. In order for a contact patch to be most efficient, the tread lugs need to remain in contact with the road surface as designed.
Contact Patch Size
The size of the contact patch will vary based on the tire design, including the size of the tire. Wider tires will possess a greater contact patch, as do most high-performance tires, which possess a smaller sidewall ratio. Some off-road tires are capable of running at very low PSI, which allows the vehicle operator the opportunity to expand the contact patch for more traction in low speed, off-road applications.
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