Contact patch is the portion of a vehicle's
tire
A tire (American English) or tyre (British English) is a ring-shaped component that surrounds a Rim (wheel), wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide Traction (engineering), t ...
that is in actual contact with the
road surface
A road surface (British English), or pavement (American English), is the durable surface material laid down on an area intended to sustain vehicular or foot traffic, such as a road or walkway. In the past, gravel road surfaces, hoggin, cob ...
. It is commonly used in the discussion of pneumatic (i.e. pressurized) tires, where the term is used strictly to describe the portion of the tire’s tread that touches the road surface. The term “footprint” is used almost synonymously. Solid wheels also exhibit a contact patch which is generally smaller than the pneumatic “footprint”.
Contact patch size
The contact patch is the only connection between the road and the vehicle.
Pneumatic rubber tires
The size and shape of the contact patch, as well as the
pressure distribution within the contact patch, are important to the
ride qualities and
handling characteristics of a vehicle. Since the wear characteristics of tires is a highly competitive area between
tire manufacturers, a lot of the research done concerning the contact patch is considered highly proprietary and, therefore, very little is published on the subject.
Because pneumatic tires are flexible, the contact patch can be different when the vehicle is in motion than when it is static. Because it is so much easier to make observations of the contact patch without the tire in motion, it is more common to conduct studies of the static contact patch.
Statically, the size, shape, and pressure distribution are functions of many things, the most important of which are the load on the tire and the
inflation pressure:
* The larger the load on the tire, the larger the contact patch.
* The larger the inflation pressure, the smaller the contact patch.
These two properties are not
linearly
Linearity is the property of a mathematical relationship (''function'') that can be graphically represented as a straight line. Linearity is closely related to '' proportionality''. Examples in physics include rectilinear motion, the linear re ...
proportional to the
area
Area is the quantity that expresses the extent of a region on the plane or on a curved surface. The area of a plane region or ''plane area'' refers to the area of a shape or planar lamina, while '' surface area'' refers to the area of an op ...
of the contact. For example, a 10% change in load or inflation pressure usually does not result in a 10% change in the contact patch area because the load or pressure on a tire can be altered freely, and the contact patch area is affected by the tire geometry and stiffness. Further, the size of the contact patch cannot be simply calculated as load divided by inflation pressure, and the average contact pressure a tire has with the road surface is not equal to the inflation pressure.
Solid tires
The contact patch size of solid materials is described by the equations of
contact mechanics. It is mainly related to the stiffness of the material in terms of
Young's modulus
Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied ...
.
Railroad steel wheel on rail
The contact patch of a railroad wheel is much smaller than for pneumatic rubber tires; it is only about the size of a
dime (252 mm
2).
[ Eric Magel, Joe Kalousek, Mike Roney, "Stress reduction, railroad style: the contact area where wheel meets rail is about the size of a dime. Making it as stress-free as possible can save big dollars". Railway Age, July, 2002]
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See also
* Camber thrust
* Cornering force
* Frictional contact mechanics
* Ground pressure
* Pneumatic trail
* Slip (vehicle dynamics)
References
{{Sim racing
Tires
Motorcycle dynamics