Traction refers to the friction between a drive member and the surface it moves upon, where the friction is used to provide motion.

For the purposes of driving a wheeled vehicle, high friction is generally desired, as it provides a more positive connection between the driving and driven members. In contrast, motion in a geared mechanism is provided by interference, and friction is usually detrimental because the gear mechanism has intrinsic sliding, and sliding under friction causes heating losses.

Traction between two surfaces usually depends on several factors including

• Material properties of each surface.
• Macroscopic and microscopic shape or "roughness".
• Force of contact.
• Area of contact.
• Contaminants at the material boundary including lubricants and adhesives.

Formula for friction

A common approximation is F=μF_N. Here, μ summarizes material properties and roughness and is called "the coefficient of friction". F_N is the normal force, which is applied perpendicular to the contact. In a simple system in equilibrium, such as a mass sitting on a surface, the normal force is equal to mass multiplied by gravitational acceleration (F_N = mg). The statement of this expression is that friction is directly proportional to the intrinsic friction caused by the materials and contaminants; and that friction is also directly proportional to the force.

In practice, this is a good approximation but in many situations other factors, e.g., the area of contact, play a role.

Friction trade-offs

In most applications, there is a complicated set of trade-offs in choosing materials. For example, soft rubbers often provide better traction but also wear faster and have higher losses when flexed -- thus hurting efficiency and sometimes causing early failure due to heat build-up. Subtle choices in material selection may have a dramatic effect. For example, tires used for track racing cars may have a life of 200 km, while those used on heavy trucks may have a life approaching 100,000 km. The truck tires have worse traction and also thicker rubber, but the race car tires cannot simply use thick rubber without compromising weight, heat build-up, and so on.

Traction also varies with contaminants. A layer of water in the contact patch can cause a substantial loss of traction. This is one reason for grooves and siping of automotive tires: most water must be displaced from the contact, but inertial effects limit the speed with which it can be displaced. Grooves hurt dry traction but reduce the distance the water travels to escape. Note there are applications where the distances are already short, for example bicycle tires have a narrow and pointed contact and so even slick tires give good traction on wet pavement. Where the roadway surface is substantially flexible or malleable, tread can also form divots in the road, leading to interference-type traction (as in gears) rather than friction.

Traction applies across a wide variety of materials and scales. For example, railroad locomotives use steel wheels on steel rails to provide traction; slot cars use rubber on plastic; and so on.

Traction boundary condition

Particularly in the context of the finite element method, a traction boundary condition is a portion of the boundary of a body for which forces—tangential, normal, or both—is prescribed. See also Navier-Stokes equations.

Traction forces in a system

The traction force is given by:

Traction Force = Driving Torque/Radius of Wheel.

Using conservation of energy, we are aware that F=ma and hence P=Fv or rate of work done. In order to calculate power:

$PE\; =\; dTF/dt\; +\; dPL/dt$

where Pe = Efficient Power, PL = Power Loss during mechanical conversion, and TF = Traction Force.

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