Traction (engineering)
Encyclopedia
Traction refers to the maximum frictional force that can be produced between surfaces without slipping.

The units of traction are those of force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

, or if expressed as a coefficient of traction (as with coefficient of friction) a ratio
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...

.

## Traction

Traction is defined as:
..a physical process in which a tangential force is transmitted across an interface between two bodies through dry friction or an intervening fluid film resulting in motion, stoppage or the transmission of power (Copyright: "Mechanical Wear Fundamentals and Testing" by Raymond George Bayer)

The traction produced by a vehicle if expressed as a force is synonymous with tractive effort, or tractive force, and closely related to the term drawbar pull.

## Coefficient of traction

The coefficient of traction is defined as the usable force for traction divided by the weight on the running gear (wheels, tracks etc.) i.e.:
Usable Traction = coefficient of Traction x Weight

As the coefficient of traction refers to two surfaces which are not slipping relative to one another it is the same as Coefficient of static friction, also known as limiting friction.

### Factors affecting tractive coefficient

Traction between two surfaces depends on several factors including:
• Material composition of each surface.
• Macroscopic and microscopic shape (texture; macrotexture
Macrotexture
Macrotexture is a family of wave-shaped road surface characteristics. While vehicle suspension deflection and dynamic tyre loads are affected by longer waves , road texture affects the interaction between the road surface and the tyre footprint. Macrotexture has wavelengths from 0.5 mm up to...

and microtexture
Microtexture
Microtexture is the collative term for a material's crystallographic parameters and other aspects of microstructure: such as morphology, including size and shape distributions; chemical composition; and crystal orientation and relationships...

)
• Normal force
Normal force
In mechanics, the normal force F_n\ is the component, perpendicular to the surface of contact, of the contact force exerted on an object by, for example, the surface of a floor or wall, preventing the object from penetrating the surface.The normal force is one of the components of the ground...

pressing contact surfaces together.
• Contaminants at the material boundary including lubricants and adhesives.
• Relative motion of tractive surfaces - e.g. a wheel on gritted ice when in motion may displace the grit and melt the ice - causing loss of traction. Current traction control systems do not work on untreated ice.

### Traction coefficient in engineering design

In the design of wheeled or tracked vehicles, high traction between wheel and ground is more desirable than low traction, as it allows for more energetic acceleration (including cornering and braking) without wheel slippage. One notable exception is in the motorsport technique of drifting
Drifting (motorsport)
Drifting refers to a driving technique and to a motorsport where the driver intentionally over steers, causing loss of traction in the rear wheels through turns, while maintaining vehicle control and a high exit speed...

, in which rear-wheel traction is purposely lost during high speed cornering.

Other designs dramatically increase surface area to provide more traction than wheels can, such as in continuous track and half-track
Half-track
A half-track is a civilian or military vehicle with regular wheels on the front for steering, and caterpillar tracks on the back to propel the vehicle and carry most of the load. The purpose of this combination is to produce a vehicle with the cross-country capabilities of a tank and the handling...

vehicles.

In some 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 reducing efficiency. 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 less traction and also thicker rubber.

Traction also varies with contaminants. A layer of water in the contact patch
Contact patch
Contact patch is the portion of a vehicle's tire that is in actual contact with the road surface. It is most commonly used in the discussion of pneumatic tires, , where the term is strictly used to describe the portion of the tire’s tread that touches the road surface...

can cause a substantial loss of traction. This is one reason for grooves and siping of automotive tires.

The traction of trucks, agricultural tractors, wheeled military vehicles, etc. when driving on soft and/or slippery ground has been found to improve significantly by use of Tire Pressure Control Systems (TPCS). A TPCS makes it possible to reduce and later restore the tire pressure during continuous vehicle operation. Increasing traction by use of a TPCS also reduces tire wear and ride vibration.

There seems to be some confusion in this statement about tracked vehicles and traction. A tank or similar tracked vehicle uses tracks to reduce the pressure on the areas of contact. A 65 ton M1A1 would sink to the point of high centering if it used round tires. The tracks spread the 65 tons over a much larger area of contact than wheels would and allow the tank to travel over much softer land.

• Anti-lock braking system
Anti-lock braking system
An anti-lock braking system is a safety system that allows the wheels on a motor vehicle to continue interacting tractively with the road surface as directed by driver steering inputs while braking, preventing the wheels from locking up and therefore avoiding skidding.An ABS generally offers...

• Friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

• Force (physics)