# Traction (engineering)

Traction refers to the maximum frictional force that can be produced between surfaces without slipping.[1][2]

The units of traction are those of force, or if expressed as a coefficient of traction (as with coefficient of friction) a ratio.

## 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[3] (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.)[4][5] 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 and microtexture)
• Normal force 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, 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, for example in continuous track and half-track 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 flexedâ€”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 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.[6]

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.

## References

1. ^ Estimating Excavation, Craftsman Book Co, Deryl Burch, 1997, ISBN 0-934041-96-2 [Amazon-US | Amazon-UK], ISBN 978-0-934041-96-6 [Amazon-US | Amazon-UK], Page 215. Google books link: [1]
2. ^ http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html
3. ^ Mechanical Wear Fundamentals and Testing, Raymond George Bayer, CRC Press, ISBN 0-8247-4620-1 [Amazon-US | Amazon-UK], ISBN 978-0-8247-4620-9 [Amazon-US | Amazon-UK], Page 3. Google books link: [2]
4. ^ Construction Management Fundamentals, Clifford J. Schexnayder, Richard Mayo, McGraw-Hill Professional, 2003, ISBN 0-07-292200-1 [Amazon-US | Amazon-UK], ISBN 978-0-07-292200-4 [Amazon-US | Amazon-UK]. Page 346. Google books link [3]
5. ^ Theory of ground vehicles, Jo Yung Wong, ISBN 0-471-35461-9 [Amazon-US | Amazon-UK], ISBN 978-0-471-35461-1 [Amazon-US | Amazon-UK], Page 317. Google books link [4]
6. ^ Tire Pressure Control on Timber Haulage Vehicles, The Roadex III project

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