Traction, traction force or tractive force is a

force In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...

used to generate

motion In physics, motion is when an object changes its position with respect to a reference point in a given time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed, and frame of reference to an o ...

between a body and a tangential surface, through the use of either dry friction or

shear force In solid mechanics, shearing forces are unaligned forces acting on one part of a Rigid body, body in a specific direction, and another part of the body in the opposite direction. When the forces are Collinearity, collinear (aligned with each ot ...

. It has important applications in

vehicle A vehicle () is a machine designed for self-propulsion, usually to transport people, cargo, or both. The term "vehicle" typically refers to land vehicles such as human-powered land vehicle, human-powered vehicles (e.g. bicycles, tricycles, velo ...

s, as in ''

tractive effort In railway engineering, the term tractive effort describes the pulling or pushing capability of a locomotive. The published tractive force value for any vehicle may be theoretical—that is, calculated from known or implied mechanical proper ...

''. ''Traction'' can also refer to the ''maximum'' tractive force between a body and a surface, as limited by available friction; when this is the case, traction is often expressed as the ratio of the maximum tractive force to the

normal force In mechanics, the normal force F_n is the component of a contact force that is perpendicular to the surface that an object contacts. In this instance '' normal'' is used in the geometric sense and means perpendicular, as opposed to the meanin ...

and is termed the ''coefficient of traction'' (similar to coefficient of friction). It is the force which makes an object move over the surface by overcoming all the resisting forces like

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...

, normal loads (load acting on the tiers in negative ''Z'' axis),

air resistance In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the direction of motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers, two solid surfaces, or b ...

rolling resistance Rolling resistance, sometimes called rolling friction or rolling drag, is the force resisting the Motion (physics), motion when a body (such as a ball, tire, or wheel) Rolling, rolls on a surface. It is mainly caused by Plasticity (physics), non- ...

, etc.

Definitions

Traction can be defined as: In vehicle dynamics, tractive force is closely related to the terms

and drawbar pull, though all three terms have different definitions.

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'' × ''normal force''.

Factors affecting coefficient of traction

Traction between two surfaces depends on several factors: * Material composition of each surface. * Macroscopic and microscopic shape (

texture Texture may refer to: Science and technology * Image texture, the spatial arrangement of color or intensities in an image * Surface texture, the smoothness, roughness, or bumpiness of the surface of an object * Texture (roads), road surface c ...

; macrotexture and microtexture) *

Normal force In mechanics, the normal force F_n is the component of a contact force that is perpendicular to the surface that an object contacts. In this instance '' normal'' is used in the geometric sense and means perpendicular, as opposed to the meanin ...

pressing contact surfaces together. * Contaminants at the material boundary including lubricants and adhesives. * Relative motion of tractive surfaces - a sliding object (one in kinetic friction) has less traction than a non-sliding object (one in static friction). * Direction of traction relative to some coordinate system - e.g., the available traction of a

tire A tire (North American English) or tyre (Commonwealth 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 (engineeri ...

often differs between cornering, accelerating, and braking. * For low-friction surfaces, such as off-road or ice, traction can be increased by using traction devices that partially penetrate the surface; these devices use the shear strength of the underlying surface rather than relying solely on dry friction (e.g., aggressive off-road tread or snow chains)....

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 higher 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 Continuous track or tracked treads are a system of vehicle propulsion used in tracked vehicles, running on a continuous band of treads or track plates driven by two or more wheels. The large surface area of the tracks distributes the w ...

and

half-track A half-track is a civilian or military vehicle with wheels at the front for steering and continuous tracks at the back to propel the vehicle and carry most of the load. A half-track combines the soft-ground traction of a tank with the Car handl ...

vehicles. A tank or similar tracked vehicle uses tracks to reduce the pressure on the areas of contact. A 70-ton M1A2 would sink to the point of high centering if it used round tires. The tracks spread the 70 tons over a much larger area of contact than tires would and allow the tank to travel over much softer land. 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 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.

References

Force Vehicle technology Mechanics

Definitions

Coefficient of traction

Factors affecting coefficient of traction

Traction coefficient in engineering design

See also

References