Traction (engineering)

Traction (engineering)

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.

In the case of a wheeled vehicle, when the motor and/or transmission turns the axles, a reaction torque on the axles is created by the traction of each wheel on the ground. Without traction, there would be no torque (other than that required to overcome the friction and inertia of the transmission and wheels themselves), and hence no movement of the vehicle.

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=mu F_N, where F is the frictional force and F_N is the normal force. Here, μ summarizes material properties and roughness and is called the coefficient of friction.

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 less traction and also thicker rubber, but the race car tires can simply use soft rubber without compromising weight and heat build-up.

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 patch, but inertial effects limit the speed with which this can happen. Although the grooves on a tire decrease dry traction, they reduce the distance water must travel to escape the contact patch in wet conditions. In some application, the distance water must travel is already short; for example, bicycle tires have a narrow and pointed area of contact, so even slick tires give good traction on a 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.

Vinegar can be used to increase traction on Concrete floors.Fact|date=May 2008

Traction applies across a wide variety of materials and scales. For example, railroad locomotives use steel wheels on steel rails to provide low rolling resistance and long wear; 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/dtwhere Pe = Efficient Power, PL = Power Loss during mechanical conversion, and TF = Traction Force.

Maximizing multi-wheeled vehicle traction

It is important, due to broad application, to point out the specific case of multi-wheeled vehicles or vehicles with multiple contact patches between the tire and the road surface.The constant coefficient of friction ("COF") approximation is not adequate to describe real world maximum traction situations. If the normal force is increased, per given area of contact patch, the "COF" decreases and as the normal force decreases, the "COF" increases. If this were not true, then lowering tire air pressure or installing wider tires, which increases the area of the contact patch, would have little effect on traction.

The importance of having a "COF" with the above properties has significant implications in multi-wheeled vehicle handling. The case of two wheels sharing a given normal force is particularly important in vehicle design. Two identical tires sharing a common load achieve maximum traction when they share the load equally. Likewise, an unequally loaded pair of tires sharing a common load will not be able to achieve the same maximum traction. Consider the "traction pair". If each tire's "COF" remained constant, the increase in load on one tire, and consequent increase in traction, would perfectly offset the decrease in load on the other, and the consequent decrease in traction. However, in fact, the less laden tire’s "COF" increases slightly, and the more laden tire's "COF" decreases "but the decrease is proportionally greater than the increase". This results in a net reduction in total traction, compared to a balanced load.

A vehicle has balanced or neutral handling when the front and rear pairs of tires achieve maximum traction proportional to the normal force on each pair of tires. Example: If 60% of a vehicle's total normal force is at the front of the vehicle, then 60% of the traction should also need be in the front for balanced handling. Achieving this is non-trivial due to the dynamic forces involved such as changing corner radius, bank, braking, acceleration, aerodynamic loading and "COF"-changing factors such as road surface debris, moisture, temperature etc. Automotive engineers attempt to minimize the effect of non-linear forces as much as possible in order to simplify design considerations.

Loss of traction in road vehicles

Hydroplaning is a common reason of significant loss of traction.

Loss of traction in low water situations

Hydroplaning most often occurs when there are large volumes of water on a road surface. Even slight wetness on a road, however, can cause a car to lose traction. This effect differs from hydroplaning.

Tires maintain traction on the road by using a mechanism called bulk friction, where the rubber of the tire pushes down into tiny pits in irregularities of the road surface. When a road becomes slightly wet, water can fill these pits, thus the water tops them off without overflowing. As the narrow strip of tire contacting the road rolls over these miniature puddles, the rubber of the tire seals the edges of the pits. Because water does not easily compress, each pit essentially has a barrier over it that prevents the rubber from pressing into it. The result is a reduction in traction. A complete loss of control, however, is unlikely.

Another form of loss of traction in low water situations is called mudplaning. It occurs during the first rainfall on an area that has not seen rain for a period of time, so that the road is covered by a thin layer of dust. When the rain first falls, the thin layer of dust turns into a nearly transparent layer of mud, so that the color of asphalt remains as usual. However, the layer of mud may cause a critical loss of traction, comparable to driving on rime ice. When additional rain totally washes the dust off the pavement, the effects of mudplaning cease. The whole phenomenon is similar to the formation of black ice, which has been responsible for many unavoidable accidents on highways.

Loss of traction due to leaves in the Fall (season) and pollen in the Spring (season)

See also

* Rail adhesion
* Road slipperiness

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