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Traction (engineering)
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- This article uses "traction" as defined by classical mechanics. For other meanings see traction.
Traction is defined by dictionaries as adhesive friction, another name for static friction (non-sliding friction). Traction is never properly used to mean kinetic friction (sliding friction). Specifically, traction refers to the maximum static friction that could be produced between given surfaces without slipping, rather than the actual static friction produced between those surfaces (which may be less than the possible maximum at any particular time).
The term traction is most commonly found in limited contexts where static friction is used to produce and/or prevent independent motion of some system against the ground and kinetic friction is less desirable, such as "these new hiking shoes give me great traction on the rocks," or "my truck wheels keep slipping because I can't get any traction in this mud." In contrast, one wouldn't speak of the traction of a toboggan, since even though a toboggan is designed to move over the ground, slipping is desired.
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- This article uses "traction" as defined by classical mechanics. For other meanings see traction.
Traction is defined by dictionaries as adhesive friction, another name for static friction (non-sliding friction). Traction is never properly used to mean kinetic friction (sliding friction). Specifically, traction refers to the maximum static friction that could be produced between given surfaces without slipping, rather than the actual static friction produced between those surfaces (which may be less than the possible maximum at any particular time).
The term traction is most commonly found in limited contexts where static friction is used to produce and/or prevent independent motion of some system against the ground and kinetic friction is less desirable, such as "these new hiking shoes give me great traction on the rocks," or "my truck wheels keep slipping because I can't get any traction in this mud." In contrast, one wouldn't speak of the traction of a toboggan, since even though a toboggan is designed to move over the ground, slipping is desired. According to the dictionaries, traction is also used in the context of pulleys and ropes.
In the design of wheel-propelled vehicles, higher traction between wheel and ground is generally more desirable than lower traction, as it allows for more energetic acceleration (including cornering and braking) without wheel slippage, giving the driver more control over the vehicle. One notable exception is in the motorsport technique of drifting, in which rear-wheel traction is purposely lost during high speed cornering. All else being the same, higher traction also allows for steeper ground inclines without wheel slippage, whether the vehicle is moving or parked. Other designs dramatically increase surface area to provide more traction than wheels can, such as in continuous track and half-track vehicles.
Traction between two surfaces depends on several factors including:
- Material composition of each surface.
- Macroscopic and microscopic shape or "roughness" (although this is debated ).
- Normal force pressing contact surfaces together.
- Contaminants at the material boundary including lubricants and adhesives.
Coefficient of Traction
The coefficient of traction is identical to the coefficient of static friction except maximum static friction is named traction. It's used typically in look-up tables comparing surface material combinations.
Traction 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.
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:
where 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.
Multiple wheels do not increase a vehicle's traction, because the friction force is independent of the contact area.
The friction force (Ff) is dependent upon the coefficient of friction (COF) between the contact surfaces and the normal force (N = a force perpendicular to the contact surface).
Ff= N(COF)
A greater number of wheels will allow a vehicle to carry a larger load, thus increasing the resulting normal force. More traction is achieved with a greater load because the potential frictional force has been increased.
The case of wheels sharing a given normal force is particularly important in vehicle design. Two identical tires sharing a common load achieve maximum stability 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 stability.
The simplified mathematics behind wheel slip
Suppose a FWD vehicle weighs 1000 kg. Its engine produces 100 Nm torque at some RPM. It wheels have radii of 0.2 m.
Assuming 60% of the car's weight is on front wheels, each wheel carries a load of 1000*0.6/2 = 300 kg.
Approximating g = 10 m/s2 and coefficient of friction of dry road = 0.9, the tractive force on each wheel comes at
= u*m*g = 0.9*300*10 = 2700 N
Assume the car's 2nd gear ratio 0.5 and final drive is 0.25. Then 100 Nm engine torque will appear on drive shaft as = 100/0.5/0.25 = 800 Nm
Torque at each wheel = propulsive force * radius of wheel
For FWD cars, torque is applied at two front wheels. So propulsive force at each wheel = (800/2)/0.2 = 2000 N
The wheel slip occurs if propulsive force is greater than tractive force (i.e. available traction).
In this case, traction is 2700 N and propulsive force is 2000 N - which is less than traction. So no wheel slip occurs.
The value of u falls to ~ 0.4 on wet pavement. So traction in this case = 0.4*300*10 = 1200 N only, which is much lower than propulsive force (for that particular car using 2nd gear). So wheel slip will result if brake is applied while engine is producing 100 Nm torque as shown in the example.
However, if you apply the brakes while the car is moving slowly (i.e. RPM is lower and much less torque say 40 Nm is being produced), the propulsive force will also be lower (eg. reduced by 100/40 = 2.5 times => 2000/2.5 = 800 N which is less than traction) and the car will have a correspondingly shorter stopping distance.
This simple calculation shows:
- Why it is said that you can easily skid on wet and on ice (where friction coefficient is very low)!
- Why it is advised that you should use higher gears (thus less available torque on wheels) while driving on snow.
- Why 4WD has greater traction (its torque is shared by 4 wheels instead of 2 so propulsive force at each wheel is 50% less compared to 2WD, leads to less prone to skidding)
- Why FWD cars are generally more drivable in snow compared to RWD cars (FWD cars have engine above driven axle so more traction available)
In our example, if the car is 4WD, the propulsive force on dry road will be 1000 N only, which is still lower than 1200 N (traction in wet example) - so no slip will happen.
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.
Loss of traction due to leaves in the Fall (season) and pollen in the Spring (season)
See also
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