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An automobile skid is an
automobile handling Automobile handling and vehicle handling are descriptions of the way a wheeled vehicle responds and reacts to the inputs of a driver, as well as how it moves along a track or road. It is commonly judged by how a vehicle performs particularly durin ...
condition where one or more tires are
slipping Slipping is a technique used in boxing that is similar to bobbing. It is considered one of the four basic defensive strategies, along with blocking, holding, and clinching. It is performed by moving the head to either side so that the opponent ...
relative to the road, and the overall handling of the vehicle has been affected. Subtypes of skid include: *
fishtailing Fishtailing is a vehicle handling problem which occurs when the rear wheels lose traction, resulting in oversteer. This can be caused by low friction surfaces (sand, gravel, rain, snow, ice, etc.). Rear-drive vehicles with sufficient power can ...
, where the vehicle yaws back and forth across the direction of motion. * ''spin'' or ''spinout'' where a vehicle rotates in one direction during the skid. *
understeer and oversteer Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Oversteer is what occurs when a car turns (steers) by more than the amount commanded by the driver. Conversely, understeer is what occu ...
where front or rear wheels lose traction during cornering, causing a vehicle to follow a larger or smaller turning radius. * ''burnout'' where a vehicle slips or spins its tires during acceleration. * skidding during braking (with or without directional or yaw changes). __TOC__


Slip and skid

Tire slip, and related
slip angle In vehicle dynamics, slip angle or sideslip angle is the angle between the direction in which a wheel is pointing and the direction in which it is actually traveling (i.e., the angle between the forward velocity vector v_x and the vector su ...
(angle of motion relative to tire), describe the performance of an individual tire. Important concepts about slip and skid include
circle of forces The circle of forces, traction circle, friction circle, or friction ellipse is a useful way to think about the dynamic interaction between a vehicle's tire and the road surface. The diagram below shows the tire from above, so that the road surface ...
or circle of traction, and
cornering force Cornering force or side force is the lateral (i.e., parallel to wheel axis) force produced by a vehicle tire during cornering. Cornering force is generated by tire slip and is proportional to slip angle at low slip angles. The rate at wh ...
. To a first approximation, the tire can withstand approximately the same absolute force relative to the road surface in any direction. Graphically represented, a circle (or ellipse) of force magnitude represents the maximum tire traction, and the force
vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...
can be in any direction up to the limit of the circle without tire slip. A tire that can withstand 0.8 G of force in braking can also withstand 0.8 G of force in turning or in acceleration, or for example approximately 0.56 G of cornering and 0.56 G of braking simultaneously, summing to 0.8 G at a 45 degree angle. Once the force exceeds the limit circle, that tire starts to slip. Skidding is the vehicle's response to one or more tires slipping. The vehicle dynamics during a skid will depend on whether some or all of the tires are skidding, and whether the car was rotating or turning when the skid began.


Road conditions

Road surface conditions such as moisture on the road, snow, ice (particularly
black ice Black ice, sometimes called clear ice, is a thin coating of glaze ice on a surface, especially on streets. The ice itself is not black, but visually transparent, allowing the often black road below to be seen through it. The typically low level ...
), debris or sand, oil or other fluids, can cause skidding at much lower force levels or velocities than under normal conditions. Moisture can cause
aquaplaning Aquaplaning or hydroplaning by the tires of a road vehicle, aircraft or other wheeled vehicle occurs when a layer of water builds between the wheels of the vehicle and the road surface, leading to a loss of traction that prevents the vehicle fr ...
, also known as hydroplaning, where water builds up in front of and under tires and causes loss of tire grip.


Types


Fishtailing

Fishtailing is a cyclical skid combining alternating oversteer (rear wheel skidding) with overcorrection, leading to oversteer/skidding in the opposite direction.


Spin out

Spin outs are where the vehicle starts to skid while rotating, or develops significant rotation while skidding, and rotates out of control. Once the vehicle is rotating sufficiently rapidly, its
angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
of rotation can overcome the stabilizing influence of the tires (either braking or skidding), and the rotation will continue even if the wheels are centered or past the point that the vehicle is controlled. This can be caused by some tires locking up in braking while others continue to rotate, or under acceleration where driven tires may lose traction (especially, if they lose traction unevenly), or in combining braking or acceleration with turning.


Understeer and oversteer


Burnout

A burnout is when a car intentionally locks the front wheels to hold the car in place while spinning the rear wheels. The dynamic friction of the spinning tire against the road causes significant amounts of the tire's rubber to be deposited onto the road surface, and increased temperature from friction usually creates dense white smoke. It is common in
drag racing Drag racing is a type of motor racing in which automobiles or motorcycles compete, usually two at a time, to be first to cross a set finish line. The race follows a short, straight course from a standing start over a measured distance, most c ...
to heat tires to a more desirable temperature in order to increase traction. Burnouts are usually illegal on the street; drivers engaging in them may be considered a
hoon A hoon is an Australian term describing a person who deliberately drives a vehicle in a reckless or dangerous manner, generally in order to provoke a reaction from onlookers. Hoon activities (or hooning) can include speeding, burnouts, dough ...
.


Skidding during braking

This is the simplest type of skid, where directional changes are not relevant, and the vehicle merely locks up the tires moving forwards in a straight line. If all four tires start to skid approximately evenly, then a vehicle will not start rotating due to the skid, and can come to a stop with locked up tires at a somewhat longer distance than threshold braking might have achieved.


Avoiding and managing skid


Manual avoidance

Threshold braking Threshold braking or limit braking is a driving technique most commonly used in motor racing, but also practiced in road vehicles to slow a vehicle at the maximum rate using the brakes. The technique involves the driver controlling the brake peda ...
and
cadence braking Cadence braking or stutter braking is a driving technique that involves pumping the brake pedal and is used to allow a car to both steer and brake on a slippery surface. It is used to effect an emergency stop where traction is limited to reduce the ...
are two manual techniques used to extract maximum deceleration from a vehicle. Threshold braking maintains a steady braking force with slight (10-20%)
slip Slip or SLIP may refer to: Science and technology Biology * Slip (fish), also known as Black Sole * Slip (horticulture), a small cutting of a plant as a specimen or for grafting * Muscle slip, a branching of a muscle, in anatomy Computing and ...
, around or just below the point of maximum tire grip force. Cadence braking accepts that holding the threshold braking limit is exceptionally hard, and relies on manual manipulation of braking force to rapidly go just above and below the skid point, essentially oscillating between unlocked rolling and locked skidding around the point at which threshold braking would be done. This technique is less effective than threshold braking but much easier to learn.


Lockup

For deceleration straight ahead, where turning or maneuvering are not required, one technique is to simply accept a skid and lock up the brakes. While ABS or ESC brake systems may perform better, and reduce risk of loss of control, many less skilled drivers will stop faster while locked up than any alternative they can realistically perform. This is not true if the vehicle has to be steered while stopping.


Automated systems

Electronic stability control Electronic stability control (ESC), also referred to as electronic stability program (ESP) or dynamic stability control (DSC), is a computerized technology that improves a vehicle's stability by detecting and reducing loss of traction ( skiddi ...
or ESC systems, and the older
anti-lock brake An anti-lock braking system (ABS) is a safety anti-skid braking system used on aircraft and on land vehicles, such as cars, motorcycles, trucks, and buses. ABS operates by preventing the wheels from locking up during braking, thereby maintai ...
or ABS systems, perform an automated braking (and for ESC, steering) function using wheel-by-wheel rapid brake pumping, similar to a mixture of threshold and cadence braking on a tire by tire basis. ABS senses wheel rotation compared to ground velocity, and if the wheel starts to lock up or slip will then rapidly moderate the brakes to let it begin to rotate again. This is done separately for all 4 wheels, and without regard for the rotation of the vehicle. ESC does the same, but combines that with sensing the steering and yaw or rotation velocity of the vehicle (for example, rotating as it goes around a corner). ESC will go beyond simply avoiding lockup in each tire, to dynamically braking other tires to maintain the existing path of the vehicle.


See also

*
Braking distance Braking distance refers to the distance a vehicle will travel from the point when its brakes are fully applied to when it comes to a complete stop. It is primarily affected by the original speed of the vehicle and the coefficient of friction be ...
*
Directional stability Directional stability is stability of a moving body or vehicle about an axis which is perpendicular to its direction of motion. Stability of a vehicle concerns itself with the tendency of a vehicle to return to its original direction in relation ...
*
Road slipperiness Road slipperiness is a condition of low skid resistance due to insufficient road friction. It is a result of snow, ice, water, loose material and the texture of the road surface on the traction produced by the wheels of a vehicle. Road slipperi ...
*
Vehicle handling Automobile handling and vehicle handling are descriptions of the way a wheeled vehicle responds and reacts to the inputs of a driver, as well as how it moves along a track or road. It is commonly judged by how a vehicle performs particularly durin ...


References

{{reflist Driving