Skid (automobile)

Last updated

An automobile skid is an automobile handling condition where one or more tires are slipping relative to the road, and the overall handling of the vehicle has been affected.

Contents

Subtypes of skid include:

Slip and skid

Tire slip, and related slip angle (angle of motion relative to tire), describe the performance of an individual tire. Important concepts about slip and skid include circle of forces or circle of traction, and cornering force. [1] 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 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), 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, 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 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 to heat tires to a more desirable temperature in order to increase traction. [2] Burnouts are usually illegal on the street; drivers engaging in them may be considered a hoon.

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 and cadence braking are two manual techniques used to extract maximum deceleration from a vehicle. Threshold braking maintains a steady braking force with slight (10-20%) slip, 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 or ESC systems, and the older anti-lock brake 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

Related Research Articles

<span class="mw-page-title-main">Anti-lock braking system</span> Safety anti-skid braking system used on aerospace and land vehicles

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 maintaining tractive contact with the road surface and allowing the driver to maintain more control over the vehicle.

<span class="mw-page-title-main">Brake</span> Mechanical device that inhibits motion

A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction.

<span class="mw-page-title-main">Electronic stability control</span> Computerized safety automotive technology

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 (skidding). When ESC detects loss of steering control, it automatically applies the brakes to help steer the vehicle where the driver intends to go. Braking is automatically applied to wheels individually, such as the outer front wheel to counter oversteer, or the inner rear wheel to counter understeer. Some ESC systems also reduce engine power until control is regained. ESC does not improve a vehicle's cornering performance; instead, it helps reduce the chance of the driver losing control of the vehicle.

A traction control system (TCS), is typically a secondary function of the electronic stability control (ESC) on production motor vehicles, designed to prevent loss of traction of the driven road wheels. TCS is activated when throttle input and engine power and torque transfer are mismatched to the road surface conditions.

Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of the vehicle to changes in steering angle associated with changes in lateral acceleration. This sensitivity is defined for a level road for a given steady state operating condition by the Society of Automotive Engineers (SAE) in document J670 and by the International Organization for Standardization (ISO) in document 8855. Whether the vehicle is understeer or oversteer depends on the rate of change of the understeer angle. The Understeer Angle is the amount of additional steering that must be added in any given steady-state maneuver beyond the Ackermann steer angle. The Ackermann Steer Angle is the steer angle at which the vehicle would travel about a curve when there is no lateral acceleration required.

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 during cornering, acceleration, and braking as well as on the vehicle's directional stability when moving in steady state condition.

<span class="mw-page-title-main">Highsider</span> Motorcycle calamity

A highsider or high-side is a type of motorcycle accident characterized by sudden and violent rotation of the bike around its longitudinal axis. This generally happens when the rear wheel loses traction, skids, and then suddenly regains traction, causing the rider to be thrown head-first from the side of the motorcycle or over the handlebars.

<span class="mw-page-title-main">Locking differential</span> Mechanical component which forces two transaxial wheels to spin together

A locking differential is a mechanical component, commonly used in vehicles, designed to overcome the chief limitation of a standard open differential by essentially "locking" both wheels on an axle together as if on a common shaft. This forces both wheels to turn in unison, regardless of the traction available to either wheel individually.

<span class="mw-page-title-main">Aquaplaning</span> Loss of traction due to water buildup under tires

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 from responding to control inputs. If it occurs to all wheels simultaneously, the vehicle becomes, in effect, an uncontrolled sled. Aquaplaning is a different phenomenon from when water on the surface of the roadway merely acts as a lubricant. Traction is diminished on wet pavement even when aquaplaning is not occurring.

<span class="mw-page-title-main">Electronic brakeforce distribution</span> Automotive braking technology

Electronic brakeforce distribution or electronic brakeforce limitation (EBL) is an automobile brake technology that automatically varies the amount of force applied to each of a vehicle's wheels, based on road conditions, speed, loading, etc, thus providing intelligent control of both brake balance and overall brake force. Always coupled with anti-lock braking systems (ABS), EBD can apply more or less braking pressure to each wheel in order to maximize stopping power whilst maintaining vehicular control. Typically, the front end carries more weight and EBD distributes less braking pressure to the rear brakes so the rear brakes do not lock up and cause a skid. In some systems, EBD distributes more braking pressure at the rear brakes during initial brake application before the effects of weight transfer become apparent.

A beadlock or bead lock is a mechanical device that secures the bead of a tire to the wheel of a vehicle. Tires and wheels are designed so that when the tire is inflated, the tire pressure pushes the bead of the tire against the inside of the wheel rim so that the tire stays on the wheel and the two rotate together. In situations where tire pressure is insufficient to hold the bead of the tire in place, a beadlock is needed.

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 effect of skidding from road wheels locking up under braking. This can be a particular problem when different tires have different traction, such as on patchy ice for example. Its use in an emergency requires a presence of mind that the situation itself might preclude. Cadence braking is supposed to maximize the time for the driver to steer around the obstacle ahead, as it allows the driver to steer while slowing. It needs to be learned and practiced. For most drivers of modern cars, it has been entirely superseded by ABS, however it is still a valuable skill for drivers of non-ABS equipped vehicles such as classic cars.

<span class="mw-page-title-main">Burnout (vehicle)</span> Practice of spinning wheels while keeping vehicle stationary

A burnout is the practice of keeping a vehicle stationary and spinning its wheels, the resultant friction causing the tires to heat up and smoke.

<span class="mw-page-title-main">Fishtailing</span> Vehicle handling problem

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. Rear-drive vehicles with sufficient power can induce this loss of traction on any surface, which is called power-oversteer.

Lift-off oversteer is a form of sudden oversteer. While cornering, a driver who closes the throttle, usually at a high speed, can cause such sudden deceleration that the vertical load on the tires shifts from rear to front, in a process called load transfer. This decrease in vertical load on the rear tires in turn decreases their traction by lowering their lateral force, making the vehicle steer more tightly into the turn. In other words, easing off the accelerator in a fast turn can cause a car's rear tires to loosen their grip so much that the driver loses control and drifts outwards, even leaving the road tailfirst.

<span class="mw-page-title-main">Bicycle and motorcycle dynamics</span> Science behind the motion of bicycles and motorcycles

Bicycle and motorcycle dynamics is the science of the motion of bicycles and motorcycles and their components, due to the forces acting on them. Dynamics falls under a branch of physics known as classical mechanics. Bike motions of interest include balancing, steering, braking, accelerating, suspension activation, and vibration. The study of these motions began in the late 19th century and continues today.

A wheelspin occurs when the force delivered to the tire tread exceeds that of available tread-to-surface friction and one or more tires lose traction. This leads the wheels to "spin" and causes the driver to lose control over the tires that no longer have grip on the road surface. Wheelspin can also be done intentionally such as in drifting or doing a burnout.

In (automotive) vehicle dynamics, slip is the relative motion between a tire and the road surface it is moving on. This slip can be generated either by the tire's rotational speed being greater or less than the free-rolling speed, or by the tire's plane of rotation being at an angle to its direction of motion.

All Wheel Control (AWC) is the brand name of a four-wheel drive (4WD) system developed by Mitsubishi Motors. The system was first incorporated in the 2001 Lancer Evolution VII. Subsequent developments have led to S-AWC (Super All Wheel Control), developed specifically for the new 2007 Lancer Evolution. The system is referred by the company as its unique 4-wheel drive technology umbrella, cultivated through its motor sports activities and long history in rallying spanning almost half a century.

Crosswind stabilization (CWS) is a relatively new advanced driver-assistance system in cars and trucks that was first featured in a 2009 Mercedes-Benz S-Class. CWS assists drivers in controlling a vehicle during strong wind conditions such as driving over a bridge or when overtaking a semi-truck. CWS uses yaw rate, lateral acceleration, steering angle, and velocity sensors to determine how much assistance to give the driver in a certain scenario whether it be at different speeds or while turning. Using different components throughout the vehicle like brakes, differentials, and suspension, CWS can implement the readings from force sensors to properly assist the driver in a given situation.

References

  1. "Adams", "Herb" (1992). Chassis Engineering.
  2. Holstein, Jared. "The Burnout Kings". Car and Driver. Hearst Digital Media. Retrieved 18 February 2021.