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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 occurs when a car steers less than the amount commanded by the driver.
Automotive engineers define understeer and oversteer based on changes in steering angle associated with changes in lateral acceleration over a sequence of steady-state circular turning tests. Car and motorsport enthusiasts often use the terminology more generally in magazines and blogs to describe vehicle response to steering in a variety of manoueuvres.
Standard terminology used to describe understeer and oversteer are defined by the Society of Automotive Engineers (SAE) in document J670  and by the International Organization for Standardization (ISO) in document 8855.  By these terms, understeer and oversteer are based on differences in steady-state conditions where the vehicle is following a constant-radius path at a constant speed with a constant steering wheel angle, on a flat and level surface.
Understeer and oversteer are defined by an understeer gradient (K) that is a measure of how the steering needed for a steady turn changes as a function of lateral acceleration. Steering at a steady speed is compared to the steering that would be needed to follow the same circular path at low speed. The low-speed steering for a given radius of turn is called Ackermann steer. The vehicle has a positive understeer gradient if the difference between required steer and the Ackermann steer increases with respect to incremental increases in lateral acceleration. The vehicle has a negative gradient if the difference in steer decreases with respect to incremental increases in lateral acceleration.
Understeer and oversteer are formally defined using the gradient “K”. If K is positive, the vehicle shows understeer; if K is negative, the vehicle shows oversteer; if K is zero, the vehicle is neutral.
Several tests can be used to determine understeer gradient: constant radius (repeat tests at different speeds), constant speed (repeat tests with different steering angles), or constant steer (repeat tests at different speeds). Formal descriptions of these three kinds of testing are provided by ISO.  Gillespie goes into some detail on two of the measurement methods. 
Results depend on the type of test, so simply giving a deg/g value is not sufficient; it is also necessary to indicate the type of procedure used to measure the gradient.
Vehicles are inherently nonlinear systems, and it is normal for K to vary over the range of testing. It is possible for a vehicle to show understeer in some conditions and oversteer in others. Therefore, it is necessary to specify the speed and lateral acceleration whenever reporting understeer/oversteer characteristics.
Many properties of the vehicle affect the understeer gradient, including tyre cornering stiffness, camber thrust, lateral force compliance steer, self aligning torque, lateral weight transfer, and compliance in the steering system. Weight distribution affects the normal force on each tyre and therefore its grip. These individual contributions can be identified analytically or by measurement in a Bundorf analysis.
While much of this article is focused on the empirical measurement of understeer gradient, this section concentrates on road performance.
Understeer can typically be understood as a condition where, while cornering, the front tyres begin to slip first. Since the front tyres are slipping and the rear tyres have grip, the vehicle will turn less than if all tyres had grip. Since the amount of turning is less than it would be if all tyres had traction, this is known as under-steering.
The opposite is true if the rear tyres break traction first. The front tyres will continue to accelerate the front of the vehicle laterally, tracing a circle. The rear tyres will have a tendency to continue along the tangent of that circle but cannot because of their attachment to the front of the car, which still has traction. The result is that the rear tyres will swing outwards relative to the front of the vehicle. This turns the vehicle towards the inside of the curve. If the steering angle is not changed (i.e. the steering wheel stays in the same position), then the front wheels will trace out a smaller and smaller circle while the rear wheels continue to swing around the front of the car. This is what is happening when a car 'spins out'. A car susceptible to oversteer is sometimes known as 'tail happy', as in the way a dog wags its tail when happy and a common problem in negative-k vehicles is fishtailing.
A car is called 'neutral' when the front and rear tyres will lose traction at the same time. This is desirable because while the vehicle may slide towards the outside of the turn, it maintains the effective steering angle set by the driver. This makes it 'safer' to drive near the limit condition of traction because the outcome of breaking traction is more predictable.
In real-world driving (where both the speed and turn radius may be constantly changing) several extra factors affect the distribution of traction and the tendency to oversteer or understeer. These can primarily be split up into things that affect weight distribution to the tyres and extra frictional loads put on each tyre.
The weight distribution of a vehicle at standstill will affect handling. If the center of gravity is moved closer to the front axle, the vehicle tends to understeer due to tyre load sensitivity. When the center of gravity is toward the back of the vehicle, the rear axle tends to swing out, which is oversteer. Weight transfer is inversely proportional to the direction and magnitude of acceleration, and is proportional to the height of the center of gravity. When braking, weight is transferred to the front and the rear tyres have less traction. When accelerating, weight will transfer to the rear and decrease front tyre traction. In extreme cases, the front tyres may completely lift off the ground meaning no steering input can be transferred to the ground at all.
Tyres must transmit the forces of acceleration and braking to the ground in addition to lateral forces of turning. These vectors are added, and if the new vector exceeds the tyre's maximum static frictional force in any direction, the tyre will slip. If a rear-wheel-drive vehicle has enough power to spin the rear wheels, it can initiate oversteer at any time by sending enough engine power to the wheels that they start spinning. Once traction is broken, they are relatively free to swing laterally. Under braking load, more work is typically done by the front brakes. If this forward bias is too great, then the front tyres may lose traction, causing understeer.
While weight distribution and suspension geometry have the greatest effect on measured understeer gradient in a steady-state test, power distribution, brake bias and front-rear weight transfer will also affect which wheels lose traction first in many real-world scenarios.
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When an understeer vehicle is taken to the grip limit of the tyres, where it is no longer possible to increase lateral acceleration, the vehicle will follow a path with a radius larger than intended. Although the vehicle cannot increase lateral acceleration, it is dynamically stable.
When an oversteer vehicle is taken to the grip limit of the tyres, it becomes dynamically unstable with a tendency to spin. Although the vehicle is unstable in open-loop control, a skilled driver can maintain control past the point of instability with countersteering, and/or correct use of the throttle or even brakes; this can be referred to as drifting.
Understeer gradient is one of the main measures for characterizing steady-state cornering behavior. It is involved in other properties such as characteristic speed (the speed for an understeer vehicle where the steer angle needed to negotiate a turn is twice the Ackermann angle), lateral acceleration gain (g's/deg), yaw velocity gain (1/s), and critical speed (the speed where an oversteer vehicle has infinite lateral acceleration gain).
For motorized vehicles, such as automobiles, aircraft, and watercraft, vehicle dynamics is the study of vehicle motion, e.g., how a vehicle's forward movement changes in response to driver inputs, propulsion system outputs, ambient conditions, air/surface/water conditions, etc.
Steering is a system of components, linkages, and other parts that allows a driver to control the direction of a vehicle.
A traction control system (TCS), also known as ASR, 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.
Quattro is the trademark used by the automotive brand Audi to indicate that all-wheel drive (AWD) technologies or systems are used on specific models of its automobiles.
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.
In automotive design, a front-engine, front-wheel-drive (FWD) layout, or FF layout, places both the internal combustion engine and driven roadwheels at the front of the vehicle.
In both road and rail vehicles, the wheelbase is the horizontal distance between the centers of the front and rear wheels. For road vehicles with more than two axles, the wheelbase is the distance between the steering (front) axle and the centerpoint of the driving axle group. In the case of a tri-axle truck, the wheelbase would be the distance between the steering axle and a point midway between the two rear axles.
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.
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A swing axle is a simple type of independent suspension designed and patented by Edmund Rumpler in 1903. This was a revolutionary invention in automotive suspension, allowing driven (powered) wheels to follow uneven road surfaces independently, thus enabling the vehicle's wheels to maintain better road contact and holding; plus each wheel's reduced unsprung weight means their movements have less impact on the vehicle as a whole. The first automotive application was the Rumpler Tropfenwagen, later followed by the Mercedes 130H/150H/170H, the Standard Superior, the Volkswagen Beetle and its derivatives, the Chevrolet Corvair, and the roll-over prone M151 jeep amongst others.
A tilting three-wheeler, tilting trike, leaning trike, or even just tilter, is a three-wheeled vehicle and usually a narrow-track vehicle whose body and or wheels tilt in the direction of a turn. Such vehicles can corner without rolling over despite having a narrow axle track because they can balance some or all of the roll moment caused by centripetal acceleration with an opposite roll moment caused by gravity, as bicycles and motorcycles do. This also reduces the lateral acceleration experienced by the rider, which some find more comfortable than the alternative. The narrow profile can result in reduced aerodynamic drag and increased fuel efficiency. These types of vehicles have also been described as "man-wide vehicles" (MWV).
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Trail braking is a driving and motorcycle riding technique where the brakes are used beyond the entrance to a turn (turn-in), and then gradually released. Depending on a number of factors, the driver fully releases brake pressure at any point between turn-in and the apex of the turn.
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.
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.
S-AWC is the brand name of an advanced full-time four-wheel drive system developed by Mitsubishi Motors. The technology, specifically developed for the new 2007 Lancer Evolution, the 2010 Outlander, the 2014 Outlander, the Outlander PHEV and the Eclipse Cross have an advanced version of Mitsubishi Motors' AWC system. Mitsubishi Motors first exhibited S-AWC integration control technology in the Concept-X model at the 39th Tokyo Motor Show in 2005. According to Mitsubishi Motors, "the ultimate embodiment of the company's AWC philosophy is the S-AWC system, a 4WD-based integrated vehicle dynamics control system".
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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.
The brake balance or brake bias of a vehicle is the distribution of brake force at the front and rear tires, and may be given as the percentage distributed to the front brakes or as the ratio of front and rear percentages. The braking balance affects the driving characteristics in terms of how fast the vehicle can brake, how the vehicle can take corners, and tire wear. The optimal brake balance can vary between circuits, weather conditions and driving styles. On race cars, the brake balance is often part of the racing setup, and in formula car racing it is regularly adjusted during the course of an entire lap. In some cases, the brake balance may be adjusted to match the traction (grip) of the vehicle during braking, which usually means distributing a greater braking force to the front. In other cases, it may be desirable for the brake balance to be the more similar at the front and rear for the tires to last longer, which may be beneficial in endurance racing. Adjustment of the brake balance is often done by adjusting a proportioning valve which determines the distribution of the brake force between the front and rear brakes. The adjustment can be made via mechanical couplings or with the help of a small electric motor.