Oversteer

Oversteer is a characteristic of an automobile while attempting to corner or while already cornering.

The car is said to oversteer when the application of cornering force (lateral force) also applies a rotational torque (or moment) to the car in the direction of the turn.

The effect is opposite to that of understeer.

An oversteering car is referred to as "loose" or "free."

Oversteer is a dynamically unstable condition; in other words, if control is lost, the vehicle will spin.

 

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 Causes

The tendency of a car to oversteer is affected by several factors such as mechanical traction, aerodynamics and suspension, and driver control, and may be applicable at any level of lateral acceleration.

Generally, oversteer is the condition when the slip angle of the rear tires exceeds that of the front tires, even when they are both small. Limit oversteer occurs when the rear tires reach the limits of their lateral traction during a cornering situation but the front tires have not, thus causing the rear of the vehicle to head towards the outside of the corner. The driving technique called opposite lock is meant to cope in this circumstance. Trailing Throttle Oversteer (TTO), a.k.a. "snap-oversteer" is induced by the weight balance of the car shifting from the rear to the front, this may happen if the car is cornering under throttle, causing the car to settle on the rear, if the throttle application would be removed—e.g. as to reduce the radius of the turn—the balance would suddenly shift to the front, giving less traction on the rear, if the car was already at the traction limit before the driver lifted the throttle it is very likely to cause a TTO. Rear wheel drive cars are more prone to oversteer, in particular when applying power in a tight corner. This occurs because the rear tires must handle both the lateral cornering force and engine torque.

[edit] Yaw rate

The terms oversteer and understeer are related to yaw rate and not to sideways movement. A car undergoes a circular spinning motion (yaw) as it turns, as well as sideways movement (towards the inside of the corner). Understeer and oversteer refer to the yaw motion. The difference between yaw and sideways movement is best demonstrated by practising turning an aircraft, because separate controls control each of the two movement types in aircraft. Consider a car with its steering wheel turned part way to one side and locked in that position. Now imagine that car rolling forward very slowly on a flat surface. It will move along an arc of a circle whose radius is determined solely by the position of the wheels, since centrifugal force is minimal. Its sideways motion and yaw rate are hence interlinked and set by the steering wheel position. However, the wheels can only provide a limited amount of sideways force before they slide. This sliding will happen at a larger radius as either the speed increases, the friction coefficient decreases, or the normal force decreases. once this sliding occurs, the sideways movement and yaw rate may become unlinked. If the yaw rate of the car tends towards a larger radius than the radius set by the wheels, it is said to understeer. If the yaw rate radius is smaller (spinning too fast), it is called oversteer. During oversteer or understeer the sideways movement of the car may also follow a different radius to that set by the steering wheel, but this does not affect the definition of oversteer or understeer.

[edit] Critical speed

Oversteering cars have an associated instability mode, which occurs at and above the critical speed^[1]. As this speed is approached, with the car on an approximately straight course, the steering becomes progressively more sensitive. At the critical speed the yaw velocity gain becomes infinite, that is, the car will turn violently in response to the slightest steering input or external disturbance. Above the critical speed analysis shows that the yaw response will be reversed for a given steering wheel input, such as a car turning left in response to turning the wheel to the right. This is an oversimplification, however, as the model used is linearised in many important ways.^[2] Understeering cars do not suffer from this, which is one of the reasons why high speed cars tend to be set up to understeer.

[edit] In road cars

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A Mercedes-Benz CLS AMG 55 oversteering on a wet surface

Contrary to popular opinion, modern rear-wheel-drive cars are much more user-friendly in regard to oversteer. Their suspension is not balanced heavily toward understeer, in fact with today's experience in making cars, most manufacturers try to achieve neutrality from the respective configurations so that they are largely capable of oversteering especially when the driver attempts to invoke it on purpose.

The natural reaction of most drivers to the perception of loss of control during oversteer is to immediately lift their foot off the gas pedal. Cutting the power mid-corner can induce more oversteer, known as lift-off oversteer. The correct reaction to oversteer is to gently steer into the slide and take the power away as needed without pitching the car forward. Indeed, "trail braking", or continuing to apply brake pressure after turning into a curve, can induce oversteer by transferring weight off the rear tires, regardless of whether the car is front, rear or all-wheel drive.

Braking may or may not improve the situation. Most modern cars have a brake bias which tends to straighten out the car. However, there are two factors working against this. Most drivers must lift their foot from the gas pedal in order to press the brake, inducing the spin as described above. The second is that braking transfers more of the vehicle's weight forward which tends to worsen oversteer. Even so, the brake bias may be enough to help or at least not make it worse.

[edit] In race cars

A car that tends neither to oversteer nor understeer when pushed to the limit is said to have neutral handling. It seems intuitive that race drivers would prefer a slight oversteer condition to rotate the car around a corner, but this isn't usually the case for two reasons. Accelerating early as the car passes the apex of a corner allows it to gain extra speed down the following straight. The driver who accelerates sooner and/or harder has a large advantage. The rear tires need some excess traction to accelerate the car in this critical phase of the corner, while the front tires can devote all their traction to turning. So the car must be set up with a slight understeer or "tight" tendency. Also, an oversteering car tends to be twitchy and ill tempered, making a race car driver more likely to lose control during a long race or when reacting to sudden situations in traffic.

Carroll Smith, in his book "Drive to Win", provides a detailed explanation of why a fast race car must have a bit of understeer.^[3] Note that this applies only to road racing. Dirt racing is a different matter.

Even so, some successful race car drivers do prefer a bit of oversteer in their cars, preferring a car which is less sedate and more willing to turn into corners (or inside their opponents). The judgement of a car's handling balance is not an objective one. Driving style is a major factor in the apparent balance of a car. This is why two drivers with identical cars on the same race team often run with rather different balance settings from each other. And both may call the balance of their cars 'neutral'.

[edit] Aerodynamic stability

The importance of the position of a fast car's aerodynamic centre of pressure to its directional stability was not understood at first. In the late 1950s, cars such as the 120 mph Jaguar 3.4-litre saloon / sedan were reported to feel directionally unstable at high speeds, and were badly affected by gusts.

Simple streamlining tends to lift the back of a car, reducing the downforce on its back wheels relative to the front wheels, resulting in oversteer. Streamlining also moves the centre of pressure well forward, causing directional instability in cross winds.

At first, aerodynamic oversteer was counteracted by setting the cars up with strong mechanical understeer, resulting in excessive understeer at lower speeds. Various means of achieving aerodynamic stability have since been developed, such as tail fins to move the centre of pressure back, the Kamm tail and the spoiler to reduce lift, rear wings to generate downward acting lift force, and air dams and skirts to reduce air pressure under the car, causing down force due to ground effect. Most of those features improve stability but increase drag, reducing top speed and increasing fuel consumption. However an early example of a fin used for directional stability without reducing top speed is provided by the Jaguar D-Type.

Usually these features are little more than styling gimmicks, the cars not being fast enough to benefit from them.

In modern race cars, especially open-wheel race cars, oversteering in high speed turns is caused mainly by aerodynamic configuration.^{[citation needed]} A heavier aerodynamic load on the front of the car relative to the rear causes it to oversteer. Oversteer in low speed turns is often reduced or eliminated electronically through traction control (if the sanctioning body allows their use). The front/rear balance required to make the cars fast through corners is obtained by setting up the aerodynamics and balancing the suspension. The car's tendency toward oversteer is generally increased by softening the front suspension or stiffening the rear suspension in roll. The suspension's roll stiffness may be adjusted independently of pitch stiffness by means of adjustable or interchangeable anti-roll bars at one or both ends of the car. Camber angle, ride height, and tire pressures can also be used to tune the balance of the car.