Jump to: navigation, search

Oversteer

Ideally, the vehicle will travel along the line marked with green dots. If the vehicle oversteers, the rear wheels lose traction and the vehicle tends to follow the red dotted line.


Oversteer is a phenomenon that can occur in an automobile which is attempting to turn. The car is said to oversteer when the rear wheels do not track behind the front wheels but instead slide out toward the outside of the turn. Oversteer can throw the car into a spin.

The tendency of a car to oversteer is affected by several factors such as mechanical traction, aerodynamics and suspension, and driver control. The driving technique called opposite lock is meant to cope in this circumstance.

Limit oversteer happens when the rear tires exceed the limits of their lateral traction during a cornering situation before the front tires do, thus causing the rear of the vehicle to head towards the outside of the corner. More generally oversteer is the condition when the slip angle of the rear tires exceeds that of the front tires.

Rear wheel drive cars are generally 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.

In modern race cars, especially open-wheel race cars, oversteering in high speed turns is caused mainly by aerodynamic configurationTemplate:Fact. In this respect, 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). Nevertheless, the required front/rear balance 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. Camber angles, ride height, and tire pressures can also be used to tune the balance of the car.

An oversteering car is alternatively referred to as 'loose' or 'tail happy'.

How do you differentiate Oversteer and Understeer?
The racing driver's joke is "When you see the tree you're about to hit, it's called understeer. When you can only hear and feel it, it's oversteer." Another is "Oversteer is when the passenger is scared; Understeer is when the driver is scared." To put things in even simpler terms, when you turn into a corner, oversteer is when the car turns more than you expected and understeer is when it turns less than you expect.

Critical Speed

Oversteering cars have an associated instability mode, called the critical speed. As this speed is approached the steering becomes progressively more sensitive. At the critical speed the yaw velocity gain becomes infinite, that is, the car will continue to turn with the wheel held straight ahead. Above the critical speed a simple analysis shows that the steer angle must be reversed (counter steering), but this may be an oversimplification, as the model used is linearised in many important ways. (see Gillespie :Fundamentals of Vehicle Dynamics, or any basic vehicle dynamics text). Understeering cars do not suffer from this, which is one of the reasons why high speed cars tend to be set up to understeer.

In road cars

Contrary to popular opinion, modern rear-wheel-drive cars are much more user-friendly in regard to oversteer. Their suspensions are not balanced heavily toward understeer, in fact with today's experience in making cars, most makers try to achieve neutrality from the respective configurations so that they are largely capable of oversteering especially when the driver attempts it on purpose. This is especially evident in the motoring show "Top Gear", where you rarely see a car that doesn't oversteer when tested by Jeremy Clarkson. However, there are some exceptions (eg. Smart Fortwo and Toyota Prius) that display marked understeer. In particular the Smart failed to even turn into the second curve of an S shaped section. More powerful cars typically have on-board computer systems which can automatically brake the wheels or override the driver's throttle inputs to prevent an oversteer condition (whether the driver wants it or not). This is because understeer is generally much safer for novice drivers, whereas oversteer is much more difficult to correct when one is not prepared for it. Many sports cars allow these systems to operate in more liberal modes or turned off completely by experienced drivers. Often this is performed by a special key sequence or other little known means so that inexperienced drivers will hopefully never try it.

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. Unfortunately, this is exactly the wrong thing to do - it can lead to the treacherous phenomenon of lift-off oversteer. Releasing the throttle pitches the car forward, causing a weight transfer towards the front of the car, thus reducing rear traction even further. The nose of the car rotates sharply toward the inside of the turn as it pitches into a spin.

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.

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, cutting the power mid-corner can induce oversteer even in a front wheel drive vehicle. This is known as lift-off oversteer. "Trail braking," or continuing to apply brake pressure after turning into a curve, can induce oversteer by transferring weight off of the rear tires, regardless of whether the car is front, rear or all-wheel drive. Note that in a front wheel drive car it is often better to simply accelerate hard to correct an oversteer slide.

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. Note that this applies only to pavement 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). It should be noted that 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'.

Drifting

The act of deliberately sending a car sideways through a series of corners is actually a popular form of motorsport that originated in Japan known as drifting.