Oversteer

Oversteer – an overview
Oversteer – factors involved
Symptoms of oversteer
Causes of oversteer
Oversteer and different drive / engine layouts
Preventing and correcting oversteer – rules of thumb
Simple modifications to make a car less prone to oversteer

When you reach the limits of grip on a corner, two scenarios can result known as understeer or oversteer.

Oversteer – an overview

Oversteer, like understeer, results from the car reaching the limits of traction on a corner. In this case, the rear tyres reach the limit of adhesion before the front. This leads to 'the back coming out' (see the flash movie or Figure 1). The good thing about oversteer is that you normally go through the hedge backwards, thus preventing expensive repairs to the front of your vehicle! If you manage to performed sustained, controlled oversteer this is know as drifting.

Flash movie: Oversteer

The figures below outline exactly what is happening here...

Figure 1: Oversteer in action

A. The car has turned in to the corner normally, all wheels are gripping effectively
B. The rear wheels are starting to lose adhesion, the driver compensates by steering left to keep steering aligned to the blue line
C. The car is at the limit of left hand lock, a spin is likely at this stage

Figure 2: A close up of the latter stages of oversteer

Oversteer is more exciting than understeer and like most exciting things (such as jumping off cliffs) there is an element of risk involved. Most 'driver's cars' have a tendency to oversteer when on the limit around corners, and this property can be found in a variety of vehicle layouts and drive formats.

Oversteer – factors involved

Passive factors at work

• Weight distribution
• Drive layout

Active factors involved

• Cornering speed
• Throttle
• Braking
• Steering inputs
• Weight transfer

Symptoms of oversteer

• Rear of the vehicle unstable
• Car rotating so driver is facing the inside of the corner

Causes of oversteer

There are four major causes of oversteer, but what you're likely to encounter depends on the car being driven (for more information on this please skip to the next section). Causes include:

1. Entering the corner too fast
2. Accelerating into the corner, too early or too aggressively
3. Braking into the corner or mid corner
4. Lifting off the throttle mid-corner. This scenario is also known as:
   
   
   • lift off oversteer
   • snap-oversteer
   • trailing-throttle oversteer
   • throttle off oversteer
   • lift-throttle oversteer

To view a blog post on lift off oversteer, please click here

Oversteer and racing / performance cars

Most performance cars have a tendency to oversteer as they are usually developed to provide a large amount of grip at the front wheels (to turn into corners well at speed), and a large amount of power (usually at the rear wheels). This means that if traction is lost it will usually be at the back first. See the physics of grip section for more information. Thus in most cases the ideal cornering technique in a performance car needs to try and reduce or control oversteer.

And how you do this depends on the spec of the car you are driving...

Oversteer and different drive / engine layouts

Rear wheel drive
In a rear wheel drive car you have the benefit of sharing the demands on adhesion between the front and rear wheels. The front wheels do the steering and the rear do the accelerating (and deceleration under engine braking). This usually results in a more balanced vehicle. In most conventional front engine, front wheel drive vehicles such as the Ford Fiesta it is rare to ever experience anything other than understeer.

Rear wheel drive cars can experience oversteer for different reasons depending on where the engine is located.

Rear wheel drive, front engine e.g. BMW M3, Mazda MX5

In a front engine car, most of the weight is over the front wheels. This usually results in higher natural levels of grip at the front (which is useful for steering, braking and accelerating). The comparatively unladen rear wheels may have lower natural levels of grip, which can lead to oversteer especially when accelerating round a corner. This is the reason some very high powered cars have wider wheels and tyres at the back.

Rear wheel drive, rear engine e.g. Porsche 911

Rear engine, rear wheel drive vehicles such as the Porsche 911 can oversteer for different reasons to front engine varieties. In this case most of the vehicle weight is over the rear wheels, leading to greater levels of natural grip at the back, which should lead to an inherent low risk of oversteer. However, there is another force at work here – momentum. Objects with greater mass carry more momentum and are harder to change direction at speed. In the case of rear engine cars, the rear has more momentum than the front which can lead to a greater risk of oversteer as a result of braking or lifting off mid corner.

Mid engine cars can also oversteer for similar reasons, but tend to provide a better balance of under and oversteer.

Front wheel drive (front engine) e.g. Peugeot 205 GTi, Renault Clio Cup

Sporty front wheel drive cars are more likely to experience oversteer than a standard car due to the vehicle setup. Dial out the inherent understeer tendencies of a front wheel drive car using clever engineering, and the result is a better 'turn in' and an increased ability to oversteer. In this case, it is usually possible to accelerate out of the oversteer situation, using the rearward weight transfer to actively increase levels of grip at the back. Front wheel drive cars are especially prone to lift off oversteer due to the forward weight transfer combined with light rear end..

Four wheel drive e.g. Subaru Impreza, Nissan Skyline

Four wheel drive vehicles can oversteer as much as rear wheel drive cars (depending on setup). However, due to the sharing of drive forces over all four wheels, there is less risk of oversteer due to too over-exuberance with the throttle pedal. Treat as a rear wheel drive vehicle and you won't go far wrong. With particularly powerful four wheel drive vehicles it may be possible to enter a 'four wheel drift'. This is a particularly spectacular way of exiting the road!

Preventing and correcting oversteer - rules of thumb

To correct any form of traction loss, you need to consider why you've exceeded the limits of grip at the rear wheels….

Note: Certain models of car may behave differently on the limit. The following rules of thumb are guidelines only. A front wheel drive Clio Cup doesn't have the same set of criteria as a BMW M3.

1. Entering the corner too fast

Enter a corner too fast and you're asking for trouble. Unless of course you have a lovely long run off to play with or if you're driving Silverstone with a go-kart. It’s not the quickest way to take a corner and leads to increased risk. If you have entered a corner too fast, ensure that every input you make is incredibly smooth, and take the easiest route.

2. Accelerating into the corner, too early or too aggressively

If you manage to break traction at the back when applying throttle, you're probably in a powerful car and need to be less aggressive. If you're spinning wheels, the power is not transferring to the road and you're not benefiting from the many horses you have sitting under the bonnet. Gently ease off the gas and you should regain adhesion at the rear wheels.

3. Lifting off the throttle mid-corner

If you are on the power mid-corner and close to the limit, do not lift off suddenly (or at all if you can help it). The resulting forward weight transfer can upset the balance of the car and allow the rear wheels to break loose.

4. Braking into the corner or mid corner

You should avoid braking in corners in most situations, however there are circumstances which require the use of the anchors. Ensure braking inputs are especially smooth, gentle and progressive, and if a squirrel has just run out in front of you, try steering around rather then doing an emergency stop. To correct brake induced oversteer, smoothly release the brake and adhesion should be reintroduced.

In all conditions

Whatever the cause of oversteer it is important to keep the front wheels pointing in the direction you're hoping to go (please see the blue line in FIgure 2. If you fail to do this, the most likely result is a spin. This technique is known as counter-steering.

Simple modifications to make a car less prone to oversteer

If you have a track car and find oversteer a problem, you can complete some relatively easy modifications which can make the handling more neutral. These include:

• Reducing the rear tyre pressure
• Softening rear springs or anti-roll bar
• Use softer rear tyres
• Increase rear down force (if aerodynamics fitted)

Disclaimer: Always stick to the manufacturers recommended limits when altering tyre pressures, and never modifiy a road car.