Tech-Talk: What is understeer/oversteer?

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What is understeer/oversteer? What causes them to happen? Which one is better?
What is understeer/oversteer? What causes them to happen? Which one is better?

The terms ‘understeer’ and ‘oversteer’ are tossed around in motoring circles on a regular basis, with arguments on which is more fun, and which is safer, but many people aren’t always 100% sure what exactly they mean.  In this week’s Tech-Talk we’ll explain what understeer and oversteer are, what causes them, what the advantages and disadvantages are, and which is ‘better’.

What is understeer/oversteer?

Simply put, ‘understeer’ is when a car turns less than the amount commanded by the driver.  ‘Oversteer’ is when the car turns more than the amount commanded by the driver.  A frequently used saying is that understeer is when the front end of your car hits the tree, and oversteer is when the back hits the tree.

The more complex and more accurate meaning of those terms is that ‘understeer’ is when the front wheels carve a wider turning arc than intended, and ‘oversteer’ is when the rear wheels carve a wider turning arc than the front wheels.

Understeer and oversteer simply displayed: The green line on the diagrams on either side is the correct line of the vehicle, and the red line is the incorrect line caused by either understeer or oversteer.
Understeer and oversteer simply displayed: The green line on the diagrams on either side is the correct line of the vehicle, and the red line is the incorrect line caused by either understeer or oversteer.

What causes understeer/oversteer?

Understeer and oversteer are both caused by loss of traction, from different pairs of wheels, either front or rear.  Understeer is caused by the front wheels losing traction during a turning manoeuvre, whilst the rear wheels maintain grip, and oversteer is caused by the rear wheels losing grip during turning whilst the front wheels retain their grip on the road surface.

Any factors can contribute towards understeer/oversteer, such as wheel camber, suspension setups, driven wheels, and tyre stiffness to name a few.  For example, low profile tyres have a much stiffer sidewall that is less prone to flexing.  As there is less ‘give’ they are less prone to losing traction, which would result in either understeer or oversteer.

Another example and one of the most common factors in understeer/oversteer situations is the driven wheels of a vehicle.  Front wheel drive cars, for example, place an extra load on the front wheels as not only do these have to steer the vehicle, but also drive the vehicle.  As such, they often lose grip and understeer in a cornering situation.  Rear-wheel drive cars however let the front wheels focus solely on steering, while the rear wheels are driven.  As such, the rear wheels will often lose grip easier than the front wheels due to heavier throttle inputs, resulting in oversteer.

How can oversteer/understeer be prevented?

Mechanically – not relating to driver error/instruction, but instead the vehicle itself – understeer/oversteer can be prevented in several ways:

  • A limited slip differential – when a vehicle corners, the outer wheel has to cover a greater distance than the inner wheel.  Differentials allow the wheels to spin at different speeds to accommodate this.  However, under strenuous cornering at higher speeds, one of the wheels may lose traction, this is when a LSD (Limited Slip Differential) feeds power to the wheel with the most grip, allowing traction to be retained overall and avoiding understeer/oversteer situations.
  • Low profile tyres – as mentioned above, higher profile tyres have more flex in their sidewalls.  Low profile tyres, being stiffer, do not flex and lean as much, and as such maintain better grip, preventing understeer/oversteer.  This same principle applies to run-flat tyres which have better handling characteristics than regular tyres.
  • Camber adjustments – some cars, such as BMW’s RWD cars often have slightly negative camber on the rear wheels.  This allows the weight and power to shift when cornering enthusiastically and apply more grip to the outer wheel, aiding prevention of oversteer.
  • Torque vectoring – this system, generally via electronic control methods, calculates wheel slippage and applies brakes to individual wheels in order to aid cornering.  Commonly used in modern AWD applications, this system will correct understeer by braking the inner front wheel during a corner and allowing the car to essentially pivot around in a tighter cornering arc.  In the same way, it can also prevent oversteer by braking the outer front wheel.  Depending on the computer calculations, some systems may also brake individual rear wheels at times.

From a driver point of view, understeer and oversteer can be avoided by not cornering too hard and fast for the capability of the vehicle, by controlling throttle inputs, braking selectively, and managing weight transfers accordingly.  Understeer and oversteer can also be controlled and maintained using these same techniques, however this should never be attempted unless in a closed environment with professional teaching and assistance.

What are the advantages and disadvantages of understeer/oversteer?

Understeer and oversteer each have separate pros and cons, as such we’ll deal with them individually:

Understeer:  Advantageously, understeer is generally seen as a safety net.  Should a driver go into a corner too hard, a car will understeer rather than spinning out as this is deemed safer by car manufacturers, as a safety function.  Also, understeer is more easily corrected than oversteer by a less-skilled driver as by coming off the power, the car will slow and the wheels will regain traction and continue turning.

However, understeer is not conducive towards performance-based driving.  Understeer prevents tight cornering and results in slower lap times and less accurate placement of the vehicle on its handling limits.  It is also seen to be not as fun or entertaining.

Oversteer:  The advantages of oversteer is that it can be controlled and utilised more than understeer by actions such as correct throttle inputs and counter-steering or applying opposite lock.  Oversteer is also generally seen as more fun than understeer, as there is something quite enjoyable about sliding the back end of a car about.

However, the techniques used to control oversteer are only to be used by capable drivers.  As such, inexperienced drivers may find themselves in way over their heads without the ability to control an oversteering vehicle.  Learning to control oversteer should only ever be done in a closed environment under expert supervision.

Which is ‘best’, understeer or oversteer?

For the vast majority of inexperienced or ‘amateur’ drivers, understeer is far safer.  As such it is generally a better characteristic to be found at a car’s limits. For performance based vehicles, however, oversteer is generally better as it is more controllable with a capable driver at the helm.

Ideally a car that responds to steering commands 100% accurately without understeering or oversteering is a better car in all scenarios.

Conclusion:

Now you know what understeer and oversteer are, you may be intrigued as to what each feels like, and some of you may be driven to play around with these handling characteristics.  We strongly urge all drivers to partake in an advanced driving course that will not only demonstrate both these characteristics in a safe environment, but that will teach you how to control either of them should they occur in the real world.

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