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Credit to 'Competition Car Suspension' by Greg Simmons for the original image
When you reach the limits of grip on a corner, two scenarios can result known as understeer or oversteer.
Understeer occurs when traction is lost at the front wheels while cornering, forcing you wide on a bend despite applying the correct steering angle (Diagram 1). When viewed by an observer, this action looks as if the driver has applied insufficient steering lock (or under steered). If you're car is understeering, you're scrubbing off speed and missing the optimum line, so it's not a quick way to take a bend.
OVERVIEW
The diagram below shows the path that that is travelled during an episode of Understeer.
The Stages of Understeer
A. The car has turned in towards the apex
B. The driver has hit the apex but has found the car is pushing wide of the desired line (dotted)
C. Despite increasing the steering angle, the car has taken a line which is not tight enough to take the turn
D. The car has been forced off the track by understeer
Close up view of Understeer
Understeer is most likely to result from the following scenarios (which are more difficult to correct as you move down the list):
- Accelerating into a bend
- Braking into a corner
- Ploughing into a corner too fast
- Low traction conditions on the corner such as ice or oil
Having lost traction, understeer is actually a fairly stable state for the car to be in, and thus many manufacturers 'engineer in' this behaviour.
UNDERSTEER – FACTORS AT WORK
PASSIVE FACTORS INVOLVED
- Weight distribution
- Drive layout
- Suspension & chassis setup
- Tyre type, wear and pressures
ACTIVE FACTORS INVOLVED
- Cornering speed
- Throttle
- Braking
- Steering inputs
- Weight transfer
SYMPTOMS OF UNDERSTEER
- Light steering
- Drifting towards the outside of a bend
- Possible tyre noise from the front wheels
CORRECTING UNDERSTEER
To correct any form of traction loss, you need to consider why you've exceeded the limits of grip at the front wheels….
1. ACCELERATING THROUGH A BEND
Picture the scene, you're in a front wheel drive car on a roundabout about to take the third exit but you think it might be a laugh to go round again. You keep accelerating through the bend and find that the car has a tendency to run wider and wider. The available grip at the front wheels is being used in equal amounts to accelerate and to steer. As you accelerate more, you have less grip to steer – simple. So, reducing either of these inputs will correct the understeer.
This is the easiest form of understeer to correct, and a slight, smooth reduction in power will free up more grip (with the added benefit of a forward weight transfer), and a small corrective input to the steering will get you back on line.
In a rear wheel drive car, in theory the front wheels will be able to resist understeer for longer due to the division of steering and drive between the front and back wheels. However, understeer can also be engineered into a car for safety, and most rear wheel drive cars will also understeer if power is progressively applied mid corner. If you do decide to accelerate aggressively mid corner you're likely to cause oversteer.
2. BRAKING INTO A CORNER
When you apply the brakes, most of the braking effort is exerted on the front wheels due to the forward weight transfer. So if you're braking into the corner you're already using most of your available grip trying to scrub off speed. If you then apply some steering lock, the addition of these lateral forces on the tyre can cause the limits of grip to be exceeded. So, correcting understeer seems simple - stop trying to turn the corner (a better plan might be to get your braking out of the way while you're in a straight line). However, if you happen to be in the middle of a bend as your car starts to understeer, continuing straight on might not seem like the best plan. An alternative strategy could be to reduce your braking effort, freeing up more grip for steering and hopefully allowing you to take the bend successfully.
3. PLOUGHING INTO A CORNER TOO FAST
If you have attempted to take a corner too fast, have turned the steering wheel, and find yourself running wide, you my friend are in a spot of bother. But before you close your eyes and hope for the best, all may not be lost. You've exceeded all of the available grip, yes, but it may be possible to actually increase the level of grip by the slightest, smoothest dab on the brakes. "The brakes?!", I hear you shout, "but surely that is adding to the demands of the tyres, not reducing them?". This is very true, but if you're not totally out of control by pressing the brakes you're causing the weight to transfer to the front, and thus artificially increasing the levels of adhesion at the wheels. This may, however, not work. The moral is to enter the corner at a slower speed, then get on the power early on the way out.
4. LOW TRACTION CONDITIONS
If you've entered a corner at speed and notice a sudden reduction in traction due to oil, black ice or a banana skin, the best course of action (in the immortal words of Douglas Adams) is to consider how lucky you are that life has been kind to you so far.
If life hasn't been kind to you, consider how lucky you are that it won't be bothering you much longer.
Alternatively, be careful, think about the conditions and adjust your entry speed accordingly.
AVOIDING UNDERSTEER – RULES OF THUMB
- Be as smooth as you can
- Don't enter corners flat out, and accelerate as you exit
- Don't brake in a corner. The only exception to this is if you are using trail braking...
TRAIL BRAKING
In some situations on the track, it may be possible to get a better time by leaving your braking to the very last minute, forcing you to maintain braking into the turn. If this is the case, ensure most of the braking effort has been carried out in a straight line, and progressively release the brakes as you approach the apex. The resulting forward weight transfer can reduce understeer and improve 'turn in', however, it can also make the car more prone to oversteer. This is an advanced technique and should only be used once you are very confident with your car, the track and the conditions.
SIMPLE MODIFICATIONS TO MAKE A CAR LESS PRONE TO UNDERSTEER
If you have a track car and find understeer a problem, you can complete some relatively easy modifications which can make the handling more neutral. These include:
- Reducing the front tyre pressure
- Softening front springs or anti-roll bar
- Use softer front tyres
- Increase front down force (if aerodynamics fitted)
Might be worth consulting an expert before doing anything too dramatic, but if you're feeling adventurous, the chart below can help
ADVANCED UNDERSTEER DIAGNOSIS AND MODIFICATIONS
Below is a flowchart which can help diagnose and treat the symptoms of understeer.
Diagnosis of understeer and modifications to solveCredit to 'Competition Car Suspension' by Greg Simmons for the original image
When you reach the limits of grip on a corner, two scenarios can result known as understeer or oversteer.
Oversteer occurs when the rear tyres reach the limit of adhesion in a corner before the front. This leads to 'the back coming out' (see Diagram 1). The good thing about oversteer is that you normally go through the hedge backwards, thus preventing expensive repairs to the front of your car. If you manage to performed sustained, controlled oversteer this is know as drifting.
OVERVIEW
The diagram below shows the path that that is travelled during an episode of oversteer.
A. The car has turned in normally and is aiming towards the apex
B. The rear wheels have started to lose adhesion, the driver compensates by steering left to keep steering aligned to the desired driving line (counter steering, explained below)
C. The driver has maintained control of the car and continues to follow the desired line
D. The car is at the limit of left hand lock, a spin is likely at this stage unless dramatic corrections are made.
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
Oversteer results from a number of factors, some of which involve the natural handling characteristics of the car, and some result from the way it's being driven.
PASSIVE FACTORS AT WORK (THE NATURAL PROPERTIES OF THE CAR):
- Weight distribution (front or rear bias)
- Engine and drive layout
- Suspension & chassis setup
- Tyre type, wear and pressures
ACTIVE FACTORS INVOLVED (THE WAY IT'S BEING DRIVEN):
- Cornering speed
- Throttle
- Braking
- Steering inputs
- Weight transfer
SYMPTOMS
It's unlikely you'll ever experience oversteer unless you're driving a car near the limits of grip. You can recognise oversteer if...
- The rear of the vehicle becomes unstable and 'light' due to lack of grip
- The car starts to rotate so the driver is facing towards the inside of the corner
CAUSES
There are four major active 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:
- Entering the corner too fast
- Accelerating into the corner, too early or too aggressively
- Braking into the corner or mid corner
- 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
LIFT-OFF OVERSTEER
Lift-off oversteer is a phenomenon which can occur when reducing the throttle mid corner. This will only happen when driving close to the limit so only experiment when on the track! Sporty front wheel drive drive cars can be especially prone to this due to the heavy front end and light rear. Reducing the throttle input results in a forward weight transfer, which increases the grip at the front tyres, but reduces levels at the rear. If this is performed during cornering, the combination of the heavy front end and the reduction of grip can cause the rear wheels to break traction and start to slide towards the outside of the corner.
In the image below, the driver of a front engine car lifts off the throttle mid corner at [A] which results in lift-off oversteer at [B] due to a forward weight transfer
In the image below, the driver of a front engine car lifts off the throttle mid corner at [A] which results in lift-off oversteer at [B] due to a forward weight transfer
Lift-off oversteer
Luckily, lift off oversteer can usually be corrected by reapplying the throttle and accelerating. This should pull the front of the car forwards and straighten out the car, but continuing to reduce throttle application can cause the car to spin. Steering in the direction of the intended direction of travel will help keep the car on track.
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 our 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.
PREVENTING & 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 wheels. In all cases of oversteer, counter steering is also required (explained here).
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 in a go-kart. It’s not the quickest way to take a corner and leads to increased risk of oversteer. If you have entered a corner too fast, ensure that every input you make is incredibly smooth, and take the easiest route. Next time make sure your entry speed is slow enough to maintain grip, you can build up speed as you gain experience.
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 the throttle. The resulting forward weight transfer can upset the balance of the car and allow the rear wheels to break loose. In a front wheel drive car, re-applying the throttle can often help in this situation.
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 (but rapidly) release the brake and adhesion should be reintroduced.
CORRECTING OVERSTEER - COUNTER STEERING / OPPOSITE LOCK
Whatever the cause of oversteer it is important to keep the front wheels pointing in the direction you're hoping to go. If you fail to do this, the most likely result is a spin. This technique is known as counter-steering or applying opposite lock (see Diagram 3). You should apply enough steering lock to point the wheels in the direction of the slide as shown below. Too little and you're likely to spin as the back continues to come round, too much and the car will rapidly over-correct, often resulting in a spin in the opposite direction. The skill can only be mastered with plenty of practice and should become instinct if you're planning to drive fast on a track.
Tip: Always look and focus on the direction you're hoping to go - this makes the steering correction almost automatic. Conversely - don't concentrate too hard on that tree you're hoping to miss as you will be more likely to head in that direction.
Counter-steering, applying opposite lock
Applying corrective steering needs to be done rapidly to catch the back of the car before it slides to a point which may be difficult to control. Once the slide has been controlled and the back starts to fall back in line, it's also important to get the steering correction off quickly too, otherwise you might find your self with oversteer in the opposite direction due to the resulting pendulum effect.
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]
Causes, and likelihood of oversteer for a real wheel drive, front engine car Cause of oversteer Likelihood of oversteer Entering the corner too fast High Accelerating early or aggressively High Lifting off the throttle Medium Braking Medium
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 and braking). 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.
Question: When a front-engine rear-wheel drive car accelerates hard in a corner, there is a high possibility of oversteer. I also understand that weight is transferred to the rear under acceleration, therefore i would like to know how the rear wheels can still lose traction when weight is transferred to the rear and on to the rear wheels.
Answer: There are many factors at work when cornering and it’s a fine balance of forces. It’s important to understand that when you’re cornering there is much less grip available for acceleration and braking. For a rear weight transfer to occur in the first place, you need to have the grip available to put the power down – if you try to accelerate hard you’re likely to overwhelm the rear tyres and spin one or both of the wheels. This reduces the physical grip dramatically which can then lead to oversteer.
Rear wheel drive, rear engine
Causes, and likelihood of oversteer for a rear wheel drive, rear engine car Cause of oversteer Likelihood of oversteer Entering the corner too fast High Accelerating early or aggressively Medium Lifting off the throttle High Braking High
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.
[e.g. Peugeot 205 GTi, Renault Clio Cup]
Causes, and likelihood of oversteer for a front wheel drive car Cause of oversteer Likelihood of oversteer Entering the corner too fast More likely to understeer initially Accelerating early or aggressively Low Lifting off the throttle High Braking High 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 due to the naturally light rear. 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.
Question: When a FWD car accelerates, weight is transferred to the rear. Does this mean that the front of the car where the engine is located becomes lighter than the rear during acceleration?Answer: Firstly, the weight transfers which result from acceleration are perhaps not as great as you think. This is because you're unlikely to be able to accelerate as quickly as you can brake or change direction with the steering - thus the advantages or rear weight transfers are usually slightly less than in other directions. It's very unlikely any front engine car could accelerate to the point that the rear weights more than the front. There will be some transfer of weight onto the rear wheels, but not that much.Four wheel drive (Front Engine)
[e.g. Subaru Impreza, Nissan Skyline, Audi Quattro]
Causes, and likelihood of oversteer for a four wheel drive car Cause of oversteer Likelihood of oversteer Entering the corner too fast Medium Accelerating early or aggressively Medium / High Lifting off the throttle Medium Braking Medium 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!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)
If you want to try something a little more daring, the chart below can help.ADVANCED OVERSTEER DIAGNOSIS AND MODIFICATIONS
Below is a flowchart which can help diagnose and treat the symptoms of oversteer.
Oversteer diagnosis & car modification guideCredit to 'Competition Car Suspension' by Greg Simmons for the original image
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