Drivers Fact Sheets
Four Principles Of Safe Cornering
Cornering is a potentially dangerous driving activity. To remain safe, the driver must assess the severity of the bend. By using these four key principles of safe cornering, the manoeuvre can be accomplished without undue risk.
- The vehicle should in the right position on the approach.
- It should be travelling at the right speed for the corner or bend.
- The right gear should be selected for that speed.
- The vehicle should be able to stop on its own side of the road in the distance the driver can see to be clear.
The driver must plan and exercise good judgment when applying these four principles to the variations in traffic conditions, the road surface, visibility and the bend itself.
Before we look in detail at these four principles comprising the system of vehicle control for cornering, it is helpful to consider other key factors that affect the ability of a vehicle to corner safely.
It is obvious that a moving vehicle is at its most stable when travelling on a level course at a constant speed in a straight line. Unless some other force changes or alters its direction it will continue to travel in a straight line. Steering the vehicle creates the turning force from the action of the front tyres on the road enabling it to change direction. Now the important bit; if the front tyre grip is broken, the car will continue in a straight line. The tighter the bend, the heavier the vehicle and the higher the speed, then the demands on that tyre grip are greater.
As you corner your body feels it is being pushed towards the side of the car. Actually, it is still moving in a straight line and only turns round the bend because it is forced to by the car.
Three forces compete together to affect tyre grip.
The more you accelerate or brake, the less tyre grip you have for steering. Therefore, the faster you go into a bend, the greater the tyre grip needed to keep you on course round it.
The practical outcome of these forces is to cause vehicles to continue in a straight line rather than turning when tyre grip is lost.
Understeer And Oversteer
Understeer is the tendency of a vehicle to turn less whilst oversteer is the tendency to turn more in response to a given turn of the steering wheel. This tendency to oversteer or understeer is a characteristic of the vehicle itself and depends primarily on what sort of drive the vehicle has.
Therefore a driver should always take extreme care when taking charge of a vehicle which is unfamiliar to him or her. In general, front-wheel drive vehicles understeer and rear-wheel drive vehicles oversteer. To summarise it is a driver’s responsibility to make a point of knowing whether his/her vehicle understeers or oversteers and to adapt to the characteristics of the vehicle on corners and bends.
In a front wheel drive vehicle you will increase understeer if you:
- enter the bend at too high a speed
- apply power in the bend
- steer too sharply
It follows you can reduce this understeer by partially reducing power and/or steering. However, if power is reduced too much too suddenly, you may convert the understeer to oversteer with potential disastrous consequences.
A rear –wheel drive initially behaves in the same way. But if excessive poweris applied on a slippery surface the understeer may convert quite suddenly to oversteer. This requires a prompt steering correction in the opposite direction to the bend.
Of course, four – wheel drive vehicles provide better road adhesion all round but when driven to extremes, they will behave in a similar way to the front or rear wheel drive model from which they are derived.
Camber And Superelevation
Across its width, a road’s surface is not normally level but slopes to improve drainage. The slope across the road affects steering. The normal slope falls from the crown of the road to the edges and is called camber.
- on a left hand bend camber increases the effect of your steering because the road slopes down in the direction of turn.
- on a right hand bend camber reduces the effect of steering because the road slopes away from the direction of turn.
Of course, this only applies if you keep to your own side of the road. If you cross over the crown to the other side of the road, the camber will have the opposite effect on steering.
There are many instances, especially at junctions where the slope across the road surface falls at an unexpected angle. Whatever the slope, if it falls in direction of your turn it will increase the effect of your steering; if it rises in the direction of your turn it will reduce the effect of your steering. You need to consider the slope across the road when deciding your speed for a bend.
Superelevation is where the whole width of the road is banked up towards the outside edge of the bend, making the slope favourable for cornering in both directions.
The factors which affect a vehicle’s ability to corner are as follows.
- the amount of steering you apply
- the amount of acceleration
- the amount of braking
- the characteristics of the vehicle
- camber and superelevation
- the road surface and the effects of weather on grip
The System Of Vehicle Control And The Limit Point
The system of vehicle control assists planning in approaching and negotiating corners and bends. There are five phases of the system:
As you approach a bend using the system you should be putting together as much information as possible about the severity of the bend. You should look out for the clues available to you for the following:
- road surface
- road signs
- road markings
- the line made by lamp posts and trees
- the speed and position if oncoming traffic
- the angle of headlights at night
This is necessary to anticipate and plan for the severity of the bend. A valuable aid to observation is the limit point because it gives you a systematic way of judging the correct speed through the bend.
How To Use The Limit Point To Help You Corner
The limit point is the furthest point along a road to which you have an uninterrupted view of the road surface. On a level stretch of road this will be where the right-hand side of the road appears to intersect with the left-hand side of the road. This point of intersection is known as the limit point. To drive safely you must be able to stop on your own side of the road within the distance you can see to be clear – that is, the distance between you and the limit point.
The ability to stop on your own side of the road in the distance you can see to be clear determines how fast you can go. The more distant the limit point the faster you can go because you have more space to stop in. The closer the limit point the slower you must go because you have less space to stop in.
As you approach and go through a bend the limit point appears at first to remain stationary as you approach it, then to move away at a constant speed and finally to sprint away to the horizon as you come out of the bend.
The technique of limit point analysis is to match your speed to the speed at which the limit point appears to move. If it is moving away from you, accelerate, if it is coming closer to you or standing still, decelerate or brake. Even if the bend is not constant, you can still match your speed to the apparent movement of the limit point, because this will vary with the curvature of the bend.
Approaching The Bend
At first the limit point appears to remain at the same point in the road. Reduce your speed to be able to stop safely within the remaining distance.
As you approach the bend take information about the sharpness of the bend and carefully assess the appropriate speed for cornering.
Going Through The Bend
Just before you enter the bend the limit point begins to move round at a constant speed. Adjust your speed to the speed of this movement.
You now have the correct speed for the bend. Select the gear to match this speed before entering the bend.
Coming Out Of The Bend
As the bend starts to straighten out, your view begins to extend, and the limit point starts to move away more quickly. You then accelerate towards the limit point in proportion to the straightening out of your steering.
As the bend comes to an end, continue to accelerate to catch the limit point until other considerations such as speed limits or new hazards restrict your acceleration.
The Advantages Of Using The Limit Point Together With The System
- it ensures that you observe the driving safety rule of matching your speed to your ability to stop within the distance you can see to be clear
- it gives you the appropriate speed to approach and negotiate the bend
- it gives you the appropriate speed to go round the bend, and therefore the appropriate gear to be in
- it gives the point at which to start accelerating
- it is self-adjusting: as road visibility and conditions deteriorate you need more distance in which to stop, and so your speed must be reduced to compensate.
Using The System For Cornering
This section takes you through the five phases of the system identifying key considerations at each phase and explaining how to use limit point analysis in the speed phase. As with any other use of the system, you should work through it methodically selecting the phases that are appropriate.
On the approach to a corner or bend you should be constantly scanning the road for information, but specifically you need to look for:
- the traffic in front and behind
- the road surface and the effect of weather conditions on it
- the severity of the bend
- the limit point
Seek out opportunities to look across the bend through gaps in hedges or between buildings. Look at the line of curvature of hedgerows and lamp posts to give you more information about the severity of the bend. Avoid becoming preoccupied with the bend – look for early warning of other hazards as well.
You need to consider three things when deciding where to position your vehicle for cornering:
- information needs
- reducing the tightness of the bend
Position yourself so that you are least likely to come into conflict with other road users, taking into account the length of your vehicle. On tight bends be aware of the need to allow for trailer cutting. Look out for pedestrians to your nearside and oncoming traffic to your offside. Safety is the overriding consideration. If you can safely adopt one of the positions suggested below do so, but never sacrifice safety for position.
Your road position will determine how much you can see when you enter a bend. The position which gives you the greatest view depends on whether the bend is a left-hand bend or a right-hand bend. For a right bend the best viewing position is towards the nearside and for a left bend it is towards the offside. But always put safety first.
Right-hand bends – position yourself towards the left of your road space. Be wary of parked vehicles and pedestrians and give them sufficient clearance. Other dangers to consider are blind junctions or exits, adverse cambers and poor condition of the nearside road surface.
Left-hand bends – position yourself towards the centre line so that you get an early view round the bend. Before you adopt this position consider:
- approaching traffic and other offside dangers which require a greater margin of safety
- whether your position might mislead other traffic as to your intentions
- whether any advantage would be gained at low speed or on an open bend
Reducing The Tightness Of The Bend
The other thing to consider is reducing the tightness of the curve through which you drive. By moving your vehicle from one side of the available road space to the other you can follow a shallower curve and thereby improve stability. The path you take is different for a right-or a left-hand bend, but always consider safety first. Do not take a straighter course unless you can see ahead clearly. Often you will not be able to do this until the road begins to straighten out.
Reducing The Curvature Of The Right-Hand Bends
If you have a view across a bend and there is no oncoming traffic, take a gradually curving path towards the centre of the road. Then ease the vehicle back towards a more normal position on the other side of the bend. Under no circumstances must any other road users be endangered.
Reducing The Curvature Of Left-Hand Bends
Keep towards the centre line until you can see clearly ahead. Then drive your vehicle through a gradual curved path towards the nearside of the road, moving into a more normal position on the other side of the bend
When you have adopted an appropriate position, the next phase of the system is to consider and obtain the appropriate speed to enter the bend.
Use the limit point to judge the safe speed to drive round the bend. Where the curve round the bend is constant, the limit point moves away from you at a constant speed. This gives you the speed for the bend unless the curvature changes. If the bend tightens, the limit point appears to move closer to you, and you should adjust your speed accordingly to remain within the safe stopping distance.
When assessing the speed to go round a bend, you need to consider:
- your vehicle’s characteristics
- the road and road surface conditions
- the traffic conditions
- the weather conditions
When you have achieved the right speed and before entering the bend, engage the appropriate gear for that speed. Select the gear that gives you greatest flexibility.
Think also about your expected acceleration on the far side of the bend. For instance, if you expect to come out of the bend in a 30mph area, gentle acceleration would be appropriate. If the speed restriction on the other side of the bend is the national speed limit, consider entering the bend in a gear that will provide maximum acceleration out of it. The condition of the road surface needs to be included in these considerations: in wet or slippery conditions, harsh acceleration in a low gear may well result in wheel spin and a loss of steering control.
Depress the accelerator sufficiently to maintain a stable speed round the bend. Providing there are no additional hazards, start to accelerate when the limit point begins to move away and you begin to straighten your steering. (Check no vehicles are attempting to overtake you first).
As you continue to straighten your steering, increase your acceleration to ‘catch’ the limit point. Accelerate until you reach the speed limit or other speed limiting considerations.
I confirm I have read the fact sheet ‘Cornering’
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