A vehicle is only completely stable when stationary or travelling in a straight line at constant speed (Mr Newton told us this). However, we get into our vehicle in order to travel, hence the vehicle must move and we must risk some instability � the roads we travel along change direction by means of curves, bends, corners or even �hairpins�! In other words, our vehicle has a mind to go straight on, but we want to follow the road; hence bends (we�ll use this word to describe all types of variation in direction) are a problem. So we must deal with them as safely as possible. How do we do this?
Obviously we must approach at a speed at which we know, by learning and experience, we shall be able to get round them. The IAM System of Car Control gets us to prepare for this by: observing all the time, correct positioning, changing speed, then selecting a gear � all this before finally accelerating through the bend once it is clear that we can safely do so. But why, when negotiating a bend, do we need the acceleration phase of the system? Surely this could make us go too fast and not negotiate the bend at all. Yes, if overdone, but experience tells us that gentle acceleration is what is needed. Why? This article will address this in simple terms.
We have said above that the vehicle will want to go straight on in a bend, but once we have got it turning there will be a tendency for this turning action to continue and the vehicle to spin i.e. the rear wheels going towards the outside of the bend. We need all the help we can get to control this by keeping these wheels firmly planted on the road. To help with this, it would be good to find a way of putting more weight onto the rear wheels to improve their grip. Whilst the road surface, its condition, the characteristics of the tyres and the loading of the vehicle cannot immediately be changed by the driver, there is a way of getting more downwards force onto the rear wheels to increase grip and this is by getting some �weight transfer�.
For a given situation, the total grip of a tyre is, as we know, dependent upon the down-force acting upon it. We therefore need to increase that down-force. To do this is easy. As a vehicle accelerates, the forwards force acting on the bottom of the two or more driven wheels tries to pull the vehicle forwards faster and as the mass (bulk) of the vehicle is reluctant to do so, it tends to hold back (Mr Newton again). This causes the mass of the vehicle, somewhat above the road, to try to swing towards the rear (sometimes the front end can be seen to rise as the rear presses down), giving what is called �weight transfer� � an increase in the down-force on the rear wheels.
Hence, for increased stability, and thus safety, in a bend, accelerate slightly through it to get increased grip at the rear wheels. Remember, too, not to do anything which might tend to decrease stability of the vehicle in any way � avoid applying any additional braking (may be impossible on a steep downhill bend), avoid changing gear or steering erratically. Doing any of these things will make the vehicle less stable and safe grip is more likely to be lost. However, all these problems will have been avoided if we are correctly using the IAM system.
It must be added that, talking of everyday driving, we are not concerned here with whether a vehicle �over-steers� or �under-steers�. The principles described above apply no matter what the vehicle characteristics are. In the main, these characteristics will affect how and how fast the vehicle responds to steering inputs. Nor does it matter whether the vehicle has front, rear or all-wheel drive.
One might also add that accelerating through a bend will increase passenger and driver comfort. Instead of, at a constant cornering speed, being thrown directly sideways across the seat by the cornering forces, acceleration will tend to hold the occupant into their seat, giving more support and thus less discomfort. Advanced driving should always aim to provide a comfortable journey.
Remember � complete the braking and gear changing phases of the System fully before entering a bend, then with both hands on the steering wheel, gently accelerate round the bend once you know it is safe to do so.
(Article reproduced from the IAM Newsletter)