The system of car control - Part 8

Following our examination of each phase of the System of Car Control, and a general review in the last issue, it is now time to discuss potential problem areas – particularly those that cause difficulty for associates preparing for the IAM test.

Most advanced drivers soon become able to put the System into practice when approaching road hazards.  However, the part of the System that associates have most difficulty with, and the most difficult thing that observers find to teach, is brake-gear overlap – or rather, how to avoid it.

What is brake-gear overlap?  It simply means changing gear whilst still applying the foot-brake; in other words, overlapping the Speed and Gear phases.  ‘Why is that such a bad thing?’ I hear you ask.  Read on – that’s what the rest of this article is about.

A car (or any vehicle) is most stable when it is moving at constant speed in a straight line, because there are no unbalanced forces acting on it.  You might recognise that as Newton’s First Law of Motion.  As soon as you apply a driving input such as steering, braking or accelerating, you upset this balanced state and part of the car’s weight will move to the front, the back, one side or one corner.  For example, if you accelerate, the car’s weight moves to the rear wheels; if you brake, it moves to the front; if you steer left it moves to the right; and vice versa.  If you now do more than one of those at the same time, you cause a more complex weight transfer: e.g. braking and steering left transfers the weight to the right front wheel, and so on.  That’s a bad thing, because it destabilises the car, is upsetting for passen­gers and, taken to extremes, can cause a skid.  It’s why we emphasise the importance of braking in a straight line before a corner or bend, driving round the bend under gentle power to keep the car balanced and stable, and acceler­ating again on the straight road after the bend.

But what about changing gear?  When you press the clutch you disengage the engine power from the driving wheels, so the force moving the car forward is no longer balanced by the opposing forces – the frictional forces in the engine, transmission and the road surface.  It’s a bit like gentle braking – the weight transfers to the front of the car.  You can test this for yourself (in a car park or a very quiet road) by accelerating gently in any gear and then depressing the clutch – the car lurches as the weight transfers forward, and the lower the gear the more it will lurch.  Add proper braking to that and the weight transfer will increase.  Depressing the clutch also means that you are then no longer in full control of the car because the engine’s power or retardation (engine braking) is no longer being applied to the driving wheels.

So, we learn to do one thing at a time – braking, steering, accelerating or changing gear.  If you follow the System correctly, you will do all your slowing down by first releasing the accelerator and then braking, if necessary, before you select the correct gear for the hazard.  That might mean braking harder and for longer than you normally do, in order to get the speed right down before changing gear.

Brake-gear overlap is usually caused by not getting the speed low enough, soon enough.  If you fail to do that you might find yourself simultaneously braking, changing gear and steering into a bend.  Picture that – most of the weight is on the outside front wheel, the opposite rear tyre is scrabbling for grip, the car is probably understeering and so needs more steering input, you have only one hand on the steering wheel because the other is on the gear lever, and boy are your knuckles white!

I can hear another question forming: If brake-gear overlap is such a bad thing, you ask, why do the diagrams in Roadcraft show the Speed and Gear areas overlapping a little bit?  That’s a good question.  It recognises that there are some – a few – situations in which brake-gear overlap is necessary.  Such examples are turning tightly into a junction, usually to the left, from a main road that is downhill.  However soon and however hard you brake, as soon as you press the clutch to change gear the car speeds up again because there is no longer any engine retardation.  So to avoid that you might need to keep the braking on longer, and start to change gear during the end of the braking phase.  Another example is when you select a ‘rolling first gear’ in the final stages of the approach to a Give Way line.  You select first gear whilst still in the last stages of braking, ready to stop completely or to move off at a low speed.  There is a good demonstration of exactly that on the Roadcraft video.

We must recognise, however, that although brake-gear overlap is some­times required, it’s only in special situations like the ones described, and even then the overlap should come towards the end of the braking phase.  If you routinely brake and change gear together (or if you have to brake again after completing the gear change, which is then called secondary braking) you are doing it wrong.