The word acceleration, according to the English dictionary, means, ‘to cause to move faster’ but it really goes deeper than that. A physicist will say that acceleration is a terms used to state that an object has been set in motion, as opposed to being in stationary, but the more well known meaning of that term would be to say that an object has been subjected to the progressive increase in speed. When talking of cars, and other motor vehicles, acceleration is a word used to describe the act of increasing speed, either through an increase in the pressure applied to an accelerator pedal, thus bringing about an increase in the amount of engine power delivered to the driven wheels, or through the force of gravity, i.e. when descending an incline. We understand it more as the increase from one speed upwards to another.

If you plonk your right clog on the gas pedal the car surges forward to a higher speed. Well if you drive around with your brain in neutral then yes that is all there is to it, but what is going on within a car when you apply that power?

For a start the car doesn’t actually want to move.  It is quite happy where it is, standing still.  In fact all the time it is standing still it is in its most stable state as far as balance is concerned. It is only once it begins to move does the balance change. To get the car to move you have to apply a considerable amount of energy, and it takes more energy to begin that movement than it does to maintain it.  That is because you have to overcome something called inertia, which isn’t a force, it is just a natural state that any object will be in when standing still.  We know that it is harder to get the car moving than it is to keep it moving, because if anyone has ever tried to physically push a car along the road, perhaps when it has broken down, this phenomenon will be very well demonstrated.

Of course, vehicles like any other inanimate object, do not have emotions or feelings, so when it is said that a car does not want to move, this is not to be taken literally.  It is just a term of expression.

As a vehicle moves away from rest, one gets the impression that much of its weight moves backwards in response

• the power output of the engine relative to the mass of the vehicle.
• the degree of throttle opening applied.
• the speed of the vehicle at the time the requirement is made for it to accelerate to a higher speed.
• the amount of adhesion that exists between the tyres of the driven wheels and the road surface.
• the ratio of gearing that exists between the output of the engine and the driven road wheels.
• whether the vehicle is on a gradient, and in which direction the force is being applied relative to that gradient.

It’s all very well talking about all of this, but do we really need to think about what is going on beneath us when we drive, and do we need to know to what extent? Well actually yes, because what you do with the accelerator will affect the balance and stability of the car, and whether that effect is healthy or otherwise.

As long as the force involved in the act of acceleration remains parallel to the direction of travel of the car, that car will remain sufficiently well-balanced so as to be easily kept under control by the driver. However, if that angle of force becomes out of alignment to the direction of travel of the vehicle then the car will become less stable, and when a car becomes less stable it is one step closer to crashing. At Ride Drive we put this in simple terms by saying that the best policy is to keep the loading applied to the road wheels evenly matched between wheel-pairs, so in the case of an accelerating car this would involve the two rear wheels, or if you prefer to think of it in this way, the rear axle. Weight equally distributed over matching pairs of wheels is a good situation, but an imbalance is a bad situation.

It also pays to load up those forces on your car smoothly and progressively so that it has a chance to deal with them. To illustrate this we use an analogy that we call the Heavy Box story. If you had to lift a heavy box the most controlled method you would choose would be to have the object sitting in a table-top and at about waist height. You would then take a firm and comfortable grip and progressively take up the strain until you had overcome the gravitational pull (weight) acting upon that box as you lift it upwards. However, if someone tossed that same box into your arms you would probably drop it – not because you were not strong enough to hold that amount of weight, but because it was loaded upon you too quickly for you to cope with it.

The heavy box story applies to vehicles all the time. Throw the loading onto a component part suddenly and it may not cope, but allow that component to take up the strain progressively and it will deal with it more easily – and probabaly last many times longer too. The accelerator pedal is not an on/off switch, but a device that facilitates progressive adjustment in its application; just in the same manner as a volume control on a radio. Apply power smoothly and the car will react favourably. Treat it with indifference and without empathy, and the car will protest in some way. You will immediately notice the difference.

There is another force caught up in all of this, a force that is trying to keep it all together for you, and that is tyre grip. If any other force becomes greater than the grip of the tyres on the road surface then you will skid, and if you don't do something to redress that balance quickly enough, you will lose control and may well crash - it's that simple.

Julian Smith

Ride Drive Limited