Gears and brakes

A friend of mine took the IAM test (without the benefit of joining the local Group and gaining guidance from observers) and failed. He told me that the examiner had regarded it as a good safe drive and that he had failed because he had "used the gears instead of the brakes". Now this set me thinking, if only because this is one of the grievous sins which I too was in the habit of committing.

The planned system of driving (page 16 of "Pass your advanced driving test") is quite clear on the subject: Information, Position, Speed, Gear, Acceleration - get it in the right order. Under Speed the text is "Adjust your speed to the correct level for the hazard by using the brakes or engine braking...". The "Gears or brakes?" issue is specifically addressed on page 26: "Long ago, when brakes were far less efficient, 'sequential downchanging' provided a useful engine-braking effect, but slowing down in a modern car should be achieved with brakes alone, except when descending a steep hill, on icy surfaces (see page 58), or in an emergency caused by brake failure".

Then comes the gem: "It is cheaper to wear out brake pads rather than the gearbox". Pardon me for sniggering at this point, but has anyone ever asked the car manufacturers what they thought of the likelihood of wearing out a gearbox in this fashion? A gearbox is designed to transmit loads under acceleration (particularly in these modern cars we are talking about) that are far greater than the loads generated in the reverse direction under conditions of engine braking. I doubt that the reverse faces of the teeth on the gearbox cogs have any property that is magically different from those on the faces taking the acceleration loads, or that the bearings are unable to accept any ensuing rotational force changes. All I will say further on this gearbox matter is that I started driving "long ago" enough for my driving style to take account of inefficient brakes. It has been my habit to keep cars for 10 years or more - for 80,000+ miles each.

I have yet to replace a gearbox.

OK, engine braking means a potential strain on the engine. Selecting, or remaining in, a lower gear implies higher engine speeds for the same road speed. Is this potentially damaging? To answer this one needs to examine what may happen at excessive engine revs, and also see how nearly the limits are approached in the course of normal driving. One advanced driver suggested to me that if I frequently approached maximum revolutions there was a risk of the plug cores blowing out, an event about as likely as my running into a dinosaur on the way home from work. In fact it is the valve gear that is most immediately at riskof malfunction, with damage to the connecting rods and pistons some way behind. This is the reason for specifying maximum engine speeds, and, for those vehicles with rev counters, marking them with a red sector. I have looked closely at these limits on my three current vehicles, with interesting results. The Rover 414 has a red sector on the rev counter from 6500 revs/min, and a warning in the handbook never to allow the needle to enter this red sector. However, even before that point it is capable of an 80 mph road speed in third gear and, perhaps surprisingly, of 60 mph in second gear. Clearly, remaining in third gear up to 60-70 mph to take advantage of engine braking is not a strain on the engine. The Polo 1.3 boasts a rev counter with a red area starting at 6000, and the manual states that the maximum permissible speed for continuous operation is 6300 rpm; this is a bit of a laugh because the red zone can barely be achieved in third gear, let alone fourth or fifth. (Shades of the VW Beetle, in which the cruising speed was the maximum speed!) However, even the Polo will manage 70 in third gear. For comparison the BMW R80 RT, while not a patch on current Japanese superbikes, will run to over 70 mph in third gear and over 50 even in second. The engine braking effect is huge by comparison with any car (large capacity engine, little vehicle weight), to the extent that I virtually never need the brakes on my twisty road home. The parallel IAM manual for motorcyclists "Pass your advanced motorcycling test" says on page 28: "Another braking aid is available when surfaces are treacherous - the braking effect of the engine when the throttle is closed. In fact the truly skilled rider uses anticipation and good judgement of speed to carry out most retardation on engine braking alone".

Perhaps people just do not realise how high engines can rev without coming to harm? If not the engine and not the gearbox, what then may be damaged by using a low gear for better engine braking? The answer may be the clutch plates, as a slow release of the clutch pedal during a deceleration manoeuvre will certainly cause more rapid wear on these items. On the other hand, a timely gear change made at a high constant speed should not result in any greater clutch wear than a change at a lower speed. If the sentence had read "It is cheaper to wear out brake pads than clutches" I would have been bound to agree.

Copyright PHP Harris 1999

Chief Observer's reply

I think that some of the above comments are well made especially the bit about wear on the gearbox which, I agree, should read clutch.

However there may be some parts of this that indicate more confusion with the system of car control, or the author's interpretation of it. I mentioned last time that the information phase goes through the whole system i.e. a signal could be put on at any time during the hazard negotiation.

Moving on, the next part is Position, no problems here (as yet) and then BRAKES followed by GEAR. When the system is applied perfectly these should only overlap during changes down to one, usually on the approach to an inclined giveway.

What you appear to have confused is the term "engine braking". This does not of course mean stick it in a lower gear and bring the clutch up. Engine braking means to release the accelerator to some degree causing the engine to decelerate and give the effect of slowing you down This is laudable if done properly and demonstrates, good if not excellent planning.

1 am a little concerned about your advocation of attempting to over rev the engine. Part of good driving is car sympathy; regular red lining antics should be left to the suburban boy racers. Would you buy a car off one of them?

Your last comment appears to be somewhat flawed also. A timely gear change, if it is to be timely, should only be made when the engine revolutions match that of a subsequent, lower gear. It is quite permissible to do this at a speed higher than the gear would normally be used but would need to be accompanied by what is known as "sustained gear changes". This is where the accelerator is trailed into the gear change keeping revs constant, allowing for a smooth and seamless gear-change. This is best achieved in accompaniment with double de-clutching'

Touching on the subject of double de-clutching and your comment about gearbox wear not being a problem. A common condition on many cars is loss of synchro on one or more gears. Synchromesh is a friction device and given a number of years of usage certain gearboxes will give up the ghost on one or more gears, especially first.

Ask any Austin Metro servicer if you doubt me.

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