Anti-lock: it was not such a shock
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May I suggest two possible reasons for the large number of applications of the Sleidchek anti-lock system during its road trials (CM, September 12), which seem to have so surprised the Girling engineers.
The first is psychological, and seems to me to be likely, but hard to verify ; it is that drivers generally exploit any device which eases their work load, whether physical or mental, so that, once they had come to trust an anti-lock system, they would expect it to do its share of the job, and might let each braking situation go a little further without precautionary braking. It would not take much to exceed Girling's rather low expectations.
Even so, a further factor may be needed to explain the very large difference in results, but this is a situation which I have prophesied (but not in writing, I fear) ever since becoming involved with brakes as they actually are in service, some years ago. This missing factor,
which is never mentioned at demonstrations of the latest offering in anti-lock technology, is the trueness of the brake drums to their running centres. Adding many years' experience of automatic control systems in transport and industry to that of the rough end of brake servicing, I would expect a system sensing individual wheel speed to be very sensitive, near the cut-in point, to variations in the deceleration of the wheel caused by uneven braking effort during each revolution. It is well known that this effect can be a primary cause of uneven tyre wear (CM's Workshop series, about three years back, I think), and anyone who machines drums true for a living, as I do, knows the high proportion of drums in service which are out of true, whether eccentric, oval, hard spotted, pinched by wheels, or whatever. In my experience, unacceptable judder of front brakes can be caused by a run out of only .020in (20thou) ; it could take much less than that to fox a wheel speed sensor.
The answer, fortunately, is easy, cheap and quick — brake trueing, which is available as a same-day service in most parts of the country now, from either one or two of the brake lining companies or specialists like my own. It may be significant that in America, Where anti-lock systems are most widely used, brake trueing is much more common, and may have been taken for granted. New drums are not in themselves an answer ; the judder case quoted above occurs more often with new drums, which at least one major British group is quite good at supplying cam-shaped.
So, for anyone who does feel inclined to spend a few hundred pounds to fit an anti-lock system of any make, an extra £20 or less per axle could save a lot of system uttering. Or, for anyone who does not, that same expenditure could make their braking so much better balanced as to eliminate the need for an anti-lock system on all but a few occasions.
PETER RIVERS, Rivers Trustop Ltd, Barnsley.