Where are we going on brakes
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WILL AN INCREASE IN FRONT-AXLE BRAKING REALLY PROVE IN PRACTICE TO BE THE SAFEST SOLUTION FOR ARTICS?
IT is becoming increasingly obvious that the production versions of many of the new vehicles designed for the increased weights and featured at Earls Court will have different braking systems from those on the Show vehicles. Already there have been revisions by the Ministry of Transport regarding mechanical handbrakes and the performance required of them and, at the moment, there are considerable differences of opinion among manufacturers about what is the best secondary or emergency system for artics."
The main point of argument is which axles are to be braked by the "secondary" system. In the case of rigids it is universally accepted that the rear axles should beselected for this. In other words, no one is considering putting the secondary brakes on the front wheels. But on artics semi-trailer makers have put forward the suggestion that the front axle of the tractive unit, not the driving axle, should be braked at the same time as the semi-trailer or axles under the " secondary " system.
This may have influenced Fodens Ltd. who displayed a four-axle outfit with this layout at the Show, and now Scammell have announced that they are to do the same. They may be right, and on the grounds of Obtaining stability under wheel-locking conditions they probably are. But what about braking when the wheels do not become locked? And if the brakes are applied when the outfit is going round a bend and the front axle locks is it satisfactory that the vehicle should go straight on, very likely into oncoming traffic? What is hard to understand is why rigids are not to be dealt with in the same way, for the same arguments of stability apply.
STABILITY
Before considering the situation it will be well to examine what is meant by "stability ", a word which has been very often used in connection with artics and braking since the Institution of Mechanical Engineers sympositirn on vehicle control during braking and cornering held in June, 1963. As I see it, stability in the context of brak a50 ing means that the vehicle will not go out of the line that it is on under the changed forces that are then acting upon it. It was said at the symposium by one of the speakers that a vehicle will make the greatest deviation from its normal path when the rear wheels lock before the front wheels. This is undoubtedly the reason why there has been a good deal of talk in the last year or so about fitting heavier front brakes. But whilst this can give advantages on some cases—and some goods models are grossly underbraked at the front —it is nonsense to suggest that an improvement in the situation will be produced by making the front wheels lock as well as the rears.
UNSUITABLE LINE
If all wheels are locked, the " stability " which results can mean that, although the vehicle will not go out of the line it is on, the actual line taken can be a most unsuitable one. In other words, if the vehicle is going round the bend it can, as stated earlier, carry on tangentially to the curve, straight into the oncoming traffic or into the ditch depending on which way the road bends. Another thing is that the point at which a wheel will lock depends on adhesion between the tyre and road. If the road surface varies in this respect from one 'side of the vehicle to the other, then the wheels on one side can be locked whilst those on the other are not and I cannot see how a vehicle will stay in line in these conditions.
In artics, the main concern appears to be the prospect of jack-knifing and here it is certainly true that locked tractive unit rear wheels will tend to bring about this most undesirable phenomenon. Some people say that locked front wheels will not cause a jack-knife; others, including Mr. H. Perring of the Ministry of Transport (at The Commercial Motor Conference), say they will. Locked trailer wheels will not cause a true jack-knife —the tractive unit t will continue straight but it is likely that the trailer will "come round" on it. But what about the situation where the front wheels are braked to a point just
before locking, or the maximum for the particular road surface, with the driving axle unbraked and the trailer wheels locked? It is impossible to know exactly, but is it unreasonable to imagine that the outfit will do a pirouette around its front axle?
This may be a considerable way from the truth, but the main cause for concern about the decision of manufacturers to go on to a secondary system, in which the front axle and trailer axles only are braked, is that they seem to be jumping without having really tried to find out what will happen in all circumstances with this layout. There has not been enough time for this, and there is so much that is not fully understood about braking. This could Well mean that the first time the theories will be put to the bitter test will be when a poor driver uses his secondary system and finds "outer-axle braking" is not as safe as he thought it was. The resultant accident will not be added to the list of those caused by brake failure, but probably put down to another case of jack-knifing by one of those " dangerous beasts, artics Tests carried out by the Road Research Laboratory at M.I.R.A. are said to have confirmed the greater safety of outer axle braking on artics. But has any testing been done on normal roads? It is all very well using a perfect surface, and no doubt a perfectly maintained vehicle for brake tests, but can these logically be related to the normal situation? Roads generally have uneven, cambered surfaces with varying adhesion characteristics and vehicle brakes are not always in tip-top condition. I have carried out maximum-braking tests on normal roads with goods vehicles having the theorists' dream—greater braking effort on the front wheels. On tests when all the wheels locked I did not find these vehicles any more stable than is the case with rears only locked.
STOP LOCKING
In all discussions on braking it seems to be taken for granted that wheel locking must occur. Would it not be better if the time spent on actually testing to find out what happens when wheels lock were devoted to the development of a means of preventing wheel locking---or making anti-locking devices such as the Maxaret work effectively? Why is a situation that is known to be dangerous tolerated as it is? One of the most important things about braking is balance between the axles and, whilst the design of a system can provide for this in the fully laden condition (and normally does), when running unladen. the balance is completely gone. Light-laden valves which reduce the braking effort at an axle when the load is taken off overcome this "problem. These are available from British air-pressure equipment manufacturers, but no vehicle maker fits them—although similar units are used, apparently successfully, on the Continent. Is it not about time that steps were taken to fit these light-laden valves here?
ON ALL WHEELS
Another point is that there seems a good case for having the secondary braking system applying to all the wheels, possibly with less effort at each wheel than is applied by the service brake. This would answer all the arguments about which axle should be braked and also reduce the chance of fade which becomes an increasing problem when only one axle is braked. There would not be much increase in cost; only a few feet of plumbing and slightly more expensive brake chambers and this would be largely outweighed by the advantages.
What is amazing about all this is that the decisions on braking systems are being made without full regard to experiences in other countries. America and the Continental countries have been running vehicles incorporating safety braking systems for some time and so far as I know none of these use the principle now being considered for artics here. It is, in fact, known that in America the front-wheel braking of artics is deliberately reduced or removed when there is ice or snow about, to improve the controllability of the outfit. A very strong argument advanced in favour of this is that steering control is retained; but locked front wheels mean that steeringwheel movements have no effect. In France, where a secondary braking system has been a requirement for some time, in every case the tractive unit driving axle, not the front axle, is braked by the secondary circuit. Surely the practical experience obtained in other countries should be investigated before theories are allowed to dictate a "safety" system over here. Or, if not, exhaustive testing should be carried out before these designs are incorporated on production vehicles.