BRAKES
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let's get down to it
Better standards must be accepted for commercial vehicles By JOHN F. MOON, A.M.I.R.T.E.
FEW subjects these days raise blood pressures quicker than that of lorry brakes. The general public, conditioned by factual news stories about run-away vehicles inevitably give more credence to the less factual " hate " treatment given to commercial vehicles in newspapers and on television.
Admittedly, legislation is now at hand to deal with braking standards; but its full implementation will be difficult and, in any case, what about standards concerning the quality of braking systems and their components?
It is all very well—and only too easy—for a new vehicle running at its designed gross weight to meet certain braking standards; but it is quite a different matter to ensure that this standard is maintained throughout the life of the vehicle. The fault can lie equally well in the braking equipment as in the vehicle's operator or driver. I do not make this remark idly; during my days as technical editor of The Commercial Motor there were occasions during road-tests I have made when braking-system failure has brought myself and the vehicle into some danger.
It is harsh to blame only operators for faulty braking systems. Until plating becomes effective, every manufacturer of medium-duty vehicles knows that a percentage of his products is going to be overloaded. His reason for not providing braking adequate to cope with such overloads is that it would cost too much and put his products at a disadvantage in terms of price compared with his competitors. So he provides brakes to cope with his recommended gross weight. On the face of it the indications as I see them are that 'too many British goods vehicles are operated in an under-braked condition, whatever the reason.
Excessive Braking Bad On many occasions when a vehicle submitted to The Commercial Motor for road testing is brake tested, the ensuing report condemns excessive braking at one or more of the axles. This condition can be every bit as dangerous as that associated with underbraking when the drums are cold. When the drums are hot, the " overbraked " vehicle frequently becomes underbraked—even at its design load— whilst the " underbraked " lorry becomes brakeless. In both cases there is still the ever-present risk of brakingsystem failure It must be remembered that such safety systems as dual circuits are not demanded by British law. With our braking systems, the failure of one little item can be disastrous, particularly as handbrake performance in so many cases is almost useless in an emergency.
One encouraging note, at least, is that when Lord Chesbam spoke on the subject of heavy-vehicle braking at the M.I.R.A. annual luncheon in London last November, he said his Ministry was aware that it was useless to legislate for better braking efficiencies whilst at the same time c4 ignoring the equally important subject of preventing payload from moving forward and crushing the driver under conditions of severe braking.
Incredible as it may seem, ordinary platform bodies are still being made without the headboard stanchions being adequately braced to the underframing—or to each other by means of load-carrying capping strips—whilst plywood headboards of a mere half-inch thickness or so have been put on maximum-capacity tandem-axle semi-trailers. With bodywork such as this, good brakes are more trouble than they are worth. I know, because I have had to abandon a number of brake tests because of faulty headboard design. Body manufacturers obviously still have a lot to learn and could profit from re-reading the article published in this journal on May 18, 1962, on the subject of body strength, this having been specially written by Guy Tidbury, of the Advanced School of Automobile Engineering.
Exhaust and Transmission Brakes More than eight years ago I wrote a two-article survey of British braking systems in use at that time, in which reference was made to likely future developments. In one of these articles was mentioned the advantages to be gained from the use of a hand-controlled exhaust brake and a transmission handbrake, whilst other pointers to the future made were that growing use would be made of full-air and air-hydraulic braking systems on medium-capacity vehicles. This latter prophecy has been proved right; but the exhaust brake still seems to be as far away as ever from the general British scene, whilst transmission handbrakes are used by only one large manufacturer as yet.
At the same time the future objects of brake manufacturers were quoted as being to increase the reliability of brakes generally, to reduce maintenance, to combat fade, and to diminish physical effort "without necessarily increasing deceleration rates to the danger of goods or passengers".
I remarked that: "These four characteristics are likely to be achieved without recourse to completely new systems although work on new units, such as disc brakes, will continue to be done so long as there are possible advantages to be gained."
Well, it doesn't look as though the position has changed very much since mid-1955, does it? We are still waiting for universally reliable brakes; brake maintenance is still a regular chore which too many operators seem to ignore; fade is still with us (although perhaps not to quite the same degree) except that the raising of the speed limit to 40 m.p.h. and the unrestricted speeds permitted on motorways have brought new problems; and physical efforts have been reduced in many cases only to create new difficulties in the shape of over-sensitive brake reaction and excessive wheel locking.
It is high time something drastic was done. Legislation in parts of the Continent, and in Germany in particular, has succeeded in raising braking standards. If German manufacturers and operators can accept as obligatory such devices as load-sensitive valves to reduce rear-wheel locking, and fit effective exhaust brakes as standard, why should an equally civilized country such as Britain be reluctant • to follow suit?
Brake manufacturers appear to be aware of the need for improved braking standards; I have been told often enough that efficient brakes promising good anti-fade performance, long life and the minimum of adjustment were being designed. The trouble is that, according to these informants, once such brakes have been made available vehicle manufacturers have been reluctant to pay the higher cost in case their competitors will not buy them, thereby putting the more conscientious chassis maker at a price disadvantage.
This all seems a bit weak to me, if it is true. For one thing, there is at least one British manufacturer of lowpriced vehicles that designs its own brakes with very good results; so obviously in this case there is no price disadvantage to deter prospective customers. And if the new brake designs are so good, why don't the brake manufacturers (if what they say is true) completely drop their old types so that the chassis makers have no choice but to buy the more efficient ones?
I have discussed this problem of the supply of brake units at great length with one of our principal brake manufacturers, and I must confess to having come away from these discussions in a rather depressed state. This particular company draws up design specifications for the brakes of new chassis, which are presented to the chassis manufacturer as recommendations. Unfortunately, it appears that often these recommendations are not complied with; and the brake-manufacturer's engineering chiefs do not appear to be informed that their recommendations have not been followed.
Brakes Compared
This was proved during one particular discussion session, when the brake specifications of various vehicles with particularly bad retardation performances were compared with the original theoretical recommendations. In every case the actual specification was different from that recommended, and the test results I had obtained showed this. At least, the brake manufacturers could not be blamed in these instances if the vehicles were potentially unsafe.
This question of "finding out' raises another weak link in the chain. Very often, when I arrive at a factory to carry out a road test, I am told that the brakes "seem all right "; but nobody has actually made any simulated crash stops, let alone tried to measure the performance. Indeed, there are still manufacturers whose brake-test equipment is limited to some primitive form of meter, whilst their idea of a " crash-" stop seems not to involve anything as drastic as depressing the brake pedal hard and as quickly as possible.
This being so, such vehicle makers have only the brakemanufacturers' recommendations to go on as to whether their designs arc safe. If they don't follow these recommendations they are liable to get into trouble.
Of course, if there really was full co-operation between brake and vehicle manufacturers, the braking people would insist on testing new designs before they agreed to supply brake units for production purposes; but competition being what it is, I suppose there is always the fear that if sale conditions were made too difficult, the contract would go elsewhere. Thus, even if the brake Manufacturer should happen to be right with his original design he is virtually powerless to do anything but supply the units ordered.
Even when he does supply the right units, there is still no guarantee that the vehicle manufacturer is going to operate them correctly. When it comes to articulated outfits the position becomes even more involved, with so many different thoughts on the subject from both tractive-unit and semi-trailer makers that it sometimes seems to me to be little short of a miracle that articulated vehicles ever manage to stop in a straight line, One of the principal causes of dangerous braking is excessive braking effort at the rear axle or axles. In my own test reports when with The Commercial Motor I was inclined to be unkind about designs that suffer from rear wheel locking under emergency-stop conditions. Some engineers take me to task about this, saying that if they balanced a braking system so that the rear wheels did not lock on a dry road when the brake pedal was fully applied, the front wheels would lock on a wet road with the same pedal effort. So what? A vehicle with locked front wheels continues in a straight line. Furthermore, if a vehicle locks its rear wheels on a dry road, it is obviously going to lock these wheels much more quickly on a wet road; and in many instances a rear-wheel skid is more dangerous than a front-wheel slide.
The general line of thought seems to be in favour of locking rear wheels, rather than attempts being made to produce a simple and economical braking system that will resist wheel locking, It seems that many engineers still belong to the old school which feels that a vehicle hasn't got any brakes unless it locks its wheels. Of course, it goes without saying that these people usually don't have to wrestle with laden (or, worse, unladen) articulated vehicles under emergency-stop conditions on a wet road.
I am not suggesting that the answer to the problem of providing a good all-round braking system on a commercial vehicle is an easy one. What I am saying, however, is that it is high time the industry stopped thinking in terms of shillings and pence, and got down to making a serious attempt to evolve a reasonable solution. Some really strong legislation could set the ball rolling: As someone who has the greatest respect for most of Mr. Marples' work, I .am surprised that nothing has been done already beyond tentative proposals.
Among the things that a really well-braked and safe heavy vehicle will need are a braking system that is responsive to individual axle loadings, an exhaust brake or—better still—a lightweight hydraulic retarder to relieve the-wheel brakes of normal slowing-down and hill-descending duties: a really efficient, independent handbrake system; ample reservoir capacity with a big compressor, and maximumgrip tyres incorporating either high-hysteresis rubber or an effective non-skid pattern,
None of these things is impossible. They only need paying for.