AT THE HEART OF THE ROAD TRANSPORT INDUSTRY.

Call our Sales Team on 0208 912 2120

COMAIIERCIA

8th February 1963
Page 58
Page 59
Page 60
Page 63
Page 64
Page 65
Page 58, 8th February 1963 — COMAIIERCIA
Close
Noticed an error?
If you've noticed an error in this article please click here to report it so we can fix it.

Which of the following most accurately describes the problem?

VEHICLE BRAKES

Proposals for heavier goods vehicles, the raising of the

speed limit tomorrow to 40 m.p.h., and the knowledge that the Ministry of Transport is considering new standards for commercial-vehicle brakes, give added topicality to a subject which has provoked controversy in recent months. "The Commercial Motor" therefore arranged an informal luncheon recently at the Royal Automobile Club in London, after which five experts discussed current aims and problems in brake design. Taking part were John Alden, chief product engineer, Vauxhall Motors Ltd., Don Ballard, technical director, Clayton Dewandre Co. Ltd., John Cullen, chief engineer, commercial vehicle brakes, Girling Ltd., Hubert Perring, chief mechanical engineer, Ministry of Transport, and John Moon, technical editor of this journal. Alan Havard, the editor, took the chair. Their views were captured on a Grundig tape recorder.

Havard: with the subject of excessive rear-wheel 1 feel it would be a good idea to start off locking at design load John Moon might like to say something here, as he has suffered a lot of this on recent road tests.

Moon: It has definitely been my experience that the majority of commercial vehicles that we test at design load have too much braking on the back end and not enough on the front, resulting in the whole system being out of balance. Some of the few exceptions are John Alden's vehicles, which have an equal front/rear braking ratio.

Cullen: The whole thing is a compromise and I don't think it is anything like as simple as

it at first appears. The front/rear ratios on trucks manufactured throughout the world vary, with from about 28 per cent to 50 per cent of the total braking on the front axle, and these ratios are selected as a result of operating experience under different conditions.

Alden: We have been into this for years. At one time the proportion of braking on the front wheels was as low as 30 per cent; but, as John Moon has said, our present ratio is 50 per cent, and I think we can claim to be the first people to have put so much braking on the front axle of a commercial vehicle. We are considering putting even more on the front, but for the present I think it is right for the rear wheels to lock slightly before the fronts, although the ideal thing is to have all four wheels locking at the same time. The unladen condition presents difficulties, of course, and I think some type of anti-locking device must be in our future somewhere.

Ballard: evidence that on most I agree there is definite vehicles you can get rear-wheel locking at design load, particularly at retardations in the B32

neighbourhood of 06g. The problem is concerned not only with static loading but also load transfer, and I entirely agree with John Alden that we must think of methods of controlling rear-wheel locking which can be considerably more dangerous than front-wheel locking. There are two possible methods of controlling this: either the anti-wheel-locking unit, or the device which you can apply to air-braked vehicles known as a " lightladen " valve, and which is controlled by the amount of weight on each axle. It does seem that both devices could usefully be investigated for the future.

Perring: What surprises me a little is that we are talking about rear-wheel locking at design load because, as Don Ballard has pointed out, locked rear wheels can result in loss of control of the vehicle, and if the rear wheels are going to lock at design load when maximum braking is used, they will lock long before the maximum braking effect has been produced in the case of an unladen vehicle. This suggests to me that the balance at which the rear wheels should lock first should be somewhere within the designed loadigg range and not at the ultimate design load.

Alden: I agree with that, and still feel that there

is a case for more braking on the front end. I think that one point of interest which affects this, and which is not always stated, is the effect on transfer that the height of the centre of gravity of the load has. I believe this is very important. Some good results can be obtained by limiting the air pressure of a fully air-braked chassis, but I think this is only partially the answer. On braking generally the phenomenon of when, or just before, the wheels lock must be studied more closely.

Moon: I am surprised to hear John Alden talking

about wheel locking, as this gives the impression that John is resigned to it. Two vehicles I have tested within the past 12 months stopped in remarkably short distances from 30 m.p.h. without any wheel locking whatsoever. One of these was a 12-ton-gross rigid and the other a 24-tongross articulated outfit, both with full airpressure braking. This suggests to me that under design-load conditions it is possible to achieve a high deceleration in terms of stopping distance without wheel locking and, to my mind, rules out any excuse that other manufacturers might offer for getting wheel locking with similar types of vehicle. If the manufacturer says he has got a different braking system, my answer to that is that he has got the wrong type. We have shown it is possible to produce very good average decelerations without a dangerously high maximum, and with the minimum of delay in the system. If some people can do this, why can't others?

Anybody can do this: the brakes are

available. It must be remembered, though, that you have an extremely large envelope of fore-and-aft ratio for any set conditions of load coefficient. For example, if the centre of gravity of the vehicle is moved from 2 ft. behind the design position to 2 ft. in front, at 0.6 g the braking required at the front axle changes from 30 per cent to 45 per cent. I think it is technically right that more braking should be put on the front, particularly with the trend away from normalto forward-control vehicles, but this is only one answer to a lot of varied conditions which the designer of the vehicle has not completely under his control. I would like to refer to this question of locking the rears before the fronts. This is accepted, I think, so far as cars are concerned, but I do not think it necessarily refers to a truck. It certainly is not the unanimous feeling of the commercial-vehicle industry that you must lock the fronts before the rears.

This brings us to the question of what is maximum retardation. The very fact that some vehicles lock their wheels earlier than others means that their maximum retardation is going to be lower. Concerning load distribution, we do find that operators by and large spread their loads evenly. You can say this is one result of loading vehicles up to maximum weight and beyond, as the limitations of platform space force them to use the whole available area. Therefore you do get a reasonable distribution condition with full load. Alden: .

Controllability of rear-wheel locking is considerably more complex, in my opinion, on commercial vehicles than it is on passenger cars. Although we are talking about design load, we cannot ignore the unladen vehicle, and this plays quite an important part in the whole situation. Nor must we think that devices for controlling wheel locking are the complete answer because, with an unladen vehicle, you can get wheel locking at as low a pedal effort as 30 lb. and this gives a very narrow limit of braking control over the vehicle. For this reason I am pushing the idea of the necessity for some control which varies the braking according to the axle load. Ballard:

Perring: John Moon has said that vehicles on test at design load tend to lock their rear wheels first, and usually his tests are conducted with a low load, giving a low centre of gravity and, therefore, the minimum of weight transference. Yet the rear wheels are still locking. This seems to me to be getting towards the edge of the envelope that Mr. Cullen spoke of, instead of operating somewhere nearer to the centre of conditions. Regarding controllability, I could not agree more with Mr. Ballard when he says we have got to move farther in that direction because of variations in load which, particularly on the rear wheels of an articulated outfit, are so enormous that it is just impossible to envisage any one fixed distribution which gives tolerably satisfactory braking over the whole loading range. I feel very strongly that we have got to adopt some system of distributing the braking between axles according.to the loading of those axles. One scheme that I have seen is a control giving three preset positions. One objection to this, of course, is that the control can be left in the wrong position.

I agree with that, but I must say that most of my remarks were concerned with rigid vehicles. Maybe we can discuss articulated vehicles in more detail later. Alden: Having moved from rigids to artics., the matching of the tractive unit and the semi-trailer is a constantly discussed and criticized subject on which John Moon may have some ideas. Havard:

Here, again, I return to the 24-ton-gross articulated vehicle I referred to earlier. This could stop in about 50 ft. from 30 m.p.h. and the two parts of the combination had not in any way been matched prior to the test. This shows that, possibly by luck, the outfit consisted of a perfectly braked tractive unit and a perfectly braked semi-trailer which managed to give near-ideal bra'king when joined together. This proves to me, therefore, that it is possible with a difficult outfit like an artic. to get good braking figures. I feel that 50 ft. is as short a distance as anyone can expect to stop in without the load damaging the bodywork. If this sort of braking can be obtained once, is there any hope of legislation to ensure that it can happen every time? Any tractive unit should he able to match with any suitable semitrailer to give balanced braking, though obviously we cannot expect a 15-ton-gross tractive unit to be happy with a semi-trailer designed for a payload of 20 tons, • Indeed, there ought to be some legislation to stop that sort of thing happening.

Moon:

Ballard: This is something very much after my

heart. Our company has done a lot of work in the past on the matching of tractors and semi-trailers and a reasonable amount of brake matching cart be obtained provided you fiddle about with the various component; concerned. I think that what is really wanted in the industry is some method of designing tractors and trailers so that, as it were, if you were blindfolded you could put together any tractor and any semi-trailer of a given weight class and the braking would be balanced. This actually is being considered by the S.M.M.T. at the moment, but it is a complex matter. It is very, very important, however, and becomes more important when we have to consider matching up with Continental vehicles as well.

Perring: eriabling a semi-trailer to be drawn to any one country's border present at Geneva with the object of This is something under consideration at

and picked up by the tractor of another country, the two being compatible in every respect—mechanically, electrically, etc.

The things that seem to be important are that when the tractor is being braked to, we'll say, 0-3 g, then it is giving some signal to the trailer that the trailer system can pick up and transfer to its braking system to produce 0.3 g also. Another thing is the question of time delay in the system. It is important that the trailer brakes do not come on too long after the tractor brakes, but I think the topic of how to speed the relative timings of the trailer brake coming on and the tractor brake coming on is too long to be discussed here. Some relationship between the two units must be defined so that whenever you couple-up the tractor and trailer a fixed relationship will exist in the timing.

Alden: This is all very true. In the past we have

found that the degree of braking on a trailer can vary between 15 and 75 per cent of the whole. Taking a retardation of 0.4 g, at one time we aimed to have the ratios on the tractor front, the tractor rear and the trailer in terms of 20 per cent, 40 per cent and 40 per cent, and we have never got that ratio properly yet. The question of jackknifing is very important also—I think it is one of the most important things in the commercial-vehicle picture. We have been looking at various things, and one possibility might be some form of mechanical control of the articulation of the trailer,, although another might be limiting devices in the braking-system layout. I do not think we have gone far enough, thougii the position has changed in recent years and the trailer people have got together and come up with some very good thoughts. But, certainly, it is one of those things which the tractor and trailer manufacturers must get together on, and I fully support Hubert with regard to lag: we have measured some phenomenal lapses in terms of time.

Cullen: The main thing, I think, is that whereas one can cope with the problem of tractors and trailers—even when manufactured by different people— when the outfits are in the designed laden condition, there is a definite requirement for having brake apportioning rather than anti-wheel-slide protection to cope with the vast variations that occur in axle loadings. I feel that the safety aspect will be improved with the help of legislation, although it is not as simple as all that. Apportioning the braking on each axle will give maximum retardation during panic or crash-stop conditions; but we have also got to remember brake fade on hill descents. Even with apportioning you can overload a particular brake by 50 per cent whilst under snubbing or hilldescent conditions—a semi-constant-horsepower condition. Brakes which are perfectly adequate and safe on a fixed ratio can fade and let the vehicle run away. So when you consider apportioning as against a fixed ratio you have got to consider a new specification for the brakes on each of the axles of the prime mover and the trailer.

Moon: First, I would like to contest a statement made by Hubert that he felt a signal should be given by the tractor to the semi-trailer brakes. Ideally, I feel that operation of the tractor footbrake should apply the semi-trailer brakes first, which should then signal to the tractor. The timing of most current vacuumand air-braked articulated outfits is such that the semi-trailer brakes do come on slightly ahead of those of the tractive unit, even though there is lag, of course. Turning to John Alden, I would venture to suggest that most of his experience with articulated outfits has been with some form or other of automatic-coupling tractive units. This type of coupling is liable to produce considerable delay in actuation of the semitrailer brakes because of the sheer layout and mechanical inefficiency of the linkages between the brake pedal and the semi-trailer brake unit The modern fifth-wheel coupling does not take all that long to connect and it does ensure more positive braking reaction between the two parts of the unit. It ensures, furthermore, proper electrical connection for the. semi-trailer lights!

B34

Perring: Referring to John Moon's query, what I

really meant was "something that was coming from a direct operation of the pedal, not something that resulted from the deceleration of the tractor ". In other words, if application of the brake pedal produced 50 p.s.i. in the system feeding the tractor brakes, it would at the same time produce 50 p.s.i. in the trailer system. This system would be designed so that, on receiving the 50 p.s.i. signal from the tractor, it would produce the same deceleration as that of the tractor. This is, after all, only what happens with a rigid vehicle.

Ballard: This is a very complex problem, and in connection with John Moon's comments I would say that it is difficult to obtain satisfactory brake balance and performance with the fifth-wheel type of combination; but when you come to the automatic coupling it is even more difficult! Theoretically, it should be easier, but it is the opposite of that case and, in my opinion, it is almost impossible to produce really good balance on a vehicle with an automatic coupling of the present type. So far as the fifth-wheel type of combination, where the brake equipment for the trailer is actually on the trailer, is concerned, according to European braking proposals we are allowed 0.6 of a second lag, and I think that most well-designed air-pressure systems of the two-pipeline type can operate Within that figure. It is, nevertheless, true that we want to reduce that figure to a minimum. Systthns where there is a delay mechanism to ensure that the trailer brakes &nne on first are all right from the point of view of stability of the combination but, of course, actually result in pulling down the overall retardation of the complete outfit.

Havard: I would like to suggest that we now, discuss the question of standards of braking efficiency, if necessary by legislation, and would ask whether 50 per cent efficiency is considered a good legal standard.

Alden: On this question of artics., to lend some point to the importance of trailer braking would point out that there are many articulated outfits in the world With a hand control on the steering column for operation their trailer brakes only. When you talk about brake performof the trailer brakes only, where drivers rely a lot on applying ance, of course, you are talking about retardation and you are talking about fade, which has been mentioned by all of us, I think. Regarding the maximum figure, 50 per cent might be a reasonable one to aim at and, of course, you have got to add the handbrake performance too, which could probably be 25 per cent. I do want to dwell on this question of fade because I think it is very important. We always try to engineer our brakes so that they can give adequate retardation both hot and cold, and when we talk about fade we normally think in terms of a friction coefficient loss. This has not always been our experience, particularly on trucks, where you have large drums which can expand quite considerably. Fade must be interpreted, therefore, in two ways: a loss in pedal effort due to the shoe factor falling off because of certain heat conditions, and excessive pedal movement due to expansion of the drums or non-adjustment of the brakes.

Cullen: I would like to see legislation from the

safety viewpoint. I think the tests laid down by law, as they will probably be carried out on a new vehicle or on one which has just undergone brake servicing, should aim at 60 per cent, so that 50 per cent is ensured while in general service. You will have some deterioration in your brake mechanism in service, and this is bound to happen to some degree. Future legislation should cover two main things: one, the ability to stop at 60 per cent from 50 m.p.h., and the other to cover the amount of allowable brake fade—the conditions being artificially reproduced if necessary. I also think that legislation would increase safety if automatic adjustment could be brought in as mandatory because, as John Alden has mentioned, straight " fade " is often confused with lack of shoe-centre lift, not only because of expansion of drums and lining wear but, unfortunately (in far too many cases, I think), due to lack of maintenance and adjustment. Today one can design and fit a large enough brake to control lining fade so that it is never dangerous; but you will never, even with good design, entirely overcome apparent fade if you have not got automatic adjustment and are relying solely on the human element.

Moon: I'm afraid I am going to cross swords with

John Cullen for talking about 50 per cent as I feel certain he is talking of a meter reading. May we clarify, that point first?

Cullen: I think that meter readings for the straight developments of brakes are satisfactory in that they give you a good comparative. For such purposes you don't want complex gear; but I completely agree with John Moon in that, for commercialvehicle legislation in particular, it is the stopping distance that matters. This is particularly so with air brakes, which are subject to lag. When I say 50 per cent, I mean 50 per cent as a.measured distance, not as a meter reading.

Moon: In other words, we are talking about measurement of braking efficiency in terms of distance, which is the only way to do it. Now, if 1 may go back a few minutes to the remark made by our friend from Clayton Dewandre about this 0-6 sec. lag. This does not sound very much, but a vehicle travelling at 30 m.p.h. is doing 44 ft. per second, so 0-6 of a second is roughly equivalent to 30 ft—just under 30 ft. before anything really starts to happen at the brakes. This is far too long a figure, So I think the regulations referred to, which are those drawn up by the E.C.E. in May, 1961, are rather ridiculous. These regulations give different requirements for vehicles of different weights, and to a certain extent I agree with this, but I do not feel that a distance of more than 60 ft. should be allowed for a 30-ton-gross vehicle when stopping from 30 m.p.h.

Ballard: I did, as John Moon said, mention 0-6 of a second in accordance with the E.C.E. proposals, and I also qualified that by saying that we want to get that down to a smaller figure if possible. In other words, I do agree with what John Moon says, and so far as braking efficiency is concerned I was going to speak most definitely about measuring on a stopping-distance basis, because it is most important that we take system lag into account. The suggested legislation gives a stopping-distance figure equivalent to somewhere between 0-4 and 0-45g, but I do feel that this should be carefully coupled with the fact that we are considering vehicles of a definite laden weight and also, in my opinion, consideration should be given to limitation of maximum pedal effort: I think this should be somewhere around 120 lb. Another thing in connection with the design weight at which the brakes are tested is that it must be made clear that the weight should be correctly distributed in accordance with design figures.

The question of laden performance should be coupled with the unladen performance because of the factors we have already mentioned, though lack of wheel adhesion may make it impossible to obtain these figures in certain cases. Talking about the performance of hand brakes, I think that this performance is difficult to relate to load. Our experience is that a vehicle of over 10 tons gross needs some sort of power assistance for the handbrake. Referring to the question of self-adjusting brakes, I think this is

Alden:

all good, but we have got to make sure that the self-adjusting mechanisms are reliable. Returning to handbrakes, I would mention the importance of the handbrake performance when hot. In many vehicles the handbrake is coupled to the same shoes as the footbrake. I would make a case, in many instances, for a separate cold transmission brake. After all, we are talking about the safety of the vehicle, the load, and the occupants. of both the vehicle and the road.

So far as legislation is concerned, it would mean a departure from what we do at the moment, but one which is quite conceivable. To include a fade test and a stopping-distance test is quite practicable, 1 think, if we have " type testing ". For roadside testing, however, we must stick to the meter reading as the most simple way of recording efficiency. There is one point I would like to throw to the meeting, and that is that the E.C.E. proposals for fade tests give a definite gradient which one has to simulate somehow, and they quote speeds which are substantially similar for all classes of vehicle. This seems to me to be wrong. I feel that the heavier vehicles would automatically be driven down long gradients at lower speeds than lighter machines. We have simulated some tests, and the brakes of light vehicles hardly warm up when subjected to the E.C.E. fade tests. But, on the other hand, it has proved impossible to simulate a fade test with a heavy vehicle using its own engine as a source of power on level ground: to do this you need about 15 b.h.p. per ton, and we rarely have more than 5 or 6. So I would like to hear views on whether it is a desirable thing to quote different speeds for different classes of vehicle for fade tests.

Perring:

Alden: Yes, I think there is quite a bit of support that can be given to this argument about different speeds in terms of horsepower per ton figures.

I think that fade on hill descents is what we are mostly concerned with. One has got to decide whether you want to blend better legislation with what is easy to do and can readily be done, or whether you should say that legislation must be equally safe for all vehicles and by legislation enforce a higher cost and greater complexity on to the brakes of heavy vehicles in order to have the same fade performance as lighter vehicles. The technical problem is that you have roughly the same space to install this energy dissipator on a 2-ton payload truck as on a vehicle suitable for a 16-ton payload and, because of that, it is technically difficult to get the same performance from both types of vehicle. I would add that a vehicle's brakes must work under panic-stop conditions at all times. For this reason you want a dual system so that the split second you hit the pedal you get scente braking. I do not think the handbrake, from a safety viewpoint, comes into this, because the human time lag after you have pumped the brake pedal several times and given up and gone for the handbrake is too long. On the other hand, I think John Alden's remark concerning a cold brake--which presumably, on a pure cost basis, will be on the transmission— is a good thing, not so much to stop the vehicle at high retardation but to keep control if other measures have failed.

Cullen:

I feel any discussion on brake fade would be incomplete without mention of retarders of some form or another, of which the exhaust brake is in existence and gives a moderate amount of assistance to this brake-fade matter, whilst, of course, the more expensive forms of transmission retarder can give an even greater amount of assistance.

Ballard:

I have several comments to make, taking Don on retarders first. The exhaust brake. we know, gives a certain limited retardation but it can also B37

Moon:

reduce engine life, particularly of the valves. So far as transmission retarders are concerned, the electric type can be bulky, heavy and expensive, but a small hydraulic retarder can be jutt as effective without any of the three main disadvantages. Such retarders are under development. As regards automatic adjusters, these are all very well in general use, but on a long Alpine descent an automatic adjuster could take the shoes out to catch up with drum expansion to such an extent that when the drums cool down again the brakes are locked on. Regarding Hubert's point of speed differentials for light and heavy vehicles going down a gradient, I feel we ought to aim at a constant road speed and take the minimum speed which would preclude the drivers of lighter vehicles getting impatient and trying to overtake under adverse road conditions. So far as pedal travel is concerned, I agree that with a hydraulic system pedal travel does give an indication of drum expansion, but this is not so with a pure airpressure system. The same remarks apply with regard to pedal effort because, after all, the pedal of an air-pressure system is merely a valve and the amount of pedal effort required depends on the strength of the spring you put underneath the

pedal. I think that, in a way, quotations of pedal effort are red herrings. To me, pedal effort and maximum braking efficiency do not give a true indication of a vehicle's capability• of stopping.

Moon: " . . too much braking on the back end and not enough on the front."

Cullen: Let's get back on to automatic adjustment.

It will theoretically tackle a long descent in that your brake drum expansion, as the drum reaches a high temperature, will balance out because its heat-transfer properties improve as its temperature gets steadily greater than the ambient temperature. On a long Alpine descent you have about 30 thou. of radial expansion of the drum to cope with and you can do that quite happily on shoe lift alone if it is reasonably adjusted when cold or, alternatively, with an automatic adjuster. One stops adjustment occurring under the difficult snubbing-descent condition by having an automatic adjuster which operates under reverse-torque conditions only, so that it does not come on during a long hill descent.

I would like to hear a few opinions on disc brakes, transmission brakes and other sorts of auxiliary brake.

Havard:

I think there is a definite case for retarders. I think they are coming and I think they will be a big help to us_ In terms of auxiliary brakes? Well, we have gone one stage by fitting a transmission brake, and the retarder may be the second, I do not think the exhaust brake is the complete answer by any means. Talking about disc brakes, I suppose we ought to start from the context of the popular attitude towards disc brakes. One subtle principle of a passenger-car disc brake is that it is usually fitted with a power servo, as compared with a drum-braked car which has no servo. Thus, on passenger cars they certainly improve various braking characteristics by decreasing the overall leverage ratio and putting the effort back in terms of power assistance, which immediately gives a better pedal feel, etc. When we come to disc brakes on commercial vehicles we have already taken advantage of using a servo and most of us have, I think, tested disc brakes on trucks and we are not as yet particularly impressed with them. It would appear, right now, that for overall conditions we can make a better drum brake than a disc brake, but that is as far as we've gone.

B38

Alden:

Cullen: I agree with what John Alden says in that disc brakes have been thought of from their passenger-car performance and their passenger-car duties. They do, however, have certain advantages to my mind for commercial vehicles; there are disadvantages as well. I think the advantages are that they are comparatively fade-free, or at least more fade-resistant than a drum brake, when both brakes are being used under adverse conditions. Overall, I think they have a definite advantage in respect of reduced down-time for a lining change. They are already in limited use on certain heavy vehicles as well as on p.s.v.s, and I do know that operators can tell the difference in that discs are, as well as being fadefree, a smooth progressive brake. I feel that there is a limited use for them, particularly as a disc-drum combination with discs on the front axle only; but there are limitations. On rear axles there is the problem of heat dissipation because, if the heat gets to the hubs, it can write off the seals and bearings. There is a definite problem in getting them to live under dirty conditions with the open-spot-type brake. The rate of wear, comparing clean and dirty road conditions, can vary as much as 5 to 1 on a rear axle. There is also the problem, still, of tbe handbrake. But I think that slowly and steadily there will be a future for disc brakes on medium, and eventually on heavy, commercial vehicles. But we have a long way to go before we can get mechanically actuated versions which can be operated by direct air pressure, hydraulic actuation not being very popular on heavy chassis.

Ballard: All I have to say is that direct air pressure

is going to be the requirement on disc brakes for commercial vehicles in the future, and there is not so very much difficulty in applying direct air except that a

satisfactory mechanical calliper design is necessary. Space requirements come into this, of course. Also the disc brake has a low brake factor and requires a large amount of boost to operate it: this is much easier to obtain with high-pressure hydraulics than it is with air pressure, as the size of airpressure actuator needs to be large. There is definitely work to be done for the future.

Alden: I think that discs will probably, if they are successful at all, weigh more and cost more than equivalent drum brakes and 1 think I would rather put my money on some kind of independent retarder. I would just like to ask something. Surely it is possible with the present type of disc brake for the brake to fade out quite suddenly, whereas with a drum brake you do get some warning of fade?

Can we move to a slightly different subject? Would you like to discuss whether you think operators really appreciate the danger involved in running overloaded vehicles, especially when faced with emergency-stop conditions, and also whether you consider that designers necessarily take into account all types of service conditions from the operators' viewpoint.

Havarti:

There are a number of operators who believe in overloading and accept lower performance and safety factors. This is a problem which I think will have to be dealt with by other means. One of the problems is that some vehicles go to pick up a load and there are no weighing facilities so that there can be no check on whether the correct weight of payload is being carried. A satisfactory answer will have to be found to this.

Perring:

I am quite convinced that many operators do not realize that their brakes are dangerous until it is too late. After all, many braking systems are insufficient when the vehicles are new and running at their design weight, and quite a number of these vehicles are bought with the deliberate intention of being overloaded. Furthermore, as far as maintenance is concerned, some operators will tend

Moon:

to put on quite unsuitable linings merely because they can get these at a reasonable discount and because it saves them the trouble of checking to see which the correct type of lining should be.

Overloading is, of course, a continuous problem, and has been to us for years. We have been forced to a certain extent to over-engineer, if you like, and we do quite a few of our tests above the normal designed load; but this is a situation which an engineer cannot face fairly and squarely. If we have to design for a certain gross vehicle weight we will; but we say that this vehicle is good for a certain weight and by and large most people—and this is improving all the time—do adhere to that weight. There are, however, quite a few people who do not. There is also the question of the dump-truck and tipper operator, who not only overloads his vehicles but also goes into the mire and contaminates his brakes, and when he gets out on the road various things can happen. I think, in all fairness to everyone, that there must be some kind of proper control over overloading, and I am confident that the Ministry of Transport will come to a very sensible recommendation on this.

Alden:

I completely agree with all that John Alden

has said. I would say that a large majority of our service problems are as a result of brakes being overloaded, and not problems associated with operating at designed weight ratings. The problem is sheer overloading of the vehicle; but sometimes it is enhanced by the operator trying to get a more efficient brake by increasing the system boost until he can get the equivalent of a meter reading of up to 0.6g. He is then overloading not only the vehicle but the brakes also and, of course, gets into trouble because the brakes are not designed to operate in the way that he is using them.

I think the remark made by John Moon regarding fitting linings of the wrong type is very relevant, because a lot of what we have been saying is largely nullified if completely different linings are fitted on to a brake.

Ballard:

I would ask whether manufacturers are working on new designs to cope with higher speed and weight potentials which will soon be available for goods vehicles? Havard:.

?erring: are referring I presume you here to possible changes in permissible weights. Clearly we will be under an obligation to try to get loads and sizes common with those in other European countries. The sizes are so big compared with the width of a number of our roads that we would, I think, have to have some form of preferred or nominated route; but I believe that this is a problem which other countries are having to face as well. I do not think any country can allow these very big vehicles to circulate anywhere. So we want in every way to try to get on a common footing with the other countries. On maximum dimensions the problem is not quite as acute as it is on maximum weights because, as you know, in this country we are bedevilled with bridges over railways which were built a long time ago and which may preclude us from getting up to Continental standards.

Cullen: " . . a large majority of our service problems are as a result of brakes being overloaded, ."

Alden: My company, of course, does not actually engineer vehicles for the maximum legal weights. We are looking at this for the future, and we will look at the probable or possible increase in total load that is now coming to be on the Continent. I do not believe that speeds will go up all that much: I think these sort of comic-strip stories of vehicles going from continent to continent at 70 m.p.h. are over-rated: I would say a maximum speed in the region of 60-65 m.p.h, is quite adequate. In actual fact, we are one of the few countries in the world

to have a motorway without any speed limit. I do think_ there is a very big case for an improvement in powerto-weight ratio to reduce traffic congestion on the roads, and in terms of engine power I would far rather use it to help that along by increasing acceleration rather than maximum speed.

I entirely agree

with what John Alden has said. I would put the maximum cruising speed at a lower figure, however. Some Continental makers of vehicles in the 3035-ton category feel that 50 m.p.h. is quite enough and that, even if they were to design for higher speeds, they could not guarantee equipping the vehicIei with entirely adequate brakes. As they can already brake their vehicles satisfactorily from 50 m.p.h., I feel there is no reason why a British heavy-vehicle manufacturer should not be able to do the same, although there is no evidence that British makers can do so at the moment.

Havard: " Do you Mink operators appreciate the danger. „ in running overloaded vehicles

Cullen: Power-to-weight ratios, improved roads,

higher speeds. They do not all go up by the same degree; some remain static while others climb. But all, of course, put a greater demand on the brake. As brake manufacturers, we are constantly recommending bigger and better brakes and we are quite genuinely seeing these being readily accepted and are being asked for them by vehicle manufacturers, so I contend that the know-how and product are there. The problems are the specification and the sorting-out of the actuation to improve performance under varying axle-load and road-surface conditions. I feel that increases in cruising speeds and so forth can be coped with by the brake manufacturers.

Ballard: I would just like to go back to clarify one point. John Moon did refer to my comment about pedal effort. I agree that, of course, with a full air-pressure system you can make the pedal effort whatever you want. The limitation of maximum pedal effort for a given retardation has also to take into account the powerassisted brake system. One final comment: I think that any discussion on commercial-vehicle braking would be incomplete without saying that anything that can be done to improve tyre adhesion and to improve the standardization of good road surfaces will, of course, materially assist in this braking problem. I would just like to go back to the first point in the discussion-fore-and-aft ratios. I agree that the trend has been for vehicle designers to call for more braking on the front end; but I think there is too much variation in respect of operating conditions, the country in which the vehicle is being operated and the wheelbase, to say that there is a figure within an envelope which will suit all types of vehicle operation.

Havard: Has anyone any final points to make?

Moon: I only wish we could have got on to the

subject of third-axle conversions. .


comments powered by Disqus