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The Stability of Articulated

flavor& Can we start with what seems to be rather

a vexed question, that of matching tractive units to semi-trailers? The criticisms seem to be directed at tractive unit manufacturers in particular. Now this may or may not be fair, but I would like to hear your opinions generally on matching—whether you think sufficient attention is being paid to it and, if you think that sufficient attention is not being paid to it, who you think is at fault.

Hope. Well, take the question of stability. Who

is responsible for the stability of an articulated vehicle—the trailer maker or the tractor manufacturer? I say that 80 per cent of this responsibility belongs to the trailer manufacturer, because he knows what payload the trailer is likely to carry, whereas a tractive unit can go under alternative types of trailers, such as semi-low-loaders, lowloaders or box vehicles, with varying heights. If you liken semi-trailers to chariots, then I'd say we are no further forward with their control than the Romans were 2,000 years ago.

Elliott: From the operator's point of view, we

always want things made as easy as possible and as cheaply as possible. The essence of our business is to make the absolute maximum use of the motive unit and the driver. That is the nub of the whale situation. We want that ubiquitous tool, a tractor that can hitch on to anything; this is our ideal.

The difficulty really is of matching any semi-trailer with any tractive unit and this boils down to standardization—the standards that are not—or mote exactly, the lack of implementation of standards. Wilding:

Havarti: Well, Tony, you used a good phrase—" the

standards that are not ". We all know the S.M.M.T. went into this in a tremendous way and a certain amount of standards, I understand, have been agreed. Have you. Tony, come across any operator who is satisfied that he can take any semi-trailer and tractive unit and match them together?

No; you cannot do this, can you? degree, but not all the way.

Well, to a great extent this question of interchangeability has been overcome. There are quite a number of S.M.M.T. standards and if operators. tractor manufacturers and semi-trailer manufacturers work to these standards the main difficulties should not arise. Afarenbon: Now there are full standards for coupling-up. We have the normal dimensional standards which set the height and position of the fifth wheel and the position of the landing gear, and ensure that the semi-trailer can couple-up to the tractor satisfactorily without any fouling. We also have standards for the braking and lighting connections, so that side of things has also been looked after.

It is rather up to the operator to see that the tractor is compatible, because trailer makers have no jurisdiction over that. In other words, you could have a tractor which had the fifth wheel in the right position, but was very much underpowered or wasn't suitable for the super-imposed load.

There is also, of course, the question of matching up the brakes, which is being considered at the moment. But I would say that the line pressure question has been more or less settled, and in due course you will find all manufacturers working to a common line pressure. The degree of braking has not yet been established; but that, of course, is something that will be dealt with later on.

Havarti: Arthur, I am prepared to accept that the

machinery is there and working; but what To a . facilities are there for ensuring that the decisions of the S.M.M.T. are carried into effect?

Alatenbon: That, of course, is the difficult part. The

. standards are purely voluntary, and have no legal force as such. That, possibly, is the weakness of the present situation, but it is one I would much rather have than be saddled with compulsory legislation. ries of after-lunch discussions, Mr. H. W. Elliott, managing 'cords) Ltd., Mr. W. L. Morgan, chief engineer, B.R.S., Mr. F. J. e Transport Ltd., Mr. L. D. Watts, director and chief Lorries Ltd., Mr. A. IVIarenbon, technical and sales director, d., and A. J. P. Wilding "The Commercial Motor" talk about in braking and driving articulated units. Alan Havard, the r.

Surely it is up to the operator to specify

the standards? 1 am interested in this 1. when we started to build our articulated fleet fifteen ,go, we foresaw these great difficulties of matching and, ie makers' co-operation, we did arrive at a high degree dardintion and matching. We determined dimensional .cls for both semi-trailers and motive units. which we lhered to ever since. As far as our fifth-wheel articulated concerned, we are interchangeable. With the other he automatic coupling ot Scamrnell type—they, of are interchangeable within their own type. So we have 3 non-interchangeable types, admittedly, but there is no ty within either fleet.

insist on not only dimensional specifications being ined, but also the matching of braking performances. ample. a new type (to us) of motive unit or semi-trailer irough brake performance tests and has to comply with requirements.

great difficulty with articulated vehicles is the variation in which make braking performances very different from ad condition to another; but under like-for-like con. we are satisfied we have an excellently matched and angeable fleet.

Is stability satisfactory?

11: Yes, as far as possible. I do not think that

the stability of any articulated vehicle can I to be absolutely satisfactory, or equal to a rigid. But the things we did in these early discussions with the icturers was to insist thai the motive unit manufacturer mounted the fifth wheel on his chassis. It had been the practice, and probably still is the practice in many cases, for a customer to buy a motive unit from a manufacturer and then send it to a trailer maker to mount the fifth wheel. Weil, we prefer to ask the motive unit manufacturer to accept the full responsibility for any mountings or fittings on his chassis, so that the king-pin is fitted in the right place in relation to the axle, and so on.

Hope: But you have not answered my question.

Morgan: in what way?

Hope: Well, control of the vehicle. 1 depend on

artics for my living; but they are a killer vehicle--there is no doubt about it I am so sick of manufacturers telling me, when accidents occur, to get rid of the.driver; that it is the driver's fault. Now even with the most professional driver, at some time during the life of that articulated vehicle, it may become uncontrollable, and no one can state the reason why.

Morgan: The articulated vehicle, no matter how well designed, is less safe, or more critical, than a rigid. We have gone a long way towards making it safe, but there are inherent difficulties in this type of vehicle.

You have been successful in dictating a policy in what you require as far as certain specifications are concerned, but you have not been successful in dictating a policy where you can specify an articulated vehicle that would be absolutely stable in its operations?

Hope: Morgan:

No, because I don't think an absolutely stable vehicle is possible.

Ha raid:

Fred, I think I am right in saying that you have been over in America studying articulated units.

Hope: Yes, that is so. The roads today are very

congested. and we are working at very faster speeds than we were five years ago—twice the speed, in fact. We use the hand control, which is a necessary piece of equipment as far as articulated vehicles are concerned; but I don't like it to be over-used. In America, where you have longer roads, less congestion and so on, the drivers of artics have more time to think They can lay on all the hand controls

VIorgan Fred. you mentioned jack-knifing. I regard :

o the factors which lead to a motive unit and trailer assuming an undesirable angle to each other as being two separate sets of conditions. One is when you are on a bend; you brake and the overrunning of the weight of the semi-trailer forces the motive unit rear wheels to skid sideways and thereby the motive unit makes an angle with the semitrailer. That I regard as true jack-knifing. The other circumstance is when the semi-trailer itself skids. The same angle occurs, but it is due to a totally different cause. They are two different things. calling for different measures.

Mention was made of the overriding trailer handbrake. While we fit these. I think their main use is when descending long straight hills in good weather conditions. There is no doubt that if you apply braking to the semi-trailer wheels and not to the motive unit, there is a tendency to get a trailer skid which, of course, becomes more marked on greasy roads. When there is ice and snow on the roads, I have said to an artic driver: "-How are you getting on?" The reply from a good driver is usually: "It is all right if I slow her down on the gears, and do not touch the brakes ". 'He is really saying: "1 am all right if I retard my motive unit but do not brake the trailer wheels".

Wilding: First of all on this question of stability. At a recent I.Mech.E. symposium on vehicle control, one of the speakers, Mr. Hales, said the artic was basically an unstable vehicle, and proved it by some very mathematical formulae. Now I was surprised that no manufacturers spoke out in their defence. Another thing with B.R.S. is that they are big operators. But can a smaller operator go to a manufacturer and say that he must work to the S.M.M.T. standards? So, as far as the small operator is concerned, he has to accept what he can get, and basically it is very difficult for him to get a vehicle to match.

You quoted air-line pressures. Arthur. We know of one tractive unit manufacturer who has a line pressure much higher than the others. Those vehicles give a different braking efficiency, with identical irailers. I am not concerned whether the thing will fit together—they can make it fit simply and easily; but when they do fit together will the axle loadings be right, will the braking efficiency be right, and so on?

Marenbon: First of all, did Mr. Hales actually say that the articulated vehicle was basically unstable, or that it might be unstable? He did conclude his paper by saying that the conditions to avoid jack-knifing can be obtained relatively simply? I don't think, in a lot of cases, that the artic need be basically any more unstable than the E22 rigid, for the simple reason that it is possible to lower the frame of the semi-trailer so that the centre of gravity of the load is lower than it would be if carried on a rigid vehicle.

Tony asked whether the small operator has all the benefits that B.R.S. have. The answer is "'Yes ". because the S.M.M.T. standards were so arranged that the B.R.S. standards came within them. Any operator can go to a semi-trailer manufacturer and he can say: "I want my semi-trailer to be built in accordance with S.M.M.T. standard No. 50; I want the braking and lighting to be as per standards 80 and 81 ", and so on, and the whole thing is more or less tied up. I admit there are certain minor things that are not tied up, such as this question of braking pressures. But in our standards we have even tied up what the manufacturer has to supply; how he is to terminate his braking. so that the semi-trailer manufacturer can do the rest. And, incidentally, with the S.M.M.T. standards the onus of extending the tractor braking and lighting connections is upon whoever fits the fifth wheel.

Watts: If you want to match a tractor with a semi-trailer there are at least four things you have to match. First, the combination has got to be within the overall legal length; so your tractor length, back of cab to front of tractor, back of cab to fifth wheel and fifth wheel to driving axle must all be standard; your king-pin must also be a prescribed position from the front of the semi-trailer. Also, you have to standardize the height of the fifth wheel. The S.M.M.T. standards do give the height of the fifth wheel according to tyre size. But there is nothing to stop you coupling a trailer which may well be designed for carrying quite a big load to a tractor fitted with 7.50-20 tyres, and in that I think it is wrong. You have also to standardize your braking, as we have already discussed, and, of course (what is equally important) your axle weights.

It is not possible, therefore, to ever achieve a true state of interchangeability unless the tractor itself is of a prescribed dimension. Until we have such a standard, will you ever get really true interchangeability? As regards stability, I think we want to be clear what we are talking about.

Hope: I say controllability.

Watts: I think we must realize this. I think it is true to say that there are no more accidents with articulated vehicles than there are with rigids.

Elliott: Not by good drivers.

The insurance rates for articulated vehicles are no higher than for rigids. You cannot jack-knife a rigid, but you can have another type of accident which, I am afraid, we all too frequently get—when a rigid cannot get round a corner and overturns. I think the penalty we pay on an articulated vehicle for that extra manceuvrability is the possibility of jack-knifing, and I think you have to accept that. It is fundamental of the artic, but it can be reduced by good engineering. Watts: think we are still on technicalities, but largely I agree. Given a good standard of driving, articulation is no more dangerous than rigid working. What we have found is people coming up to roundabouts too smartly and turning over; but that is the driver's fault and not the vehicle's. It is pretty certain there is no more danger in an artic. badly driven, than in a rigid badly driven. Elliott: ' Hope: Surely this must be the problem of the manufacturer of the tractor or trailer. The be so engineered as to counteract the human artic should clement. I don't disagree with that, but there is a limit to which the manufacturer can go.

That limit is a lack of professional drivers.

Canwe switch to this question of braking?

Over the past 18 months, in particular it probably because of the increased pressure for payloads, is to be getting more criticism of braking results on ted units. This leads me to ask whether you feel that y all artics should have (a) a standard system and (b) r we have reached the stage where we can achieve a dized braking system. Into this question I would throw 'easing influence of Continental aspects.

o n: Well. to throw the cat amongst the pigeons,

P

I think that the majority of the blame at inent is upon the average tractor manufacturer, because in from our good friend here—doesn't usually manufaeractor. What he does is to take a four-wheeled chassis is basically designed as a short-wheelbase tipper. He ys: "This will be a suitable motive unit ", but forgets ; circumstances are rather different.

I he is operating it as, say, an 8-ton tipper under

c braking conditions. it still carries an 8-ton load. Any at is transferred from the back axle is transferred to the xle, and so he finishes up with 8 tons. But supposing t that tractor beneath a semi-trailer so that an 8-ton superimposed. Under braking conditions you get very nore than eight tons on the tractor. The total loading tractor may go up to 10 tons, or even more. That, s one basic reason why in some cases braking has been actory. But I would say that we have obtained excellent with some tractive units. We have had braking :ies as high as 60 or 65 per cent, or even higher for the .e articulated vehicle.

generally speaking, the kind of braking efficiency that is 1 in this country at the moment is not very high. Let flop that. As manufacturers, we would feel very much if someone would tell us the standard of braking :yr that should be attained, Lewis Morgan asks for ng like 40 per cent efficiency. Other people would put ' hands in horror at this. It is true that this 40 per cent I at something like 20 m.p.h. so that, because of time lag, ested it at a rather higher speed it would work out at a higher figure—but it would not be very much more, ig the past few years we have had our speed limits d from 20 m.p.h, up to 40, which means that for the raking efficiency the vehicle will take four times as long —that is to say. four times the distance. I think we a to recognize that some of the standards which have :cepted in the past for braking efficiency are rather old-fashioned, and we should move up a little and try to achieve rather higher standards.

But until someone decides what braking efficiency is acceptable, it is rather difficult to standardize. In the meantime, as far as most people are concerned, we are trying to persuade them to have semi-trailers with the largest and most powerful brakes that they can. And then, as far as possible again, when we fit up the tractive unit, we test out the brakes to try to make sure that the tractor and the semi-trailer are reasonably matched.

Watts:

I just can't accept your opening comment, Alan. that brakes on articulated vehicles

are getting worse.

Havarti: Although the articulated unit can now run at 40 m.p.h. and up to 24 tons gross, 1 don't feel that the brakes are of the same standard as they were when its limits were much lower. This, I feel, is because the speed plus the carrying capacity of the articulated unit has increased and the braking efficiency has not increased by the same proportion; but I will be very happy to be proved wrong. Very happy indeed.

The braking efficiency today of our vehicles is much better than it was. That is to say, in, stopping distance. It isn't so many years ago whgn we would positively refuse to brake an articulated vehicle at 60 per cent. We do that commonly now. But I think you have got to realize the penalties of doing that, and if you have a 24 tons g.v.w. articulated vehicle, your weight distribution (if it is a bonneted machine) will probably be something like 12 tons on the back, about 9 tons on the driving axle and maybe 3 tons on the front axle.

When you brake that vehicle to, say, something over 50 per cent, your trailer axle loading will drop to a figure of something like 10 tons. The tractor's driving axle will only go to just over 9 tons, hut the odd 2 tons or 24tons has gone on the front axle, The front axle will go up from about 3 tons to very nearly 5, which I imagine is the point you are making, Alan. But, that is all very well when you are braking to that degree of retardation of 50 or maybe 60 per cent. If you braked your front axle in that proportion, you will find that when you are braking to a lesser figure of. say. 30 per cent on a greasy road, you lock your front wheels. The vehicle will then be completely unsteerable and then you will have trouble.

So I think we want to be quite careful in what we say is a good brake on an articulated vehicle. And I don't regard a good brake on an artic as something that locks all the wheels in a cloud of smoke. I regard a good brake as one that maintains control under the widest conditions. And I think we shall never achieve that until we get a commercial anti-skid device which will physically prevent any of the wheels on an artic (or any other vehicle) locking. And when you do that, then you can do what Arthur Marenbon says. You can beef up your front axle brakes. You can overbrake the back axle. and you won't worry then about the ratio between a light and a laden trailer; you will then have, what you can call a very well-braked vehicle.

Watts: Hope:

Arthur was waiting for somebody to say what is good brakes. Well, he need not him now. They must be fade-proof and

wait. I can tell foolproof.

did not mean it quite in that sense. 1 meant it rather from the point of view of what is a satisfactory efficiency. I would like to go along with Laurie about the importance of the overall picture; that is to say, you must have a vehicle which is well-braked when it's empty, well-braked when it is medium-loaded, and well-braked

Marenbon:

when it is loaded. And therefore, under present circumstances, obviously our braking has to be a compromise.

Morgan: Arthur Mentioned that in B.R.S. we ask

for 40 per cent efficiency. I must make it clear that this is not laid down as a minimum requirement, but is part of a much more comprehensive specification, and the relevant paragraph is that "the vehicle must be stopped in less than 33 ft. from 20 m.p.h. without locking any of the wheels. with a pedal pressore of 80 lb. in the fully laden condition, and 50 lb. in the lightly laden condition ".

What we want is that with normal deceleration and reasonably balanced brakes, we are reasonably controllable at unloaded and lightly loaded conditions on wet roads. If you skid one wheel under this condition, you risk a really serious skid. There is great danger in having such a braking arrangement that, in lightly loaded conditions, you can easily skid the trailer axle. That is the one thing we want to avoid. That is why we have specified what we think is a reasonable and maintainable figure.

Havard: Do you feel it is sufficient, Fred, or would you like more?

Hope: Well, it would be sufficient if the brake was synchronized.

Morgan: That is the point. A brake can always be

synchronized. The difficulty is that adhesion between the road and tyres varies because of the load variations.

Hope: Morgan: You mentioned time lag, earlier. Yes, there is a bit of time lag.

Hope: Well, whilst you have time lag, you have not got synchronization.

Morgan: actuation. You must have some time lag. It is inevitable, unless you have mechanical

Hope: As fax as braking is concerned, with the

higher speeds, this is how it looks to me. The vehicle manufacturer was at the starting post without his r24 shoes on, and the trailer manufacturer was dozing at the startini post when they introduced these 40-mile limits. You can ge the braking, but you have not been able to obtain thi synchronization to get equal braking or get a good, cfficien brake.

I have designed a self-energizing disc brake. It mechanically operated and it uses the bulk of the movemen of the vehicle to apply the braking. My own money is goini into this. With this, I hope to enable the trailer to be able te look after itself by drawing its power from the motion energy And then. because I feel this is an absolute necessity foi braking, to leave the driver's mind completely clear and let tht vehicle look after itself, much as it does on a passenger car.

A very small amount of time lag in youi braking system is not disadvantageous provided it is not an undue amount of time lag which will upsei your overall picture. In other words, from the stability poini of view, a very small amount of time lag does not upset the vehicle, make it tend to jack-knife, or anything like that Another aspect of the matter is that—as has been pointed Out— you get this wide variation of loads on the axle, between lader and unladen conditions. I think it should be possible, in the case of air suspension, to link the braking system to the ail suspension. because if you use air suspension, the pressure in the bellows is proportionate to the load on the axle. so that you could then get variable braking. The braking would vary in accordance with the loading on the axle.

Marenhon: Putting this in hauliers' language, the Hope: vehicle offers its greatest retardation just this side of skidding. So operators need not be impressed if they are shown a vehicle that skids very well.

I don't quite understand why Fred Hope is

Watts: • so adamant that the balance of the brakes between a tractor and a semi-trailer is bad. I would not have said so, not in a well-engineered tractor and semitrailer, particularly if they are two-line air pressure. I would agree if they are vacuum brakes, of course. Loads and speeds are getting to a point now where vacuum brakes are just not on, for heavy vehicles. But provided the vehicle has a two-line air-pressure system, I would not have thought he had all that amount to complain about, provided that the trailer brakes are kept in good order.

But I would put in a plea here. Trailers do suffer from a lack of any real maintenance attention. The trailer is the unmechanical bit that is left out in the yard. Operators do all the maintenance on the tractor, and they just send the tractor out to pick up the trailer. The numbers of times we have vehicles in to our factory with the complaint of bad brakes— purely because the trailer is in such an appalling condition, either the brakes have not been adjusted or the linkages are seized up or there is a hub-seal leak. It is not really our fault at all. It isn't anybody else's fault but the operator's.

I reinforce what others have said. I do not think we have yet anything like an adequate lightly laden or load-proportioning braking device. I don't believe one at present exists and I still don't think that that is the answer, anyway. I would put my money on some seasonably cheap and commercial antilocking device, because I think that, in the end, is what will count. It will cover a multitude of other sins that inevitably arise in operation—that is to say, bad adjustment or grease on the linings, or a hundred and one other things—and will at least keep the vehicle safe. I think that that will be the ultimate, and I look forward to the day when we will get such a device.

The same thing was said in our after-lunch

Havard: discussion on brakes which we conducted about six months ago (The Commercial Motor, February 8, 1963),