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Care & Maintenance Sure way to stop jack knifing

13th May 1977, Page 140
13th May 1977
Page 140
Page 141
Page 140, 13th May 1977 — Care & Maintenance Sure way to stop jack knifing
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Which of the following most accurately describes the problem?

Girling demonstrates a jack-knife situation.

SKIDS can result just as easily from an incorrect adjustment or bad loading from driver error, and thesi causes fall into the domain of the engineer. But before rushing off to order some anti-skid systems when it appears that skidding is on the increase, the engineer should first look at his tyres.

It goes without saying tha4 for a lorry to skid, the tyres must lose grip, but it is sometimes forgotten that as tyres wear, so their wet-grip gradually deteriorates, so tha at 100km/h (6 2mph) the gri of a half-worn tyre is about two-thirds that of a new tyre. Once the tyre gets to the leg limit there is hardly any grip all — so running right to the limit can be false economy.

Another point that is wortl reminding the drivers of artic about is the way to part-load the vehicle. Quite often, two short containers are being carried, or there are a numb€ of packing cases that cover only half of the platform, ant the question arises: -Should the load be at the front or al, rear?"

The answer is that the load ould always be at the front, if part of the load is to be Dipped off first, make that the ar half.

The reasons for placing the ad at the front are connected th braking, cornering and iction. If the load is placed at 3 rear, then there is very :le load on the driven axle, d so in an emergency, the akes on the driven axle are ely to lock up first — and at means a jack-knife.

Then, when the vehicle is rnering, the driven wheels II also carry very little weight :he load is at the rear, and if 3 tyres carry little weight,

they cannot generate ich cornering force. In other ards, to make sure that the es grip during cornering, u need to put some vertical id on them.

If the tyres of the driven le cannot generate as much rnering force as the tyres of a trailer, then again, the hide could jack-knife.

tally, if there is not much id over the driven axle, the es will have a job gripping slippery surfaces, so ction will be poor.

So, on all counts, the load should be at the front on an artic —_and it often is not. On a rigid, the situation is not so critical, and depends to some extent on the weight of the load. If it is heavy and short, then placed at the front of the platform, it may overload the front axle. If possible, though, the load should still be placed forward of the rear axle, but biassed towards the rear.

But what, you may ask, about load-apportioning valves? Shouldn't they compensate for all this? They should, but if we go back to basics, we need to remember that just as skidding starts at the tyres, so locking brakes start with the foundation units. Therefore, brake maintenance needs to be to a high standard if the possibility of a skid is to be avoided. A badly grabbing brake will always be a problem whatever equipment you have on the vehicle.

The need for maintenance applies equally to load-apportioning valves, which vary the braking force according to the deflection of the springs. First, of course, the springs settle during the early life of tile vehicle, and so the valve needs re-setting once the vehicle is run in, and secondly, whenever the system is dismantled, the load-apportioning valve is likely to be disturbed.

The principle is that when the springs are fully deflected by the full plated load on the axle, then maximum braking effort is applied. When the vehicle is unladen, the minimum effort is applied.

Therefore, if the load-apportioning valve is adjusted correctly, and it is a good match with the vehicle, then it should not matter too much where the load is placed.

Unladen axle

But that's a lot of ifs, and they don't help traction or cornering, and as one braking expert said: "There is many a jack-knife that has got nothing to do with the brakes, but results from too fast cornering with an unladen driven axle."

Even an anti-skid system won't stop a vehicle skidding in this way, but it is definitely the answer to skidding or jack-knifing under braking. A number of these systems are now available to operators, and on the maximum capacity artics, they would seem to be worthwhile, and cost-effective. There is the Dunlop Maxaret, the Girling Skid-Chek, Lockheed Anti-Lok and Bendix-Westinghouse EBC, all of which are now available.

The addition of an anti-skid system does, of course, complicate the braking system further, and EEC regulations may seem to make brakes complicated enough as it is. In operation, these systems sense when a skid is imminent, and they then reduce the pressure in the brake chamber momentarily, and then increase the pressure very quickly. Therefcre, the brakes are pulsed on and off until the danger of a skid is avoided.

In practice, these systems don't seem to have any real problems, despite the furore in the USA. The fact is that the problems encountered there have been eliminated from the British systems, by and large, before they were put on the market.

Naturally, the anti-skid systems need checking over now and again. But, of course, if the braking system is not maintained properly, then you had better expect some accidents anyway . .

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