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roll-over can be caused by going too slowly

31st January 1969
Page 44
Page 44, 31st January 1969 — roll-over can be caused by going too slowly
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Which of the following most accurately describes the problem?

continued The front wheels of a truck usually bear a relatively small proportion of the load's weight, an,d so the front suspension cannot contribute so much to roll stiffness. It could do so only if the chassis had plenty of torsional stiffness, but it has not. Putting independent suspension on the front of a truck therefore would not improve matters greatly unless the chassis frame were made torsionally stronger; its torsional stiffness could be doubled, and perhaps even trebled, by using cruciform cross-membering, but the stiffness would still be only about a tenth of a chassisless car's. Tankers, or buses for that matter, do have very high torsional stiffness, however. In such cases independent front suspension could be valuable, and it is important for the springs to have extra travel so that diagonally opposite wheels can be deflected without imposing terrible stresses on the practically untwistable structure.

There could be some benefit from independent front suspension so far as ride comfort is concerned, however. Because of its inherently great roll stiffness the springs themselves of an if.s. system can be made softer without entailing extra roll. A cheaper, though admittedly less effective way of improving ride quality without increasing roll is to use singleor twin-leaf springs, which dispel all the friction-jarring of a leaf spring.

On articulated vehicles with more than three axles the major part of the load is carried by the trailer wheels. Therefore, here, the greatest improvement in roll stiffness would be obtained by putting independent suspension or outrigged springs on the trailer. Lighter and cheaper independent suspension can result from locating the wheels by transverse links rather than by trailing arms.

Two other weapons are available to combat the low-speed roll problem. One is to "scramble" the roll frequency of the suspension so that the whole rolling-mass system gets confused, virtually, and never manages to get the steering frequency and the roll frequency to coincide for long enough to be critical. This can be done by using highly progressive springs—readily achieved with both air and leaf springs. If there is a big increase in a spring's rate as it deflects there is every chance of the critical roll frequency being passed through without harm. This aspect of suspension design actually seems to offer the best prospects of salvation from the low-speed roll-over danger.

The other weapon is the suspension damping. Really effective dampers could do a lot to prevent side-sway on the rebound from developing a dangerous resonance.

Also, any chance of running a step-frame rather than a straight-frame trailer should be seized, because knocking only a foot off the load height would do much to raise roll frequencies outside the critical band.

So once the problems are appreciated properly there seems no reason at all why some fundamental research should not enable the designers to produce some answers.

In the meantime, drivers exposed to risk of roll-over have no guide-lines. They have always been taught that you roll over only if you go round corners too fast. That is quite true, but it is also now known that in a slow-swerve manoeuvre you can also roll over if you go too slow—and that is why lorries generally overturn on the far side of roundabouts, not at the entry turn.

The trouble is that the critical speed range varies according to the height and weight of the load and, to a lesser extent, the suspension characteristics (only because at the moment there is not much difference between one vehicle and another of similar type as far as roll frequency is concerned). All that can be recommended is to try to make the first strong movement of an S-bend manoeuvre quickly and to make the second movement slowly—then you "scramble" the steering frequency and make resonance-overturning unlikely.

Big operators with a narrow variation of types and only one traffic are in a position to get some recommendations drawn up. Those who carry a wide variety of loads are in a much more -difficult position, except that it is high loads which are the problem and some useful calculations could be done quite quickly by the Cranfield College or the Motor Industry Research Association to determine a band of potentially dangerous speeds around roundabouts.

Although this is a serious and growing problem, one must bear in mind that this type of accident is still comparatively infrequent. But the subject is one which should certainly be brought into driver training programmes.


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