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roll-over can be caused by going too slowly

31st January 1969
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Page 42, 31st January 1969 — roll-over can be caused by going too slowly
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STARTLING RESEARCH ON HEAVY-TRUCK STABILITY

ROLL-OVER of big trucks is not so often due to cornering at too high a speed as going through an S-bend manoeuvre (as at roundabouts) at a speed which is actually too low.

This is the surprising conclusion of preliminary research, particularly at the Cranfield College of Automobile Engineering, which has been making an investigation into roll-over on behalf of British Rail.

Rolling over at low speed? At first sight it seems incredible, but there are now many drivers—of heavy articulated vehicles in particular—who can testify that their trucks rolled over on roundabouts even though they were driving gently (though the miserable men have rarely been believed).

What has been discovered is this: that with high and heavy loads, which have a ponderous pendulum effect round corners, the natural roil frequency of the. vehicle is within the band of frequency of steering-wheel movement at quite low speeds—and that if the change of steering from one direction to another is at a frequency which coincides with the roll-frequency, nothing will stop the vehicle turning over. In other words, if, in going round a roundabout, you catch a load on its natural rebound with your change to the opposite lock, you're in trouble.

Tests with a British Rail artic showed that, just leisurely weaving in and out of traffic, the frequency of change of steering direction was typically three cycles in 50 seconds. This is within the band of natural roll frequencies of such a heavy vehicle with centres of gravity between 6ft and $ft from the ground—a common range of load heights these days.

Further checks with other vehicles with high loads showed that, in fact, the slower the vehicle went round corners the bigger the roll angle became. However, going round single corners is not much of a problem; it is the slow, swerving manoeuvre which can be dangerous—occasioned usually when negotiating roundabouts.

Roll has been a problem with heavy commercial vehicles for years, but it is only lately that it has developed into overturning frequently enough to cause worry in operating circles.

The reasons are varied and arise from a build-up of adverse factors in modern operation. And while suspensions stay as they are the problem is getting worse all the time instead of improving. Today, vehicles are carrying bigger loads than they have ever done. At the same time the changing nature of loads has often involved more bulk—pallets, tanks, containers—so that the average centre of gravity height of loads has gone up. The incidence of high loads has been further increased by the bigger proportion of large vehicles in use (aggravated by the effect of the plating regulations) with a minimum tyre size of 10.00-20, deeper chassis frames and higher body floors (exceeding 4ft). Articulated vehicles have fared even worse because the body-floor height of the trailer is fixed by the height of the fifth-wheel coupling on the tractive

unit and by the depth of the trailer-frame neck above the coupling; as a result, body floor heights of big articulated vehicles now approach 5ft. Add 4ft to the centre-line of an ISO container and you can see that the load's centre of gravity is nearly as high as the roof of many single-decker buses, and is almost certainly above the roof of the tractive unit cab.

Nor is that the end of the roll-inducing factors. Truck chassis frames, and trailer frames in particular, are torsionally weak. If an articulated vehicle has a fifth wheel coupling which slopes backwards, a compound angle is induced during articulation so that the front end of the trailer is twisted in just the most unhelpful direction, the direction of roll.

Moreover, with an articulated vehicle it is not so easy for the driver to sense roll, because the bearing width of the coupling is limited and there are clearances in the king-pin fit which are small enough in themselves but are amplified by the time they are translated into up-and-down movement at the side of the trailer.

What can be done about this roll problem? Well for one thing a complete re-think must be done on heavy-vehicle suspensions as a matter of some urgency. And an intensive mathematical investigation is needed to see whether it is in fact possible to raise roll frequency outside the range of steering frequencies met in service.

There is an apparently conflicting need for more flexible springing to allow the tyres to follow road-surface contours more

faithfully and in any case to give drivers and cab structures a better ride. However, this can be made compatible with the neeci for much greater roll stiffness by putting an anti-roll mechanism in the suspension layout. The challenge for designers will be to do ail this without adding prohibitive cost—it is inevitable that sonic extra cost will result from a greater degree of sophistication in suspension.

As roll stiffness is proportional to the square of the spring base, if the spring base is doubled the roll stiffness is quadrupled. 1 hat is a tremendous prize. And it can be stepped up even more by the use of an anti-roll bar.

With semi elliptic springs a heavy truck's normal spring base is only about 38in. This figure can be doubled if the effective spring base can be widened to about the wheel track of the axle. This can be done in two ways. The springs can be positioned on either side of the tyres in other words outrigged. Or independent suspension can be fitted.

Because taking such measures to increase roll stiffness has the greatest effect on the axle bearing the most weight, the rear axle of a truck is the place on which to concentrate. For a driving axle, therefore, the outrigged-spring layout seems the more practical for rear suspensions. Unsprung weight would be higher, but this is not so important as many road transport engineers are inclined to believe.


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