Care & Maintenance
Page 58
If you've noticed an error in this article please click here to report it so we can fix it.
NOT UNNATURALLY, fleet engineers tend to consider that jack-knifing, rolling over and shedding loads are none of their business — until they have to rectify the damage.
It is also often assumed thai these accidents, which are generally avoidable, happen only to• incompetent drivers or -rogueoperators.
But for a number of reasons, the avoidance of these accidents does come into the realm of the engineer, not the least because these accidents cause a lot of disruption in the workshop.
In addition, though, the engineer should make sure that the drivers are trained to avoid these dangers, and to understand some of the problems; then, he has to decide whether the cost of an anti-skid system, for example, can be justified by the savings resulting from the avoidance of these accidents; finally, he must remember that a poorly-maintained vehicle is far more likely to be involved than one in good condition.
There has, of course, been some controversy over what causes an artic to roll over at a roundabout, but the basic cause is that the vehicle is travelling too fast.
One reason for this is that drivers tend to base their cornering speed on their experience with rigids, but for some
reason, they tend to underestimate their speed with the artic
— but not with the rigid.
This is likely to remain a cause of accidents.
There is also some evidence that at large roundabouts, artics are more likely to roll over 90' after entry — as the driver goes from the leftto the right-hand lock — but on small roundabouts, the artic is more likely to roll over when turning back on to the left lock to leave the roundabout.
Surprising . . .
At first sight, this is surprising, since the radius of the larger roundabout is larger, so one would expect the driver to be able to negotiate that part with ease, and perhaps to misjudge his speed on leaving the roundabout.
In practice, though, it seems that on larger roundabouts, the smoothness of the curve deceives the driver, whereas on a smaller roundabout, he enters much more slowly, and again rolls over when he thinks he can speed up.
In other words, it is not the judgment of speed as the driver
enters the roundabout that causes the trouble, but the lack of feel of what has happened on one curve, that leads him to speed up too much for the next one.
This is the crux of the matter; with an artic, the driver has precious little feel of what is happening. On a rigid, he can feel how much the vehicle is rolling, and he can also feel any incipient loss of adhesion through the steering.
It is worth bearing in mind that steering feel is much more obvious with manual than with power steering, although the manufacturers of the power assisted systems are continually trying to improve this all-important characteristic.
With an artic, the driver is aware how much the tractor is rolling, but owing to the flexibility and movement of the coupling, he has very little idea of what is happening to the trailer.
In fact, skilled drivers have cornered with the outer trailer tyres well clear of the road surface, on test, and been blissfully ignorant of what was happening.
Even apart from the lack of rigid connections between the tractor and trailer, there are other reasons why the feel is deceptive As an artic corners, so the body rolls outwards, and quite obviously, the higher the centre of gravity, the more the vehicle will roll, and the more likely the vehicle is to turn over.
During roll, weight is transferred from the inside to the outer wheels. Since the rear wheels are more heavily loaded, more weight is transferred there, but as the vehicle accelerates and decelerates, so the tractor driven wheels are subject to fore-and-aft weight transfer.
Acceleration . . .
For example, as the driver accelerates around the main portion of a large roundabout, so weight is transferred to the rear, and this can upset things. That also happens with rigids, but the big difference is this feel.
Another point is that the steering on an artic, whether 'power assisted or not, is usually much lighter than on an old rigid, where the heavy steering more or less dictated the speed at which the vehicle was taken around the roundabout.
Some years ago, the theory was advanced that the reason an artic overturned was that it behaved like a pendulum. First it rolled one way, and then rolled in the other direction as the next part of the roundabout was negotiated, so a pendulum effect would be set up with the vehicle continuing to rock from side to side, say, once every five seconds.
It was then argued that if the next roll to the left coincided with the application of left-ham lock, then the vehicle would rot over just like that. In othe words, there was a critical speer for each roundabout, and thi would cause a particular vehicli to turn over, if it entered at it critical speed.
While there is no daub something in that theory, wonl at the Transport and Roar Research Laboratory shower that this didn't really happen ir practice, although the theorist are probably still arguing abou it all.
They showed quite simpl■ that if an artic entered a come too fast, it would turn over. Thi main reason, as was pointed ou earlier, is that the driver has nc real idea here what is happeninc. to the trailer — and the onli solution to that one is a comple tely new type of fifth-whee coupling.
Experience . . .
In the meantime, driver5, must just learn what speeds are safe by experience — and that is generally a little slower thar with a rigid. Then, they musi remember to steer smoothly around the curve, and not jerk the wheel.
If the roads are wet, o' course, the speed should bE reduced, to prevent the wheek sliding. If they hit the kerb, thE artic is likely to turn oval anyway.