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CLARITY AND GUIDANCE We were very pleased to read John Dickson-Simpson's letter about tipper stability (CM 28 Nov-4 Dec).
Page 22
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We agree with him absolutely when he says that the guide is as relevant today as it was at the time of publication. One of the simple beauties of the guide is that it is independent of gross weight and is equally applicable to 3.5tonne gross vehicles as it is to 44-tonne GCW artics.
It continues to provide an empirical companson which operators may rely on when deciding their future plans. Providing, of course, that all the parameters are the same for all tests and for all marques of vehicle. That is: all wheels on the ground, fully loaded and fully tipped.
On a related subject, the code of practice issued and amended by SA F and published in CMraises some interesting issues, quite apart from the questionable practice of dumping the air when tipping.
There is no doubt that the fabricated-arm' genre of running gears provide an attractive feature when applied to tippers. They are significantly lighter than their more conventional spring-trailingarm counterparts.
If being paid by the tonne carried, maximising payload is a worthy pursuit. This reduction in unladen weight is made possible by the use of the fabricated trailing arm being firmly and permanently fixed to the axle which usually has a larger diameter than the normal 127mm or so, and is, therefore, permitted to have a thinner wall.
The rigidity of the assembly is compensated by having pivot bushes performing a dual role in that they are also vertically deformable. This allows the normal roll and side-to-side bump movement to be absorbed.
However, one never receives `ought for nought' and the drawback, as far as tippers and stability is concerned, is that the bushes are not linear in their rate of deformation as the spring trailing arm is.
The rubber bush is more relaxed in the early stages of roll and stiffens up as the roll increases. But in general the roll stiffness in the range required by normal road travel is less than the springarm system.
This does not help the tipper When it is being subjected to the PIE test and it is most likely that a spring-trailing-arm suspension will produce a higher degree of stability, chassis for chassis, than the fabricated-arm. The difference can be significant.
If operators have a target I include the highest degree of safety in their specifications it will pay them to discuss the running gear options with their regular vehicle supplier. Angus Spooner,
General 1 ra,lers, Fruehauf Dereham, Norfolk.