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THE STABILITY OF TIPPERS

21st November 2002
Page 24
Page 24, 21st November 2002 — THE STABILITY OF TIPPERS
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Which of the following most accurately describes the problem?

As the producer of the tipper stability code for the IRTE I feel I should comment on the recent Commercial Motor articles revisiting the subject.

The code took more than two years to research and agree with the Road Haulage Association tipper group and the major manufacturers of tipper bodies, chassis and tipping gear.

The leading author, Norman Simmons, drew on masses of background data an tipper stability performances from his trailer-design experience. His mathematics for predicting stability were reinforced by investigations by Bristol University. All pointed out the critical influence of chassis torsional stiffness.

The fundamental conditions under which tippers operate have not changed; Until they do there is no case for altering the approval angles of sideways tilt.

The state of loading that is specified provides consistency that is repeatable from test to test. This and the minimum tilt angles take account of the inservice distortions of load distribution that arise in practice, such as partial discharge and sticking toads.

No tinkering with the code's basic requirements should be contemplated, therefore. However, refinement of the guidance is desirable to recognise charges in vehicle construction. For example, although most rigid tippers still have steel leaf suspension, most trailers now have air suspension—and it would be helpful to explain its dynamics and their relation to stability.

A supplementary exercise in data collection is needed, preferably including physical tests as well as mathematics.

There seem to be misunderstandings about the action of trailer air suspensions when resisting side sway. Nearly all the roll stiffness of a trailing-arm air suspension comes from trying to twist the axle tube and compressing the bushes. By comparison, the air springs themselves contribute very little to roll stiffness—it follows that it makes no significant difference whether they are inflated or not.

The lowering of the vehicle when the air springs are exhausted is of little practical help. Typically the tipped height of the centre of gravity is reduced by only about 1.5% and the body front-end height by just 1%.

With the air dumped the chassis slopes, so only the last axle is likely to be on its bump stops—the other axles are left floating. That probably means the load imposed on the last axle soaring to 24 tonnes or more. Even if axle and suspension can recover from that, it brings excessive tyre deflection to decrease the attainable tilt angle.

might not hold the combination.

Only with the springs inflated can there be load equalisation between the axles to spread about 30 tonnes of ground force when the body is fully tipped. John Dickson-Simpson, TPS Design, London.


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