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'Ideal' conversions do not exist

5th February 1983
Page 22
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Page 22, 5th February 1983 — 'Ideal' conversions do not exist
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Which of the following most accurately describes the problem?

MANY OF the engineers and managers at Woodcote Park were keen to hear about the relative merits of different conversions to suit operation at the higher weight, and the session led by Keith Buckby and Graham Thomson, representing respectively York and Crane Fruehauf, gave them the opportunity they wanted.

But it soon became clear that anyone who was expecting to be told categorically that one type of conversion was best for a particular operation was going to be disappointed.

Keith Buckby admitted: "I don't want to talk about costs. If I give you a cost somebody will undercut it." He emphasised though that operators should make sure they know exactl what they are getting for the money and proceeded to ider tify a number of pitfalls for ine) perienced convertors.

"The most popular convel sion," he suggested, "is likely t be a tandem axle semi-trailer t a tri-axle. The two most impoi tant points here are the bogi position and imposed low through the king pin, becaus there is only 3/4 tonne leewa between the sum of the axl, weights and the gross combina tion weight."

Also, if the brake chamber are not changed when the semi trailer's axle weight goes dowi from perhaps 91/4 to 71/2 tonne: then clearly the axles will bi overbraked. "And don't overlook the cost sideguards in your calcuions," he added.

While most of the attention on mi-trailer conversion has been cussed on 12.2m (40ft) Ddels, Keith Buckby also sees -wed arising for conversion of orter models. The warning he d for operators on this subject 3s that they should watch the itical axle spacings, changed r the new legislation from riner axle spacing" to '"releint axle spacing" and he reinded operators that the fifthheel coupling's position on the ]ctive unit would also be more itical on 38-tonne combinems.

The problems associated with re equipment for 38-tonne rribinations had been touched earlier, and during Keith Jckby's presentation were exained in more depth. York's chnical consultant cautioned aerators to check that any new heels fitted were compatible ith the axles used.

He said that Michelin's new )mm negative offset wheel ee page 16), which by effecrely increasing track width by tOmm (51/2in) would also inease wheel bearing loads and ierefore axle ratings, and lould be carefully checked.

This was challenged by Colin leller of Heavy Transport (ECC) ho reminded us that at the ime time that track width was .ing increased, individual axle adings were going down by most two tonnes, and the net suit would be that axles would mtinue to work well within eir design limits.

Keith Buckby's call for a stenard fifth-wheel coupler height cm tractive unit manufactur.s, to overcome the problems isoci a ted with braking reaction n severely sloping semiailers, was responded to by aul Wain, Leyland's vehicle enineering manager.

He explained the manufacturr's dilemma of having to satisfy ther conflicting requirements good ride characteristics for xample — and concluded that us meant there would never be standard coupler height.

As the workshop prepared to lose for lunch Steve Abel, roup managing director of oadline, could contain his isappointment over the lack of Iformation on costs no longer. "I would like to make some cost comparisons between, for example, tandem axle conversions and complete tri-axle replacement and I've heard nothing yet to help me do that. Also there has been no mention of running cost comparisons: how, for example, do tyre costs compare?"

Keith Buckby referred Steve Abel to a recent CM Drawing Board for information on conversion costs but admitted that he was unable to provide any running cost data for 38tonne combinations. "I think these simply are not available," he said, and nobody would contradict him, though some engineers contributed to the discussion with their individual experience of tri-axle semitrailers in general and tyre running costs in particular.

Colin Kneller had had mixed experience with Michelin wide singles. "I changed 120 semitrailers from twin wheels to 1819.5 Michelins and they were a disaster, but the 18-22.5 tyres which replaced them have approximately halved my tyre costs compared with twins," he said.

He had also identified other operational benefits, namely much improved road spring and air tank life, due, in his opinion, to the better ride resulting from the 18-22.5 tyres' more flexible sidewalls compared with twins.

Like every other contributor to the discussion, Colin Kneller had no data to quantify the tyre wear due to scrubbing associated with tri-axle bogies. "But," he said, "I suspect that the scrubbing problem won't be as serious as some people fear."

It became clear at the Woodcote Park workshop that transport engineers in the brewing industry have real concern about the roll stability of 38tonne combinations. Andrew Davis, transport development engineer with Whitbread, described how his company's research at the Motor Industry Research Association had resulted in a standard for roll stability with which all new Whitbread vehicles must comply.

The basic requirement of that standard is that at a centre of gravity height of 2.3m (7ft 6in) lateral acceleration must not ex ceed 0.3g. • Andrew Davis said that, in his opinion, "in the next two years we should be moving towards a code of practice standard for roll stability. My fear is that increasing use of tri-axles will lead to more semi-trailers turning over."

Representing Crane Fruehauf at the workshop, Graham Thomson claimed that tyre scrub on a tri-axle trailer will not be the problem it was in the sixties. Commenting on. CF sales where the vast majority of new trailer orders are for tri-axles, Mr Thomson emphasised that, in CF's opinion, a 4x2 tractive unit and a tri-axle trailer represents

the cheapest way to achieve 38 tonnes gcw.

It was suggested that because of the use of wide single tyres the increase in weight over a tandem would be in the region of half a tonne. Mr Thomson went on to say that the roll stability of a tri-axle on wide singles will be better than an existing tandem because of the wider frame centres and because the trailer will -have six springs resisting the roll and not just four.

Nigel Trotman, group transport engi.ieer at Amey Roadstone Corporation, queried the compatibility of tyre equipment across the fleet but Mr Thomson replied that once wide singles were specified then compatibility was lost anyway. Mr Trotman also queried the tyre scrub with a tri-axle, a point also taken up by Walter Batstone who asked about the costeffectiveness of self-tracking axles.

Mr Thomson replied that CF had looked at such axles but the feeling was that the complications outweighed any possible advantages, and the saving in tyre wear was insufficient to balance the increase in first-costs.


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