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To dump or no to dump?
Page 22
Page 23
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The long-running debate over whether it's safer to raise the body of a tipping trailer with its air suspension inflated or exhausted looks set to rumble on for some time yet. Brian Weatherley sifts through the latest salvoes.
• The great 'dumping' debate has flared up again. Last month, Newton Commerdials, UK importer for Stas Trailers and a leading campaigner in the search for the definitive answer, fired a fresh salvo.
During a visit to the recent Lyon Eurexpo Show (CM1-7 May) Stas's sales director Rik Tanghe noticed a label on a Kogel tipping trailer fitted with BPW axles. It recommended that the trailer's air suspension be dumped when tipping to ensure better stability.
Newton director Andrew Smith was on to CMin a flash. He also fired off a letter to the IRTE citing the decal as "further evidence" for the IRTE's Vehicle Stability Working Group to consider, adding: "ft might be of interest to note that BPWprobably the largest axle supplier in the UK—also suggests dumping the air as recommended by the label on the chassis. It would seem to be quite clear that what SAF and BPW really believe is correct."
While both axle manufacturers have gone on record to advocate dumping air when tipping, Smith can't understand why the factory view has not been adopted in the UK. However, his recent comments in CM certainly generated a strong response from both readers and trailer makers.
Spot on
Malcolm Millard of MGM Haulage in Southampton, a former tipper operator now on container work says: 'Newton is 100% spot on. I've been in haulage 50 years-80% of the time on tipping—and there's only one way to tip when you have air bags: you dump the air after you've dropped any lift axles."
And he insists: '"When you dump the air, the trailer is 6-10in lower to the ground. I've seen an air bag burst on the back axle when toping feed, and were it not for the fact that the vehicle was in a steel frame it would have gone over."
However, owner-driver Hugo Hoyle from Abingdon, Oxford, asserts: "I write as a tipper operator of twenty years plus experience and have strong views on the current debate. I'm very anti-dumping! I first became aware of the problem a couple of years ago when my trailer lost its suspension air over the course of a weekend. When the air had escaped, the entire load was taken by the rear axle and landing legs, and I feared mainly for the landing legs as they were taking well over half the overall weight"
He adds: "The only way I can see that it would be safe to tip with the air dumped would be if the trailer chassis was per
fectly parallel to very level ground when the air had been dumped. In practice, this would never happen. The trailer will always be low at the back. Even if the tractor air was to be dumped, most trailers run slightly 'front high'. I could see dangers with all the weight on the rear axle."
Sticky load
With a sticky load like scalpings or sugar beet nuts, Hoyle concludes: "The majority of the load would be imposed on the rear axle 0 suggest that this would be well over 20 tonnes) and weight like this must invite the failure of springs/trailing arms, tyres or supporting ground leading to a rollover. Keep the suspension inflated!"
Angus Spooner, Fruehauf's tipper product manager, sees three issues at stake. Namely exhausting the air in the springs for legitimate purposes: exhausting the air to enhance stability in a tipper; and finally the effects of exhausting the air on wheels and tyres when a vehicle is loaded.
In certain circumstances, notes Spooner, it's both legitimate and desirable to exhaust the air in a trailer's running gear when unloading. "Skeletal trailers go from full load to no load in an instant. This loads the restraints heavily and if those restraints are the shock absorbers then 'after sales costs' for the supplier often follow!" For the record, all Fruehauf skeletals are fitted with suspension exhaust valves.
However, he adds: "We've never seen a tipper unload instantly! Yes, a lot of the load plunges down the body—but unless it's being discharged into an under-floor hopper it usually comes to rest in a big heap, waiting for the driver to carefully draw forward and let the remainder out. That's not exactly instantaneous and gives plenty of time for the levelling valve to have exhausted the requisite amount of air pressure."
"It is absolutely true," continues Spooner, "that exhausting the air does enhance stability when tipping—but in much the same way that propping up the rear of the trailer on bricks would be. However, we certainly don't recommend the pile of bricks!"
On the effect of exhausting air from the trailer's suspension when tipping, Spooner says the trailer bogie load will normally be close to 24,000kgs.
"Most trailer configurations will put about 18,400kg on the rear axle when the air is dumped (taking into account the two unloaded axles 'resting' on the ground)," he says.
Bonus loading
"At 9,200kg on each tyre they are already close to their absolute 'bonus loading' of 10,350kg. Meanwhile, the axle is 60% over its design, plus 30% load that most suppliers allow for low speed work (eg traction enhancement)."
However, he warns that when the body is raised the centre of gravity (CoG) moves backwards and the bogie load increases. "When the load is restrained, the bogie load will increase from 24,000kg to about 32,000kg. With air dumped, the load on the rearmost axle will be around 26,400kg with the two unloaded axles 'resting'. The tyres now exceed the bonus load by 28% and the axle is 225% over its low speed load. The wheel load/strength is another question. Moving along while tipping into a road paver in this condition is not an attractive proposition," he reports.
However, last week Smith e-mailed CM with the tipping instructions from body manufacturer Neville Transport Company: Newton recently sold five Neville-bodied trailers to an operator based on new Weightlifter chassis and Mercedes axles. 'They run to four pages in total," says Smith, "and you'll note that
they expressly insist on air dumping in two different places."
IMeanwhile, Graham Ellis, chairman of the IRTE's Vehicle Stability Working Group has written to Newton with the offer to subject a Stas trailer to an independent test at the Qinetiq facility at Chertsey with the IRTE "acting as independent mediators and ensuring the tests are carried out in a totally impartial and independent manner", in order to "validate" the pro-dumping claims.
For the moment, the IRTE's working group is still deliberating, with Ellis reporting that it's looking at 'all related aspects of tipper stability including axles, suspension systems, chassis structure and manufacture, and wheels and tyres. Once this work is complete and relevant tests conducted a revised Tipper Stability Guide will be published".
But he says: "Early indications suggest that, by deflating the suspension system, the stability and safety of both trailer and unit are compromised."
Mixed messages Until we get a new guide it's hard not to feel sympathy for Newton Commercials. According to Smith, -there are mixed messages out there being given by the manufacturers and even within a manufacturer's literature there are contradictions."
While insisting that Newton remains neutral on the issues and does not make recommendations on safe tipping, he says his company is nevertheless under a duty to pass on the full instructions for the products they supply. "Can you see why we are confused?"
Responding to the claim in Ellis's letter that the IRTE has not received any tangible independent evidence to support the argument to deflate the air suspension system prior to tipping, Smith points to the literature published by Stas and now Neville—and the aforementioned Kogel sticker.
Amongst the tipping gear suppliers, UK sales manager for Harsh Kevin Johnson says: "Although I'd be happy to do a tilt test with the suspension up, down or even half-way in between I believe that when you dump the air, the whole weight of the body is on the bumpstops so there's no form of self-levelling and the axle doesn't react in the way it is designed to react."
Lateral stability
Indeed its successful IRTE tilt tests at both 38 and 44-tonnes with Harsh underfloor and Penta front end gear were both done with the suspension inflated, says Johnson, who adds that the underfloor gear configuration offers signficantly superior lateral stability compared with a front-end ram.
As the argument rocks back and forth and we all await a clear steer from the IRTE's working group, it's worth bearing in mind the comments from both Hoyle and Millard on the subject of safe tipping. "Most tipping mishaps are due to bad, or no training," maintains Millard. "And not remembering the first rule of tipping: 'You tip uphill, you tip downhill—but you don't tip on the side of a hill!'" Likewise the assertion from Hoyle: "Choose your site, and use your common sense!"
Until we get the definitive answer to the question "to dump, or not to dumpr, this seems like sound advice.