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Tilting to ards an answer
Page 16
Page 17
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To dump or not to dump? Since tilt tests were conducted as part of a large tipping trailer contract for Lafarge Industries, an intense debate has blown up over whether it's better to exhaust air from a trailer's suspension before discharging the load, or to leave it inflated. an Weatt. finds out what all the fuss is a
• This summer, construction giant Lafarge announced that it was in the market for a substantial fleet of tipping semi-trailers. Not surprisingly, interest from the manufacturers was keen. However, all potential bidders had to submit their products for a tilt test to show they met the IRTE's "Class A" stability standard for tipping on uneven ground. This states that a trailer should be able to tit sideways by 7° with the body fully loaded and raised.
Many manufacturers and operators see this 1992 standard as the definitive test of a tipping trailer's stability, but the IRTE has always said it's not a target to aim for when tipping on uneven ground. It was developed as a minimum to ensure a safety margin within a guide to the specification of more stable tippers. The trials, conducted at the pinetiq test track in Chertsey, involved CT Fruehauf, Wilcox and Stas. All three passed the 70 requirement "with varying degree of panic", according to CT Fruehauf's product manager, tippers, Angus Spooner.
However, not long afterwards a fierce argument blew up over whether it was better to tip with the trailer's air suspension Inflated or dumped. Firmly in the anti-dumping camp was CT Fruehauf, with Spooner declaring: "We sfRcificaeY say 'don't do it'. With the suspension inflated you get a more-or-less even load across the bogie. if you dump the air you can get up to 28 tonnes on the rearmost axle. We don't think that's desirable and as far as possibie any test should replicate what you'd do in real life."
In the pro-corner was Newton Commercials, the UK importer for Belgian trailer maker Stas. Its stance was based on the advice it received from the German axle manufacturer SAF which recommended dumping the air from the trailer's suspension for maximum stability, not least when tilt testing.
Indeed, not long after the tilt tests Sars UK distributor IMS issued an OEM update stating: "SAF strongly recommends that during static tipping use, air is exhausted from the air suspension before tipping. This offers greater stability and also lowering the trailer of course lowers the centre of gravity height"
It went on to say that when "fast discharge" loads like barley were tipped without dumping the air "the levelling valve can't exhaust air from the inflated airbags quick enough, [and] the stored energy is released quickly and bounces the trailer up. This condition is not only potentially very dangerous but also can damage shock absorbers, air bags an body finings." The notice concluded the the ideal design combination "is a [ai dump] valve which is activated when th ram is operated". And IMS confirme that it was acceptable to operate trailer at up to 20km/h "on reasonabl level surfaces (eg in combination wit road pavers) in this condition."
Excessive loads When CM reported on the SAF notice i October it attracted strong feedback— not least from readers who felt that tir ping with the trailer's air suspensio deflated would put excessive loads o the rearmost axle.
Among them was John Dicksor Simpson, former chairman of the IRTE' tipper stability study group. Writing to CM, he said: "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 1.4% and the body front end by a mere 1%.
Tyre deflection
"And because with the air dumped the chasm slopes, only the last axle is likely LO be on its bump stops—the other axles are left floating. That probably means the load imposed On the last axle is 24 tonnes or more. Even if the axle and suspension can recover from that it brings excessive tyre deflection to decrease the attainable tilt angle." He concluded: "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."
Not long after Dickson-Simpson's letter appeared in CM, and in response to a request for further clarification from CT Fruehauf, IMS declared: "Since the issue of our OEM update on this topic we have received comments from several knowledgeable industry sources that require SAF/IMS to review operating recommendations."
That review led to IMS modifying its view on fast discharge loads: it now recommends dumping the air from the suspension during, rather than before tipping: "In ideal circumstances a valve should be fitted that begins to exhaust air from the suspension as the ram starts to extend. In this way the air is exhausted gradually in parallel with the extension of the ram ano discharge of the load, and this prevents the 'trounce' effect, while avoiding unacceptable overload of the rear axle."
Travelling slowly
Moreover, while it originally stated that operators of tipping trailers with SAF running gear working with a Barber Greene could safely dump the air from the suspension, it now says: "When carrying a gradually discharging load, and the unit is travelling slowly eg a road paver discharging asphalt, load transference caused by chassis deflection can cause rear axle overload, so air should be maintained in the suspension system."
Confused? To be fair IMS has reacted to input from the industry by acknowledging that this issue isn't as black and white as was first stated. It now says the OEM bulletin it sent earlier should be disregarded.
Other axle manufacturers have taken their own stance, Tom Hughes, European sales director of Arvin Mentor says: "It's not for us to tell operators what to do. We back the IRTE code all the time because we feel it reflects real-life operations which are that tippers have the suspension inflated. That doesn't load up the back axle."
SO WHERE DOES THIS LEAVE OPERATORS?
Newton Commercial's Andrew Smith says: "When we openly asked axle manufacturers what they recommended they said 'dump the air' so now they've backed away from what they originally said. Historically, operators running large trailers going on to landfill sites have asked for a dump valve—but if it's so strange now, what's changed?"
The whole episode has left Smith feeling somewhat exasperated: "1 don't mind what they say as long as it's best practice, but I did feel they gave that advice In the first place and then backed off. It's been frustrating for us trying to get the definitive answer We had a directive in writing from the [SAF] head of engineering in Germany which was issued to Stas and also communicated directly to Lafarge. They also say they mane other [trailer] manufacturers aware of this. Then the local bulletin a month later changed that. We don't want to get into an argument with anyone—we just want to determine best practice definitively."
Tipper operators are left none the wiser and most will probably continue to tip their trailers with the suspension inflated. However, Smith maintains: "I don't make trailers, I'm a buyer of trailers. I just want people to operate safely. I can't see that there can be two sides to this—the definitive answer is out there: we just believe we had it first time round."
The advice from Dickson-Simpson is to do a comparative test: "The doubters need facts. Record individual axle loads while tipping—air In, and air out of the springs."
Smith also suggests setting up a forum attended by all the trailer manufacturers "so we can thrash it out for once and for all". That sounds like a good idea—indeed CHwould be more than happy to host such an event. Any takers?