A step ahead y:
Page 16
Page 17
If you've noticed an error in this article please click here to report it so we can fix it.
on ston times
Wabco's electronic brake actuation system (EBS) is fitted to Newport Pagnellbased Roy Lawrence's new Wisbech Bulk Systems step-frame Maxie semi-trailer, which also sports Discos disc-braked axle/suspension modules.
Discos has been developed jointly by DaimlerChrysler and Hendrickson using Daimler's disc brake engineering expertise and Hendrickson's vast knowledge of trailer suspensions.
For the time being the Wabco EBS package is only compatible with the Actros. An electrical signal actuates the trailer brakes so they can be actuated more quickly than with air, although air is retained to apply the brakes.
The EBS/Discos trailer has been in service since last September. It's hauled by a Mercedes-Benz Actros 1843 430hp 4x2 tractor which has EBS and discs all round.
Lawrence is more than satisfied with its performance so far: in an emergency I reckon my truck will stop more quickly than a Range Rover," he says.
Foolhardy
He argues that truck and trailer braking performance has to improve, not least because of the foolhardy behaviour of car drivers on our overcrowded roads. Truckers allow enough braking distance between themselves and the vehicle in front so they can stop in an emergency— only to have motorists nip into the space.
"That means your braking distance is cut from 30 or 40 metres to just five metres, and the cars in front of you will be able to stop far more quickly than you can," says Lawrence. "The performance of your brakes has to be just as good as theirs, although you're in a 38-tonner."
Lawrence acquired his latest trailer as a consequence of his experience running an earlier WBS step-Frame triaxle fitted with Discos, but without EBS, which was also married to an Actros 1843 4x2. This one has been in service for almost 18 months. "Its performance is good but the trailer equipped with EBS provides a slightly quicker braking response," he says. "And we've had no problems with the braking being biased."
Hauling bulk grain and animal feed nationwide for Fermac, almost always at full maximum gross, the Discos-only trailer, like its stablemate, is definitely saving Lawrence money. Aside from safety checks, he reckons he will only have to take it off the road once every two-and-a-half years.
"There are no slack aqiusters to worry about, and no S-cams to grease," he points out. "So far it's come off the road for half a day so Hendrickson could report on it, and at that stage it had covered 135,000km. The report told me we're on for 300,000km for the pads."
However, regular driver Mickey Dixon says: "We may have to move the rear pads to the front to equalise the wear. The front axle gets less wear because it's a lift axle, and overall there's a braking bias towards the rear axle."
"You're talking about 4mm of wear a year on the pads, so they don't wear that quickly," says Lawrence. "I'll be changing them when I've got about 2mm of wear left."
Dixon reckons that the discs should last for around 600,000. "We've already put it in for its first test, and there's no problem with them," says Lawrence. There's no evidence of scoring or cracking, and Dixon and Lawrence believe that Discos—especially when used with EBS—should help extend the life of the tractor unit's brakes because they're not having to work so hard to compensate for the inadequacies of trailer drum brakes. "And I think we may be improving tractor unit tyre life on an older Actros," Lawrence adds.
Discos marketing manager Terry Green says some customers are now achieving a trailer pad life approaching 400,000km, so 600,000km could be feasible. And he believes that the discs could last for all of 1.2 million kilometres.
Lawrence's interest in the potential of trailer disc brakes was sparked off when he went to an Actros ride-and-drive event at the Millbrook proving ground in Bedfordshire. It was there he got talking to representatives from Hendrickson.
"Disc brakes looked like the right idea, and we were told that a trailer fitted with a Discos package could go for 300,000km without needing replacement pads; a claim which looks like being borne out," he says. "However, they added that there would be a cost—about £1,500-i1,800 more than drums."
They also quoted a maintenance cost of 21,432 over a million kilometres with three services, compared with a cost of 22,567 and 12 servicing stops with drum brakes over the same distance. Some operators will spend more, depending on their operation. Given the maintenance savings that can be achieved, Lawrence was willing to pay a premium for discs.
He also runs a WBS taper-frame trailer with drum brakes hauled by a 420hp Volvo FH12. "We have to take that trailer off the road twice a year to reline the brakes, and it costs us 1700 a time," he points out, "so with discs you see your money back very quickly."
Disc brake pads have a long life, and if EBS results in the brakes being applied more evenly, their existence could be prolonged even further. But they won't last for ever, and replacement is likely to cost about 2150 per axle at current prices, says Lawrence. He believes prices will come down as disc brakes become more popular and independent parts suppliers start targeting the aftermarket.
Brake shoes
If you buy from independent suppliers you'll have to make sure the pads last as long as OE products, Dixon warns: it's a bit like brake shoes. You get some poor-quality linings that tear the drum out because they're too hard and glaze, or wear out quickly as they're too soft." He estimates that replacement discs will cost around 2300 apiece.
Lawrence says: "We haven't had much success with drum brakes, wear-wise. We used to do a lot of work in the Welsh hills. We've had several Mercs over the years, and the exhaust brake on some of the earlier models was inefficient so we had plenty of trouble with overheating service brakes going down hills.
"No matter how you drove, if you went down the box you still had to use your brakes to stop the engine from over-revving," he adds. "The brakes were getting very hot, and as a conse quence we were getting bearing trouble. The S-cam bushes had to be changed every 12 months, and all-in-all it was a lot of work."
Dixon says: "When you think about it, drum brake efficiency hasn't improved for the past 20 years, yet the power out put of the tractor units we run has gone from 290hp to 430hp. And a lot less effort is required when you're applying disc brakes—just resting your foot on the pedal will do. There's none of this grabbing hold of the steering wheel and pushing down with your foot as hard as you can. You feel safer, and you feel you've got a lot in reserve."
Lawrence predicts that within the next 10 years "trailer disc brakes will become as common as air suspension".