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BANGING THE DRUM

25th August 2005, Page 64
25th August 2005
Page 64
Page 65
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Page 64, 25th August 2005 — BANGING THE DRUM
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Which of the following most accurately describes the problem?

Trailer drum brakes are back in fashion.

Sharon Clancy examines why disc brakes have often failed to live up to expectations.

The arrival of disc brakes on trailer axles in the 1990s seemed like a utopian dream for operators promising long-life, low maintenance and greater safety.

Trailer axle manufacturers liked disc brakes too because the longer life complemented their million-kilometre, maintenance-free axle designs. Other operators took a long-term view, believing disc-braked trailers would have higher residual values.

By Mk axle manufacturers were reporting a 70:30 ratio in favour of discbraked trailer axles.This rush to embrace trailer disc brakes was a UK phenomenon,says BPW sales director Peter Wray, and now the honeymoon seems to be over, with many operators reverting to drums on their trailers.

So why are they abandoning a technology that promises lower operating costs and greater safety? Wray explains: "There have been problems on early models with pad and disc life, and a lack of understanding about servicing requirements. Not only did costs not come down as expected, on some operations they became unacceptably high.

Making the switch

He reports that the drum-disc ratio has moved back to a 55:45 split in favour of drums not least because it counts some major trailer leasing fleets among its customers, including Hill Hire and TIPTrailer Services, both of which have switched back to drums.

TIP marketing leader Damien Wall says: "Before we specified our latest trailers we did an extensive analysis of cost-benefits. For dryfreight trailers there's no doubt that drum brakes are the most cost-effective solution. For most rental contracts we simply do not have enough reliable information on what will be pulling the trailer and the type of cargo it will be carrying."

The axle and brake manufacturers say trailer brake specifications are now more closely linked to operational patterns."Offroad operations and those where the trailer owner cannot predict what type of tractor will be pulling the trailer are prime candidates for switching to drum brakes," says Tom Hughes, ArvinMeri tor's European sales director.

"Disc brakes are ideal if you have a dedicated tractor-trailer fleet tramping up and down the motorway," says Richard Veasey, fleet sales manager of Knorr-Bremse: "Used with an Electronic Braking System (EBS) the braking can be balanced and the brakes will give long service life, provided they are serviced regularly."

But for trailers going off-road, or those that are pulled by a drum braked tractor unit, it's a different story.

Off-road duties invariably mean more dirt and dust. If brake seals are damaged, that dust can work its way into the brake assembly where it becomes a grinding paste, accelerating wear rates,There have also been reports that the wheel baths that trailers pass through before leaving a site contributed to early disc-cracking problems as cold water hit hot discs.

"Although the latest generation of disc brakes are more robust and reliable, we think there will be a demand for drum brakes for the foreseeable future," says Hugh es."Operators will be cautious until disc brakes are proven in more arduous applications."

Lower whole-life costs were a major factor in persuading operators to accept discs and even pay a premium for them.Typical lives of 400,000km for pads and 800,000km for rotors were quoted, but these figures have proved optimistic for many operators. But while manufacturers might indulge in a bit of marketing hype they are not in the business of actually misleading operators, so why this discrepancy?

First-generation disc brakes certainly had some design issues with factors such as heat dissipation. Brake balance, pressure and temperature all affect brake wear rates and they all contributed to reports of poor pad and rotor wear on that first generation of trailer disc brakes.

Product developments

Heat has been the big enemy for disc brake life. Drum brakes operate at between 10 and 200°C; disc brakes at between 200 and 350°C. Disc brakes have fewer moving parts than drum brakes and they are less prone to fade from overheating. However, thermal stress can lead to shortened pad life and premature rotor cracking.

Rotor size was also an early issue. Smaller (380min) rotors made it possible to change pads without removing the hub, but that compact size meant there was also less metal to dissipate the heat.ArvinMeritor has always maintained that 380mm rotors are perfectly adequate provided the design ensures enough cooling air gets into the wheel envelope. Other manufacturers preferred 430mm rotors on 22.5in wheels.

A spokesman for friction material manufacturer Ferodo points out that trailer brake wear problems can also be caused by mismatched brake sizes. Trucks are invariably on 22.5in wheels, whereas trailers can be on 19.5in or 22.5in," he says.

"The smaller trailer brake will get hotter, simply because there is less area over which the heat can dissipate. It's a major reason why operators on heavy-duty operations such as tipping and off-road work have wear problems with disc brakes."

The thorny issue of tractor-trailer compatibility is also implicated in high rates of discbrake wear. Wray says: "Discs are not as tolerant towards imbalance as drums, so unless EBS is fitted the tendency for trailer brakes to do more than their fair share of the work will increase wear rates.Drum brakes absorb a lot of the energy through mechanical movement; on disc brakes there are fewer moving parts so less energy is absorbed.

Bernard Clancy, sales director at Haldex, says operating disc-braked trailers without EBS is open to abuse: !t is easy to put a greater proportion of the braking demand on the trailer. On some operations that will lead to high wear rates."

BPW says one solution to the compatibility problem is to delay the application of the trailer brakes until a higher pressure has been reached, m aking the tractor brakes do more of the work.This is easier to do with ERS. which can be specified on drum-braked trailers Brake actuation is faster with EBS so stopping distances will be shorter— although not as short as on a disc-braked trailer, all things being equal. Each axle brakes its own load using the available adhesion between road and tyre. EBS can help cure drum-brake imbalance problems— the brake can be made to do some work, even when hot and beginning to fade.

EBS offers better distribution of braking effort, provided the tractor unit is also equipped with this technology It is easier to adjust the braking predominance if wear is an issue, or if the operation changes One reason trailer manufacturers have adopted EBS as standard is that it is easy for them to set up the braking system at the end of the production line: they simply programme the unit with the correct data.

For the same reason EBS systems are relatively simple to retrofit to a drum-braked trailer because the main components are the emergency relay valve and the trailer modulator. There is little doubt that a lot of the problems with trailer disc brakes have been caused by lack of knowledge on correct servicing.Trailer brakes should be inspected every six weeks, or at intervals agreed with the Vehicle and Operator Services Agency.

Product developments

Drum brakes are more tolerant to less-thanideal maintenance, according to the axle manufacturers "Inspection procedures are more critical with disc brakes," says Veasey."Failurc to pick up seal damage, for example,can result in a costly bill for caliper replacement."

Major replacement parts for disc brakes are pricier than for drums. A seal costs pennies compared with £300-plus for a caliper. Some operators say you can kit out a trailer with a whole set of drums for the price of one caliper.

Although drum brake development is not a hotbed of activity, manufacturers continue to support it and designs are tweaked periodi cally to incorporate relevant developments. All the major trailer axle manufacturers offer a drum-brake alternative, apart from DaimlerChrysler whose trailer axle design is based on using identical components, where possible, to the truck axle range.

Bedding in of drum brake linings is not always necessary. BPW, for example, has eliminated the need to machine brake drums to guarantee maximum braking efficiency by grinding the brake shoe radius once the lining has been attached to the drum.

ArvinMeritor continues to invest in drumbrake design. Its latest LM2 drum-brake axle has longer service intervals thanks to improvements in the cam rollers and bearing pack.

Drum brakes are also benefiting from continuing research into friction materials Although this research is driven by disc pads, there are often spin-offs for drum linings..


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