Aug. 1, 2003
Not so long ago, the conventional wisdom on heavy-duty brakes held that air disc brakes (ADB) and electronic brake systems (EBS) would work together to ensure future trucks would meet upcoming stopping-distance requirements that will be significantly shorter than today's. That made sense at one time. But technology and the supplier engineers who drive it forward have a knack for not standing still.

Not so long ago, the conventional wisdom on heavy-duty brakes held that air disc brakes (ADB) and electronic brake systems (EBS) would work together to ensure future trucks would meet upcoming stopping-distance requirements that will be significantly shorter than today's.

That made sense — at one time. But technology and the supplier engineers who drive it forward have a knack for not standing still.

It turns out air discs and electronic braking will still be in the picture. But so will something new — “enhanced” drum brakes and advanced friction materials. Electronic braking will play a role as well but not an all-consuming one.

Welcome to the brave new world of braking, replete with high technology and — good news for fleets — a high degree of choice.

But first those all-important regs. The National Highway Traffic Safety Administration (NHTSA) is considering the reduction of mandated stopping distances for commercial vehicles from the current limit of 355 feet by as much as 30% — which would bring that mark down to just 248 feet.

Whatever new stopping-distance limit NHTSA does mandate will become effective in 2007. That means brake makers have to be ready with product for OEM installation and just as important, explanations for fleet owners of how they will best meet the new regs.

Leaving brand names aside, the general consensus of suppliers appears to be that by ‘07 fleets will be able to spec so-called second-generation air discs brakes or specially enhanced drum brakes and either way end up with vehicles that meet the government's requirements.

Beyond the now familiar antilock braking systems (ABS), for the foreseeable future any more electronic braking will come in the form of new add-on functions controlled by the ABS computer brain.

Getting back to the choice fleets will have in foundation brakes, neither air discs nor S-cam drums are of course new. However, manufactures have vastly redesigned ADBs in recent years to bring their reliability in line with their high performance capability.

That makes the new generation of enhanced drums the new kids on the block. Makers call them wide or high-output or high-performance drums so call them “enhanced” for a uniform generic term. They have been engineered to last longer and stop shorter than standard drums.

A crucial point to bear in mind is that leading brake suppliers suggest there will be a place for drums, discs and electronics in future brakes — and that it will be largely up to fleets to decide what gets applied where.

Here is a look at where top suppliers see brakes heading:


According to Rick Romer, director of electronic products and Tom Martineau, marketing manager for MeritorWABCO, it is best to think of “electronic braking” as a wider term that goes beyond EBS which implies total brake-by-wire systems.

“The concept of electronic braking is not new,” points out Martineau. “First there was ABS, then automatic traction control was added. Now we are bringing out our RSC (Roll Stability Control) system for tractors as well as RSS (Roll Stability Support) for trailers.

Romer says RSC intervenes when critical lateral acceleration thresholds are exceeded to regulate deceleration. MeritorWABCO testing has demonstrated the system's ability to slow the vehicle to give the driver increased ability to avoid a rollover.

Intervention is done electronically as the system takes control as needed of the engine, retarder and drive and trailer brakes via an accelerometer mounted directly to the ECU of the antilock braking system. RSC constantly monitors driving conditions and comes on only if critical lateral acceleration is detected, Romer explains.

He says RSS is an independent trailer system that also lends electronic assistance to reduce rollovers. If a critical acceleration limit is exceeded, the system automatically applies the trailer brakes.

“We feel RSC on tractors and/or RSS on trailers vs. full-blown EBS will deliver benefits at costs appealing to fleets,” says Martineau. He says RSC is already available on some Freightliner models and will be offered by other OEMs while RSS is available directly from MeritorWABCO but has not yet been released to any trailer OEMs.

Further out, Romer and Martineau expect other electronic functions will be added to vehicles using ABS as a platform. These include brake control and monitoring as well as tire-pressure monitoring.

“The new stopping-distance regs have not yet been written,” observes Romer, “but when we can better control brake pressure through these technologies, we will balance brake performance across the different axles and that will improve stopping distances and stability.”

Addressing the stopping-distance regs, Prakash Jain, ArvinMeritor's technical support manager for braking systems, points out that current vehicles have the capability to stop within a band from 260 to 280 ft. — or “a whisker away from the 250 or so feet that may be required” by '07.

“Disc brakes on the front axle used with standard drum brakes on the drive axle are one way to get shorter stopping distances,” says Jain. “Another way is to place a high-output drum brake on the front axle with existing drive and trailer drum brakes.” Either way, he says the idea is to raise braking output at the front axle.

“Fleets know manufacturers will meet the law,” says Jain. “So most fleets will view brakes by their acquisition cost or total maintenance cost and a few will judge them by how much they are on the cutting edge of technology.

“OEMs will tell you that as long as drums can meet standard requirements,” he continues, “they will offer discs optionally to promote technology and provide fleets a choice.”

Jain does allow that since today's ADBs are in wide use in Europe, their production volume has brought their acquisition cost down a bit here.

“The only way the ADB will move forward here is if OEMs choose to make it standard, as happened in Europe. They must either absorb the cost difference with drums or ask fleets to pay it,” Jain reasons.

He says the other approach to shorter stopping distances is high-output drums that are larger (16.5×5 in.) than standard (15×4 in.) ones and use an advanced high-performance lining to help reduce the brake pull that counted against larger drums in the past.

“New lining technology,” Jain reports, “mitigates the left/right variation of 30% typical on drums to just 15%. That compares favorably to the 10% variation common with discs.

“Larger drums, of course, provide longer brake life and that is why some fleets already spec them,” he adds.

“The general perception a few years back was that shorter stopping distance would be met by EBS and air discs,” says Jain. Now we know they can be met with air discs up front and no EBS or with a high-performance drum with advanced lining.”


According to Dennis Losh, director of engineering for Bendix Commercial Vehicle Systems, ABS is an electronic braking system that can serve as a platform to bring advanced braking features to trucks without the need for a “brake by wire” EBS setup as used in Europe. “Adding full electronic controls on top of pneumatic ones would create a braking system that would be cost-prohibitive in this market,” he says.

Kevin Romanchok, Bendix's ABS product line director, says electronic vehicle stability is the first brake enhancement that will be added onto antilock systems.

Losh says to think of such a system as “advanced ABS or ABS with added features” so that buyers know it does more than basic ABS.

The company already offers ABS with SMART ATC (Automatic Traction Control), which helps deliver more traction when a truck is accelerating uphill and more stability when it is being driven into a curve.

Bendix plans to roll out its new RSP (Roll Stability Program) and ESP (Electronic Stability Program) as options on Bendix ABS in the '04-'05 timeframe.”

Romanchok says RSP will help avoid rollovers by electronically engaging all brakes on dry, high-friction surfaces, such as roadways and exit ramps. He says ESP will do the same but also alleviate instability on low coefficient surfaces — ice, that is — by engaging brakes at individual wheels as needed.

Less complex than EBS, such integrated electronic-braking solutions would also be transparent to drivers. “Driving is done the same,” says Romanchok. “The system supplements or aids the driver as needed.”

Romanchok also contends that enhancing ABS rather than moving to EBS will ease maintenance and retain familiarity for fleets, “keeping braking functions all within the same system architecture.”

Losh says this add-on approach will enable pneumatic brakes with ABS to remain the preferred braking system for “years to come.”

However, points out Losh, advances on the ABS front do not impinge on fleets choosing to spec drums or discs as foundation brakes. “The pneumatic system does not have to be changed to use air disc brakes — that is, EBS is not needed,” he notes.

Ron Bailey, Bendix's ADB technical sales manager, says the shorter stopping distances expected from NHTSA will be met with today's air disc brakes or with “upsized” drum brakes.

However, he says increasing drum brake size to meet a 248-ft. stopping distance may reduce driveability compared to using ADBs.

Bailey points out that ADBs have changed substantially from the ones sold here 20 years ago. A key difference is a lower degree of hysteresis that makes them more compatible with drum brakes on the same vehicle.

Robustness and durability have been improved, too, thanks to a design he says allows the disc calipers “to remain free to float.” Bailey reports that recent Bendix testing shows that its SB7000 air discs, developed with its parent firm Knorr-Bremse, lasted 87% longer than drums in 300 miles of cross-country driving with a series of standard braking applications. “Depending on application,” he notes, “disc lining life can be 30 to 80% longer than that of drums. And it's easy to replace disc pads in just 15 minutes.”

According to Bailey, fleets that are most interested in ADBs today include emergency vehicles as well as hazmat tankers.

Bailey expects the price differential between discs and drums to narrow as disc volume increases. “Eventually,” he notes, “price will decline with volume and North American production.”

Dana Spicer

According to Jim Clark, chief engineer-brakes for Dana Corp., electronic braking is a catch-all term that covers everything from brake by wire, which means an electrical signal instead of air pressure activates the brakes, to ABS with various enhancements.

“What we are seeing,” he says, “is the ABS controller getting ‘smarter’ so more functions could eventually be added, such as anti-rollover systems, brake-wear and temperature monitoring and tire-pressure monitoring. It is possible that ABS will be enhanced to the point it becomes a true chassis control system incorporating these functions and interacting with the electronic controls of the engine and transmission.”

As for the new stopping rules coming out, Clark says brake by wire will not likely be required, nor will fleets have to replace drums with discs.

However, there will be change. “For shorter stopping distances, the steer axle will need more braking capacity because the weight is transferred there,” he explains.

“Steer axle brake torque will have to more than approximately double to meet the anticipated stopping distance,” Clark states. “But this can be done with larger drums and new linings or with air disc brakes.”

Clark calls the ADB the “more elegant but more expensive solution” to shorter stopping. “The disc does have a nice feature — no fade. It does not lose torque as it heats up. But its price will never get as low as a drum and it will not last as long as newer wide drums.

“The drum brake, on the other hand, is going through significant changes,” Clark continues. “Longevity is being increased dramatically thanks to larger sizes being offered, as well as new longer-life lining materials.

“A combination like Dana Spicer ES 16.5×7 steer and new 16.5×8.625 drive-axle brakes with our ES450 lining can provide double the life of earlier drum brakes — which in some cases would mean the first owner of a truck would no longer have to do a brake job.

“And this brake package would also produce better stops while fading less,” he adds. “So it is one concept we are exploring to meet the stopping distance expected in ‘07.”

As for discs, Clark expects them to continue to appeal to fleets seeking a high degree of safety performance as well as those seeking to attract d rivers by offering the most technologically advanced features on their trucks.

“In the end, stopping distances have to be met,” Clark adds. “But we know what also excites fleets is getting brakes with the lowest initial and lowest maintenance cost.”


Randy Petresh, vp-technical services for Haldex, says that most vehicle applications will meet the expected changes in the stopping regs through foundation brake/wheel end spec changes.

“We would expect an increase in the size of steer axle foundation brakes, especially on those vehicles now using very small 15×4 brakes,” he says.

“In cases where larger steer brakes, brake power changes and higher friction linings will not be adequate for compliance, OEMs may be forced to derate various axle configurations,” he advises.

Petresh does not believe the new regs will create more demand for discs in most cases. “The primary solution will be ‘enhanced’ drum brakes that are larger or wider as well as brake power changes and use of higher linings,” he states.

“However, there may be some applications, i.e. high C.G. straight trucks, that may not be able to meet the new regs with enhanced drum brakes. And fleets which place a greater premium on braking performance may still prefer to spec air disc brakes. These include fire trucks, refuse haulers, bulk and hazmat haulers and other special applications.”

Petresh says the industry will be changing friction material specs to increase foundation brake torque output but will do so using materials which are already available. “Haldex is not currently developing any advanced friction materials nor have we been asked to,” he reports.

According to Petresh, Haldex does expect certain electronic functions, such as rollover protection, will be incorporated into current ABS setups. “Electronic enhancements to ABS will continue to be developed and implemented to provide increased value and features to the user. And Haldex is developing other electronic brake system features, such as air drying and air treatment systems, which will become available in the near future.”

Judging by what these brake experts expect, trucks are going to stop better in years to come and with technologies with which fleets are already familiar.

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