With braking demand on heavy-duty trucks increasing, steer-axle brakes bear closer examination.

Installing front-axle brakes on an over-the road Class 8 rig was once about as popular as slapping one on an artfully chopped Harley hog.

But just as Harley-Davidsons now wow doctors, lawyers, and stockbrokers as status symbols, steer-axle brakes are gaining acclaim as trucking equipment races to keep pace with changing times

It used to be that truckers felt front brakes lessened their driving control in lock-up situations. And many fleets didn't mind not paying to install and maintain them, either. Besides, proof was lacking to override driver objections or to demonstrate the safety value of having steer-axle brakes on tractor-trailers.

"One reason the idea that front brakes took away control is now defunct," points out Larry Strawhorn, vp-engineering, American Trucking Assns. (ATA), "is that antilock braking systems (ABS) have put steering control back in." He says the old view held enough sway that for years some fleets only installed the "smallest possible" front brakes.

In 1988, federal law finally mandated that steer-axle brakes be installed on heavy trucks. But trends favoring the adoption of front brakes began well before then, and will continue into the foreseeable future.

The impact on fleets of higher fuel prices dating to the '70s and of industry deregulation that hit in the '80s led OEMs to rapidly update the design of Class 8 vehicles, which until then had been evolving at glacial swiftness.

Sleekmobiles Over the last decade, most heavy-duty linehaulers have been reengineered for better aerodynamics, lower weight, and higher horsepower -- all of which has affected vehicle-braking requirements.

The slipperier, lighter, and faster a truck is, the harder its braking system must work. And that makes brakes heat up even more, which can reduce lining life as well as stopping distances.

Starting in March, fleets will witness the arrival of federally mandated ABS. And there's early talk of "brake by wire" controls that promise further improvements. Though they can't put timetables to it, individual engineers indicate electronic braking system (EBS) technology will make mixing disc- and drum-type brakes on the same tractor-trailer more feasible for fleets.

According to ATA's Strawhorn, a combination of ABS and "longer-service" drum-brake packages will better offset panic stops while increasing service intervals. "However," he cautions, "the weight of larger drums on a lightly loaded vehicle can pose a problem when one side gets wet."

Strawhorn says the solution might be to use disc-type brakes, which have a "wiping action" that removes water to restore balance between brakes. "Up till now," he remarks, "this approach hasn't been successful with `mixed marriages'-- vehicles equipped with both disc and drum brakes. But the advent of electronic braking systems may allow getting over the incompatibility issue."

While ABS regs will roll out over the next few years and EBS will eventually come off the drawing board, in 1997 several new or improved steer-axle brakes will be on the market. At least two suppliers are bringing out new air-disc models, while others plan to release revamped S-cam drum products aimed at front-brake applications.

In the U.S., the de facto industry standard has called for 15x4-in. brakes on steer axles and 16.5x7 brakes on drive and trailer axles. Now, according to Prakash Jain, Rockwell International's director of stopping systems, there's a trend to engineering wider foundation brakes to take advantage of the increased lining widths they allow.

However, there are limits to consider. Jain recommends not going bigger than 15x5 on front brakes and using them with wider 16.5x8 drive/ trailer-axle brakes. He says 15x5 front brakes, in lieu of 16.5x5 versions, "provide the flexibility" to use either 22.5- or 19.5-in. wheels.

Dating back to the 1930s, cam brakes are regarded as tried and true by U.S. truckers. Familiarity hasn't bred contempt. Quite the contrary, the ubiquity of the S-cam has made compatibility an immense stumbling block to the acceptance of disc brakes by over-the-road fleets.

That's why discs have long been perceived as niche products, typically found in duty cycles, such as bulkhauling or refuse transfer, characterized by high brake loads and multiple stops.

Proponents list these among the chief advantages of using air-actuated discs in place of drum brakes: little or no fade or side-to-side pull when applying brakes; consistent braking performance on wet or dry surfaces; and low operating costs, due mainly to built-in automatic adjustment, no required preventive maintenance, and relining that can be completed in about the same time it takes for S-cam brakes.

If, as many predict, EBS technology vaporizes the compatibility issue, a slugfest pitting discs against drums may one day erupt. Before EBS can be successfully marketed, FMVSS-121 will have to be revised and fleets will have to embrace the new technology.

At least two major suppliers of disc brakes, AlliedSignal Truck Brake Systems and Rockwell International Automotive, aren't waiting to cash in the EBS chip. According to AlliedSignal, its next-generation disc offering for North American fleets will capitalize on technology developed with Knorr- Bremse, its German joint-venture partner. As for Rockwell, it's the first out of the gate with a brand-new disc brake, having rolled out its DiscPlus just last month.

Other major suppliers, including Eaton Corp. and Spicer Heavy Axle & Brake Div. of Dana Corp., report they'll be concentrating in the immediate future mostly on further improving their front drum-brake offerings.

According to Rockwell, its new DiscPlus model "incorporates the latest technology to offer significant performance and maintenance advantages" over its previous air-disc products. Rockwell also contends that DiscPlus improves stability, fade resistance, and modulation ("feel") compared to current drum-brake offerings.

The first DiscPlus model is the DX195, a 413-mm-dia. steer axle brake that fits 19.5- or 22.5-in. wheels. It will be available from OEMs starting this month. The DX195 will be offered initially in a package with Rockwell 16.5x8 cam Q Plus drive/trailer drum brakes engineered for Class 7 and 8 linehaul operations seeking "extreme-duty" braking performance.

This combo, according to Rockwell, delivers "increased performance, similar to that currently available on passenger cars, at a fraction of the cost" of running air discs all around. "DiscPlus can provide up to 16% shorter stopping-distance performance over conventional drum brakes," states Ken Myers, general manager-brake systems.

Because DiscPlus is designed to eliminate "hysteresis," it is more compatible with both drive-axle cam brakes and antilock braking systems, according to Rockwell's Jain. He says that by improving brake stability, enhancing feel, and reducing fade, DiscPlus can also "reduce the potential for front-wheel lockup in non-ABS situations."

"We haven't promoted air-actuated discs as a front-axle option, but as a braking system worthy of adopting across the board," says David Knight, an AlliedSignal director of brake engineering. "In North America, fleets typically view discs as independent of their front-brake choice."

While Europe is often cited as the model air-disc market, Knight points out that over there technical advantages are weighted more heavily in brake selection, and fleets have much less involvement in spec'ing vehicles.

On this side of the pond, according to Knight, discs have been hampered by a higher initial cost that makes it harder to justify their lifetime cost. "Today's air disc is primarily being sold in specialty niches, including tank, refuse, and motor-coach applications," he says.

"But we're working on lowering the cost barrier," Knight reports. "Our next-generation air disc for trucks will boast a more favorable cost of ownership, making it more acceptable to both OEMs and fleet users. He notes that this new disc brake capitalizes on AlliedSignal's relationship with Knorr-Bremse, its German joint-venture partner.

Knight expects EBS to play a role, too, in increasing the disc share of market. "The potential for greater compatibility through electronic control is also being considered as we develop new air-disc products," he advises.

"Considering the long range," says Paul Pollock, Spicer's chief engineer-vehicle braking systems, "trucking is still a very evolutionary market. But the need to increase braking power will accelerate change. Further reducing stopping distances will mean increasing brake-torque capability on front axles," he continues.

Pollock expects that once ABS is mandatory, fleets will realize that still more can be done with braking, which will help move along other improvements.

Right now, Pollock reports that Spicer is readying a new steer-axle model to join its line of XtraLife II S-cam brakes. Set to be available in the first quarter of this year, the 15x4 front brake will feature a double anchor pin and incorporate "industry standard" shoes and lining blocks.

Wide angle "There's a significant push on for fleets to adopt air-disc brakes," says James Clark, Eaton's product line manager-foundation brakes. "But the U.S. market is not Europe."

By contrast, Clark says Eaton's latest brake packages leverage S-cam technology by offering wider designs. Introduced just last year, a "Wide Brake" option for certain Extended Service (ES) brake models and a "Severe Service" package for steer- and drive-axle brakes on heavy straight trucks offer 23% more wearable lining, according to Clark. He says this lowers operating temperatures enough to greatly lengthen brake life.

"The Wide Brake package should virtually eliminate steer-axle brake relines for the first owner in typical on-highway fleets," Clark states, "and significantly reduce required tractor and trailer maintenance. The Severe Service package has been used to solve brake problems on refuse vehicles," he continues, "including short lining life and tire-bead failures due to excessive heat buildup.

"Eaton has been promoting larger front cam brakes for years," Clark points out. "In fact, we require straight trucks with 12,000-lb. steer axles to be fitted with a 16.5x5 brake vs. the 15.5x4 size that other manufacturers generally allow."

No brake maker denies the value of discs to some fleets, nor that EBS may prove in due time to be the great equalizer. On the other hand, even the disc spinners agree with the drum beaters that the venerable S-cam design still offers room for improving both lifecycle-cost and safety performance.