Are air discs in your future?

by Lou Smyrlis

TORONTO, Ont. – The advantages of air disc brakes have been touted for some time now, but is it a certainty that these higher performing brakes will become a common spec’ among Canadian carriers and owner/operators?

Without a doubt, according to the three brake manufacturer representatives speaking at a seminar on brake trends at the recent Canadian Fleet Maintenance Seminar.

In Europe, air disc brakes are already the norm, being used in over 80 per cent of heavy trucks.

“The conversion was led by the vehicle OE’s themselves,” said panelist Michael Bryan of ArvinMeritor.

“But unlike the European heavy truck market where the OEM defines vehicle content, North American trucks are defined, largely, by the end user, he said.”

And although the North American end user has to this point proved reticent to invest in air disc brakes, there are now several inescapable market factors pushing a migration towards these brakes.

Chief among these is the U.S. Department of Transportation’s (DoT) goal of reducing heavy vehicle related accidents by 50 per cent by 2010.

To reach that goal the U.S. DoT is looking to significantly narrow the gap between heavy-duty vehicle stopping capabilities and those of passenger cars.

This will require a 30 per cent reduction in stopping distance for heavy trucks – from the current 355 feet at 60 mph to 248 feet. The requirement for a 107-foot reduction in stopping distance is expected to go into force by 2007.

“Since drive and trailer axles are at or near wheel lock today, most of the improvement will have to come from the front axle brakes. And in doing so, we must be aware of driver sensitivities to brake noise, feel, and handling, especially side-to-side pull,” said Bryan.

While the cam brakes currently in wide use can stop a truck travelling at 60 mph in 292 feet, which is 63 feet better than the current requirement, brake experts say they would likely not be able to meet the more stringent stopping distance requirement coming in 2007.

Testing at Bendix Commercial Vehicle Systems found cam brakes able to deliver a 283-foot stopping distance – 35 feet short of the target, according to Ron Gervais, who heads up the Freinmeister Group but who stepped in to represent Bendix at the CFMS panel session.

They may produce as much as 30 per cent variation from the left to the right side.

“This may become more of an issue if larger front cam brakes are applied to meet the proposed shorter stopping distances,” Gervais cautioned.

Air disc brakes on the other hand have a lower brake factor and so are less sensitive. They may produce only about 10 per cent variation from the left to the right side providing “passenger car feel” according to Gervais.

Testing shows they also considerably outperform cam brakes on stopping distance. Trucks equipped with air disc brakes only on the front axle are able to stop within 236 feet or 12 feet better than the proposed stopping standard for 2007.

A truck equipped with air disc brakes on all the axles can stop within 215 feet.

Another distinct benefit of air disc brakes is their superior fade resistance.

This is due to their friction material characteristics as well as the fact that as the pads and rotor heat up, they move closer to one another instead of moving apart, explained Bryan.

Gervais pointed to mountain fade testing conducted in Colorado on the I-70, west of the Eisenhower tunnel. Tractor-trailers equipped with cam and disc brakes were run along an eight-mile long, seven per cent grade that has a downhill truck speed limit of 30 mph.

“Bendix conducted testing by snubbing the brakes from 35 to 30 mph at five feet/sec2 decelerations down the hill. Bendix made approximately 50 to 55 brake applications down the grade and experienced a pressure increase on the cam brakes of 25 PSI, while the air disc brakes experienced only a four PSI increase,” Gervais said. “At the bottom of the grade, Bendix completed an exit lane stop from 45 mph at 12 ft/sec2 and found the cam brakes required 60 to 70 PSI in comparison to 29 to 40 PSI on the air disc brakes.”

By this point the cam brakes had heated to 600F while the air disc brakes had heated to the mid 800F range.

The tractor-trailers were then driven to a back road where their maximum braking capabilities were tested while the brakes were still hot.

“The cam-braked tractor trailer could achieve only 20ft/sec2 deceleration, which is not enough to lock the wheels or to make the ABS system operate. The brakes were severely faded,” said Gervais. “The air disc braked tractor trailer achieved 24 to 25 ft/sec2 deceleration, which is sufficient to lock the wheel and make the ABS system function. The air disc brakes were nearly still as effective as when cold.”

Ease of maintenance is another area where air disc brakes can outperform their more established cousins. Gervais said air disc brake pad relines can be completed in 15 minutes, compared to about 60 minutes for cam brakes. Bendix has also designed a new splinted rotor to reduce rotor replacement time.

The rotor disc is mounted directly to the spleens on the hub and can be removed without first having to take out the caliper, carrier, axle shaft or bearing set. Rotor replacement time: 20 minutes – a 66 per cent reduction in maintenance time over conventional rotor designs.

Steve McCallum, territory service manager for the Greater Toronto area for Roadranger Field Marketing, preached caution when it comes to combining air disc brakes with older braking technologies.

“Although you may have a new high performance tractor, when used with an older trailer, the chances of an extreme brake imbalance are quite possible,” he said. The best strategy is to introduce air disc brakes on only the steer axles, greatly reducing the potential for compatibility problems, he added.

“During hard stops vehicles experience significant load transfer to the steer axles and unloading of the drives. This … creates an environment that allows for greatly increased brake torque at the front axles. Air disc brakes can provide added torque and a variety of other braking characteristics desirable for front axles, such as reduced pull and fade.”

McCallum also pointed out the two brake systems could be adjusted to have the same single stop torque output characteristics. The inferior fade characteristics of a drum brake, for example, can be adjusted through mechanical and friction techniques to boost its performance closer to that of air discs.

“With proper set-up, disc brakes can be compatible with drum brakes … Disc brakes only on the steer axles provide the biggest gain for the dollars spent while giving drivers better control. It’s also a system that is more easily balanced. An all disc tractor can create additional balance problems and although those problems can be overcome, it will create additional fit-up and cost penalties, with minimum additional braking advantages,” McCallum said.

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