Master Stroke

On the fifth of September, truck inspectors from San Diego to St. John’s conducted Operation Air Brake, a well-publicized brake-inspection blitz that again had the enforcement community voicing concerns about misadjusted binders.

Big news? Hardly. Brake-adjustment maladies have been the chief cause of out-of-service violations for years. Depending on which target is the most convenient at the time, the blame has been laid on lazy drivers, spotty training, overzealous inspectors, and the design of the brake equipment itself.

When it comes to looking for solutions, however, efforts often come up short. Except for one: lengthen the stroke of the spring brake actuator.

A typical Type 30 brake chamber, common on trailers, has a 2-1/2-inch maximum pushrod stroke. A normal brake application of 100 psi will impose about an inch and a half of stroke even with the unit in full adjustment. However, inspectors say two inches of stroke is considered the point at which the brake should be readjusted. That leaves you a half-inch to play with, hardly a comfortable margin and one that causes just about every trucker out there to approach roadside inspections-and mountain passes-with less than full confidence.

There can’t be anybody out there who wouldn’t like a little more brake performance and some leeway in the demands of brake adjustment, like an additional half inch of useable stroke. That’s what a long-stroke brake chamber delivers, and it’s becoming an increasingly standard spec on air-braked trucks and trailers.

A long-stroke chamber design has a rated stroke greater than a conventional chamber by a minimum of a quarter-inch, and as much as three quarters of an inch. It will provide higher pushrod force at or near the recommended re-adjust position than a conventional chamber, as well as a higher stroke margin for CVSA (Commercial Vehicle Safety Alliance) inspection out-of-service criteria. Most often it delivers a stroke advantage of a half-inch, meaning the 2-1/2-inch maximum stroke of a Type 30 brake chamber becomes three inches.

Long-stroke chambers offer an additional half-inch of pushrod stroke-good insurance against variations in measurement techniques, brake fade, and drum expansion. Among the causes of these problems is temperature, “a huge factor in loss of braking effectiveness,” says brake consultant Dick Radlinski, a former U.S. Dept. of Transportation research engineer. At the DOT, Radlinski did tests on a 16.5-by-7-inch full-scale brake to measure braking force output (torque) as a function of adjustment, at a number of different temperatures-from 200°F to 600°F. He says he’s seen 1000°F brake-drum temperatures at the bottom of a mountain grade.

Even at a “relatively cool” 200°, when a brake’s stroke gets out to that critical two-inch limit, the amount of torque available has dropped to 85% due to lining fade and drum expansion. Push that to a temperature of 600° with the same stroke, and you’ve got a brake working at only 50% of its capacity.

“On-road tests of stopping distance show what this really means,” Radlinski explains. “If you start out with a cool brake that’s in adjustment, you could stop a fully loaded truck [80,000-pound GVW] going 60 mph in about 342 feet. If the brake stroke is at the legal limit, even at that same temperature, the distance becomes 458 feet, or an increase of 34%.

“However, with hot brakes, the ‘fully adjusted’ distance is 393 feet, and the ‘at legal stroke limit’ distance jumps up 76%, to 692 feet,” he says. “So, even though the brake may still may be technically in adjustment, that heat has actually reduced braking force by 50%, and doubled the stopping distance.”

The other problem caused by a hot brake drum is an increase in stroke, due to natural drum expansion. At 600°, even with the brake fully adjusted, tests have shown that the stroke can increase about six tenths of an inch. So, if the brake was originally at 1-1/2 inches of stroke, you’d then be up to slightly above the two-inch legal limit just from the heat. And you’d fail an inspection.

“Heavy trucks need all the help they can get in terms of improving thermal capacity, and improving performance when the brakes heat up,” Radlinski concludes. “All the more reason to put more stroke in the brake chambers, to handle the increased stroke that inevitably occurs as the drum expands.”

While there was some confusion and controversy about long-stroke brakes when the idea was hatched in the early 1990s, all of that has since died down.

A couple of years ago, truck owners were given a clear signal by the U.S. National Highway Traffic Safety Administration (NHTSA) that retrofitting vehicles with long-stroke brakes is acceptable without any re-sizing of the braking system’s air tank. The brakes on non-drive axles also generally require no modification when retrofitting long-stroke chambers.
However, on drive axles, brake-chamber mounting brackets may need to be “reclocked” in order to avoid clearance problems between the slack adjuster and axle housing. Generally, if you can fit a Type 36 chamber on your vehicle, you’re OK with a Type 30 long-stroke.

And how about making sure that inspectors know they’re dealing with a long-stroke brake?

To help ease identification, a three-inch long-stroke chamber will have, in the case of spring-brake chambers, square port bosses into the flange case where the air lines connect to it. This will enable an inspector to quickly identify whether it’s a standard or long-stroke brake.

In addition, three-inch long-stroke service chambers will have a half-inch-high square embossment raised from the top (under the centre port) of the pressure cap, and all chambers will have a trapezoidal stroke secured at the clamp bend bolts. These will further help inspectors to identify what type brake chamber they’re measuring.

That’s important as long-strokes become prevalent on truck and trailer models. And given the performance benefits, we should only see more of them.