Despite mandatory antilock brake systems and automatic slack adjusters, the trucking community is still having trouble keeping its brakes up to snuff. According to the Commercial Vehicle Safety Alliance, brake problems still account for about half of all safety defects resulting in heavy vehicles being put out of service at roadside inspections.
The most common violation remains out-of-adjustment brakes, as determined by measuring brakes-applied, brake-chamber pushrod stroke. But this measurement doesn’t determine how much brake force is actually being applied to the road. Even if pushrod stroke is within acceptable limits, a brake system could still suffer from an uncovered balance or stability condition. In other words, the industry’s brake problems could be worse than we know.
That’s why there’s federal rulemaking pending that would require the use of performance-based brake testing. With this type of testing – using a roller dynamometer or drive-on sensor plate – actual brake force can be measured at individual wheels and compared, side-to-side and front-to-rear. And such a test can be performed in a fraction of the time it takes to measure the 10 pushrods on a typical tractor-trailer. Moreover, with the right equipment, it can be performed regularly by fleets, leaving CVSA inspectors no brake problems to find.
However, action on the proposed rulemaking is proceeding at a true, legislative (read: slow) pace. “I can’t understand why [PBBT] hasn’t been adopted already,” says Dick Radlinski, president of braking consultants Radlinski & Associates, East Liberty, Ohio, and former chief of the Crash Avoidance Research Branch of the National Highway Traffic Safety Administration. “It’s not controversial at all,” he insists. “This is an easy one.”
Radlinski is firmly convinced that PBBT, particularly with a brake dynamometer, will improve highway safety, and I’ve got to believe him – he knows more about truck brakes than I ever will.
“Even on brand new vehicles,” Radlinski says, “undetected problems can significantly compromise braking performance. We recently had a new, six-brake dump truck in here because of a driver complaint about insufficient braking. Statically, everything checked out fine, and there was no ABS light. But the dyno showed that two ABS sensors were cross-wired. This confused the ABS control unit, effectively causing the loss of two wheels’ braking under hard application. The wiring harness was actually mislabeled…I have no idea how many trucks are out there like that.”
The problem, explains Radlinski, is that ABS self-diagnostics can only perform static tests, such as checking for open or shorted circuits in cables to sensors and valves. It can perform a “chuff” test, to check that all valves are operating, and it can store static-malfunction codes, which can be blinked out on the fault lamp, or read with a handheld scan tool or PC.
But the self-diagnostics can’t verify component locations, check wheel speed-sensor outputs or detect mechanical damage. For example, the ABS electronic control unit (ECU) can’t verify air or electrical circuit routing, and can’t detect installation problems, such as left/right or front/rear reversals.
As long as the electrical cables are connected, the ECU turns off the failure-warning lamp at the end of the self-test, which is automatically initiated whenever a vehicle is started.
In addition to finding “no-light” ABS faults, concludes Radlinski, a good brake dyno can measure brake drag, rolling resistance, valve crack pressures and braking-force variations due to drum ovality. It can find everything from kinked air lines to cracked drums. And it has tremendous potential to reduce accidents.
Like Dick says, this is an easy one.
Paul Richards is editor of Commercial Carrier Journal. E-mail [email protected].
