PBBT can find unsafe trucks

user-gravatar Headshot

According to the Commercial Vehicle Safety Alliance (CVSA), brake problems still account for about half of all safety defects, resulting in heavy vehicles being put out of service (OOS) at roadside inspections. And any procedure or technology that can identify unsafe trucks, such as performance-based brake testing (PBBT), certainly deserves consideration.

While the most common brake defect still is misadjusted brakes, adjustment doesn’t tell the whole story. Brake-chamber pushrod stroke, which is the measurement used to determine adjustment, might say very little about how much brake force actually is being applied to the road. That value could be next to zero, for example, if the brake linings were soaked with oil from a leaky wheel seal, or if a brake is “cammed over.”

PBBT, on the other hand, uses a roller dynamometer, drive-on sensor plate or breakaway torque tester that can measure actual brake force at individual wheels. And since that’s what really stops a vehicle, such testing would go right to the heart of the purpose of roadside inspections – which, presumably, is highway safety.

The final rule on PBBT, which took effect in 2003, established pass/fail criteria for test results. To pass, any vehicle over 10,000 GVW or GCW must have a total brake force equal to 43.5 percent of its weight as tested, which equates to a deceleration rate of 14 ft/sec/sec. However, the rule didn’t require inspectors to use PBBT, didn’t establish pass/fail criteria for side-to-side brake balance, and didn’t require that failing a PBB test be grounds for placing a vehicle OOS. And after a recent close vote by CVSA membership, PBBT still won’t be used for that purpose.

Why on earth not? Largely, it’s the result of lobbying efforts by the American Trucking Associations, which may have some legitimate concerns with PBBT and OOS. For example, weather conditions can affect the accuracy of PBB testers. Moreover, a PBBT inspection can’t identify problems such as a severely worn brake hose that remains functional, or loose or missing components. What officials see is brake force data – not necessarily truck frames, axles, springs and other hardware.

Partner Insights
Information to advance your business from industry suppliers

For those and other reasons, ATA wants PBBT to be used as a screening tool only, coupled with inspectors’ good judgment. But PBB testers, especially dynos, have proven valuable in diagnosing hard-to-find brake problems. “Dynos can expose poor drum/lining fit, contaminated linings, broken park springs, broken or out-of-round drums, kinked air lines and defective valves,” says Dick Radlinski, principal of braking consultants Link-Radlinski and former chief of the Vehicle Stability & Control Division of the National Highway Traffic Safety Administration.

“We’ve found cross-wired ABS sensors on several vehicles,” he adds. “That confuses the ABS control unit, effectively causing the loss of two brakes under hard application, and it’s not something that would trigger the ABS fault light. I have no idea how many trucks are out there like that.

“Plus, PBBT results can back you up in court,” he continues. “If asked, say, in litigation, ‘how do you know your brakes were doing their job?,’ you can point to test results that mean your trucks can stop in accordance with DOT requirements.”

So, PBBT can find faults missed by visual inspection and vise-versa. The obvious answer – at least to me – is to use both. Let inspectors take a few extra minutes and use any means possible to reduce the industry’s embarrassingly high number of brake problems. Whether or not PBBT alone should be used to place vehicles OOS, or just for screening, really doesn’t matter – as long as inspectors have all the tools to identify unsafe trucks and get them off the road.