MONTREAL, Que. – If your rig crashes because of a mechanical failure, the chances are better than 50/50 that your brake system was partially or completely to blame.

A malfunctioning brake system is also the most dangerous truck mechanical system when not working properly, is the most likely to fail as your equipment ages, and it is the most likely system to suffer a major defect in the weeks after a mandatory vehicle inspection.

These were the main findings of a 1997 study carried out by Ecole Polytechnique’s department of mechanical engineering road safety team, which looked at 195 accidents involving heavy vehicles.

It will be presented as one of the major papers at the International Brake Safety Conference, held this month in Toronto.

The team discovered that in 30 of the accidents, mechanical defects were wholly or partially responsible. And the finger of blame pointed directly at malfunctioning brakes in more than half of those crashes.

Two more conclusions published by the team complete the big picture: First, of all of the truck accidents that happen in a given year, 13.2 per cent are at least partially due to mechanical defects – much higher than the three to five per cent estimate that police reports suggest. Second, 23 of those 30 accident-causing defects found by the study team could have been caught during pre-trip inspections.

What this seems to boil down to (although this is not one of the team’s stated conclusions) is that good pre-trip inspections should be able to catch many brake system defects and could reduce the number of tractor-trailer accidents by perhaps six per cent.

The team recommended truckers should be better-trained in pre-trip inspections, and should use visual stroke indicators to verify the adjustment of push rods.

The study also called for brake tests using dynamometers, rather than relying on visual inspections alone; preventive maintenance inspections after every 12,000 km or 300 hours of service; and a special focus on braking systems at the shop level. And it adds that more work should be done to improve braking systems.

The team also conducted a detailed inspection of 293 heavy vehicles involved in accidents. It tallied the minor and major defects in all of the truck systems and found that brake system defects topped the list: There were 98 minor brake defects and 60 major brake defects, which together accounted for 20.57 percent of all of the defects found in the trucks. The only other system that came close, with 133 defects in all, was the lights and signaling system.

Of the 68 crashed trucks that still had intact braking systems after their accidents, 15 of them did not meet the minimum stopping standards set by the federal government.

The study also looked at the incidence of major defects following mechanical inspections required under Quebec’s Mandatory Mechanical Inspection Program (MMIP). And brake-related problems began to emerge more quickly than problems with other systems.

Within two months of a mechanical inspection, the major defect rate for brakes was 15 per cent. Other systems averaged a rate of about 10 per cent. Three to five months after the mechanical inspections, the percentage of major defects in brake systems climbed to just over 26 per cent. But the major defect rate for other systems remained stable, at or below 10 per cent, in the nine months following a mechanical inspection.

Six to eight months after the mechanical inspections, the major defect rate for brake systems dropped to 11.5 per cent, then climbed back up to nearly 28 per cent at the nine-month milestone.

There was also a very strong relationship between the percentage of major defects in brake systems and vehicle age: Trucks up to four years old had a major defect rate of 8.33 per cent in their braking systems, compared to 6.94 per cent defect rate involving other systems. At five to eight years of age, brake systems accounted for 18.67 per cent of major defects, versus just eight per cent for other systems. n