The following is the second of a two part series on brakes.TORONTO, Ont. - There is irony in the fact that brakes can be at their worst when components are newest. Do a complete brake job - replace an...
SHOELESS: Top shoes can wear down befroe their counterparts.
The following is the second of a two part series on brakes.
TORONTO, Ont. – There is irony in the fact that brakes can be at their worst when components are newest. Do a complete brake job – replace anything worn, anything suspect – and your equipment faces a host of new stopping problems.
If one brake is more aggressive than another is, the newly created imbalance can swing your trailer into oncoming traffic. For that matter, the wrong replacement parts can leave you without any stopping power at all.
1. The burnishing myth
Brakes aren’t working at their full capacity unless the entire contact surface of a brake shoe conforms to the shape of a drum. But only part of the shoe will make contact if a spider is bent or welded off center. The lining itself might not even be ground to the proper shape.
Engineers insist friction material will naturally conform to the required shape after 500 miles of use in a process known as “burnishing” (basically, the rubbing away of any high spots). But Dale Holman of Georgetown, Ontario’s Truck Watch Services, and Ontario Trucking Association technical guru Rolf VanderZwaag question that figure.
“Any suggestion that brake burnishing takes place within 500 miles is highly suspect,” VanderZwaag says. According to one testing standard, brakes would have to be used for more than 200 hard stops at temperatures between 350 and 500 Fahrenheit, the latter of which would be the equivalent to trying to stop on a mountain grade.
“A good driver is never going to reach this temperature,” says VanderZwaag.
“We have 17,000 km of use right here. Does it look like the brakes have been burnished?”
Holman asks, pointing to a picture of a brake shoe that still has visible machining marks, despite its supposed service in stopping a B-train.
But there is a solution, specifically brake lathes that are commonly used in Europe.
Brake assemblies are simply attached to wheel ends, and the lathe scrapes away uneven friction material until the shoe conforms to the shape of the drum. Holman currently sells the equipment and the service, but it’s only now beginning to emerge in North America.
2. Mixed linings
A mixture of different brake linings could cause problems with brake balance. Aggressive brake shoes will wear away more quickly than their counterparts; tires on the associated wheel end will develop flat spots; and the vehicle will be twisted and pulled by the different forces, leading to potential cracks in the frame and suspension.
According to the Technology and Maintenance Council, aftermarket materials can offer anywhere between 45,000 and 74,000 lb-ft of stopping force. And you can’t buy the exact friction material used by an Original Equipment Manufacturer – only an equivalent.
“Don’t let the parts man buy lining from wherever it’s on sale,” VanderZwaag warns. And ensure similar linings are used on every wheel end. The class of the friction material is shown in letters on the far right side of the edge code.
3. All valves aren’t equal
Holman pulls out two air valves that look virtually identical, right down to the blue paint. The fittings for airlines are even in the same position.
But a metal tag on one of them reads “RV000,” indicating a pilot valve that offers a steady supply of air as soon as brakes are applied. The other reads “RV040,” signifying the four psi crack pressure of a relay valve.
That valve will need about five psi of air before it overcomes the resistance of an internal spring and sends any air to trailer brakes.
“The crack pressures create balanced timing and delivery,” Holman says, explaining the reason for different pressures. “The whole idea is to get simultaneous air delivery and simultaneous brake application (at every wheel end). And by putting the wrong valve in, by putting a few fittings in, by doing all kinds of things, you impede balanced braking.”
If mechanics are swapping components, ask to compare the old valves with their newer counterparts. Sometimes the wrong choice is obvious. Holman shows a favourite picture that he has distributed for years to make his point. It’s of something a wannabe plumber jammed together to get the wrong component to fit in place, and basically it’s made of a variety of brass fittings.
4. Shielded views
Holman isn’t a fan of dust shields, which can obscure views of brake components. He often sees shields on well-worn trucks that have never been pulled off for inspections. And small openings offer just a partial view of only one of an assembly’s shoes. “This thing could fracture and break off and you won’t see it,” he says.
Consider that 60 per cent of the brake jobs he performs for FedEx Ground are because the top shoe has worn down before its counterpart. That wouldn’t be seen through an inspection hole. The solution is simple. As long as you’re prepared to actually inspect the brakes, pull off the shields.