Bottom Line
Modern Trucking Techniques
Grinding to a halt
Spec'ing friction material is a matter of beating the heat

By Stephen Petit
Special to Land Line

Ask about replacement brake linings and the simplest advice you'll get is to swap in the same material that came with the vehicle from the factory. Presuming you haven't converted your long-haul tractor into a garbage truck, that's a good rule of thumb.

The factory-fitted friction material has been subjected to a battery of research and development work by OEMs and lining suppliers. It was installed to match your axle loads, brake chamber size, slack adjuster length and truck or trailer's intended use.

The truth is, the truck or trailer you have today is not the same vehicle that rolled out of the big barn doors all shiny and new, and your brake linings and drums are Exhibit A.

Brake components - comprising a vital safety system where tolerances are measured by thousandths of an inch - change when they leave the factory. Spiders, cams, hubs, drums and other parts are summoned into service dozens if not hundreds of times a day. They swell, grow, move and rub together.

You may be stressing your brake components in ways you didn't anticipate when you bought your vehicle. Perhaps you've picked up a run through the mountains instead of the flats, or you now haul trees instead of toilet paper.

But still, you should try to match the replacement lining to the original. The manufacturer of the original lining almost certainly makes an original equipment-spec'd replacement.

Several reputable companies manufacture an array of quality linings for the aftermarket. You can also buy linings from importers, some with ties to familiar manufacturers here in North America.

No matter what company's name is on the box, you want a friction material that's rated for your axle weight and formulated to wear and dissipate heat predictably.

When the time comes to replace your linings, you really need to look at how and why they're wearing out. Then you can choose a replacement lining that's suited to the way you work and drive today.

Beating the heat 
Stopping the truck is a byproduct of your brakes' true No. 1 job: to convert the energy of your moving vehicle into heat. Not to get all "Physics 101" on you, but that's what happens when the air-brake system presses the linings against a rotating drum. The friction generates heat, which is absorbed and dissipated by the steel brake drum.

The lining's ability to generate brake torque when it connects with the drum is called the coefficient of friction. Linings with a higher coefficient of friction generate more brake torque because, simply put, they're more "grabby."

That's not necessarily better.

If the coefficient of friction is too high, the brake may be over-aggressive, perform a disproportionate amount of work and run hot.

If the coefficient of friction is too low, the brake may not be grabby enough and cause other brakes on the vehicle to work harder and run hotter. In either case, lining life suffers.

Most linings used on heavy trucks tend to lose their coefficient of friction as the temperature increases, a condition called "heat fade" or "lining fade." How quickly they recover their pre-fade performance depends on the composition of the lining.

Each friction material manufacturer has its own, highly proprietary recipes for cooking up their products, and the choice of these materials and how they're combined, molded, baked and cured makes a big difference in the service life, effectiveness and value of the lining.

The better brake lining suppliers fine-tune the composition of their friction materials for the demands of specific types of work, said Allan Wright, a brake consultant in Hope, British Columbia, Canada, which is home to some of the most challenging mountain roads on the continent.

"Usually, the reason one lining wears longer, recovers faster when it's wet or really hot, or is more expensive is that the manufacturer has invested in research and testing to come up with a formula that produces specific characteristics suited to a particular task," Wright said.

Most linings use the same three basic elements: heat-resistant fiber; resin-based binders to hold the fibers together; and chemicals and fillers designed to either increase or maintain the friction properties of the lining.

When you apply the brakes, heat causes those resin-based binders to turn to liquid and burn up. Without resin to hold them together, some of the fibers in the lining wear away. That exposes a new lining surface and the process of gradual wear starts all over again the next time you stop.

The more heat a lining is subjected to, the faster this process occurs. During heavy braking, an increase of 100 degrees can double the wear rate of the brake lining.

Growth patterns
Another thing happens when you apply the brakes: the lining swells and grows. "Swell" and "growth" are technical terms.

Brake block swell is temporary - the lining expands and thickens when it gets hot and contracts when it returns to an ambient temperature.

Brake block growth, on the other hand, is a permanent expansion that happens when the friction material is first exposed to heat after installation.

The amount of swell and growth depends on the composition of the lining and the process the manufacturer uses to harden, bake and cure their products.

It's different enough among manufacturers that you shouldn't mix lining makes or models on the same truck.

Lining growth can contribute to dragging brakes or imbalance issues after a reline.

"Not many guys who turn wrenches actually drive the truck long enough to get the lining to grow before it goes back to the driver," said Tom Golden, manager of technical services for BrakePro, which manufacturers aftermarket linings at a plant in Toronto.

"The brake mechanic does his work, makes sure the slack adjuster is holding 25-thou of clearance between the lining and the drum, and then hands the keys to the driver.

"Well, the driver goes down the road, gets those new linings nice and hot, and they grow."

If they grow more than 25 one-thousandths of an inch (0.025), the slack adjuster can't keep up and the brakes may drag. Golden said a post-reline road test should include at least 10 brake snubs from 40 mph to 20 mph, and 10 snubs from 20 mph at moderate air pressure, and two stops from 20 mph using full air pressure.

Linings with minimum swell and growth have been sufficiently cured after baking. The longer the curing process, the more stable the lining will tend to be.

It also will be more expensive, said John Younger, general manager of Carlisle's Altec brake shoe remanufacturing business and Canadian friction materials operation.

"When people wonder why one lining costs more than another, the curing process is one answer," Younger said. "The more time in the oven, the more expensive the lining will be."

The payoff, Younger said, is that it's better to cure the lining in the oven than on the truck.

"With an inferior lining, for the first few thousand miles, the vehicle is providing the curing that should have happened back at the plant," Younger said. "The linings can grow and drag and before you know it you're back at the shop complaining about it."

Reading wear
Think of brake linings as your coal mine canary: they can tell you a lot about the overall health of your braking system. That's why you should inspect the old friction material while it's still on the axle and see how it's wearing relative to all the other wheel positions on the vehicle.

If you have no way to compare the temperature of each lining, or the brake force at each wheel position, the wear rate is the next-best indicator that each brake is doing its job.

If each wheel has the same mechanical input, the same linings at proper friction levels, and the air system is functioning well, there should be no noticeable differences in wear among the leading and trailing shoes on both sides of the axle.

Each lining should be wearing evenly around the circumference of the brake assembly and from the inboard to the outboard sides, and there should be no sign of heat-checking.

It's not unusual to see small fractures on the lining surface, but cracks or voids more than 1/16-inch of an inch wide; cracks that exceed 1½ inches in length; cracks across the lining face that extend through the lining edges; cracks that result in missing chunks of lining - all are potential out-of-service conditions according to Commercial Vehicle Safety Alliance inspection guidelines. The reason may be a stretched shoe. A diligent mechanic will measure the arch and shoe with a stretch gauge to determine if they're still in spec, and make sure the lining follows the contour of the shoe.

Excessive wear in the center of the lining indicates an insufficient coefficient of friction for your application - too much grab - and you need to rethink your spec. Tapered wear on the bottom, top or one side can indicate a bell-mouthed drum, a bent spider or worn bushings.

And if you're burning through linings at one wheel position only, that's probably a dragging brake or a brake balance problem.

The most important consideration as you read the wear patterns on your brakes is the total contact area with the drum.

That means front-to-back and side-to-side.

It may sound good to have 70 percent contact over the total length of the lining, but if you have 50 percent contact from side to side, the reality is less than half the surface area of the lining is coming into contact with the brake drum.

It doesn't take much wear to have an impact on your braking system. Under normal conditions, each 10 one-thousandths (0.010) of an inch of lining wear - roughly equivalent to the thickness of five pages in Land Line Magazine - will require an additional ¼ inch of chamber stroke to make drum contact.

Couple that with a drum that's out of round or oversized and you won't get the lining-to-drum contact you need to stop the vehicle.

Buying advice
When you walk into a shop and ask, "How much for a reline?" the person at the counter is going to figure your first priority is a cheap price. Instead, start the conversation by asking what linings he has on hand. Somewhere in that counter-wide bank of catalogs is a lining spec that suits your need for performance, longevity and value.

Find it. When the guy performing your reline sees that you're paying attention to what he's installing on your truck or trailer, he's less likely to reach for the brake lining that, for whatever reason, is his favorite whether it suits you or not.

If you're jobbing out your relines, take time to learn which friction material is right for you and then demand it when you go in and visit the shop, said BrakePro's Golden.

"Don't let your worn-out linings end up in the core bin. Ask to see the wear patterns so you have a clue about how your overall brake system is performing," Golden said. "Be fussy, like your life depends on it, because it does."

Golden said he worries that, with fleets and owner-operators running lean these days, they'll be tempted to cut corners.

"If you make a change at one wheel position, do the same thing at all the others," he said. "Don't mix different friction material specs or material from different manufacturers on the same vehicle because the coefficients of friction and amount of brake growth will vary among them."

The other brakes will have to compensate. They'll work harder, generate more heat, and the linings will wear out faster than you want them to.

"If you make cost-cutting or price your top priority," Golden said, "you're probably sacrificing lining performance and setting yourself up for more downtime due to relines and brake balance issues. A better lining, and one that's designed for your application, will more than pay for itself if you don't have to reline as quickly."

Allan Wright tells a story about touring a plant that produced a range of brake friction material for trucks, buses, and trailers. As he stopped to watch the process where the linings are formed, Wright noticed excess molten friction material dripping over the sides of a mold and onto the floor, like batter from an overfilled waffle iron.

The spillage didn't go to waste.

"At the end of the week, the workers swept up all the leftovers off the shop floor, put them into a pot, mixed up this concoction, poured the material back into a mold, and sold the product as a low-priced block," Wright says.

"No one knew what the ingredients were. Of course, the performance characteristics would have been unpredictable."

The only thing constant about those linings was the bargain-bin price, Wright says. And that people bought them.

Editor's note: Stephen Petit is a journalist who has been covering the trucking industry for 13 years. He can be reached atrollaudio@mac.com.

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