Opinion: New Stopping Distances, New Technology

By James Clark

Director of Engineering

TMD Friction Inc.

This Opinion piece appears in the April 23 print edition of Transport Topics. Click here to subscribe today.

I’m proud to be a member of the North American heavy-truck supplier community. Together, we’ve made significant strides in Class 8 truck brake safety, but not since the introduction of non-asbestos brake linings more than 20 years ago has the heavy-duty industry seen technological changes to brakes as significant as those we are experiencing today.

Two new products in particular — high-torque drum brakes for the new reduced stopping distance regulations and air disc brakes — have hit U.S. roads recently, and that means it’s no longer “business as usual” when it comes to selecting replacement linings. Every maintenance manager needs to understand these new technologies and to know how to make sure he or she has the information needed to avoid costly mistakes in selecting replacements.

Over the years, we’ve seen the development of brake-related technologies ranging from automatic slack adjusters to anti-lock braking systems. In the normal way of things, new technologies are introduced to the market by safety-conscious suppliers, safety-conscious truck manufacturers and safety-conscious fleets. Once they have proved reliable, standards are established and regulations created.

That’s where we are now with aftermarket brake linings — fleets are using them, but we lack regulations to standardize and control their performance. That means there are no effective safeguards to ensure that an aftermarket lining can stop a Class 8 vehicle within the same distance as the original-equipment linings.

A new truck’s brakes must pass two legal requirements as defined by Federal Motor Carrier Safety Standard No. 121 — a brake dynamometer certification and a full-vehicle stopping-distance test. This regulation includes torque output performance, fade and recovery characteristics and fully loaded stopping distance tests from 60 mph.

The latest stopping distance reduction has prompted the braking industry to make a significant effort to create new brakes, linings and vehicle air systems. But if you take a new truck out of the factory and down the road, the only regulation it has to meet at present is a rarely enforced Federal Motor Carrier Safety Administration in-service stopping distance rule, and in my 35 years in this industry, I have never heard of this test being done on an actual in-service truck as part of a safety check.

When a fleet maintenance manager goes to the parts counter looking for replacement linings, he usually focuses on price — and the cheapest brake linings are made in China or India and never have undergone a real brake test.

Now that we have higher-performance brakes for shorter stopping distances, the potential degradation of the vehicle’s stopping performance through the use of bad linings is even greater than before.

However, there is at least one system to help fleets pick aftermarket brake linings: a long-established recommended practice dubbed “RP 628, Aftermarket Brake Lining Classification,” created and tended to by the Technology & Maintenance Council of American Trucking Associations.

RP 628 lists brake linings that have passed the Federal Motor Vehicle Safety Standards 121 dynamometer test, at the very least, and it gives their torque output, fade rating and other pertinent information. It’s not widely known by fleets but is available for free online from two websites:

• www.trucking.org/Federation/

Councils/TMC/Documents/

From the list of documents on this Web page, click on Aftermarket Brake Lining Report 2012, dated 3-21-2012.

• www.pri-network.org/PRI/Brake-Lining-Program.id.29.htm

Another problem with the introduction of air disc brakes in large numbers in North America is one that ultimately could create problems for vehicle operators. Don’t get me wrong — air disc brakes are great. They have more consistent torque output, reduced fade characteristics and the potential for longer life. Europe has now converted to air disc brakes almost completely, and North America probably will follow suit when air disc brakes have proved themselves.

The performance issue isn’t really with the air disc itself; it’s when the air disc is mated with the old reliable North American air-actuated drum brake.

We operate in an extremely competitive market that drives us to expect a return, which means, frankly, that the low-cost drum brake is going to be around for a while. When there is a mix of air discs and air drum brakes, there is the definite potential for a compatibility problem — not just from the brake designs but also the friction materials they typically use. As it heats up, the drum brake “fades,” or loses torque, and the air disc brake doesn’t.

Drum brake “fade” happens for two reasons — the drum expands as it heats, actually moving away from the brake lining and making the stroke of the air chamber longer and therefore less effective. Also, the type of lining used on drum brakes usually has a resin base that will start burning off at the surface when the brake gets too hot, around 600 degrees Fahrenheit. As a result, the drum brake essentially will try to push part of the braking workload off onto other brakes on the vehicle.

The difference with the disc brake is that as it heats up, it maintains its brake torque output for the exact opposite of the reasons a drum brake will “fade.” First, as the disc rotor heats up, it actually moves toward the friction material, making the air chamber no less efficient. Second, the lining or friction material pad usually is made primarily of metallic particles that will continue to produce friction to the melting point of the metals, around 1,500 degrees Fahrenheit.

When mated with a drum brake, the air disc will naturally accept the workload the drum brake is pushing onto it, and, in a worse-case situation, prematurely wear out the pads and/or crack the rotor.

All this is, of course, amplified if the drum brake is poorly maintained — and especially if it has inadequately performing aftermarket linings.

We’re still facing some challenges, clearly, but brakes are as critical as they’ve ever been. Selecting and maintaining the brake systems correctly is the key both to safe operation and to the lowest possible operating costs.

TMD Friction of North America makes brake friction materials for the automotive and commercial vehicle industry. It has offices and manufacturing facilities in Troy, Mich., and Queretaro, Mexico.