Tractor Braking Systems

The federal government is in the middle of a three-year program to reduce the stopping distances of Class 8 tractors. The first part of its new stopping distance regulations covered three axle tractors up to 59,600 pounds gross vehicle weight ratings (GVWR) and was effective August 2011.

The program will be complete in August 2013 when all other tractors - with GVWRs over 59,600 pounds - are included in the National Highway Traffic Safety Association (NHTSA) regulations.

The goal of the NHTSA program is to make the stopping capability of trucks closer to that of a typical passenger car.


The primary change to the tractor’s brakes, as a result of this rulemaking, is on the steer axle.

During high deceleration stopping, a weight transfer occurs and adds significant load to the steer axle - an action is sometimes called “dive.” As a result, more torque can be added to the steer brakes.

These vehicles, therefore, have either larger, higher torque drum brakes on the steer axle, or higher torque air disc brakes.

These steer brake changes, and the fact that air disc brakes are now an option on all axles, is changing the landscape of both original equipment (OE) brake linings and aftermarket brake linings in North America. It is now even more important to carefully select OE brake options and aftermarket brake linings for your vehicle.


Two basic things are important to remember.

First, brake replacement linings must be selected to assure the friction level of the replacement lining produces equivalent brake torque to the OE linings. If a reduced friction lining is used, the stopping distance of the vehicle can be significantly increased, making the vehicle less safe, and even bringing on potential liability concerns for the fleet.

Second, the introduction of large volumes of air disc brakes to the North American market increases the potential for naturally occurring compatibility problems between disc and drum brakes, a problem that TMD Friction has been working to help resolve.

It is critical to understand that today’s aftermarket linings simply do not have to meet the OE legal standards in North America. Many aftermarket linings are of poor quality and poor braking performance.

It is also difficult to know whether or not you are purchasing the right friction material product. Many linings are sold as generic products and are branded by companies that know little about braking performance. Some linings even come from overseas plants in India and China with questionable quality.

Any fleet or vehicle maintenance operation with concerns over their vehicle’s brake performance must become knowledgeable about the source and performance of their brake products.


One voluntary system for rating aftermarket brake linings does exist, and is available, free of charge. It is the Technology and Maintenance Council’s Recommended Practice 628, Aftermarket Brake Lining Qualification.

This is a list of specific brake linings that have passed the original equipment Federal Motor Vehicle Safety Standards (FMVSS) 121 - Air Brake Systems’ dynamometer standard for typical vehicle parameters of gross axle weight rating, tire size, and brake size and type. If a lining is on this list, it has passed the dynamometer FMVSS 121 test.

Also listed in RP 628 is the torque output of the lining and other critical information such as fade index, quality rating of the manufacturing plant and a guarantee that the linings are asbestos-free.

A fleet or maintenance shop only needs to choose a lining with equivalent torque of the OE lining, and the stopping capability of their trucks will be assured.


Another important thing for fleets with air disc brakes on their vehicles to remember is that air disc brakes and air actuated drum brakes are basically different animals.

The disc brake is the newer product, and it has all the advantages of disc brakes on cars - consistent torque output, significantly reduced fade and potentially longer life. Two key differences, however, exist in the designs and friction materials of disc and drum brakes.

Drum brakes, the old stand-by, have a key feature that the disc brake doesn’t. They are capable of “sharing” the braking workload with other brakes on the vehicle under high-energy conditions. This is also called “fade,” which can be a good thing.

When a drum brake becomes too hot, the drum expands and the resin of the drum brake lining surface will start to “burn” away. This causes the brake to fade - basically reduce its torque for a given brake pedal application.

This also causes other brakes on the vehicle to take over more of the braking workload, essentially causing a sharing of the work.

However, when a disc brake heats up, the rotor surface doesn’t move away from the lining. Rather, it moves toward the lining surface.

Not only that, but the disc brake lining or pad is primarily made up of metal particles, meaning it can withstand significantly higher temperatures and continues to produce the same friction forces.

Put these two different types of brakes on the drive and trailer axles of a combination vehicle, and put that vehicle in any situation where the brakes are heavily utilized, and the disc brakes will simply continue to take on workload until they are overloaded.


What are those situations, and what kind of trouble can you expect?

Let’s say you’ve got a tractor that has air discs on all axles and it’s pulling a fully loaded, older trailer down a mountain. If everything is well maintained, there is a good driver at the wheel, an engine retarder that’s operating well and the driver knows how to use it, you probably won’t have a problem.

However, add inferior brake linings, manual slacks or inoperative automatic slacks adjusters, or simple poor maintenance, and you will likely have problems.

What kind of problems? Well, rotor cracking, premature wear out - since wear increases exponentially with temperature - and even trailer bump, among others.

Just as serious is an all-disc braked trailer being pulled by an-all drum braked tractor. Any time the driver uses the hand valve as he goes down the mountain adds to the above problems.

With these mixed brake technology situations in mind, TMD Friction has paid special attention to the issue of compatibility and “work sharing” between brakes. To help address this issue, they have formulated special air disc brake pads that have a capability of work sharing with drum brakes at high temperatures.

Not only does this help with compatibility, but it ultimately improves brake life and reduces the potential for rotor cracking.

TMD also actively supports RP 628 and continues to list their linings so knowledgeable fleets at least have a chance of having properly balanced brake systems.

Jim Clark is director of engineering for TMD Friction. The company is one of the world’s leading manufacturers of brake friction materials to the automotive and commercial vehicle industry and has a leading position in the global replacement parts market. TMD markets and sells brake linings in North America under the Textar brand.