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.
STEER AXLE
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 BASICS
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.
BRAKE LINING RATINGS
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.
DIFFERENT ANIMALS
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. It is 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.
BRAKE OVERLOAD SITUATIONS
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, we 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.
NHTSA's amended stopping distances
By David A. Kolman, Editor
The National Highway Traffic Safety Administration (NHTSA) has amended FMVSS No. 121 - Air Brake System by reducing the specified stopping distance requirements by 30 percent for all new air-braked vehicles, including trucks, buses, tractors and trailers.
In essence, the regulations have shortened the maximum allowable stopping distance to a minimum braking performance of 250 feet from 60 mph, down from the previous standard of 355 feet.
For severe-service tractors, the new stopping distance requirement is 310 feet, also down from the previous 355-foot distance. That is for all loaded vehicle conditions.
NHTSA considers a severe-service vehicle to be a three-axle tractor with a gross vehicle weight rating (GVWR) greater than 70,000 pounds, or a four or more axle vehicle with an 85,000-pound or greater GVWR.
In addition, under the new stopping distance requirements, all heavy truck tractors must stop within 235 feet when loaded to their "lightly loaded vehicle weight" (LLVW).
The stopping distance requirements were also shortened for other load and system operating conditions in the FMVSS 121 regulation.
NHTSA considers a severe-service vehicle as a three-axle tractor with a gross vehicle weight rating (GVWR) greater than 70,000 pounds or a four or more axle vehicle with an 85,000-pound or greater gvwr.
The new stopping distances are being implemented in phases.
Three-axle tractors with a GVWR of 59,600 pounds or less had to meet the reduced stopping distance requirements by August 1, 2011. All other tractors must be in compliance by August 1, 2013.
As would be expected, stopping distance requirements under the NTHSA's FMVSS No. 121 vary according to vehicle type. Vehicles are tested under three different test conditions: loaded-to-gvwr, unloaded and emergency braking conditions.
The need to periodically inspect antilock braking systems
By David A. Kolman, Editor
Even though antilock braking systems (ABS) generally requires no routine maintenance, they should be checked periodically like other components of the air brake system.
Here is a review of the various aspects of ABS troubleshooting. It is a good idea to share this information with your technicians.
1. Verify the problem or driver concern. Establish the connection between the symptom and the underlying cause of the problem. Use the vehicle manufacturer's recommended information collection methods for verification.
2. Perform preliminary checks. Perform operational, visual and audio checks.
3. Refer to service information. Vehicle manufacturers provide service procedures which must be followed to ensure proper repair. Be sure to confirm that the reference material is not only applicable to the specific problem or vehicle being diagnosed, but is current.
4. Perform electrical, electronic and air system checks. Systems checks found in service manuals provide a systematic approach to identifying the probable cause of a system fault.
This step is important to properly define the correct approach for the repair and to avoid unnecessary time-consuming repairs. What's more, systems checks will help to define what the problem is not.
5. Find and isolate problem. For an active problem, the diagnosis should narrow and/or eliminate possible causes. Find and isolate the faulty part of the system or circuit by breaking the problem into smaller pieces.
For an intermittent problem, attempt to simulate/recreate the conditions where the fault would exist.
Monitor suspect circuits and components to pinpoint the probable cause while the problem is occurring.
Then, review all information describing the complaint. Things such as: When did the problem occur? What conditions are present when the symptom occurs?
If necessary, contact the driver to gather additional information or to arrange a "show me" or test drive interview.
6. Repair and verify. Once the suspect component is found, carefully disconnect the old component and inspect its connections to the harness. If the component connections are okay, temporarily connect a known good component (without installing) to ensure the problem is corrected.
After the problem is corrected with the known good component, reconnect the suspect component to make sure the problem returns.
Temporarily connecting a known good component, and then reconnecting the suspect component, will help reduce replacement of incorrect components. If reconnecting the suspect component does not cause the problem to recur, thoroughly inspect the connectors and harnessing for the cause of the problem.
Reconnect the suspect component and move (jiggle) the harness while monitoring for the problem to return. If the problem returns with the connection of the suspect component, permanently install the new component.
7. Clear fault codes. Clear any codes stored in the electronic control unit (ECU) identifying the problem.
8. Implement any possible preventive measures. Review the vehicle maintenance schedule for required service intervals and perform necessary maintenance.
Visual brake-stroke indicators reduce brake inspection time
By David A. Kolman, Editor
Fleet and maintenance managers are turning to simple-in-design and easily installed visual brake-stroke indicators to improve brake safety, maintenance and compliance by providing drivers with a convenient and effective means to inspect critical brake adjustment conditions without having to crawl under vehicles to mark and measure pushrod stroke at each airbrake chamber.
With brake adjustment defects accounting for nearly 30 percent of all truck crashes and a national out-of-service violation rate of one in every 10 vehicles, exposure to costs, risks and liabilities are considerable.
This is among the reasons why, based on crash-related risk, under the FMCSA's CSA BASIC (Behavioral Analysis and Safety Improvement Categories) safety scoring system, brake-out-of-adjustment violations carries a "4" severity rating, making it one of the top equipment violations affecting a fleet's safety score.
COMMON SAFETY DEFECT
Automatic brake adjusters and brake adjustment indicators are mandated by Federal Motor Vehicle Safety Standards (49 CFR FMVSS 571.121, S5.18 and 49 CFR 571.105, S5.1).
Yet despite nearly two decades since their mandated use, air brake adjustment defects persist as the most frequently cited safety defect and out-of-service violation. The reason being: improper inspection and maintenance of these mechanisms.
Although automatic brake adjusters eliminate the need to perform routine manual adjustments, they do not eliminate the need to perform daily inspections. Unlike manual slack adjusters, automatic brake adjusters cannot compensate for other deficiencies in the braking system.
When a brake equipped with an automatic brake adjuster is out of adjustment, there is a cause. Manually readjusting the brake does not address the cause. The problem will remain and the brake will go out of adjustment again.
Since the brake chamber's pushrod stroke is physically limited and can only operate effectively within a very narrow range of movement, a mere fraction of an inch of excess stroke can mean the difference between safe and unsafe braking.
DRIVER INSPECTIONS
It's worth noting that drivers performing daily pre/post-trip inspections can visually check every item except brake stroke.
Consequently, without effective visual brake stroke indicators, the only way a driver can properly check the condition of brake adjustment is to chock the wheels, build up air pressure, release the brakes, go to each wheel position, mark each push rod, have the brakes applied at 100 psi system pressure, crawl back under the vehicle and, using a ruler, measure the applied stroke at each brake chamber to see if they fall within the legal limits.
The problem is, this procedure is seldom, if ever, done.
Instead, drivers mostly rely on the faulty assumption that brake adjustment can somehow be determined by "feel" or other forms of guesswork.
This is the main reason drivers are commonly found unaware of existing brake adjustment defects. In fact, inspection of brake adjustment by drivers is considered to be one of the major contributing factors to non-compliance.
By eliminating all the guesswork and assumptions, visual brake stroke indicators provide an inexpensive, quick and effective means to visually confirm brake stroke.
About the Author
Jim Clark
Director of Engineering, TMD Friction
Jim Clark is director of engineering for TMD Friction. www.tmdfriction.com. 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.