We are coming up on the third anniversary of the Federal Motor Vehicle Safety Standard (FMVSS) 121 reduced stopping distance regulation (RSD) start date. Fleet Equipment decided to ask brake friction manufacturers to report in on brake performance—and/or changes—since those regulations went into effect.
“Actually, the friction materials were changing long before August 2011,” says Tom Rogers, senior applications engineer, Abex Brakes, Federal-Mogul Motorparts. “Federal-Mogul was engaged by the major brake manufacturers back in early 2007. In order to meet the new RSD requirements, the fundamental change to the overall brake system is that the front brake torque had to essentially double.” He goes on to say that the brake manufacturers accomplished this through a combination of upgrades to the front brakes. The brake was made bigger (16.5 in. vs. 15 in.) and wider (5- or 6-in. vs. 4-in.), then the chamber size was bumped up a notch from T20s to T24s, and, finally, a new family of RSD friction materials were introduced. All of these things combined were needed to make front brakes powerful enough to meet the new stopping distances.
“Previous to the RSD rule for tractors, the vast majority of our friction materials were non-asbestos organic (NAO) type materials, because they offer a good balance of performance, life and initial cost,” Rogers notes.
“With regard to the new RSD friction materials, Federal-Mogul and the brake manufacturers have taken different approaches depending on the front axle load and vehicle GVW,” Rogers says. “For the more generic 12/40K lb. 6×4 tractors, we still offer NAO materials in the portfolio, and as tractor weights increase or brake manufacturers desire, we offer a semi-metallic/NAO option, and finally, at the upper end of the GVW ranges, we offer a full semi-metallic solution.
“We also developed some new drive axle materials to go along with the new steer axle RSD materials. These materials are not really any more powerful or aggressive than pre-RSD drive axle materials, but were developed to condition, or burnish, at slightly lower temperatures to be more ready to provide their torque during the FMVSS121 vehicle test, which is performed very early in the life of the lining. They are also offered in the same three basic configurations we have for the steer axles: NAO, NAO/semi-metallic combinations and full semi-met materials.”
When asked how brake lining manufacturers have balanced lining composition (thickness of friction material) so that linings last longer and meet the RSD regulations, Rogers responds, “There is actually a lot to that question. Generally speaking, the cooler a brake runs the longer it’s going to last. With that in mind, one thing that was done was to make sure we remain cognizant of the fact that although the material needs to perform as FMVSS 121 requires during 60 MPH full pressure stops, the vehicle is rarely, if ever, driven in that manner. Its normal use is in the under-20 PSI realm and usually more in the 10-12 PSI type range, so we needed to keep an eye on maintaining non-RSD type torque levels in the real world operating pressure range. If we start off being higher friction than the trailers being pulled, the RSD brakes are going to work harder than the trailer brakes, run hotter and wear out sooner. Maintaining good brake balance with mating trailers is one key.
“We talked earlier about making the front brakes larger,” Rogers continues. “One of the additional benefits to that is that the wearable lining volume also increases. In the case of going from a 15×4 block type brake to a 16.5×5 block type brake, you’re gaining on the order of 80% more lining to wear away before a reline is needed.”
Some brake manufacturers and some vehicle manufacturers took things even further on the rear axles, going from a conventional 16.5×7 brake, out to the 8 5/8-in. width, which represents a 23% increase in lining volume on the rear axle. Lining thickness is dictated by a particular brake design and that has not changed with RSD brakes compared to non-RSD brakes. Rogers adds, “There are a couple more things these bigger brakes and correspondingly heavier drums do. They spread the heat out over a larger surface area, so for any given braking action, the amount of horsepower required is the same, but with bigger brakes and drums it gets spread out over a bigger surface area and so the heat going into the drum is lower on a per-square-inch basis. And lastly, these drums stick out a bit farther from inside the wheels, which lets them cool a little quicker too.”
To keep vehicles compliant with RSD, Rogers notes that truck manufacturers are responsible for ensuring that vehicles are compliant with the FMVSS 121 RSD regulation as of 8/1/2011. On the aftermarket side, Federal-Mogul introduced RSD compliant materials into the aftermarket.
According to Fabio Jurchaks, director of sales and engineering, NAFTA TMD friction R&D center, “Most of the linings available today are still the same first generation of RSD friction launched in 2011, which were re-engineered to increase the overall performance. In order to comply with RSD requirements, some friction companies decided to focus on performance only by developing very high friction formulas while others, like TMD, decided to use a balanced approach still looking for a high friction material but yet maintaining a good balance of wear and noise, vibration and harshness (NVH). In the last three years, the OEMs, brake and friction manufacturers have been monitoring the market to get field feedback. Some fleets experienced higher wear rates and vibrations with the high performance materials. For other fleets, the transition was seamless.”
As for changes in brake lining composition, Jurchaks notes, “The lining thickness of RSD materials are the same as pre RSD. The lining design is decided by the brake manufacturer and not by the friction manufacturer. So in order to increase friction and keep the lining life, the lining thermal transmission had to be adjusted to better dissipate the additional heat produced by the higher friction formula. Also special lubricant and abrasive packages had to be used to keep abrasive wear under control.
As for keeping trucks compliant with RSD, he goes on to says that it is important for the fleets to understand that RSD vehicles require the use of RSD replacement linings in order to keep the same performance. Fleets need to look for reliable friction manufacturers that have RSD formulas in their portfolio.
Randy Petresh, vice president technical services for Haldex Brakes, reports, “The recent significant changes in friction were developed about three years ago or so in preparation for the reduced stopping distance requirements. The most significant were new/modified higher friction materials for the larger 16.5 x 5 steer axle brakes. The rear axle brakes did not change all that much except for selection of different materials to rebalance the vehicle with the larger steer axle brakes.”
He goes on to say that the new larger steer axle brakes utilize thicker linings than the previous commonly used steer axle brakes, in addition to the higher friction material. This larger steer axle brake with the thicker linings should provide better wear balance with the standard 16.5×7 drive axle brakes. Steer and drive axle brakes will now have essentially the same thickness linings. In some cases, the vehicle OEMs have incorporated wider rear axle brakes, 16.5×8.625 in. (as opposed to 7-in.), to improve lining life and/or wear balance even further. Petresh says, “Maintaining vehicle compliance with RSD is obviously the user/fleet’s choice. There are currently no requirements for in-service vehicles to maintain this level of performance. Haldex recommends replacement linings be the same as or equivalent to the original OEM spec friction material.”
Beyond the requirements
According to Joe Kay, chief engineer-Brakes, Meritor, “The new reduced stopping distance requirements provide an opportunity to install not just new friction, but also materials that deliver the braking results customers want. Meritor has re-engineered its friction materials to meet the new reduced stopping distance requirements and go beyond in some instances.”
He points out that all new Meritor RSD brakes meet the new requirements with an additional 10% margin and they are available for fleets that want to standardize on friction materials. According to Kay, “Tractor-trailer brake compatibility was a design requirement for new Meritor RSD friction materials. Our solutions have been developed to maintain tractor-trailer compatibility with existing trailer fleets. The tractor will incur approximately 5% more of the braking workload, while the trailer will experience a proportionate reduction in workload.”
As for changes related to FMVSS 1221, Kay says since front brakes now absorb more brake torque, the front brakespiders have been made heavier and the mounting bolts are a larger diameter. Front brakes now use Type 24 brake chambers, increasing the AL factor to 132.”
Reconfiguration of vehicles to meet new stopping distance regulation has impacted differently drive axles brake linings and steer axle brake linings.
“To attend accordingly, drive axles braking torque needs, brake linings have been developed to improve speed spread effectiveness by reducing in-stop fade characteristic,” says Bernard de Jong, Fras-le senior project engineer. “Additionally, drive axle brake linings have been tweaked in such a way that thermal conditioning has become considerably less important to obtain the nominal designed performance level, resulting in high stopping power since first brake applications. In the case of steer axle, significant increase of brake torque requirement is achieved by combination of larger brake systems, higher inputs and higher friction coefficient brake linings. Steer axle friction material for RSD program has been designed to show, besides high friction level, high conformability to the drum, assuring higher contact effectiveness, which has proven as being a key factor to high and stable torque response.”
He goes on to say that the RSD program has resulted in significant increase of vehicle brake capacity. To compromise this high torque response maintaining other important brake lining characteristics, such as durability and NVH, friction material composition has been redesigned in several aspects. Structural conception and tribological mechanisms have been modified, involving also raw materials specs and process parameters revision.
“Fundamentally, it is well known that for aftermarket linings, compliance to meet any performance standard is not a legal requirement,” de Jong notes. “Fras-le is very concerned about this, since relining high performance drum brakes with lower cost aftermarket linings, not specified by OEM, can significantly reduce stopping capability and lead to longer stopping distances, potentially compromising safety. Replacement lining choice is very critical, and as such Fras-le effectively supports any rulemaking discussion and implementation regarding recommended and legally enforced specs for aftermarket friction in order to ensure conformity with designed braking performance.”
It’s about new equipment
Jeff Geist, Stemco’s director of engineering—brake products group, reminds, “The recently updated FMVSS 121 requirements rulemaking addresses only new vehicles and the equipment sold on new vehicles; it does not apply to maintenance procedures once the vehicles are sold to end users.”
He goes on to say that typical drive axle brakes already achieved sufficient torque to lock the rear wheels, there was little benefit in increasing their effectiveness. In some applications, rear brake torque may have been reduced to minimize lock-up. The friction between the tire and the road became a limiting factor. The 15×4-in. steer axle brakes were upsized in diameter and width to 16.5×5-in. and more aggressive friction was specified to gain increased front brake torque and take advantage of the weight transfer that occurs during a stopping event. In many applications, front suspensions were also upgraded to manage the increased torque and load.
The Technology and Maintenance Council (TMC) is presently undertaking a revision of RP 628B to include an aftermarket classification for RSD steer axle brake friction to enable end users more freedom in product selection should they desire to maintain or improve original equipment performance. Geist adds,“Stemco LP is in the process of formulating friction materials and engineering brake drums that not only meet the necessary torque requirements for RSD compliance, but also mitigate undesirable noise, vibration and premature wear.”
Since the RDS laws were enacted, Frank Gilboy, product line manager, Bendix Spicer Foundation Brake, says, “The formulations now have to have a greater emphasis on in-stop fade and colder burnishing in order to meet the vehicle test for RSD. Although these characteristics were always part of friction development, the new vehicle test has made them one of the key hurdles to pass for a lining to be RSD compliant.