Air Discs Are Ready
There are currently three North American air disc brakes (ADBs) that are packaged to fit inside a 22.5- and 24.5-inch North American wheel. They are from Bendix, Dana and Meritor. Bendix' brake is the ADB225, a redesigned European Knorr-Bremse component featuring the well-proven internals from its German parent company. The Dana ESD225 features Haldex actuation and adjuster mechanism and other internals inside a Dana-designed and manufactured caliper assembly. Meritor's brake is designed in Europe but to accommodate North American wheels — and suspensions as far as possible — and includes the DX225. There's also the DX195 that packages inside a 19.5-inch wheel and is smaller, lighter with less torque, but ideal for American steering-axle brakes. Bendix has the SN6 19.5-inch and also a SN5 17.5-inch brake. Haldex is marketing its Swedish-made ModulX brake in both 195 and 225 sizes.. The Haldex brakes are currently European designs, although there are plans for a redesigned ADB, repackaged in line with the competitors'.
It has taken North America a while to develop its own second-generation disc brake. The older air discs, developed in the early '80s were big, heavy, awkward to package, a maintenance nightmare with unequal pad wear and rotor cracking. And they were operationally imperfect with a lot of hysteresis in the actuation mechanisms. The much-improved European disc brakes introduced over the last decade -- like the Knorr SB7 -- have successfully dealt with all these issues and have seen some application here. Unfortunately, they don't package well on North American suspensions and, because they are built to sustain spring brake compounding, are heavy and don't quite fit into a North American wheel.
Meritor DX225 and DX195 are packaged for American trucks, have been available for some time and are seeing some applications. Bendix and Dana are working with the truck OEs on packaging issues, and are used on coach, fire and rescue and specialty applications they are understood to be very close to release at truck manufacturers. The first likely to have ADBs as a regular option is Peterbilt, though assistant chief engineer Mike Dozier says Pete is working to understand all the issues and implications. One is that adding a high-torque brake to the front axle, for instance, involves redesigning suspensions to accommodate the braking reaction. That could mean it may be a year before we see any announcement.
In the interim, disc brakes will continue to be fitted to high-end chassis like fire and rescue vehicles, RVs and coaches. Bendix has been standard on Prevost since 1994. and is a major plus for Pierce.
Dana's Jim Clark says some weeks Dana ships as many as 100 ADB sets for bus and emergency vehicle manufacturers. He anticipates a need in the short term for more brake effort on some straight-truck chassis. With straight trucks there are no trailer compatibility issues and, says Clark, straight trucks are most in need of "more brake." He says heavily loaded (or overloaded) dumps, mixers, loggers/sawed lumber trucks have more braking to do with three axles (or four or five with pusher/tags) than a tractor trailer.
So while the disc brake, with or without electronically controlled braking, has significant advantages, we're still likely to see a slow phase-in as fleets first use them on steer axles and gain experience with stability enhancement systems. Likely it will take the upcoming NHTSA braking mandate to provide the justification for them.
