On March 16th. I did a write up covering some of the choices of supplement braking systems on today’s motor coaches, like Pac and Jake brakes. I also did a piece the following week, on March 23rd, on real world application and techniques of same. Well, this week’s topic will relate to those as we look at better braking systems for air equipped coaches.
Until only recently, air disc brakes were unheard of on motor homes. Now, a number of higher end coaches are coming equipped with these as standard equipment. Will this number increase in future? Yes. Disc brakes on Class 7 and 8 vehicles will increase in popularity greatly in the future. Shorter stopping distances are being mandated by the government on trucks that share the same weight characteristics as do many diesel pusher, “A” class coaches. The NHTSA/DOT wants to reduce heavy truck stopping distance by 30% with a target date some time in 2010.
Heavy trucks in Europe equipped with air disc brakes account for 76% of the total. North America scores about 5% only that are so equipped with the first one released in 1988. The first coach type vehicle to use all air discs was MCI in 1994. Today most of the heavy motor coaches are equipped with air discs, either all or at least front. While there are few class 7 ( 26,500/33,000 lbs. or so) sized coaches employing air discs, it would appear that is all about to change in the very near future.
Are they that much better than the “S” cam drum type? According to many actual field tests they appear to be far superior in the following ways.
Better Stability and Resistance To Brake Fade:
Excellent hot brake performance with less than 4% fade and superior cooling. Far better stability with 10% or less torque difference as compared to drum at around 30%.
Shorter Stopping Distance:
Stopping distances were reduced 30 to 37% when compared to their drum type predecessor.
Longer Life and Reduced Maintenance.
Bendix conducted tests using identical vehicles, one with “S” cam drum brakes and the other with air discs. These tests produced a result showing that the air disc out lasted the life of the drum type brake linings by 87%. Lining replacement may not be required until 500,000, 750,000 or even 1,000,000 miles. Additionally, servicing is reduced when compared to that of conventional drum type foundation brakes.
Increased Stability and Superiority at Higher Speed.
Braking balance and stopping distance difference increases as speed increases.
Driving a heavy vehicle equipped with these air disc brakes yields a noticeable feel as they react to less air pressure requiring less effort on the operator’s part. They also may feel smoother with less steering axle pull tendency. Unlike the automobile hydraulic calipers, the air powered disc brakes are capable of being used in a park brake mode. These use a standard or similar additional brake chamber that contains a heavy spring. They engage the same as drum type Maxi with air off.
The cost in North America of these air powered disc brakes is about 1 ½ times that of conventional drum type. This is in contrast to Europe where they can actually produce them cheaper than the heavy drum type. But, even at an increased cost, I think this newer generation of improved braking system design will benefit all, and may be very well worth the price.
Them’s The Brakes – Lug_Nut – Peter Mercer