However, larger drum brakes will be used by most manufacturers. “On a typical Class 8 6x4 tractor, we have met the new standard by upgrading from a type 20 to a type 24 brake chamber, linings and drums with greater width and diameter and larger brake mounting fasteners,” says Haggai. “The brake lining formulas have also been changed.” These larger drum brakes will add an estimated 90 lbs. to a 6x4 tractor.
Navistar is also offering drum brakes as standard equipment. “Currently, air disc brakes are optional on International branded highway tractors,” says LaBastide. “Our standard brakes are air S-cam-type brakes and we offer Bendix air disc brakes as an option. To comply with the new stopping distance regulation, we now offer a larger S-cam brake.”
Navistar selected larger front drum brakes (16.5” x 5”) with a different lining material specifically designed for the new regulation, in conjunction with larger service brake chambers (Type 24). “New lining materials were also applied to the rear to balance out the front brakes,” says LaBastide. “As another option, the customer can order larger rear brakes (16.5” x 8.62”), as well, over the standard 16.5” x 7” rear brakes. The larger front and rear brakes will increase miles between reline by increasing the usable lining volume and reducing the brake temperatures by increasing the braking area surface.”
Despite having been around for the last century, drum brakes have really evolved from a technical and performance standpoint. “They are significantly different than they were 10 to 15 years ago,” says Gary Ganaway, director of drum brakes, marketing and global development, Bendix Spicer Foundation Brake. “They are much more capable. Be that as it may, there are still some fundamental differences that favor air disc brakes. The way we position them in our product line is the drum brake is very economical. It is a basic product that allows the fleet and the OEM to meet all of the requirements. The disc brake adds an added level of performance and feel.”
There will be a weight penalty for increasing the brake size to meet the legislation. “The larger rear drum brakes will net you an additional 60 lbs. on a tandem rear tractor,” says LaBastide. “This is moving from the 7-in. brake to the 8.62-in. wider brake with the same diameter.”
But the weight penalty is relatively minor compared to the cost penalty for air disc brakes. “Traditionally, air disc brakes have been priced as a premium to the S-cam brake,” says LaBastide.
Advantages of disc brakes
Despite the initial cost difference, some customers value the benefits offered by air disc brakes.
The challenge comes from load transfer during braking. As the tractor decelerates, the load is transferred toward the front axle. As a result, the front steer axle brakes became a primary focus. This is where the increased performance of an air disc brake can have the greatest impact. “You get your biggest bang for the buck putting it on the front axle,” says Ganaway.
“We see a growing appetite for air disc brakes and expect customers to continue migrating toward that spec in the future,” says Dorwart. “Air brakes can provide an increase in service life over drum brakes in the same application, which can make for a positive return to offset the initial price increase. But be mindful that the duty cycle, vehicle weight, etc., will have a bearing on the life-cycle performance. With the wide array of applications, disc brakes are better suited for some operations than others.”
At Waste Expo in May 2011, Mack launched its initial offering of air disc brakes engineered specifically for vocational trucks. “The balance between friction material and rotor material has been optimized for the stop-and-go needs of [vocational applications], providing improved stopping distance and handling,” says Dorwart. “The air disc brakes are easier to service and the friction materials have been designed to last twice as long as the material in S-cam drum brakes.”
Maintenance is one of the key benefits of air disc brakes. “From a maintenance standpoint, the time it takes to change air disc brake pads once the wheels are off is only about one-fourth the time it takes for a drum brake,” says Ganaway. “The other advantage is the disc brake is sealed for life. It doesn’t require any periodic lubrication. With most drum brakes, you are still required to do that.”