How do they measure up?
Changes to the tractor stopping distance requirements mandated by Federal Motor Vehicle Safety Standards No. 121 (FMVSS 121) changed in 2011. At that time, it was assumed that drum brake friction material and design could not handle the new shor ter stopping distances and they could not produce enough torque, while the engineering design of air disk brakes (specifically that they produce higher more consistent torque), made them a more a logical choice. But, the makers or drum brakes systems have changed their designs and now both types of brakes are a viable choice for fleets.
Drum brakes typically use resin based friction materials that can withstand only about 600 ºF before they begin to lose torque. Disc brakes can withstand 1,000 ºF and produce consistent torque. However, vehicle manufacturers have increased the size of the drum brakes where needed, therefore improving their temperature and torque capability.
Mixing disc and drum brakes on tractors and trailers generally shouldn't create any problems if they are properly maintained with good quality linings. Some fleets have reported that disc brakes seem to have fewer problems with roadside inspectors. The disc brakes are harder to access and see when a truck is pulled over for an inspection, therefore inspectors don't necessarily see if there are problems.
Although disc brakes have a higher upfront cost, depending on the carrier's need, it may pay for itself over the long-term. Each fleet must weigh the pros and cons of each brake type in order to determine what is best for its operation.