The proper operation of a vehicle’s braking system is essential for safety. This begins with specifying the appropriate brakes for the vehicle’s application, duty cycle, service environment, etc. Then, the system must be well maintained for the life of the vehicle. Otherwise, braking performance – and highway safety – can be compromised.
When spec’ing brakes for trailers, there are different considerations to take into account, according to Marty Watterson, field service manager, trailer division, Meritor (www.meritor.com), a leading global supplier of drivetrain, mobility, braking and aftermarket solutions for commercial vehicle and industrial markets. It’s important to be aware of several key things, including:
- Trailer application and maximum payload.
- Trailer drive cycle – how it will be used – and duty cycle – how much it will be used.
- Operating considerations, such as environment, roads and terrain encountered, etc.
- Gross vehicle weight (GVW).
- Axle capacity.
Sizing the brakes to meet the specific load carrying requirements of a trailer can improve the tractor trailer’s average stopping distances. What’s more, properly distributing the work between the tractor and trailer minimizes thermal overload of the trailer brakes.
During panic stops, especially on a heavily loaded tractor trailer, the load shifts from the trailer axles to the drive and steer axles. The harder the stop, the more the load transfers from the rear of the rig to the front.
Disc versus Drum
Disc and drum brakes have been used in combination on tractors and trailers “successfully for many years,” notes Meritor’s Watterson.
The company acknowledge that the use of split systems, particularly for the cost conscious fleet, can be a practical approach.
“Split system specifications do not reduce overall performance for the vehicle. However, it is important to assure all tractor and trailer brakes are serviced to the original OEM specifications.”
Stopping power is noticeably greater when both the tractor and trailer are outfitted with air disc brakes, especially when the rig is traveling at highway speeds.
Drum and disc brakes employ the same principles of friction to slow a vehicle down.
Basically, drum brakes – sometime referred to as self-energizing brakes – are designed so that a round drum rotates along with the wheel. Friction material is attached to brake shoes. When the brake pedal is pressed, the brake shoes push against the inside of the drum to slow the wheel. With use, the friction material wears away.
Disc brakes work by compressing two friction pads against a flat rotor. Instead of a brake drum attached to the wheel hub, a rotor is attached to the hub. A caliper is contained on the brake carrier with its own brake chamber. When the brake pedal is pressed, the caliper squeezes the brake pads against the rotating rotor to slow the wheel. Because the rotor is exposed to outside air, it is able to cool rapidly, which decreases its tendency to overheat or cause fading.
Depending on how a fleet uses its tractors and trailers, Meritor’s Watterson says it is commonly accepted that disc brakes have overall better operating performance and shorter stopping distances than drum brakes in optimal conditions. Disc brakes dissipate heat better than a drum – keeping brake pads from overheating and causing breakdown. Serviceability is easier on disc brakes as there are less components and less labor time needed.
“When combining the benefits of better friction wear, decreased service intervals, less maintenance time and increased uptime, the total cost of ownership for disc brakes versus drum brakes can be lower,” he continues. “This applies to both tractors and trailers.”
Adoption Rates
Disc brake penetration is lower for trailers than it is for tractors, notes Watterson.
Traditionally, an average fleet purchases and operates more trailers than tractors each year, he says. Because trailers typically run fewer miles per year than tractors, fleets have been slower to see the advantages of disc brakes on trailers.
However, as the price of disc brakes continues to become cost competitive, and the awareness of the technology and benefits of disc brakes is more widely shared, Meritor expects more fleets to spec disc brakes on both tractors and trailers.
Adoption rates of air disc brakes have climbed from about 10 percent in 2011 to nearly 20 percent in 2016, adds Paola Carmona, product manager, Meritor. It is a trend she expects “to carry forward to upwards of 30 percent by 2020, mainly because big fleets are starting to move into that direction.”
Segments like construction applications, logging and off-highway – where there is a lot of dirt and debris – will help keep drum brakes viable, she says, but anticipates that air disc brakes could eventually overtake the market.
“We believe drum brakes will not go away,” she says. “There are places they do make sense and are the most cost effective option.”
Carmona expects 6x4 rear tandem configurations that spec disc brakes on the front with drum brakes toward the rear will rise most in popularity. This is because of the advantages disc brakes offer, including their resistance to fade and a maintenance cost reduction that can in some cases exceed the lifecycle of the unit.
In a 6x4, both rear axles are powered.
“Fleets on a short trade cycle buy the truck and never touch the brakes, without having to do anything above standard inspection and maintenance,” she says. “A lot of the market is very cost sensitive.”
Preventive Maintenance
When doing preventive maintenance for disc brakes, it is important to pay close attention to brake pad wear, Watterson of Meritor says. Also, ensure all boots and seals are in good condition to protect the internal mechanism from contaminants.
For drum brakes, he offers these recommendations:
- Make sure brake linings are operating at the legal thickness limit. If not, they should be changed.
- If changing the brake linings, also inspect the cam bushings and change them if needed.
- Be sure to regularly lubricate cam bushings and slack adjustors.
- Brake drums should be measured to assure they have not reached wear limit, and inspected for signs of abnormal conditions such as cracks or heat-checking.
- When replacing parts, make sure to always use factory service parts.
- As always, refer to the manufacturer’s maintenance manuals for proper procedures.
