Among the vehicle issues that drivers may bring to your attention, it is probable that you’ve heard complaints about brakes that require more input than usual to achieve the same stopping performance. What they’re describing is brake fade, notes Tony Ryan, SAF-Holland’s technical service and training manager.
“Brake fade can be defined as the reduction in braking force during repeated or sustained brake applications,” Ryan explains. “Drivers will immediately recognize it most commonly during steep downhill descents or in applications requiring frequent stops. We also see brake fade in high-duty cycle, repeated stop-and-go applications like garbage collection or city bus operations.”
Keith McComsey, director of marketing and customer solutions (wheel-end systems) at Bendix Spicer Foundation Brake, says that brake fade is the result of generating a significant amount of heat from repeated or sustained brake applications. “The heat eventually soaks into the surrounding brake components, such as drums and friction materials,” he says.
“It can also be brought on more easily with high GCWs and/or higher speeds,” McComsey adds. “In addition, the use of inferior friction materials in either drum or air disc brake systems can lead to a fade condition, depending on the friction material’s performance at higher temperatures.”
Though brake fade is commonly known to occur with drum brakes, it’s important to note that fade can manifest itself in both drum and air disc brake systems, relates Melanie Elliot, marketing manager at Hendrickson. “Brake fade can present itself as either mechanical or chemical fade,” she explains. “Mechanical and chemical fade are more commonly seen in drum brakes, especially in aggressive braking environments which may cause the drum to superheat and expand, preventing the brake shoe from making adequate contact with the drum.
“Air disc brakes, on the other hand, mostly experience chemical fade when the pad material breaks down,” Elliott continues. “As the pads deteriorate, they release chemicals that create a vapor barrier, requiring an increased amount of brake pressure to slow the vehicle.”
In testing of lining formulations, it has been found that lining recipes play a large role in chemical fade for both drum and disc brakes. In the coming years, as next-generation copper compliance regulations are released, this could pose a challenge to disc brake friction material suppliers.”
Another mechanical issue that can cause fade in drum brake systems, according to SAF-Holland’s Ryan, occurs when increased temperatures physically move the inner surface of the drum further away from the shoe linings, causing the physical limit of the brake chamber push rod to be reached. All of this typically occurs when the brake system reaches 300ºF, he notes.
“In a disc brake system,” Ryan continues, “when components heat up and the rotor temperature increases, it begins to expand. This time, however, the braking surface of the rotor has moved closer to the pads causing frictional brake fade where the brake pad surface contacts the rotor.
“In the majority of drum brakes, shoes are manufactured with a resin-based frictional material so that issue can arise when the lining reaches temperatures of about 600ºF, the resin in the shoes breaks down and glazes over,” Ryan adds. “However, disc brake pads are made with a mixture of metallic and ceramic friction material that does not decline until the braking system reaches approximately 1,500ºF.”
“Brake fade tends to be more prevalent in drum brake systems due to reduced air flow inside the drum, especially when dust shields are used,” says Keith McComsey at Bendix. “Disc brakes are much more resistant to brake fade because they tend to cool much more easily due to their exposure to air flow and rotor designs that help vent heat more effectively away from the friction surface.”
According to SAF-Holland’s Ryan, brake fade can have more severe consequences on semi-trailer combinations with mismatched technology. “If a tractor with disc brakes is pulling a trailer with drum brakes, the trailer will fade first leading to trailer push and potential jackknifing,” he says. “Likewise, a tractor with steer axle disc brakes and drive axle drum brakes can potentially have similar consequences. If you’re mixing brakes across a vehicle or a combination, it is preferable to have the brakes that don’t fade as readily at the rear.
“If you have mismatched disc and drum braking systems, when the drum brakes fade, the only stopping power left is the disc brakes on other axles,” Ryan adds. “That means the remaining disc brakes are left to stop the weight of the whole combination when they are not designed for that sort of workload.”
Effective maintenance practices
Technicians cannot manage many aspects of brake fade easily because fade occurs mostly due to the environment and driver style and skill level, notes Hendrickson’s Melanie Elliot. “But there are maintenance practices that could reduce possible brake fade,” she says.
“For example, technicians should regularly check the drum or rotor for cracks and signs of wear and verify that accurate brake stroke is in place,” Elliott continues. “In addition, adjusting brake lining inspection intervals to account for differences in duty cycles, application, terrain and equipment combinations are important factors to consider. Looking at those variables when determining intervals at which components like brake linings should be replaced can help ensure that brakes are serviced before exceeding their wear limits.
“To help reduce the possibility of brake fade, shops and technicians should also consider the importance of tractor and trailer brake system balance,” Elliot adds. “Additionally, when ordering replacement brake shoes and pads, be sure to always choose OEM parts, which are held to tighter standards and may achieve longer lasting performance.” θ