For decades, single-piece rotors have been the mainstay of light- and medium-duty truck brakes. Now, two-piece rotors — first developed for NASCAR and other car racing sports — are slowly making their way into the truck market. Though more than twice as expensive as single-piece rotors, the advantage of two-piece rotors lies in better brake performance, longer and lower maintenance brake life cycle, and lower replacement cost.
Whether truck owners accept two-piece rotors, however, hinges on whether they agree that the higher price is worth the advantages, says Jason Oakley, a testing and development engineer for Clover, SC-based Performance Friction.
“It all comes down to the design of the two rotor systems and the physics involved,” he explains. “As a one-piece rotor gets hot, it ‘cones’ or bends over. That coning knocks the brake pad back, causing it to wear unevenly, leading to a softer braking feel for drivers.”
The two-piece rotor, however, acts differently because the rotor itself is separate from the rotor hub used to attach it to the wheel. “Since the hub and rotor are in two distinct pieces, the rotor cones dramatically less when it gets hot; it expands outward, keeping it aligned properly with the brake pad and maintaining better braking feel and more even wear,” Oakely notes.
Another design advantage to the two-piece system is that when it comes time to replace the vehicle's brakes, only the rotor has to be taken off. The hub, which attaches the rotor to the wheel, stays in place. “It's a huge design advantage because you reduce labor and materials cost for servicing the brakes,” he says.
The upfront cost is the main issue facing fleets, because a two-piece rotor set is priced at $460. Yet replacement costs are much lower, only $140 per set, because the hub and bearing package stay in place on the vehicle.
“The real magic comes from the design characteristics of a two-piece rotor,” Oakley explains. “Because the rotor spins true even when it's hot, you reduce both brake noise and cracking that single-piece rotors experience from constant coning. By reducing the coning and thermal distortion, we can get triple the rotor life. Also, since the two-piece rotor spins true, you get longer brake pad life because they come more evenly in contact with the rotor.”
Another benefit of the two-piece design is that by eliminating the coning effect, there's less brake drag on the vehicle, which can lead to fuel economy improvement, according to Oakley.
However, Oakely thinks the two-piece rotor's ability to withstand more punishment is what will give it a real edge in the truck market. “For many contractors using F-250 and F-350 pickups, for example, GVW ratings are just a suggestion to them,” he says. “Maximizing payload and towing capacity puts huge stresses on brakes and that translates into higher thermal conditions — the exact scenario two-piece rotors are designed to handle.”
Performance Friction's patent pending two-piece rotor package is available for F-250, F-350, and E-350 trucks, with the intention to add P-30 step vans and the Sprinter to the list.
“One reason two-piece rotors haven't come onto the scene before now is that they used to have durability concerns,” says Oakley. “They were breaking and cracking at the attachment points due to wear over time or sometimes due to improper torque at installation. With our new design, we have addressed these concerns.”
Over time, Oakley thinks the physical characteristics of the two-piece rotor design should outweigh the price difference compared to one-piece rotors, but for now it's largely a process of wait-and-see.
“The cost per mile of a two-piece rotor is lower than a single-piece rotor, though that advantage varies based on the driving application.
“The main advantage to the two-piece rotor package is the reduction of coning and thermal distortion, because if you can get that rotor to spin as true as possible, you get much more even contact with the pads, resulting in longer pad life, longer rotor life, and ultimately lower brake life cycle costs,” Oakley explains. “Everyone in racing uses two-piece rotors now and we think it will be a good fit for truck operators, too.”