Description
When a brake disc is heated, it expands radially, increasing in diameter and circumference. One-piece disc designs like the standard brakes in the KTM X-BOW Comp R, run into problems when this occurs. As the disc is being heated on the track, the disc expands, the outer edges of the disc are pulling away from the center of the disc, but there are no built-in provisions to allow for that expansion. The edges of the disc pull, lift, and distort, which is called coning. Coning directly impacts the brake pads’ contact with the disc, leading to uneven wear and tapering, and even a long brake pedal.
Two-piece discs on the other hand, compensate for the expansion of the disc as it heats. This is accomplished by building ‘float’ into the disc. The mounting holes for the hat attachment bobbins are not round, but are instead an oblong shape. These channels allow the hat mounting hardware to slide as the disc is heated and expands, allowing the disc to run true in the caliper without distortion. That means less distortion, stress cracks, and pad taper.
During exhaustive R&D testing, AP’s J Hook design was found to create a constant pathway of evenly distorted material on the face of the disc. The hooks are spaced out as evenly as possible both around the circumference of the disc, as well as from the inside edge (where the hat attaches) to outer edge, with a slight overlap to promote even heat distribution/distortion. In addition to reducing cracking, the even heating of the disc also helps provide an even transfer layer of pad material on the disc when you bed them in.
The J Hook slot pattern produces a greater number of leading edges for the pads to bite into vs. a traditional curved slot pattern, and particularly a plain-faced disc. While this may lead to slightly more whirring or scraping noises from the discs when applying the brakes, the benefits of more even heat distribution, less propensity to crack, cleaner pad material transfer during bed-in, and more bite far outweigh the slight increase in NVH (noise, vibration, harshness) for the serious enthusiast.
