Quick Tech With Marlan Davis
Last time we discussed possible benefits of crossdrilled disc brake rotors, but there’s a downside, particularly on street-driven cars subject to repetitive heating and cooling cycles: it creates stress-risers, leading to crack propagation from the drilled holes. Generally, a race car has less of a problem because it has less extreme heat-cycling (the brakes get hot and stay hot). High-end race rotors are made from better materials, and racers consider rotors a disposable item and periodically replace them just like an oil filter.
To reduce crack potential, cross-holes should not be drilled with a uniform “inline” circumferential pattern on equal radii. That’s more likely to adversely affect the structural integrity and lead to cracks. Instead, drill holes in an overlapping, staggered pattern.
As to hole location, Porsche used to drill through the ventilated disc webs. AP Racing drilled into the hollow area between the webs. The late race engineer Carroll Smith recommended the Porsche pattern for curved-vane discs and the AP pattern for straight-vane discs. It is also necessary to carefully chamfer and radius the hole edges to reduce the chances of stress cracking.
There’s a way to obtain most of cross-drilling’s benefits without the danger of cracks: Slot the rotor face instead. We’ll get into that next time.
[ Baer EradiSpeed rotors are available both cross-drilled and slotted. Baer says it casts its rotors from crack-resistant materials, and the holes are placed behind the curved vanes to minimize crack migration. Note how the holes aren’t concentrically in line with adjacent holes, as well as how the slots point in opposite directions on these two directional rotors.