Anyone have drilled/slotted rotors?

[Mustang5L5]

After driving it to the shop, I took the stock, vented rotors off the vehicle, and checked them for proper thickness, runout, and dish. They were good, so I marked and drilled them, taking care to do it properly, and chamfering the hole edges correctly. I put them back on with the same pads. I did not face the rotors, bleed the brakes or anything else that would have had an effect on the braking performance. Total elapsed time about 2 hours, including letting the rotors cool before removing them.

That right there is VERY dangerous. Proper crossdrilled rotors should be cast with the holes in them as it allows internal stresses to be minimal. Machining the holes in later increases internal stresses greatly which should be releived through cryogenics.

 

[the_tool_man]

...Machining the holes in later increases internal stresses greatly which should be releived through cryogenics.

How, exactly, does adding a few holes increase internal stresses greatly?

Adding roughly-machined holes that aren't properly deburred, might leave enough surface roughness to act as a stress concentration. But as I said, I took care to leave smooth, chamfered holes. The RMS surface roughness of the cast surfaces of the rotor was far greater than the machined surfaces in the holes.

Adding holes indescriminantly can be bad. The holes should not interset any internal webs, and should not be located at the edge of the swept area where the pads touch. The web structure on these discs were simple radial vanes, and the discs were used, so both of these were easily achieved. It can be a challenge to do this with some rotors because of the internal web design (I had to model the Cobra rotors in CAD so that I could work out the hole pattern ahead of time). Also, most rotors have a prime number of webs, so getting a perfectly uniform hole pattern is rarely possible.

If you're worried about reduction in cross-sectional area due to the material removed, it amounted to about 1.75%, or about the same amount as would wear away during the life of the rotor.

If you're worried about the reduction in surface area due to the material removed, any freshman mechanical engineering student will tell you that the amount of friction developed by two surfaces in contact is only dependent upon two things: the coefficient of friction of the materials, and the amount of force (not pressure) applied between them.

What more brake surface area does do is reduce temperature rise as the vehicle's kinetic energy is dissapated during braking. All things being equal, more surface area in contact between the brake disc and brake pad helps reduce brake fade. However, more surface area of the brake disc in contact with the air increases convective cooling, which can counteract, and apparantly more than offset the lost contact area due to the holes in some cases.

Now, I have seen this taken too far. The brake discs on the Super Stallion car that Ford built had so many holes drilled in them they looked like swiss cheese. They had lots of surface cracks in them, too. I asked them if I could drive the car to test the effectivness of the brakes, but they said no.

Oh, and don't worry. The rotors going on my car have been cryo-treated. The stress-relieving effect of cryo treatment is relatively small. I had them treated to counteract warpage (hopefully) with more uniform grain structue.

Regards,
John.