Machining brake discs

[hotrodder]

Thats all new tolerances, add a bit of rust and you can move the decimal point about a bit. Not trying to be argumentative, just realistic. Not always practical to rotate the disc relative to the drive flange, due to a retaining bolt, so that negates being able to pick the position, and if a new hole was to be drilled for the retaining bolt; well that brings us neatly back to the 'is it wise to drill a disc' question.

I know they are brittle, they're cast and cast is always brittle. Hard to escape the fact that thousands upon thousands of drilled/grooved/dimpled discs are sold in the UK alone, and there isn't a row of cars down every hard shoulder with discs in pieces. I do acknowledge they will crack, and I'm sure some do, but it seems fair to say its more exception than rule...

The numbers i posted for hub runout were service specs not manufacturing tolerances. While any rust will make a mockery of those specs the tiny pits left behind after it's been cleaned off don't so much... i've not changed anywhere near as many as discs as a mechanic but unlike the majority of 'em i check the runout of the disc every time as it only takes a few minutes and potentially saves having to the job again after a few thousand miles due to "warped discs". Only once i've had problems getting disc runout within spec and that was on a neighbours car who asked me for advice as she was constantly getting brake judder and every garage she took it too just diagnosed "warped discs" blamed her driving and charged her to change 'em without finding the underlying cause

The single disc retaining screw is there to stop the disc falling off when the car's partially dismantled nothing more. The wheel bolts are what clamp the thing properly to the hub i.e. it's not even close to the same as drilling a bunch of holes through braking surface

Cracked discs don't immediately fall apart, they just need replacing before they're worn out. The reason the roads aren't littered with the broken remains of cars and discs is presumably at least partly down to things like the MOT resulting in cracks being spotted? That said most mechanics probably have more than a few stories related to people complaining about grinding noises that turned out to be broken discs or at least discs that fell apart once the caliper was removed!

As no energy can be lost or gained Brake discs work by convert kinetic energy into heat energy, by means of friction between the pads and discs. By grooving and drilling you are reducing the disc to pad contact, which will make for slightly worse braking under normal road use. Drilled and grooved discs are to dissipate the heat quicker to stop brake fade under heavy track use. So if it is for the road you would be better off with standard discs, if for the track would you want to use guestamated parts that may not crack.

Pad size/contact area doesn't directly affect braking power... piston sizes* and hydraulic line pressure determine clamping force so a larger pad has the same force applied over a larger area. Another way of saying the same thing is that friction is generally independant of surface contact area. A smaller brake pad obviously overheats more easily and wears out faster...

* only the total piston area on one side of the caliper counts, multipiston calipers are about applying more even pressure to a longer, narrower brake pad which increases the torque radius (effective disc diameter)

For anyone that's interested, metallurgy and iron brake discs... http://www.sae.org/events/bce/tutorial-ihm.pdf