Brake Discs thickness

[JohnBu]

Anyone know why they make Brake Discs, say 22mm.

Once worn down to 20mm, they should be changed.

Any reason they don't make them 25mm, so you can say machine them once or twice before they get to 20mm?

[Wraith]

Anyone know why they make Brake Discs, say 22mm.

Once worn down to 20mm, they should be changed.

Any reason they don't make them 25mm, so you can say machine them once or twice before they get to 20mm?

You can get at least one decent machining job done on most disk brakes...ie: unless they're worn right down or too damaged to do so.

It's not the o/a thickness that matters but the individual wall thickness on the vented ones...it can't be below a certain amount or you risk warping them.

As for solid disks (most rears) you can machine them quite a bit...

There are alot of good aftermarket disk rotors to replace most OEM types that are priced very well, I recently replaced the front 2 on my Calibra after just over 80,000km instead of machining them, because the new ones cost only $80 each, it cost more than machining them (around $20-30each) but I prefered to get new ones in there...

[nuggz]

Anyone know why they make Brake Discs, say 22mm.

Once worn down to 20mm, they should be changed.

Any reason they don't make them 25mm, so you can say machine them once or twice before they get to 20mm?

My guess
too much travel for the caliper piston things's
im guessing the further they stick out of the housing the more stress put on them

[gslrallysport]

^ Yep, correct to both. With vented it's the compromise between wall thickness and 'vented' area. If you look at RDA new's molycarbide rotors for the front of VT-VZ, they have VERY thin walls, and very large vented areas. And yep, too much travel in the disc thickness, combined with pad thickness variation means the piston design would be really weird. Most pads have friction material 12-16mm thick, so combined with disc thickness variation, then a piston has a working distance of roughly 14-18mm, with a mm or two for safety. You'll find adding an extra 3mm makes a huge difference to the overall shape of the caliper.

PLUS, if you made discs have large variations in thickness, you'd need very thick (like 10mm, as apposed to 5-6mm) pad backing plates, so that when the discs are low, the pad backing plate doesn't try and escape through the large gap now formed between the rotor and caliper carrier, where there previously used to be disc... make sense?

[JohnBu]

yup, makes sense

whats the risks of letting disc get below the minimum thickness?

again say 20mm.

say gets to 18mm.

heat?
warping?
cracking?

[gslrallysport]

All of the above. I let the rotors on the front of my ute get low low (cause even at my prices front rotors for an AWD Commodore variant aren't cheap! :(), and I had one massive crack across the rotor face on the passenger side from being below minimum.

Put it this way, I'm sure there's thousands of cars on the road that get neglected with well below thickness rotors, you don't exactly hear of them exploding, but I would personally replace them. They're not going to fail in a one off stop from cold, but with repeated use they can, or just warp etc. Warps not a big issue, but DVT is. Plus, anything below minimum is flat out illegal, and insurance companies will check.