Brake rotors.

[Bludger]

From another forum.
I never machine rotors. All the little grooves simply increase the surface area. Machining guarantees you'll need new rotors next change. A nice money making venture for brake shops. You actually pay them to machine your discs down to the wear limit and then buy new rotors from them. Incredible.


please discuss.

[BennyH]

Even if they only shave of a tiny amount off. it just doesnt make your brakes squeal and all. some customers are real flops complaining about squeals. but what he mentioned more surface area is also true. never thought about it like that.

[trism]

If they're taking that much off it only lasts 2 pads, I'd be going somewhere else.

The machining isnt to bring new metal to the top, its to take off all the high spots.

You should be able to machine rotors 3 times before replacing them.

The only time we sell new rotors are when the old ones have never been changed machined and are massively warped.

[stndrd]

I agree with trism om this. You will find with alot of european cars tho that the metal they use in the rotor is really soft and is only designed to last for 2 pad intervals then replace.

[na-118]

it increases surface areas, but will wear your pad unevenly.
how does machning garuntee you'll need new rotars next time when there is a limit?

machining the disc will get rid of small cracks, high spots, heat spots

[bennjamin]

Also the brake pistons have limited movement too. There is a reason for minimum thickness of disc.

[Lukey]

I agree with trism om this. You will find with alot of european cars tho that the metal they use in the rotor is really soft and is only designed to last for 2 pad intervals then replace.


we had a 1series bmw get traded in, and when they did the rwc the rotors needed changing and the pads still had heaps of meat left on em.

[Bludger]

interesting.

[curtis265]

Good info here guys.

I hear the pads deposit some stuff on the disc if too hot and that is one of the causes the warpage... the others having being cooled too quickly or parked with the handbrake after being heated to extreme temperatures

so IMO maching can remove the warpage but seriously shouldn't machined to the point where you need to replace them next time

[Jccck]

Most high performance Mercedes are designed to have Rotors and Pads wear out and be replaced at the same time.
It's still a rather costly exercise, but the stopping power is incredible!

[senna]

How does having grooves on the disc create more "useful" surface area? When you fit new pads the pad has a smooth surface, not a grooved one matched to your worn rotors! Imagine the first application - you may have as little as 30 or 40% of the new pad contacting the disc on light inputs which would then result in a less than desirable braking effect meaning the driver is braking for longer creating higher temperatures and glazing the disc and pad.

I struggle to see how anyone would think not machining a disc for a new set of pads is a good idea...

[mocchi]

Good info here guys.

I hear the pads deposit some stuff on the disc if too hot and that is one of the causes the warpage... the others having being cooled too quickly or parked with the handbrake after being heated to extreme temperatures

so IMO maching can remove the warpage but seriously shouldn't machined to the point where you need to replace them next time

cementite. i think thats what its called.
atom composition in rotor changed at few select spots due to elevated temperature, making these spots very abrasive and hard.
these cementite spots will wear less, other areas on rotor wear more. this unevenness gives warpage impression.


It gets worse. Cast iron is an alloy of iron and silicon in solution interspersed with particles of carbon. At elevated temperatures, inclusions of carbides begin to form in the matrix. In the case of the brake disk, any uneven deposits - standing proud of the disc surface - become hotter than the surrounding metal. Every time that the leading edge of one of the deposits rotates into contact with the pad, the local temperature increases. When this local temperature reaches around 1200 or 1300 degrees F. the cast iron under the deposit begins to transform into cementite (an iron carbide in which three atoms of iron combine with one atom of carbon). Cementite is very hard, very abrasive and is a poor heat sink. If severe use continues the system will enter a self-defeating spiral - the amount and depth of the cementite increases with increasing temperature and so does the brake roughness. Drat!http://www.stoptech.com/technical-support/technical-white-papers/-warped-brake-disc-and-other-myths

[na-118]

spot on!



How does having grooves on the disc create more "useful" surface area? When you fit new pads the pad has a smooth surface, not a grooved one matched to your worn rotors! Imagine the first application - you may have as little as 30 or 40% of the new pad contacting the disc on light inputs which would then result in a less than desirable braking effect meaning the driver is braking for longer creating higher temperatures and glazing the disc and pad.

I struggle to see how anyone would think not machining a disc for a new set of pads is a good idea...

[curtis265]

How does having grooves on the disc create more "useful" surface area? When you fit new pads the pad has a smooth surface, not a grooved one matched to your worn rotors! Imagine the first application - you may have as little as 30 or 40% of the new pad contacting the disc on light inputs which would then result in a less than desirable braking effect meaning the driver is braking for longer creating higher temperatures and glazing the disc and pad.

I struggle to see how anyone would think not machining a disc for a new set of pads is a good idea...

+1 if i could