Track day - rotor/disc replacement

[Dranter]

Hi guys,

For those of you who track your cars with the stock brakes (pad irrelevant), when do you change your rotors/discs:

- when they are below the recommended by BMW thickness (also how problematic is it if they are indeed below)

or

- when stress cracks appear connected, go all the way to the edge etc

Thanks in advance

[hC1001]

Thats correct. Which ever comes first.

When they get below minimum thickness, cracks form and connect much faster and the rotor can fail catastrophically when tracking.

If you can catch a finger nail on a crack, it might be another sign for replacement. This means the crack is large and when it gets heated up it can spread out faster and potentially cause the rotor to fail.

[CanAutM3]

The latter.

When cracks interconnect between the cross drilled holes, or more urgently (meaning immediately) when a crack reaches the edge of the rotor.

I've never bothered measuring rotor thickness.

[jfritz27]

Does anyone know the official criteria that BMW uses that is listed in their system? I did opt for the brake replacement program when I bought the car, want to be sure I get the most of it, but don't want to waste time taking it to the dealer to have them check if I can ascertain myself.

Thx

[swagon]

Originally Posted by jfritz27

Does anyone know the official criteria that BMW uses that is listed in their system? I did opt for the brake replacement program when I bought the car, want to be sure I get the most of it, but don't want to waste time taking it to the dealer to have them check if I can ascertain myself.

Thx

Min. disc thickness up front is 28.4 mm and 22.4 mm rear. 1.6mm wear permitted according to BMW. https://www.newtis.info/tisv2/a/en/f...als/34-brakes/

[edwinm3]

a cracked rotor on the track is a pretty good recipe for disaster

[robbo mcs]

As a general rule, the minimum thickness is generally stamped on a disc somewhere, usually near the hub, or sometimes on the edge.

The minimum thickness means you should replace at that point when changing pads. It does not mean if you are halfway through a set of pads and you measure it and it has gone under, that you need to replace them immediately. In effect, it is designed to only be measured when changing pads. What that means is there is some leeway.

Also, worth remembering that as the disc gets thinner it loses thermal mass, and becomes less effective. That means you can see higher disc temperatures on a thin disc compared to new. Similar principle on brake pads, a thinner pads works less well.

Just general information

[CanAutM3]

Originally Posted by edwinm3

a cracked rotor on the track is a pretty good recipe for disaster

This is not an absolute. A certain amount of surface cracking is normal as is not an issue.

[edwinm3]

Originally Posted by CanAutM3

This is not an absolute. A certain amount of surface cracking is normal as is not an issue.

right, should have said an out of limits crack

[mynameisplayer]

So....one more track day in these?