DDK - Die Deutschen Klassiker

Whilst furkling about with my driveshaft under the car (69 911T) today I spotted these marks on the inside of the rear brake disc where the holes are. They are very odd.
IMG_20200104_115128 by graham Ridgway, on Flickr

They almost look like cracks, but the shape isn't right. They are definitely indented like scratches.

I'm tempted to ignore them, but I'd like to work out whether to or not! They are not on the other disc and only on the inside.

Cheers
Graham

Interesting, good spot! Not sure what they are but some look like they go through the vent holes, so stress cracks may be an option. I would take them off, have a better look and replace if in doubt

The others are OK, so I would change both rears for fresh with new pads.
Do people skim discs nowadays? I would suspect corrosion in the cracks, if a skim to specs cleans them up OK, but prob cheaper to replace.

Have you checked the pads

Graham, if I saw that pattern in the natural world, I.e. rocks or a geological map i would call them "en echelon" faults or cracks. It is essentially a shear stress.

It looks like your LHS rear? Do you left foot brake at all? I.e. some braking and some power application? I am wondering if the pads are trying to grab the outer section of the disc while the drive shaft is trying to turn the hub, hence creating a shear stress. It's interesting that they have set up around the line of your drillings. Stress risers?

Were the discs bought that way or have you had them drilled?

Conjecture on my part I think. But if it were my car, i would whip them off to have a closer look... Matt

Thanks all I'll take apart and investigate further.
The discs came on the car when I got it s couple of years ago. I've not succeeded in doing many miles and only road use and nothing strenuous like left foot braking.

However I have no info on their history.

They are very regular marks and I don't think they are related to the holes particularly.

How unusual!

Graham

Come to think of it Graham, the need for left foot braking would not be needed. The area of the brake pad inside the diameter of the drilling compared to that outside is very small. So a large imbalance of force either side of the drillings. That could set up a shear stress across that line of drillings which is an potential plane of weakness.

Interesting for me as I'm a bit of a nerd.

Look forward to seeing the result of your investigations.

I have a feeling I'll take the disc off and there will be a set of marks all the way round and we'll be none the wiser!