Cracked brake discs?

F

Felixlamb

Member
Hey!

Fitted my winter wheels recently and was just checking out my tire pressures. I noticed that both rear brake discs have what looks like cracks on them. Each rear disc has 3 or so of these lines. I’ve tried to run my finger over them and I can only barely feel it.

I believe front and rear discs are the same age, and have never been changed. I’m at 50k miles so there about due anyway. I can’t see these lines on the fronts, which makes me wonder if these lines are just where the edges of the brake pads have been clamped when the parking brake has been on?

jM5ry2u.jpg
Pl4EviS.jpg
Ax4nSr4.jpg

I have a set of MTEC discs and EBC pads ready to fit, but I was hoping that I could do them at the same time as I took the winter wheels off next year. But clearly I don’t want to be driving around in a ticking time bomb either…
 
This is the outline of the brake pad chum, in the top picture you can clearly see that as the line approaches the centre hub it turns to the left adopting the same shape as the outline of your brake pads. Nothing to worry about. Dont leave your handbrake on when the car is left for long periods, you always run the (small) chance of the pad 'rusting' to the disc.......
 
Gotcha! Thank you very much mate! Checked the pics against the EBC pads I had delivered and the curve on the corners is pretty similar.

fc645ap.jpg
 
It probably is a mark from the pads but just to be sure take a laundry marker and mark the outer disc where those lines are, take the car out for a spin being sure to hit the brakes hard a few times.
If they are just marks then they will be gone when you get back.
 
Give your disc a light tap with something metallic.

If it rings you're fine, if it thuds you'll be reaching for the toolbox.
 
To be honest the close up looks like a crack, there is corrosion dust in the crack. Some discs the fining inside is radial straight out like a cart wheel and some are angle like a hoover fan (Thus the angled line). If stood a long time could be where the edge of pad has touched the disc and caused corrosion..
 
bob4333 said:
Give your disc a light tap with something metallic.

If it rings you're fine, if it thuds you'll be reaching for the toolbox.
Click to expand...
Wheel tappers and shunters.
Was that a tv programme based in a club?
 
flybobbie said:
Wheel tappers and shunters.
Click to expand...

Showing your age there bobbie! :P

FWIW I agree it looks like a hairline crack in the close-up picture.
 
If you can't be sure after ringing the disc, do you know any welders or engineers? If you do then one of them might be able to loan you a dye penetrant kit. It's an easy two minute job and would give you a definitive answer. Good luck.
 
Hmm okay, so I’ve given all 4 brakes a decent enough tap, none sound dull and they all resonate somewhat, but I wouldn’t say they ring per se.
It’s my daily driver so certainly for the past 9 months or so that I’ve been going to the office it’s been driven every weekday, so not like it has sat with the brakes engaged for ages.

Looking at the schematics of the rear brakes, it seems that the ventilated fins run in a straight line, rather than at an angle. Can someone verify this? If this is the case, I’d hope they’re not cracked as it’d be odd for the crack to not run in line with the ventilated fin?
 
OP, I can't answer your question but this is an interesting extract ref M3 / M4 braking systems. If I understand it correctly it's telling me that surface cracks are acceptable.....? This is a different braking system to the Z4 but principles may be comparable.

"Cracks form in particular wherever relatively cold discs are suddenly subjected to a large load. The cracks that occur are limited to the surface areas. For instance, if you have been driving your car fully laden on the motorway for an extended period without braking and then you are suddenly called upon to perform an emergency braking manoeuvre, the brake disc will become very hot in a very short space of time – in such a case, the temperature can increase from around 20 degrees to 450 degrees.

If the brake disc is required to perform in this manner several times in rapid succession, it is quite possible for surface cracks to form in the cast material of the friction ring – especially in the area around the perforation holes. In sports cars, cracks like these are, more than anything, a sign that they are being subjected to the use for which they were designed, and they have no negative effect on the service life of the brake disc or its braking performance. When it is time to change the brake pad, it is also necessary for the car dealer to inspect the brake disc."

I offer this for thought rather than recommendation.

Full article here:- https://www.bmwblog.com/2014/04/13/need-know-bmw-m3m4-brake-system/
 
To me they don't look like cracks but rather a line that has rusted into the disc and then had the iron oxide cleared out. The second pic particularly.
 
If it was my car, I would just change the discs to alleviate any doubt.
I wouldn't feel happy driving around wondering if my discs are cracked.
 
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