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[I_want_an_Omega]

Ok, this is a picture of the rear o/s brake disc of my 2010 Mondeo. I took the pic when I had the wheels balanced recently.



Any idea what the black marks are? The guy in the tyre centre had never seen anything like it before - neither had the manager. 

The marks dont appear to be on the surface, well, they won't wipe off anyway. The discs are original and have done 45,000 miles. Also on original pads which are about 80% worn. The other side is exactly the same - but the fronts are unmarked.

Any ideas?

Thanks - Rob

[martin42]

change pads and disks,never seen that before 

[Andy B]

....
Any ideas?

Thanks - Rob

the marks just clock wise of the caliper seem to be the shape-ish of a pad, so could just be where they're been sat when parked for a while. If the disc is still true, I'd leave it - if it aint broke ....... 

[Entwood]

Often caused by the brakes being held on when the discs are hot .. ie after heavy braking ... usually on "boy racer" motors were they drive on binary throttles/brakes and then sit with the brakes held on at traffic lights.

It is caused by the formation of "cementite" within the cast iron disk and causes "Disc Thickness Variation" (DTV)

from  a good source of info : http://www.powerbrake.co.za/tech_info/tech.htm

Once you have the development of DTV the surface of the disc will begin to heat unevenly. The high spots will get extremely hot compared to the rest of the disc. When the temperature around these high spots reaches 650 – 700°C. the cast iron in that area will change structurally and transform into a material called Cementite. Cementite is far harder than the cast iron of the unaffected parts of the disc and will therefore wear considerably less as the disc wears down with use. Cementite also has very poor heat sink properties and will therefore continue to run extremely hot resulting in the rapid spread of the Cementite formation deeper and deeper into the disc. As a result the DTV will get progressively worse with time until it becomes literally unbearable to use the brakes. Depending on the pads used and the driving style of the vehicle owner this process could take 1000 – 6000 kilometres to develop and, NO, the initial run-out will not necessarily be felt by the driver. Hence the confusion and clinging to the concept of brake discs “warping”.

Article goes on further on the web site linked ...

HTH 

[hotel21]

Caused by frequent braking to a stop and footbrake remaining applied whilst disc is hot, as far as I understand it.  Its partly the reason why you should use park in an auto rather than sit with the footbrake applied at the lights.

In my humble opinion, of course.....   

[I_want_an_Omega]

Thanks guys - to be fair, the car does get parked up for days at a time so that might explain it.

I'll take a look after doing one of my 100 mile trips to the office.

[I_want_an_Omega]

Caused by frequent braking to a stop and footbrake remaining applied whilst disc is hot, as far as I understand it.  Its partly the reason why you should use park in an auto rather than sit with the footbrake applied at the lights.

In my humble opinion, of course.....   

Fair comment, but the fronts are totally unmarked .....

[Marks DTM Calib]

Usualy caused when they are parked up for months when up for sale either when new or second hand 

[Andy H]

Does the Mondeo use the disc for the parking brake?

[Andy B]

Does the Mondeo use the disc for the parking brake?

Looks like a disc/drum set up  ..... 

[Marks DTM Calib]

Standard handbrake disc setup on Mondeos (can be confirmed by looking for the spring on the caliper)

[78bex]

I have had a  similar looking bike disc.
But this was after track day events ie continual aggresive hard brakng etc.
The properties of the metal changes in the disc surface & it becomes brittle.
potential  MOT failure? 

[tidla]

Had many a situation like these on trucks when discs first came out.Trucks which were stood in a yard for some time and then taken for test would "fail" on cracked discs. Closer examination showed it to be pad outline marks.

[Andy H]

Standard handbrake disc setup on Mondeos (can be confirmed by looking for the spring on the caliper)

So - markings indicate that disc has been overheated and then stopped with the brake applied. Disc is used as parking brake therefore odds are that the parking brake cable is too tight (or 'driver error' has resulted in the car being driven with the brake on )

[feeutfo]

Not uncommon, if you look closely you can see the exact mirror image of the pad in the disc.

Note that some of the pad edge is missing.