[eye30]

Only noticed today that both rear brake discs are slightly marked with loads of small black spot marks.

Front discs are also marked but not to the same extent.

When I ran my finger over the disc they felt smooth, no indentations.

The discs are bright silver, not rusty colour, so the brake pads are making contact when depressing foot brake.

Any ideas what may be causing this??

[bumpkin]

I have spent a bit of time thinking about this one for you eye and have come up with the following guesstimation

Everything is porous, even items which we think are completely solid have spaces between the molecules which make up their construction.

Now the next thing is that temperature makes things expand and contract so when you are driving and braking the discs are hot and expanded slightly and then they contract when cool.
We have all seen that horrible brake pad dust on our alloys, so what I suspect you have is small portions of dust going into the disc and then being compressed into small circles when the disc cools.

The reason it is more apparent at the rear than the front is probably due to the brake bias being more to the front, the braking is firmer which causes the expulsion of the dust onto the front wheels which does not happen to the same extent at the back, brake dust is a bigger problem on front alloys than the back ones.

I have not checked mine to see if it is the same and I guess no-one else has either since I am the only response.
Only a theory, open to debate and question.....................

Cheers
Brian

[Dazzler]

Could it be the salt they put on the roads over there (Still wonder if that is what upset Shambles alloys) .. I would be horrified if they put salt on our roads when it snowed 

Guess the salt spraying up from the tyres could have caused the rust in Shambles wife's fuel system too?

Without much thought, I would expect the dust molecules to be MUCH larger than any "holes" in the rotors.

[bumpkin]

Without much thought, I would expect the dust molecules to be MUCH larger than any "holes" in the rotors.

I don't think there are "holes" in the discs Pip, I think it is just the porosity of the metal used.  Also brake dust molecules on your alloys are not large, it is a very fine dust which if you collected would weigh very little.

[bumpkin]

Could it be the salt they put on the roads over there (Still wonder if that is what upset Shambles alloys) .. I would be horrified if they put salt on our roads when it snowed 

Guess the salt spraying up from the tyres could have caused the rust in Shambles wife's fuel system too?

Salt may be a factor and I would certainly agree it could be what attacked Shambles alloys, however even then I doubt it completely, I guess that he just had a badly made set of inserts which may not have been proofed properly which then allowed unusual corrosive attack.

I don't think salt would contribute to the petrol tank filler being corroded, the plastic wheel arch liners SHOULD prevent ingress of salt to the filler area.

Without much thought, I would expect the dust molecules to be MUCH larger than any "holes" in the rotors.

I don't think there are "holes" in the discs Pip, I think it is just the porosity of the metal used.  Also brake dust molecules on your alloys are not large, it is a very fine dust which if you collected would weigh very little.

I get your point but I'm suggesting that the size of the dust molecules will be larger than the space left between the metal molecules to make ingress impossible.

My bet would be heat discolouration and of no consequence.

[bumpkin]

I agree with your last point Pip, either explanation are of no consequence.

[Shambles]

Guess the salt spraying up from the tyres could have caused the rust in Shambles wife's fuel system too?

We'll never know, thankfully.

PS, I'm not married

[Dazzler]

Sorry.. I meant "significant other" , "she who must be obeyed" , "the old ball and chain",  "your old lady...." 

[Shambles]

PPS.. no pitting on my disks as I can see.

[eye30]

PS, I'm not married

https://www.i30ownersclub.com/forum/index.php/topic,2278.0.html

"The missus has been going on, and on, and on, about me having a new car while she bumbles around in her 1997 RAV4"

[Shambles]

"The missus has been going on, and on, and on, about me having a new car while she bumbles around in her 1997 RAV4"

I call her the missus but we're not married.

I'll never remarry, although it seems like it sometimes after being together for 17 years

[eye30]

I'll never remarry, although it seems like it sometimes after being together for 17 years

Ahh the modern marriage

[eye30]

Took these today of the brake discs.

Not all that clear showing the pitting:

[bumpkin]

They look pretty normal to me, my 54 plate Imprezas discs looked like that and it did not have any effect on the braking ability, just a bit unsightly.  Is this the first time you have had such "open" wheels to see what the discs look like?

[eye30]

No.  Just that today the weather was OK so gave the Beauty a good wash and clean out.

As I say the pictures don't really show the pitting but I'll keep an eye, so to speak, over the next few days and hope it was just due to salt etc on roads.

[bumpkin]

I've noticed after washing that the discs take on a pretty horrible rusty look also.