Brake discs - corrosion?

[Jings]

Plenty of life left in the front disks, well, plenty of wear left I should say. However, mechanic reckons they should be replaced as there are signs of corrosion on the surface which he suggests could be all the way through.

The corrosion looks exactly like the shape of brake pads, and I recall that when I left the bike unused for 3 weeks over Christmas, the front brakes had bound on and required a hefty 'clutch-dump' to free them. Can the corrosion be that bad after only 3 weeks?

The mechanic was suggesting that under heavy braking, the discs could fail. Is he being over-cautious?

[gfxmonkey]

How many miles have you done? I had front and rear discs replaced at 45,000. I commute on mine into London and due to the amount of braking I do they were near their limit for a mot so had them replaced. Not cheap I might add!

Glen

[Jings]

25,000 miles on these discs, almost all of it fast A-road and motorway miles, so no hard braking and hardly any wear on the disks.

Thing is I've got to make a special quick trip back to the UK next month (on the bike) just for its MOT and I don't want to either fail the MOT or keep riding with a genuine safety hazard. Just wondered what the esteemed collective's views were before I order the discs and get them fitted for the MOT.

[gfxmonkey]

You can get the discs checked, there is a minimum threshold after which they are deemed unfit for purpose.

[RocketSteve]

Minimum thickness is written on the disc; 4.5mm.

I'd be very surprised if surface rust on the disc would be an MoT fail.

My MoT guy is a bit tetchy on the brake tester needle if it starts flickering though, this can be noticed if you feel pulsing on the brake lever in severe cases.

I just give the bobbins a good clean out before the test and spin them round with a screwdriver.

[ShaunDW]

I would also be surprised if such corrosion went all the way through on otherwise road-worthy discs. I have noticed a similar corrosion pattern on mine several times, but the feel of the brakes is still fine. To be honest, I'm not sure whether it is the same corrosion I am spotting each time - but I have never thought so. I have always expected such corrosion to disappear after a few good, hard brake applications, and occasionally do just that (when its safe to do so). Jiggling the bobbins also helps keep the disc floating as it should.

[Volfy]

Sounds like brake pad material transfer: https://www.dba.com.au/tech-article-to31-friction-material-deposits/

This could happen if the rotors and pads are hot and you hold pressure on them for an extended period of time.  Or it could be caused by water/moisture between the rotor/pad while sitting in you garage.  If you have air in the brake fluid, it may expand and force the pad into the rotor even without you applying pressure to the brake lever.  Binding brake cylinder could do the same.

It isn't corrosion of the brake disc, in the sense of the stainless steel rusting.  And it sure isn't all the way through.  You need to distance yourself from that mechanic.

It's a relatively easy fix.  You can either have the discs turned lightly, or you can remove the deposit with a brand new and clean polishing wheel.  If you let it sit like that unfixed, it may worsen into the pulsing brake that a lot of TTSE riders think are warped rotors.

[ShaunDW]

Originally Posted by Volfy

It isn't corrosion of the brake disc, in the sense of the stainless steel rusting. 

Are the TT brake discs stainless steel?
Mine regularly acquire (what looks like) a covering of rust after being parked in the rain or ridden in salty conditions.
Could be metal deposits from the sintered pads I suppose...