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discs rusting after rain

Discussion in 'Technical and Troubleshooting Torque' started by ben1100, Jun 6, 2012.

  1. Hey guys,

    Been a pretty wet melbourne last few weeks and to my surprise i've noticed that the discs on the front of the bike get a little rust coating the day after a wet ride.

    The rust coating comes off within a few minutes of using the brakes while riding.

    This only happens when the discs are wet, and occurs overnight. I ride the bike everyday and notice the rusting surface in the morning, usually 8-12 hours after the bike was last ridden.

    The discs are brand new, replaced only a few months ago, and the pads were changed at the same time.

    Has anyone else experienced this?

    Just gathering thoughts before I pursue a warranty claim.



    Attached Files:

  2. Normal.

    Back in the dark ages, the Japanese tried to make disks out of stainless. That meant they didn't rust, but they were poor performers in the dry and diabolical in the wet. At the same time, European and American companies were making disks out of ductile cast iron. Great thermal and friction qualities, heavy as lead, but they'd rust - and not just a light coat of surface rust, they'd get pits and holes and flakes of it ... These days most bike brakes are made of some kind of steel alloy or other, that has a degree of rust resistance, reasonable friction and thermal qualities, is strong enough to be light, cheap enough to replace ... it's a compromise. It's not perfect at any one thing but it does the job. And it will rust a little.

    Don't go looking for something you can put on the disk to stop it rusting. Anything that has the slightest chance of working, will ruin your brake performance.
    • Like Like x 5
  3. bike under cover Ben?
  4. Yeh bike is garaged, but i don't wipe the bike dry after a wet ride, so it usually stays wet.
  5. thanks for the clarification guys. perfect answer kneedragon! cheers

    I haven't treated the disks, i'm not stupid haha. I've just never had an issue with the rust before. I guess these disks are a little more reactive than the previous pair.

    All good, i'll leave them as is
  6. If you have a compressor a quick blow off on the discs will help
  7. Will happen every time they get wet and you don't brake.
    One quick brake and they will be all shiny again
  8. Go look at parked cars after it rains overnight, same thing happens
    • Like Like x 1
  9. Hmmm, interesting when I've rode in the rain I've never had rust appear on my discs, in fact the only bit of rust I've found on my bike is a little pit in the chrome nut on the triple-clamp.

    While i agree that this phenomenon is normal its got me wondering what type of discs my bike has on it.
  10. Maybe you get on the brakes a bit just before home and the discs are hot enough to dry themselves out?
  11. I know in car brakes which isnt really relevant
    Cod they use a cast iron , but different brands
    Will have different mix and will not only perform
    Different but will differ in apearance
  12. Quick metallurgy lesson. There are roughly 150 odd different grades of stainless steels. These can be grouped into 4 main catagories. Austenitic, Ferritic, Martensitic and Duplex. They all have different mechanical and chemical characteristics. Austenitic is probably the most common, used in Architectural, chemical handling and marine applications. It is non magnetic and resists rusting/corrosion very well. But would make totally sh!t brake rotors as its thermal conductivity is abysmal to say the least.
    I would suspect brake discs would be made from a Ferritic grade of SS. It is still a SS but is magnetic, and will rust a bit. Far better thermal conductivity, as it's used in Furnace applications and the like. Just because it rusts, doesn't mean it is or isn't stainless steel. Your discs will simply be made of a different grade of SS with a slightly different chemical composition.
    Car brake disc are all made of Cast Iron as far as I'm aware. Apart from the odd exotic Ceramic brake package offered by some Euro manufacturers.That's why they will develop surface rust.
    Lots more to it than that, but will do for now :)
    • Like Like x 1
  13. @ OP = normal.

    Rust = oxidation (essentially surface chemistry changing)
  14. Got aftermarket cast iron discs on my old K100. They'll stand the bastard on its nose if asked, but go brown even with a light dewfall. They show no signs of wear after at least 80,000 km.