Why aren't discs made of stainless steel?

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  • Why aren't discs made of stainless steel?
  • teef
    Member

    After clean the bike and leaving in the garage for a few days the discs always have a small patch of rust where the discs lined up with the calliper. It got me thinking – why aren’t discs made of stainless steel?
    Is it a cost issue or is stainless steel an unsuitable material?

    Premier Icon wwaswas
    Subscriber

    I always though the rust was from the pads rather than the disks?

    Premier Icon cp
    Subscriber

    they are made from stainless steel.

    however, different grades of stainless have differing levels of ability to prevent rust.

    Additionally (and I speculate a bit here) I imagine in sintered pads there are metals which corrode… which get spread all over the disc surface and can cause spots of rust… and like you describe, when static, the rust on the pads will discolour the disc around the caliper.

    PeterPoddy
    Member

    Why aren’t discs made of stainless steel?

    They are.

    “Most bicycle brake rotors are made of stainless steel, although some lightweight rotors are made of titanium or aluminum”

    From here

    http://en.wikipedia.org/wiki/Disc_brake

    Craggyjim
    Member

    Yep it’s the brake pads leaving surface contaminants on the discs that rust. It’s like the rust you get in stainless allen bolts. This comes from contamination from a non stainless allen key.

    Premier Icon faint
    Subscriber

    unless you drag your brakes enough to get the temperature high enough to release some of the iron content

    +1 what they said —^

    But, cast Iron (a type of steel despite the name) has a higher friction coefficient than stainless, hence the use of Steel in car brakes, and (before the days of carbon/ceramic) cast iron disks on race cars/motorbikes.

    bristolbiker
    Member

    I had some rotors that came with BB7’s and they are cheerfully rusting around the cutouts and holes. Put it down to being laser/plasma cut from sheet of lowish chromium content stainless.

    gordonb
    Member

    as far as I am aware, dics are made from a “Martensitic” grade of stainless steel, because martensitic grades can be machined to shape in a soft condition, and then be heat treated to a harder condition. The down side to martensitic grades is that their corrosion resistance is not as good as the Austenitic grades ( 3-4, 316 etc). Which is also why the blades on your kitchen knives, which are matensitic, will corrode long before the forks, which are 18/10 ( austenitic ).

    ooOOoo
    Member

    some great material geekery guys 😀

    I love singletrack sometimes.

    gordonb
    Member

    sorry guys, I sell stainless steel for a miserable living!

    stumpy01
    Member

    bristolbiker – Member
    I had some rotors that came with BB7’s and they are cheerfully rusting around the cutouts and holes

    The discs on both sets of my avid brakes do this if I go on a wet ride & then forget to take the wheels out of the wheel bags when I stick them back in the shed.

    bristolbiker
    Member

    The discs on both sets of my avid brakes do this if I go on a wet ride & then forget to take the wheels out of the wheel bags when I stick them back in the shed.

    I think my bike should come and live with you!! Their road discs on my commuter. 5000 miles a year, year road, rain, snow, road salt… might get a wash and brush up once a fortnight if it’s lucky… I’m sure it dreams of wheelbags when its lurking in the shed at night 😉

    Avid and Hope rotors rust like crazy if left somewhere damp, as do cheap Shimano rotors, but XT/XTR rotors don’t rust at all…

    Premier Icon bullroar
    Subscriber

    some great material geekery guys

    And Martensitic steels have different thermal properties to the Austenitic steels which is beneficial for discs. Martensitic will tend to expand less and conduct better.

    I read somewhere that higher end rotors have a stainless braking surface and aluminium carrier to make them lighter and conduct heat better.

    stumpy01
    Member

    I think my bike should come and live with you!! Their road discs on my commuter. 5000 miles a year, year road, rain, snow, road salt… might get a wash and brush up once a fortnight if it’s lucky… I’m sure it dreams of wheelbags when its lurking in the shed at night

    The wheelbags are to try & stop my car getting covered in muck……seems stupid to have them sat in the loft with my bike bag, when they could be put to use.

    See here:

    You are welcome to deliver your bike to me though for safe keeping if you want….. 🙂

    oliverd1981
    Member

    I read somewhere that higher end rotors have a stainless braking surface and aluminium carrier to make them lighter and conduct heat better.

    See Hope floating rotors and Shimano centrelok. I thought the main aim was to allow the disc to expand and contract around it’s diameter without getting warped through the tension in the disc’s “spokes”

    geetee1972
    Member

    GordonB – love that insight! Seriously.

    Perhaps you can comment on this then. I actually managed to crack one of the arms on Formula rotor; this wasn’t an aluminium carrier version, just a regular stainless steel one. If you see in the picture below there are spiral like arms that carry the outer braking surface. One of these cracked at the base near the bolt hole.

    I’ve never seen this happen before.

    Yea, 2 part disks will be worse for conduction as the interface between the two wont be very conductive relative to a solid disk.

    New XTR has an option for an aluminium/stainless composite with a stainless surface and aluminium core which will be more efficient.

    Premier Icon bullroar
    Subscriber

    From Shimano:

    “The disc brake rotor requires the best balance between heat radiation, strength, weight and stability. Higher end rotors are constructed of two pieces; the stainless steel braking surface and the aluminum carrier. The use of two different materials allows for quicker heat radiation and a weight savings over one piece stainless steel rotors. The addition of holes in the rotor braking surface allows for greater heat radiation, weight savings, pad contact surface cooling and rotor stiffness.”

    gordonb
    Member

    GeeTee1972 – sorry bud, just a dumb-ass salesman, not a metallurgist. I know the very basic principles, but that’s it… but sometimes metals crack due to heating/cooling, and stress, so heat the brakes up on a descent, splash through deep puddles etc…

    Premier Icon Northwind
    Subscriber

    With motorbikes, you can get ductile iron rotors but I’ve never seen any for pushbikes. The benefits are supposed to be better braking, the downside is having to savagely kick your brake calipers every morning all winter because the pads have rusted onto the rotors and the bike won’t move.

    Premier Icon speaker2animals
    Subscriber

    Ask anyone who rode an early 70’s japanese motorbike with disk brakes about disks that don’t show up any rust. May look good but terrifying when you actually want to stop. As stated if memroy serves one of the hop ups for disk brakes was to fit cast iron disks that actually did allow you to slow your machine down.

    hamishthecat
    Member

    Ask anyone who rode an early 70’s japanese motorbike

    Agreed – my 400/4 front brake was appalling. And it got worse once the calliper sized up. 🙄

    Pieface
    Member

    Only Iron rusts, everything else corrodes

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