Re: DIY Brake Plumbing

[James Montebello <jamesm@domain.elided>]

On Sat, 12 Oct 2002, Allen Mitchell wrote:
>
> It is my understanding that steel fittings and lines should be used for
> brakes, not aluminum.  Also, I was under the impression that steel and
> aluminum fittings/lines should not be mixed together.
>
> I am sure that some of the digesters have strong feelings about this.
>

Aluminum can be used, as long as there's enough of it.  After all there
are plenty of aluminum brake calipers out there.  The difficulty comes
in where someone takes a steel fitting and simply makes that in aluminum.
Since many fittings, like adapters and such, can't be made in a different
size due to matching threads and simple space constraints, you end
up with a part that's 1/3 the strenth of the steel part.  However, if
it can be done, and you make the Al part 3x as big as the steel part,
it will have equal strength (this is an oversimplication, but you get
the idea).

As for mixing Al and steel, the risk you run is galvanic corrosion,
which will slowly eat the Al part.  Heat and the addition of water
accelerate this process, so this is indeed a bad thing for brake parts.
The Al turns into a white dust (Al oxide), which also happens to be
extremely flammable.  The way to fix this is to ensure there's no direct
contact with the Al and the steel.  Steel pistons in an Al caliper are
safe, since there's a rubber seal acting as an electrical insulator.
Coating brake pad pins in some sort of anti-seize does the same thing.
If you've ever pulled a pad pin and found it was white after you struggled
to free it from the caliper, galvanic corrosion is what's happening.
Usually, steel fasteners meant to be used in Al parts are plated with
some metal (cadmium, chromium, or nickel) which tends to reduce oxidation
and the galvanic effect.

james montebello
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