Re: [Amc-list] Re; Drilling brake drums.

[Tom Jennings <tomj@xxxxxxx>]

On Sun, 18 Feb 2007, John Elle wrote:

> I did a little web searching on this subject too and it comes back to me
> as a mixed review. The technical answers seem to support that it is a
> waste of time and money.

Well I don't think it's billed as a revolution, only an
incremental improvement. It's an old trick, I assume it predates
simple junkyard-parts disc brake swaps.

There may be hard braking/hot shoe outgassing improvements,
but I'm with you, they have got to be minimal, esp. with modern
linings. The holes as I recall are 1/8" or so, and a dozen
of them sweeping across the linings isn't going to improve
braking much.

Where *I* think they will improve things is in cooling off
hot drums.  Centrigugal force "should" pump air out the holes,
right where it will be hottest.  There's air intake at the
backing plate/drum flange area.

I bet it does improve wet-brake drying too.

But mainly, it's an old-timey fixup for old brake
systems. Swapping in discs has got to beat it 5 times over
though, bang-per-buck.


> As far as drum brakes failing due to heat, that was a typical failure
> mode and I have experienced that too. It was always my understanding
> that heat caused the drum to expand thus reaching a point where the
> travel supplied by the wheel cylinder was exceeded so the wheel
> cylinders capability of supplying pressure to the brake shoes was
> greatly reduced. Even under the condition of over heated, they did not
> get anywhere near the temperature of a disc brake assembly that was
> working normally or even hard.

I dunno -- in my and everyone's experience, whatever the
mechanism, one really hard, fast stop from high speed sure took
"adequate" size front drum brakes out of commission for quite
some time!

When drums get hot they "bell" -- hot metal expands a lot. The
flat face of the drum keeps it more or less the same diameter
as cold, but the outer edge -- along the backing plate -- gets
larger. Some braking force gets wasted pushing shoes sideways.

When a disc rotor gets really hot, it gets slightly larger
diameter (no effect) and slightly thicker (pushes against the
pads! I doubt anything usefully measurable). But they don't
change geometry. Also, hollowed discs pump cool air like mad
from the center out through the edges.

Disks beat drums at everything except pedal force (drum shoes
self-servoing) and looking retro :-)
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