Subject: 8f.16  Brake Squeal
From:  Jobst Brandt <jbrandt@sonic.net>
Date: Thu, 5 Mar 2009 10:13:52 -0800

Bicycle brake squeal is a problem that brake and rim manufacturers
have tried to suppress using various methods that achieve temporary
results.  One of the least effective of these has been to use harder
pad material or to embed hard fibers in pads because these harder
materials damage rims while giving poorer braking.  Squeal is an
interaction between smooth (homogeneous) metal rims and elastic brake
pads.

Atmospheric humidity adsorbs on rims and pads brake increasing
friction and is the cause of most brake squeal.  This becomes apparent
because brake squeal ceases when rims become hot from heavy braking.
Humidity increases friction by van der Waals adhesion effect, the
effect used to turn newspaper pages with a moistened finger.  How
tenacious humidity on surfaces is, becomes most apparent in vacuum
chambers that must be "baked" to rid them of adsorbed humidity to
achieve high vacuum.

Similarly "burning-in" brakes on steep descents drives off rim
moisture and stops squeal.  This differs from wet rims that offer
almost no braking until cleansed of bulk water.  This is most apparent
when riding in snow that collects on the inner rim circumference.  On
a subsequent descent, (poor) braking warms the rim enough to melt
accumulated snow, causing lubricating water to flow across the brake
track and diminish braking until all snow has melted.

Aluminum rims, made by extrusion, permit forming longitudinal grooves
into brake tracks as a means of reducing brake squeal.  The method
works until pads wear to conform to rim grooves and achieve full
surface contact.  Rims have also been made with machined grooves that
wear off to a smooth surface, especially during wet braking when road
grit aids in polishing the brake track.

Squeal is generated in the brake pad by a caterpillar-like wave motion
by stick-slip activation as the pad slides over the smooth rim.  With
grooved rims, the surface of a new pad is variably pressed against the
brake track so that no single stick-slip frequency is generated but
rather dissimilar ones that do not reinforce one another... preventing
squeal until pads conform to the contour or grooves become so small
that they no longer present variable contact pressure to an elastic
brake pad.

As brake pads wear, supported by slightly flexible brake calipers,
friction force rotates pads forward, thereby causing more wear at the
trailing than the leading edge, which causes toe-in so that brake pads
first make contact at the leading edge, making full contact only under
heavy braking that causes squeal.

Bending brake calipers to achieve toe-in does not prevent squeal
during hard braking because pads make full contact with the rim in
spite of toe-in.  Bending calipers is "cold setting" of aluminum that
readily causes structural damage.  Besides, this remedy can lead to
more bending with each occurrence of squeal that is better abated by
heating through hard braking.

The effect that causes brake squeal can also be heard when cars travel
on curving roads while crossing glossy paint or crack-filler tar
stripes.  Railroad tracks and metal utility covers have the same
effect.  Similarly, strong acceleration that causes wheel-slip causes
squealing tires from adhesion between a soft and hard smooth material.