From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Brake Toe-In: Define "Front"?!!!
Date: 11 Jul 2000 17:37:13 GMT

FAQ:

Brake Squeal

Most brakes on cars, motorcycles, and bicycles squeal at one time or
another, because they involve stick-slip friction, something that
should occur at frequencies above the audible range.  Squeal is not
only annoying, it decreases brake efficiency, especially in the lower
frequencies where the slip part exceeds the stick part of friction.

Brake noise requires elastic motion (vibration) to occur on a moving
brake surface with a component in stop-start motion.  Bicycles in
particular, not having power brakes, require a high coefficient of
friction and therefore use brake material that is soft and flexible
enough to achieve good contact on relatively rough rims.  Bicycle
brakes generally have a mechanical advantage between 4:1 and 6:1, as
described under "Brakes from Skid Pads to V-brakes."  That's not much
compared to motorcycles that have hydraulic disk brakes and almost no
pad clearance to take up.

Soft brake pads and flexible light weight calipers invite squeal and
chatter, chatter being the mechanically more detrimental version of
stick-slip behavior.  Brake chatter is caused by gummy residue on the
rim together with excessively flexible (skimpy dimensioned) brake
calipers.  Rims can be cleaned but flexible brakes can only be fixed
by using better brakes.  If the rim becomes gummy again after
cleaning, then either the rims are being contaminated by something
like riding through tar weed or the pads are no good.  My solution for
pad quality is Kool-Stop salmon red pads.

Squealing brakes, the more common problem, involves mainly the brake
pad that generates caterpillar like surface waves.  The usual advice
is to bend the brake calipers to make the trailing edge of the pad
(with respect to rim motion) contact first (the forward end of the
front brake pad).  This is not entirely without merit because this is
the natural state of a used, non squealing brake.  Elasticity of the
caliper, however small, allows the pad to follow the rim and rotate
forward about the caliper arm, wearing the heel of the pad more than
the nose, causing toe-in.  Toe-in is preferred because a pad that
makes full contact as it first touches the rim will rotate slightly
from frictional drag, reducing contact... and drag, which allows it to
snap back and repeat the action.  This causes surface waves in the
pad, especially when it is new and thick.  For this reason, some pads
are made with thin friction material to reduce elasticity.

If the pad contacts the rim, trailing end first, it achieves full
contact stably as pressure and frictional drag increase.  However, the
brake may squeal anyway.  This can occur with new rims or one with wax
or oil, or from other contaminants like those from riding over a moist
lawn.  Machined rims have machining grooves, a roughness that reduces
squeal tendencies so they don't have to be "broken in".  Some new pads
also may have a glossy skin that should be removed either by sand paper
or use.  Many types of residue can increase stiction (stick-slip) that
is easily removed by abrasive scrubbing.  This can be done by braking
at moderate speed with household cleanser on a moist rim, followed by
a water bottle squirt rinse (while still braking).  This effect is
more easily and conveniently achieved by riding through a long mud
puddle while braking.  Better yet, descend a mountain road in the rain
where there is usually plenty of fine grit and where rain supplies the
rinse.

Avoid bending brake calipers.  This is "cold setting" in its worst
form.  Aluminum in such cross sections doesn't bend far without
structural damage.  Besides, this remedy could lead to more bending
with each occurrence of squeal that is better abated by other means.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Squealing!
Date: 15 Apr 2001 00:27:28 GMT

Dallas Cowboy? writes:

> I have linear brakes in the front and even though they work like
> nobody's business, they started squeaking a few weeks after I bought
> the bicycle.  What's going on!?!?

Friction sliding is in fact, "stick-slip" motion in which drag is
generated by the friction material being locked to the rotor (rim) and
exerting its elastic restraining force on the moving surface until it
breaks its fixed bond and its melting surface slides with little drag.
At the lower drag, friction material jumps ahead and comes to rest on
a new position to repeat the sequence.

Ideally, this occurs at frequencies far above audible sound for two
reasons.  First because you can't hear it, and second because it has a
better duty cycle of stick to slip, that is, it restores its grip
sooner.  Because brake pad material is homogeneous, it generally
operates in multiple synchronus waves in the surface of the brake pad
so that there are many ripples of caterpillar like stick-slip nodes.
If the surface becomes contaminated with a material that has a lower
melting point, its shear strength diminishes drag by causing longer
slip waves at a lower frequency just as it would with too soft a
material.  With gummed up brakes, the stick-slip duty cycle degrades
braking noticeably.

While standing after coming to a stop, this phenomenon can be observed
by rocking the bicycle back and forth while lightly applying the front
brake to cause brake chatter even though in use the brake appears silent.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jobst.brandt@stanfordalumni.org
Subject: Re: Recommended KoolStops ?
Newsgroups: rec.bicycles.tech
Message-ID: <iYzae.16030$m31.146575@typhoon.sonic.net>
Date: Sat, 23 Apr 2005 22:26:22 GMT

Sheldon Brown writes:

>>> The salmon or dual material looks like the best overall choice for
>>> material.

>> Why would you want "dual material?"

> I believe the dual-compound models are less prone to squealing than
> the all salmon versions.  Some brake systems have a big problem with
> this when used with high-friction brake shoes.

It's been my experience that with freshly lubricated brake caliper
(with grease in its pivots) brakes lose their tendency to squeal.  I
stumbled onto this by accident once when my brakes were squealing on
hard applications and it was also spring cleaning time at which time I
cleaned and re-lubricate components.  No changes were made to the worn
pads, only the pivot shafts and washers were greased.  It seems that
little bit of viscous damping is enough to have a significant effect.

Jobst.Brandt@stanfordalumni.org