Theory of brake shoe positioning

To my vast collection of superflous bicycle tools I have recently added a "BrakeShoe
Tuner" from the dutch company Tacx. (www.tacx.nl) Now the doubt is nagging
me, whether it actually belongs in this collection. ;-) Because it seems
that it not only makes the brake shoe tuning slightly easier. Also better.

With this tool the brake shoes are not positioned parallel to the rim (as I
have allways done it) but slightly tilted/pointed in the forward direction.

Like this: /H\ (Imagine the H is the rim, the slashes are the
brakeshoes, up is forward.)

"Only then is the braking power optimal", the instruction sheet tells me.

Is this true? Is the braking power theoretically better with a tilted /
pointed brake shoe position?

Would the difference be significant in practical use?




Ivar of Denmark

Ivar Hesselager wrote:

Is this true? Is the braking power theoretically better with a tilted /
pointed brake shoe position?

I only angle mine if they're squealing (the dirty little rats...)

Ivar Hesselager writes:

To my vast collection of superfluous bicycle tools I have recently
added a "BrakeShoe Tuner" from the Dutch company Tacx. (www.tacx.nl)
Now the doubt is nagging me, whether it actually belongs in this
collection. ;-) Because it seems that it not only makes the brake
shoe tuning slightly easier. Also better.

With this tool the brake shoes are not positioned parallel to the
rim (as I have always done it) but slightly tilted/pointed in the
forward direction.

Like this: /H\ (Imagine the H is the rim, the slashes are the brake
shoes, up is forward.)

"Only then is the braking power optimal", the instruction sheet
tells me.

Is this true? Is the braking power theoretically better with a
tilted / pointed brake shoe position?

Would the difference be significant in practical use?

What is often overlooked is that the natural position of brake shoes
is angled inward to the front (toe-in) because when used forcefully,
brake calipers flex forward giving that orientation and wearing the
heel of the brake pad more than the toe. When worn in, brakes tend
not to squeal as readily as when new and unworn because they have
toe-in that tends to reduce chatter.

With a brake pad that is parallel to the rim, first contact with its
entire surface causes the pad to rotate forward with caliper
flexibility, thereby having less drag than when in full contact. This
allows the pad to snap back and regain full contact... repeatedly and
cause brake chatter. Brake squeal is a subset of that. Both of these
resonant motions have nearly invisible magnitude.

Brake calipers that do not have rotational adjustment should not be
bent to achieve this position. If the caliper is properly lubricated
with grease, its bearings will generally not permit squeal, that
frequency being in the range of bearing clearance that when full of
grease suppresses that resonance. If you insist on toe-in on new
brake pads, then it is better to achieve that by modifying the brake
pad surface with sand paper.

Jobst Brandt

Thanks for the input. I understand that the angling of the brake shoes
primarily serves to prevent squealing, and thereby also gives a more
effective braking.
With my road racers Ultegra brakes I have not had squealing problems
although the brake shoes till now have been adjusted parallel to the rim.
Not so with my winter commuter bike. Its longer V-type brake shoes tend to
squeal, and now I know how to deal with that.
The Brake Shoe Tuner makes it easy to make a very accurate adjustment on the
road racer's brakes with a tillt of exactly one mm. But the device can't be
used with the V-type brakes because of the angle the brake pads approach the
rim. So the Brake Shoe Tuner does not help me, where I would benifit the
most.
After all it does belong in my collection of unnecesary bicycle tools
Ivar of Denmark

Ivar Hesselager wrote:

Thanks for the input. I understand that the angling of the brake shoes
primarily serves to prevent squealing, and thereby also gives a more
effective braking.
With my road racers Ultegra brakes I have not had squealing problems
although the brake shoes till now have been adjusted parallel to the rim.
Not so with my winter commuter bike. Its longer V-type brake shoes tend to
squeal, and now I know how to deal with that.
The Brake Shoe Tuner makes it easy to make a very accurate adjustment on the
road racer's brakes with a tillt of exactly one mm. But the device can't be
used with the V-type brakes because of the angle the brake pads approach the
rim. So the Brake Shoe Tuner does not help me, where I would benifit the
most.
After all it does belong in my collection of unnecesary bicycle tools

Most linear ("V") brakes incorporate an angle adjustment in
the shoe holder assembly. Whether you have such or not,
lubricating all the fasteners and shoe mounting hardware so
the pad is held securely can stop the harmonic response.

To set up linear brakes with threaded post type shoes, slack
the shoe mounting nut, add a drop or two of oil along the
length of the spacer stack and the threads, pull the brake
tight against the rim with the shoe at the proper position
and tighten securely. Wipe any excess oil and test ride for
noise. These brakes generally do not need any other
attention to run quietly, the arm being particularly
resistant to twist during braking.

Models with an unthreaded post type shoe may need to be set
with a slight leading edge. 0.5 to 1mm front to back is
sufficient. If a slight cant doesn't quiet the brake, the
problem lies elsewhere. Wax, oil, other contaminants and
some rim surface finishes can add to shoe vibration so
cleaning the rim ( and shoes!) or a pass with emery cloth
can help. The noise is an harmonic response so changing any
one factor is enough to stop it.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

Ted Bennett writes:

Thanks for the input. I understand that the angling of the brake
shoes primarily serves to prevent squealing, and thereby also gives
a more effective braking.

Is that really true? It would seem that a friction pair that
generates a lot of noise would have less energy to dissipate as
heat.

Squeal and chatter are two forms of stick-slip sliding friction that
have a 50/50 duty cycle. To get the same frictional force under
sustained chatter takes more braking force from my experience and
agrees with the intermittent mode of friction. As far as energy in
noise, you can use your own model in an annoying child beating on a
drum with little effort and great volume... or for that matter a
trumpet player blasting out a solid note.

Does any one know how to quantify the amount of energy dissipated by
sound waves? Maybe I should ask the young men in the Civic with a
spoiler and loud, thumping energy dissipation, who have pulled up beside
me at the light.

You are confusing amplifier power with sound power. Besides, brakes
are in the higher audible range that does not move large volumes of
air such as the boom of a base drum that has a large diaphragm.

Jobst Brandt

The positioning you describe is called "toe-in". Basically it allows for
the flex in the brake calipers, so that when you apply the brakes, the
drag from the rim pulls the pads forward slightly, bringing the pads
into parallel with the rim. This maximises pad/rim contact, braking
"power" and also reduces squealing.

- -

"May you have the wind at your back.
And a really low gear for the hills!"

Chris Zacho ~ "Your Friendly Neighborhood Wheelman"

Chris'Z Corner
http://www.geocities.com/czcorner