Brake Adjustment Question

From Park Tool step #3 for dual pivot brakes, "Adjust height of right
pad to strike upper edge of braking surface. Adjust height of left pad
to strike lower edge of braking surface."

What is the purpose of this right-left offset?
Dave

http://www.parktool.com/repair_help/...ualpivot.shtml

On Tue, 21 Sep 2004 06:56:51 -0400, DaveH
wrote:

From Park Tool step #3 for dual pivot brakes, "Adjust height of right
pad to strike upper edge of braking surface. Adjust height of left pad
to strike lower edge of braking surface."

What is the purpose of this right-left offset?
Dave

It maximizes the surface area swept by the pad--utilizes the whole
width of the braking surface. This (obviously) applies only if/when
the pad is narrower than the braking surface.

DaveH wrote in message . ..
From Park Tool step #3 for dual pivot brakes, "Adjust height of right
pad to strike upper edge of braking surface. Adjust height of left pad
to strike lower edge of braking surface."

What is the purpose of this right-left offset?
Dave

http://www.parktool.com/repair_help/...ualpivot.shtml

Hi Dave, In the link that you have included, a paragraph down, or so,
it explains the reason, that one side acts a centerpull brake, and the
other side works like a standard side pull. They travel in slightly
different arcs (Slightly up, and down) towards the rim as the brakes
are applied.

Some important things that help having a very good Brake set up, and
close,proper adjustment is to have your wheels as true as they
possibly can be.
Any "wheel wobble" can cause unecessary brake pad rub when riding, and
because of this, it will not permit you to attain close tolerances
between the brake pad, and braking surface on the rim. You will then
have excessive Lever travel before the pads apply to the rim.

It does take a bit of time, and fiddling to insure you have correctly
set up your brakes, and many modern Dual Pivot desgns also employ an
Orbital Washer to permit slight Toe-In on the shoes if needed to
prevent brake squeal.

If wheels are perfectly true, the distance needed between Pad, and
shoe is very close, only around the thickness of a Quarter away from
each other. Insure/check at each location, that when either your
front, and back brakes are applied, that the brake pad contacts the
rim surface perfectly as possible. Neither too high that it can
possibly contact the tire, nor too low that the pad is not fully, and
squarely contacting the rim, and appears to be slightly overhanging.
Also too, that each pad is in perfect alignment to be paralell with
the rim braking surface.

Another feature I really like myself with Dual Pivot designs, is the
ability to fine tune the centering of the Brake Caliper in relation to
the wheel.

You won't need to keep resorting to manually centering the caliper in
relation to the wheel.

Once the pads to wheel rim are roughly centered as best as possible by
eye on either side of rim, tighten the Caliper Retaining Allen Bolt
firmly, and apply each brake, and watch very closely at the wheel
right where the caliper is.
If you see the wheel being slightly "Pushed" to the left, or right by
the brake pads on brake application, take the proper size small metric
allen wrench, and adjust the small allen set screw on the one caliper
arm, so that when brakes are applied both left, and right shoe contact
thier rims surfaces at exactly the same time, without causing the
wheel to deviate either way.

You'll see the caliper itself move slightly either one way, or the
other. Periodically, spin the wheel making sure that there is no rub
when free-wheeling, and adjust the Brake cable adjusting barrel if
necessary to attain as little cable slack as possible without causing
any pad rub. Recheck after 5-10 miles. Also make doubly sure that all
brake cables are tightly locked/retained at the Caliper locking screw.

Sheldon Brown has a good article on Brakes I believe, and I hope
someone else here will point you to it. Sorry for my long reply.
Mark

Well, to answer your question

Left side and right side dual pivot brake caliper arms move on
separate pivots. Pads move in different arcs as they approach the rim.
The left side arm acts as a sidepull. This pad swings downward as it
travels toward the rim. As this pad wear thinner, it will travel
downward even more. The right side acts as a centerpull. The right pad
will travel upward as it approaches the rim. Set the right pad lower
on the rim braking surface, and the left pad high on the braking
surface.

So, because dual pivot brakes don't act like you think they would (or,
at least, I thought they would). The object of the "offset" is to
compensate for the different arcs that the pads must travel.

Hope this helps clarify,

Matt

DaveH wrote in message . ..
From Park Tool step #3 for dual pivot brakes, "Adjust height of right
pad to strike upper edge of braking surface. Adjust height of left pad
to strike lower edge of braking surface."

What is the purpose of this right-left offset?
Dave

http://www.parktool.com/repair_help/...ualpivot.shtml

The explanation is right above the sentence you quoted -- each side of
a dual pivot arm tends to behave differently as the pad wears down.
One arm swings on a pivot directly above the rime, while another arm
swings on a pivot off to the side a bit. I hadn't heard this before,
but it does seem to make sense that the right side should be set high,
because the pad will move down as it wears. But is it really necessary
to set the left side low? I thought single-pivot brakes (of which the
left side resembles) didn't have this particular problem.

Guy F. Anderson Sr. wrote in message . ..
On Tue, 21 Sep 2004 06:56:51 -0400, DaveH
wrote:

From Park Tool step #3 for dual pivot brakes, "Adjust height of right
pad to strike upper edge of braking surface. Adjust height of left pad
to strike lower edge of braking surface."

What is the purpose of this right-left offset?
Dave

It maximizes the surface area swept by the pad--utilizes the whole
width of the braking surface. This (obviously) applies only if/when
the pad is narrower than the braking surface.

Makes no sense to me. If Park had substuted front of pad for right,
etc. Well, actually that wouldn't make sense either. Although it might
make more sense?
I dunno, John

On 21 Sep 2004 08:21:01 -0700, (Mark) wrote:

DaveH wrote in message . ..
From Park Tool step #3 for dual pivot brakes, "Adjust height of right
pad to strike upper edge of braking surface. Adjust height of left pad
to strike lower edge of braking surface."

What is the purpose of this right-left offset?
Dave

http://www.parktool.com/repair_help/...ualpivot.shtml

Hi Dave, In the link that you have included, a paragraph down, or so,
it explains the reason, that one side acts a centerpull brake, and the
other side works like a standard side pull. They travel in slightly
different arcs (Slightly up, and down) towards the rim as the brakes
are applied. ....snip
Thanks for that long reply. I'm embarrassed that I missed the
explanation on the very same page!

Actually, I'm having the same problem the poster below (Pizza Man) is
having--shuddering front brakes. The fork undergoes low frequency
vibration and thus large flexure under hard braking and slow
speeds--near the stop threshold. The problem is worse in a turn.

New Shimano 105 double-pivot long reach brakes, Shimano pads, and
Araya RC-540 non-machined Aluminum rims. I've done considerable
fiddling--pad position, sanding a taper into the pad, cleaning the rim
with alcohol. I've not altered the rim surfaces with sand paper, etc,
though I guess I'll have to. Kool-Stop black inserts aggravate the
problem.

Those 105 brake mechanisms are very stout. No slop anywhere that I can
detect.

I may replace the rims with Mavic MA3s or some equivalent since I
suspect the surface of these Araya rims is the culprit. A vexing
problem.
Dave

Dave Hartwick writes:

From Park Tool step #3 for dual pivot brakes, "Adjust height of
right pad to strike upper edge of braking surface. Adjust height of
left pad to strike lower edge of braking surface."

What is the purpose of this right-left offset?

http://www.parktool.com/repair_help/...ualpivot.shtml

A conventional single pivot "sidepull" brake has arms that pivot about
a shaft centered over the rim. the distance varies but has gotten
shorter with all this tight clearance "close coupled" tire clearance.
A typical Campagnolo single pivot brake has a pivot to pad arm of 60mm
and with a rim width of 20mm gives a contact angle of 9.5 degrees from
the center of the wheel. This means that a pad wear of 5mm results in
a vertical pad displacement of 60*(cos(9.5)-cos(4.19))=0.37mm or
essentially nothing worth considering.

Dual pivot brakes have the right (front) arm of 50mm length while the
other arm is 38.5mm long. The vertical displacement of the brake pad
on the 50mm long centrally pivoted arm
50*(cos(11.54)-cos(5.02))=0.82mm is still only a trifle.

The 38.5mm arm pivoted 28mm off center and 15mm below the central
pivot post to give an actuation angle to the rim of 27.9 degrees and
a direction of motion upward (into the tire) that is caused by lateral
displacement of its pivot. With 5mm pad wear that angle becomes 36.72
degrees.

That results in a vertical displacement of
50*(cos(27.9)-cos(36.72))=3.16mm, or a substantial change. This is
typical of centerpull, cantilever and V-brakes that have outbound
pivots for their brake arms.

A practical example is to swing your extended arm from a hand's width
away from your hip until contact and notice the vertical position
change of your hand (the brake pad). Then angle your arm away from
your side about 30 degrees with the lower arm straight down. With the
upper arm fixed, sweep the lower arm inward to make contact with your
hip and notice the vertical position change. This is the effect of
brakes with non-centered pivots. The beauty of the single pivot "side
pull" brake was that it had essentially no cosine error, the error just
demonstrated.

Jobst Brandt

Another feature I really like myself with Dual Pivot designs, is the
ability to fine tune the centering of the Brake Caliper in relation to
the wheel.

You won't need to keep resorting to manually centering the caliper in
relation to the wheel.

This seems overly complicated to me. I turn the whole caliper by grabbing
and pulling on the brake pad that's too close to the rim to center the
brake. Plenty fine adjustment, and faster, too.

I just got these 105 dual-pivots... I cannot believe just how much better
they are compared to single-pivot Exage I had before... the different in
linearity, cable-pull quality, and the stronger spring tension is
phenomenal. Part of it had to do with the pads I had on before... they
weren't old, but they were mountain pads that I had put in an ultrasonic
parts cleaner. I think the rubber had oils replenished to them, and they
became extremely grabby, sending me over the bars once.

This was after I had switched to a 110mm stem and cowhorns, putting my CG
much further forward than I was used to, so that's part of the reason, but I
can really yank on the brakes now with these 105 calipers, and the braking
is smooth, linear, and predictable.

--
Phil, Squid-in-Training

DaveH wrote:
Actually, I'm having the same problem the poster below (Pizza Man) is
having--shuddering front brakes. The fork undergoes low frequency
vibration and thus large flexure under hard braking and slow
speeds--near the stop threshold. The problem is worse in a turn.

Does the shudder happen once per wheel revolution? Check the spot
where the rim is joined. There may be a discontinuity there. For a
front wheel, you can solve the problem by flipping the wheel around
the other way.

Other possible culprits are a bulge in the rim or an out of true
wheel. While off the bike, spin the wheel and sight between the brake
pad and the rim. Do you see any variation in the clearance?


New Shimano 105 double-pivot long reach brakes, Shimano pads, and
Araya RC-540 non-machined Aluminum rims. I've done considerable
fiddling--pad position, sanding a taper into the pad, cleaning the rim
with alcohol. I've not altered the rim surfaces with sand paper, etc,
though I guess I'll have to. Kool-Stop black inserts aggravate the
problem.

Make sure the brake caliper is attached to the frame securely, and
that the brake shoes are tight and properly aligned.

Art Harris