Brake Adjustment Question

DaveH said:

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 <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

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/howfix_dualpivot.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 <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

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/howfix_dualpivot.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. <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

DaveH said:

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

(Mark) said:

DaveH <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

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/howfix_dualpivot.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 said:

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."

Quoted message said:

What is the purpose of this right-left offset?

Quoted message said:

http://www.parktool.com/repair_help/howfix_dualpivot.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
[email hidden]

Quoted message said:

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 said:

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?

Quoted message said:

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

Quoted message said:

Snipped fine description of how brake pivot arms function

Jobst Brandt
[email hidden]

BTW. I just bought your book. The Bicycle wheel.
I'm glad I did as you're always spot on as in the description above.
It was available in a Swedish bookshop which I think was really cool.

Thanks and keep the good work up.
--
Perre

You have to be smarter than a robot to reply.

(Art Harris) said:

DaveH said:

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.

This problem is not associated with that joint discontinuity, nor do I
think it is caused by bulges at the spoke eyelets. The vibration is
more like a mechanical resonance, a grab and release type scenario.

Quoted message said:

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?

The wheel is very slightly out of true, but the frequency of vibration
is much higher than (I think) would be predicted by such a distortion.
It is not a buffeting like that produced by a rim joint discontinuity
or spoke bulges, both of which I've experienced.

Quoted message said:

Quoted message said:

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

Phil Lee said:

Quoted message said:

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.

Quoted message said:

Quoted message said:

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

To make up for that, you must watch out that the left front pad
doesn't get into the tire sidewall with wear. However, this is seldom
a problem, most riders never braking enough to wear down pads that
much. The effect is explained in my earlier response.

Quoted message said:

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.

Are you talking about a dual pivot brake? If so, this will not work
unless the anchor bolt is loose. On a single pivot brake, this will
have no effect, the off centeredness being a result of dirt in the
return spring anchor on one side in comparison to the other one.

Quoted message said:

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.

That is a result of the higher mechanical advantage of these brakes,
nothing else. It reminds me of the car tune-up in which the gas pedal
return spring is made lighter. This makes the driver believe his car
has greater acceleration.

Quoted message said:

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.

What can I say? What made braking not smooth previously? I am sure
the linearity is unchanged, there being no change in ME of any brake
except the Campagnolo Delta (no longer made for that reason).
Predictability can only come from the operators grip. A weak grip
leaves the rider with insufficient braking at times. It's like the
Silca frame pump that is gone today because riders could not reach
100psi, not having the muscular strength to push a piston of that
diameter over such a long stroke.

Over the bars is a telling phrase.

http://draco.acs.uci.edu/rbfaq/FAQ/9.36.html

Jobst Brandt
[email hidden]

Quoted message said:

Quoted message said:

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.

Are you talking about a dual pivot brake? If so, this will not work
unless the anchor bolt is loose. On a single pivot brake, this will
have no effect, the off centeredness being a result of dirt in the
return spring anchor on one side in comparison to the other one.

Hmm... I discovered that you were correct yesterday. I tried to adjust my
rear brake manually, but it didn't work because I had the caliper on tight.
I loosened it up and the pull trick worked. I'd rather have them just
slightly loose and easily tool-free adjustable.

Quoted message said:

Quoted message said:

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.

That is a result of the higher mechanical advantage of these brakes,
nothing else. It reminds me of the car tune-up in which the gas pedal
return spring is made lighter. This makes the driver believe his car
has greater acceleration.

Kinda like a placebo effect? I had used the pads for 7 months prior to
cleaning the whole brakeset. It was after the cleaning in which they became
really grabby... I hadn't actually switched over to dual-pivot yet. I used
the brakes for about 3 weeks before getting the dual-pivots in.

Quoted message said:

Quoted message said:

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.

What can I say? What made braking not smooth previously? I am sure
the linearity is unchanged, there being no change in ME of any brake
except the Campagnolo Delta (no longer made for that reason).

It wasn't smooth because the friction curve of the brakes/pads were such
that when the kinetic and static coefficients of friction began to meet, the
static coefficient would spike, giving lots of jerky braking power when it
wasn't needed. It could also be in part because the kinetic friction was so
low. Also, my cables and housing needed replacement.

Quoted message said:

Predictability can only come from the operators grip. A weak grip
leaves the rider with insufficient braking at times. It's like the
Silca frame pump that is gone today because riders could not reach
100psi, not having the muscular strength to push a piston of that
diameter over such a long stroke.

I ride my trials/street bicycle with V-brakes just fine. I use Avid Speed
Dial levers with the ME as high as it can go, and they perform really well
and predictably with Koolstop pads. But I'd probably get sick of a Silca,
yes.

Quoted message said:

Over the bars is a telling phrase.

http://draco.acs.uci.edu/rbfaq/FAQ/9.36.html

Hardly. At my small frame size (5'5"😉 I don't have a large range of
rearward hang-back when I'm strung out over the front of my cowhorns with a
50cm frame, but I'm fully capable of skidding the front wheel on my MTB on
clean, dry pavement.

--
Phil, Squid-in-Training

Phil Lee said:

Quoted message said:

Quoted message said:

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.

Quoted message said:

Quoted message said:

Are you talking about a dual pivot brake? If so, this will not
work unless the anchor bolt is loose. On a single pivot brake,
this will have no effect, the off centeredness being a result of
dirt in the return spring anchor on one side in comparison to the
other one.

Quoted message said:

Hmm... I discovered that you were correct yesterday. I tried to
adjust my rear brake manually, but it didn't work because I had the
caliper on tight. I loosened it up and the pull trick worked. I'd
rather have them just slightly loose and easily tool-free
adjustable.

That's not a safe way to operate. Put a drop of 30W or thicker oil on
the return spring ends and center the brake. That should hold them for
a good time. I always cringed when I saw bicycle shop mechanics use a
hammer and drift pin on the shoulders of the return spring. That was
their solution to a problem they didn't understand.

Quoted message said:

Quoted message said:

Quoted message said:

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.

Quoted message said:

Quoted message said:

That is a result of the higher mechanical advantage of these
brakes, nothing else. It reminds me of the car tune-up in which
the gas pedal return spring is made lighter. This makes the driver
believe his car has greater acceleration.

Quoted message said:

Kinda like a placebo effect? I had used the pads for 7 months prior
to cleaning the whole brakeset. It was after the cleaning in which
they became really grabby... I hadn't actually switched over to
dual-pivot yet. I used the brakes for about 3 weeks before getting
the dual-pivots in.

Well you might call it that but if the car accelerates more easily
with light pedal pressure, there is some difference in definition of
response.

Quoted message said:

Quoted message said:

Quoted message said:

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.

Quoted message said:

Quoted message said:

What can I say? What made braking not smooth previously? I am
sure the linearity is unchanged, there being no change in ME of any
brake except the Campagnolo Delta (no longer made for that reason).

Quoted message said:

It wasn't smooth because the friction curve of the brakes/pads were
such that when the kinetic and static coefficients of friction began
to meet, the static coefficient would spike, giving lots of jerky
braking power when it wasn't needed. It could also be in part
because the kinetic friction was so low. Also, my cables and
housing needed replacement.

A higher static friction than dynamic friction is commonly called
stiction and brake pad material is designed not to have that. If you
have grabby brakes, it is the result of gummy contamination on the
rims, the same stuff that makes brake chatter.

Quoted message said:

Quoted message said:

Predictability can only come from the operators grip. A weak grip
leaves the rider with insufficient braking at times. It's like the
Silca frame pump that is gone today because riders could not reach
100psi, not having the muscular strength to push a piston of that
diameter over such a long stroke.

Quoted message said:

I ride my trials/street bicycle with V-brakes just fine. I use Avid
Speed Dial levers with the ME as high as it can go, and they perform
really well and predictably with Koolstop pads. But I'd probably
get sick of a Silca, yes.

High mechanical advantage means adjusting often as pads wear. As
winter approaches, we'll be doing more gritty wet riding and then this
becomes a pain.

Quoted message said:

Quoted message said:

Over the bars is a telling phrase.

Quoted message said:

Quoted message said:

http://draco.acs.uci.edu/rbfaq/FAQ/9.36.html

Quoted message said:

Hardly. At my small frame size (5'5"😉 I don't have a large range of
rearward hang-back when I'm strung out over the front of my cowhorns
with a 50cm frame, but I'm fully capable of skidding the front wheel
on my MTB on clean, dry pavement.

That means you CAN hang-back or you would overturn the bicycle in that
exercise. I have not seen anyone skid the front wheel in a straight
line. In fact, dropping the straddle cable of a cantilever brake onto
a knobby front tire invariably causes an unrecoverable endo. I think
you need to recreate the situation where you skid the front tire... or
is this an unusual bicycle (recumbent)?

Jobst Brandt
[email hidden]

Quoted message said:

Well you might call it that but if the car accelerates more easily
with light pedal pressure, there is some difference in definition of
response.

Ahh, I see what you mean now. The problem with the spring tension in my
single-pivots was that it was low enough to not pull the pads back out, but
my rectangular plastic spring-ends had disintegrated, so I was running the
spring against the caliper itself... this was part of the cause, and lubing
the intersection did not help until I put some really thick, strong springs.
There might have been some perspective alteration.

Quoted message said:

A higher static friction than dynamic friction is commonly called
stiction and brake pad material is designed not to have that. If you
have grabby brakes, it is the result of gummy contamination on the
rims, the same stuff that makes brake chatter.

I've had what you speak of... I had rubber tire particle contamination on my
rims once when my front quick-release failed to hold the front wheel in. My
fork had lost all of its damping, topping out very harshly. This was enough
to move the wheel out of the dropouts slightly, as I was jumping a set of
stairs. Rubber tire bits got on the rim, and the braking was REALLY strong,
similar to a friend's BMX bike with tarred rims.

The single-pivot brakes did not quite have this type of grabbiness, but it
was grabbier than my MTB and my friends' other road bikes. It is very
likely that the brake pads I got were of a poor compound, although I have
checked them fairly often and never found any aluminum bits and pieces, and
they were never really glazed over.

Quoted message said:

High mechanical advantage means adjusting often as pads wear. As
winter approaches, we'll be doing more gritty wet riding and then this
becomes a pain.

Hmm... I like Florida! 😉

Quoted message said:

That means you CAN hang-back or you would overturn the bicycle in that
exercise. I have not seen anyone skid the front wheel in a straight
line. In fact, dropping the straddle cable of a cantilever brake onto
a knobby front tire invariably causes an unrecoverable endo. I think
you need to recreate the situation where you skid the front tire... or
is this an unusual bicycle (recumbent)?

Yes... I can hang back. But I agree - traction is superior unless the CG is
behind that line that Sheldon describes as the line through which the CG of
the rider and bike together does to the contact patch or something, I
forget.

--
Phil, Squid-in-Training