Centerpull brakes

(Jeff Wills) said:

U-brakes are simply centerpull brakes with much beefier arms and shorter straddle cables.

The straddle on a U-brake is user-adjustable. Some now offer linear pull like a v-brake.

The size and cross-section of an MTB U-brake is in proportion to its reach, which must clear a 2"+
tire with room to spare.

Quoted message said:

I doubt the center "bridge" of an old-style road centerpull flexes much.

Oh yes it does. Not under cable tension, but under rim drag.

Chalo Colina

Quoted message said:

Chalo Colina said:

Quoted message said:

Centerpull and cantilever brakes approach the rim at nearly a 45 degree angle and have large
vertical change throughout pad wear life,

Quoted message said:

This trait of stud-mounted brakes can be accommodated by adjusting the pad height as it wears.
The pad rub on tire sidewalls exhibited by single-pivot sidepulls, however, is due to the
extreme fore-to-aft flex these brakes display and can not reliably be adjusted away.

Wait a minute, this is a geometric given, there is nothing you can do to the motion of the pad
because it sweeps around its pivot and that pivot lies in the wrong place.

The stud-mounted brake's pivot lies in the "wrong" place to yield pad travel that is normal to the
rim sidewall, but you can adjust the pad in its slot as it wears. In this regard the single-pivot
caliper's pivot point is "right".

The single-pivot caliper's pivot lies in the "wrong" place because it necessitates a long, flexible
arm for most applications. The fore-aft flex thus promoted can easily result in tire sidewall rub,
and it can not be remedied by periodic adjustment, because it is a result of braking force. In this
regard it is the frame-stud-mounted brake that has its pivots in the "right" place: as close as
practiceable to the rim track for maximum arm stiffness.

If one were to make a frame-mounted centerpull or cantilever with the pivots so tightly spaced that
it allowed a maximum 25mm to 28mm tire (like most short-reach calipers), then the pad sweep of such
a brake would be much closer to perpendicular to the rim sidewall.

As they are, stud-mounted brakes work well enough for skinny tire and rim applications, while
allowing really huge tires and rims to be used as well.

Quoted message said:

Quoted message said:

I have bent many forks under the braking forces generated by U-brakes. Before applying stopping
power like that, caliper brakes become so twisted out of shape that applying more cable tension
does not result in more braking.

OK, if you say so but you'll have to do some measurements to convince
me. Regardless of where they are mounted, the mechanical advantage cannot be higher than the
average human reach allows. You seem to be using hyperbole in that description. A brake that
operates at the yield point will break off in short order. I have never seen a caliper that
was bent from braking and I don't expect to see one.

I have never bent (yielded) a brake, with the possible exception of a flimsy centerbolt or two.
That's not the problem I describe.

What I am talking about is the elastic distortion inherent in the longer arms of a center-pivoted
caliper. In many cases, I apply enough cable tension that the forces exceed the "spring rate" of the
caliper within the available range of lever travel. The result is that I can pull the brake harder
until I run out of lever travel, but the brake flexes enough so as not to provide additional braking
force past a point.

With long reach calipers, 70mm or more, the point at which this happens is a pretty meager amount of
braking force for me. Even with normal road calipers, flex, and not nose wheelies, limits the
braking power I can get out of the system.

I have observed this phenomenon in particularly flexible brakes of other types, like the nasty Dia
Compe 986 cantilever and the Mafac centerpull. Mostly, though, it's an ill reserved for
single-pivot calipers.

Quoted message said:

Quoted message said:

The "ceiling" on stopping force resulting from flex-related distortion of the brake mechanism is
lower, and more abrupt, with single-pivot calipers than with any other type of brake.

To what do you attribute this claim. I'm not sure what you mean by the ceiling anyway. Could you
expand on that.

I hope that my above explanation clarifies what I meant by "ceiling". That is, the point at which
more squeeze at the lever does not provide more braking.

Quoted message said:

Somehow the stopping part is a mystery to me. I and the many riders with whom I have ridden have
taken a lot of steep and long descents over many years and have not had a problem with that, yet
you say they stop better. Better than what? I can raise the rear wheel on descents. I don't need
more leverage.

But if your bike were longer, or lower, or more heavily loaded, or tandem; or if you were heavier,
you would easily discern the shortcomings of caliper brakes compared to well-utilized cantilever or
U-brakes. It is the upright sport road bike-- alone among adult bikes-- that has its available
braking so limited by its usual weight distribution as to be adequately served by caliper brakes.

Chalo Colina

(Gary Young) said:

Aren't U-brakes still widely available in the BMX market?

Yes, in straddle-cable, linear-pull, and twin-cable variations.

Quoted message said:

If so, is there any advantage in reviving bike-boom centerpulls (as Grant Petersen is proposing)?
I suppose they would be lighter than U-brakes if mounted on braze-on studs, but the tenor of this
discussion seems to be that they were often too light (i.e., too flexy).

Lighter and smaller, road centerpulls are a more elegant fit to most road wheels and tires than
U-brakes are. Is this enough to warrant using them over U-brakes? In my opinion, no.

U-brakes work well, fit lots of different bikes, are easy to install and maintain, and still enjoy
broad commercial support.

There are even swanky new versions available like this one, with two sealed ball bearing cartridges
per arm: danscomp.comhazel.cgiOpen ↗

Chalo Colina

Marcus Coles said:

Quoted message said:

Quoted message said:

one thing Campagnolo did not address but used the same return spring shape that was the downfall
of all other sidepull brakes. This is the dual "ram's horn" shape, whose ends rotate about the
center of the "ram's horn" instead of the pivot bolt.

Quoted message said:

Quoted message said:

Actually my SunTour GPX sidepulls have one central spiral spring that doesn't slide. Stays
centered pretty well. Out of production for quite some time, OK.

Quoted message said:

Quoted message said:

Quoted message said:

I was glad to see that the new Mavic side pull brake that I saw at InterBike has solved this
problem and has a no-slide return spring.

Quoted message said:

Quoted message said:

How did they solve the problem?

Quoted message said:

My mid 80's Bridgestone SC sidepulls also lack the ram's horn springs. I believe these like the
SunTour GPX were actually made by Dia-Compe. They stay pretty well centered and appear to have a
3rd. arm, I imagine for centering.

Quoted message said:

Rear brake pictured in place, sorry no movies ;-) I didn't really want to pull it apart for pics
or to figure out how it works, until it stops working.

Quoted message said:

ody.caBrake.jpgOpen ↗ ody.caBrake%2001.jpgOpen ↗
ody.caBrake%2002.jpgOpen ↗ ody.caBrake%2003.jpgOpen ↗

Quoted message said:

Work well enough for me even with the pads near the bottom of the slots. AFAIK also long out of
production.

Since there is no picture of the brake off the bicycle with the arms free to spread to their
maximum, I can't see for sure what this brake is but it looks like an early dual-pivot brake. The
second pivot is visible in the front view and the side view that also reveals an intermediate plate.

If you could dismount the brake and photograph it from the front an rear as well as into (parallel
to the center plate) the parts, I could make more of the picture. In any case the return spring
cannot center the brake because it is not anchored anywhere other than at its ends on the two arms.

Jobst Brandt [email hidden]

Tim McNamara said:

Quoted message said:

My mid 80's Bridgestone SC sidepulls also lack the ram's horn springs. I believe these like the
SunTour GPX were actually made by Dia-Compe. They stay pretty well centered and appear to have a
3rd. arm, I imagine for centering.

Quoted message said:

Quoted message said:

Rear brake pictured in place, sorry no movies ;-) I didn't really want to pull it apart for pics
or to figure out how it works, until it stops working.

ody.caBrake.jpgOpen ↗ ody.caBrake%2001.jpgOpen ↗
ody.caBrake%2002.jpgOpen ↗ ody.caBrake%2003.jpgOpen ↗

Quoted message said:

Interesting. The spring is still curled like a ram's horn, but with only one curl and it's wrapped
around the pivot bolt. I've never seen that configuration in a sidepull brake, although it's seen
in centerpull and cantilever brakes. I assume there's some sort of spacer inside the spring's curl
so that the brake arms aren't compressing the spring.

The "rams-horn" spring has, as the animal, two coils, one on either side of the anchor bolt. The
spring in the pictures is called a "torsion" spring that articulates about the center of the single,
centered coil. It has no centering effect, not being anchored anywhere other than pushing at the two
arms, as I explained in another reply.

Jobst Brandt [email hidden]

Chalo Colina said:

Quoted message said:

Quoted message said:

> Centerpull and cantilever brakes approach the rim at nearly a 45 degree angle and have large
> vertical change throughout pad wear life,

Quoted message said:

Quoted message said:

Quoted message said:

This trait of stud-mounted brakes can be accommodated by adjusting the pad height as it wears.
The pad rub on tire sidewalls exhibited by single-pivot sidepulls, however, is due to the
extreme fore-to-aft flex these brakes display and can not reliably be adjusted away.

Quoted message said:

Quoted message said:

Wait a minute, this is a geometric given, there is nothing you can do to the motion of the pad
because it sweeps around its pivot and that pivot lies in the wrong place.

Quoted message said:

The stud-mounted brake's pivot lies in the "wrong" place to yield pad travel that is normal to the
rim sidewall, but you can adjust the pad in its slot as it wears. In this regard the single-pivot
caliper's pivot point is "right".

You can adjust vertical position with any brake that is reasonably versatile since this adjustment
is necessary for accommodating different rims and frames.

Quoted message said:

The single-pivot caliper's pivot lies in the "wrong" place because it necessitates a long,
flexible arm for most applications. The fore-aft flex thus promoted can easily result in tire
sidewall rub, and it can not be remedied by periodic adjustment, because it is a result of braking
force. In this regard it is the frame-stud-mounted brake that has its pivots in the "right" place:
as close as practicable to the rim track for maximum arm stiffness.

Mechanical function comes before light weight or we would all be riding Modolo Chronos brakes, the
smallest and lightest and most elegant brake ever made. It was a forerunner of the Campagnolo Delta,
a complete misunderstanding of brake function. My normal reach Campagnolo Record (single pivot)
brakes of the 1960's does not flex enough to interfere with its function and it requires no pad
adjustment from brand new pads until they are worn to the metal. That is what counts in a brake that
is to get you down something like the Stelvio Pass in the rain, where pad and rim wear are large.

tinyurl.comlen5Open ↗

Quoted message said:

If one were to make a frame-mounted centerpull or cantilever with the pivots so tightly spaced
that it allowed a maximum 25mm to 28mm tire (like most short-reach calipers), then the pad sweep
of such a brake would be much closer to perpendicular to the rim sidewall.

But still at a large angle from the pad contact zone, so you can't get there from here. The answer
is to make a single pivot brake with a return spring without "cosine error" (relative motion to the
brake arm) and beef up the arms a bit. Since few riders experience any of this, they don't
understand why anyone should work on it and the manufacturers see no need to work on an unmet but
unknown need.

Quoted message said:

As they are, stud-mounted brakes work well enough for skinny tire and rim applications, while
allowing really huge tires and rims to be used as well.

So what's the difference between this and the cantilever or its replacement, the V-brake?

Quoted message said:

Quoted message said:

Quoted message said:

I have bent many forks under the braking forces generated by U-brakes. Before applying stopping
power like that, caliper brakes become so twisted out of shape that applying more cable tension
does not result in more braking.

Quoted message said:

Quoted message said:

OK, if you say so but you'll have to do some measurements to convince me. Regardless of where
they are mounted, the mechanical advantage cannot be higher than the average human reach allows.
You seem to be using hyperbole in that description. A brake that operates at the yield point will
break off in short order. I have never seen a caliper that was bent from braking and I don't
expect to see one.

Quoted message said:

I have never bent (yielded) a brake, with the possible exception of a flimsy center bolt or two.
That's not the problem I describe.

Quoted message said:

What I am talking about is the elastic distortion inherent in the longer arms of a center-pivoted
caliper. In many cases, I apply enough cable tension that the forces exceed the "spring rate" of
the caliper within the available range of lever travel. The result is that I can pull the brake
harder until I run out of lever travel, but the brake flexes enough so as not to provide
additional braking force past a point.

You must both weigh much more than the average rider and have a more forceful grip. There are limits
to every mechanism, especially on a bicycle where weight is thought to play such a great role.

Quoted message said:

With long reach calipers, 70mm or more, the point at which this happens is a pretty meager amount
of braking force for me. Even with normal road calipers, flex, and not nose wheelies, limits the
braking power I can get out of the system.

I don't have much experience with brakes other than Universal and Weinmann sidepulls of the 1960's
and since then Campagnolo Record brakes with which I have descended many tens of thousands of
meters in the Alps with the only complaint being wet braking. That comes with rim brakes, a
compromise well worth it.

Quoted message said:

I have observed this phenomenon in particularly flexible brakes of other types, like the nasty Dia
Compe 986 cantilever and the Mafac centerpull. Mostly, though, it's an ill reserved for
single-pivot calipers.

I don't know which ones you mean, but as I said, I wouldn't know them to compare. I steered clear of
all that since the 1960's when I got Campagnolo brakes and haven't seen good cause to abandon them.

Quoted message said:

Quoted message said:

Quoted message said:

The "ceiling" on stopping force resulting from flex-related distortion of the brake mechanism is
lower, and more abrupt, with single-pivot calipers than with any other type of brake.

Quoted message said:

Quoted message said:

To what do you attribute this claim. I'm not sure what you mean by the ceiling anyway. Could you
expand on that.

Quoted message said:

I hope that my above explanation clarifies what I meant by "ceiling". That is, the point at which
more squeeze at the lever does not provide more braking.

I haven't experienced that.

Quoted message said:

Quoted message said:

Somehow the stopping part is a mystery to me. I and the many riders with whom I have ridden have
taken a lot of steep and long descents over many years and have not had a problem with that, yet
you say they stop better. Better than what? I can raise the rear wheel on descents. I don't need
more leverage.

Quoted message said:

But if your bike were longer, or lower, or more heavily loaded, or tandem; or if you were heavier,
you would easily discern the shortcomings of caliper brakes compared to well-utilized cantilever
or U-brakes. It is the upright sport road bike-- alone among adult bikes-- that has its available
braking so limited by its usual weight distribution as to be adequately served by caliper brakes.

Well, with all those conditions, I can see a market for brakes other than the ones used on racing
bicycles. That does not mean that the single or dual pivot caliper brake is not a good brake. I for
one, would not attempt descending steep roads on a tandem without a disc on the rear wheel.

Jobst Brandt [email hidden]

Quoted message said:

Tim McNamara said:

Interesting. The spring is still curled like a ram's horn, but with only one curl and it's
wrapped around the pivot bolt. I've never seen that configuration in a sidepull brake, although
it's seen in centerpull and cantilever brakes. I assume there's some sort of spacer inside the
spring's curl so that the brake arms aren't compressing the spring.

The "rams-horn" spring has, as the animal, two coils, one on either side of the anchor bolt. The
spring in the pictures is called a "torsion" spring that articulates about the center of the
single, centered coil. It has no centering effect, not being anchored anywhere other than pushing
at the two arms, as I explained in another reply.

Ah, thanks. I was struggling over terminology in that post and didn't have the term
"torsion spring."

Quoted message said:

[email hidden] (Chalo) wrote in message
news:<[email hidden]>... <snip>

Quoted message said:

Brazed-on pivots work. That is the definitive difference between the road bike centerpull as used
during the bike boom, and the U-brake as used during the 80s MTB craze. The stopping power of a
U-brake dwarfs that of any road bike brake ever made, and U-brakes were no more difficult to set
up than a road caliper.

Jeff Wills said:

U-brakes are simply centerpull brakes with much beefier arms and shorter straddle cables. I doubt
the center "bridge" of an old-style road centerpull flexes much. On mine, the crossed arms bend
upwards in response to brake lever pressure. On MTB-style U-brakes, the crossed arms are at least
4 times larger.

You doubt it? Why? It flexes a lot. Set up your brake as well as you can, pull the lever hard and
measure the distance change between the pivot bolts.

--
Andrew Muzi www.yellowjersey.org Open every day since 1 April, 1971

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

Quoted message said:

(Jeff Wills) said:

U-brakes are simply centerpull brakes with much beefier arms and shorter straddle cables.

The straddle on a U-brake is user-adjustable. Some now offer linear pull like a v-brake.

The size and cross-section of an MTB U-brake is in proportion to its reach, which must clear a 2"+
tire with room to spare.

Quoted message said:

I doubt the center "bridge" of an old-style road centerpull flexes much.

Oh yes it does. Not under cable tension, but under rim drag.

Chalo Colina

Allow me to backpedal- I went to the garage and squeezed the front brake lever while looking at it
from above. The pivots on the bridge moved visibly.

Still, the U-brake's arms are much larger than older "road" centerpulls, some of which
featured a very long reach- the rear on my Raleigh is a 750. (Old frame built for 27" wheels
fitted with 700c's.)

Jeff

Quoted message said:

Marcus Coles said:

Quoted message said:

>one thing Campagnolo did not address but used the same return spring shape that was the downfall
>of all other sidepull brakes. This is the dual "ram's horn" shape, whose ends rotate about the
>center of the "ram's horn" instead of the pivot bolt.

Quoted message said:

Quoted message said:

Actually my SunTour GPX sidepulls have one central spiral spring that doesn't slide. Stays
centered pretty well. Out of production for quite some time, OK.

Quoted message said:

Quoted message said:

>I was glad to see that the new Mavic side pull brake that I saw at InterBike has solved this
>problem and has a no-slide return spring.

Quoted message said:

Quoted message said:

How did they solve the problem?

Quoted message said:

My mid 80's Bridgestone SC sidepulls also lack the ram's horn springs. I believe these like the
SunTour GPX were actually made by Dia-Compe. They stay pretty well centered and appear to have a
3rd. arm, I imagine for centering.

Quoted message said:

Rear brake pictured in place, sorry no movies ;-) I didn't really want to pull it apart for pics
or to figure out how it works, until it stops working.

Quoted message said:

ody.caBrake.jpgOpen ↗ ody.caBrake%2001.jpgOpen ↗
ody.caBrake%2002.jpgOpen ↗ ody.caBrake%2003.jpgOpen ↗

Quoted message said:

Work well enough for me even with the pads near the bottom of the slots. AFAIK also long out of
production.

Since there is no picture of the brake off the bicycle with the arms free to spread to their
maximum, I can't see for sure what this brake is but it looks like an early dual-pivot brake.
The second pivot is visible in the front view and the side view that also reveals an
intermediate plate.

If you could dismount the brake and photograph it from the front an rear as well as into
(parallel to the center plate) the parts, I could make more of the picture. In any case the
return spring cannot center the brake because it is not anchored anywhere other than at its ends
on the two arms.

I know that brake. It's a sidepull with a single pivot. The return spring does not pass through a
slot in the center bolt but rather wraps around the bolt.

--
Andrew Muzi www.yellowjersey.org Open every day since 1 April, 1971

Quoted message said:

They all have one spring. The problem is that he spring has a "ram's horn" coil on either side of
the pivot bolt that constitute the center of rotation of the spring ends. If your brake does not
have such a spring, I would be interested to know what shape the spring is.

An internal spiral wrapped arround the center bolt, being fixed to both arms of the brake. Very much
like most cantilever.

--
MfG/Best regards helmut springer

Gary Young said:

Though I've been critical of Grant Peterson's effort to revive centerpull brakes, I have to admit
that the latest Rivendell Reader (#30) gave me reasons to pause.

First of all, in the letters section, a reader (Thomas Papetti) points out that dual-pivot
sidepulls won't track an out-of-true rim the way all other brake types will. I'm not sure how much
that counts in favor of centerpulls (Papetti doesn't discuss them at all). Even though most riders
would probably use long-reach dual pivots in lieu of centerpulls, long-reach single-pivot
sidepulls and cantilevers are still available if tracking is a significant worry.

Then in an article, Jan Heine makes several claims for the superiority of sidepulls.

He says, "The long lever on the cable side [above the pivot, if I understand him correctly] and
the short lever on the pad side [below the pivot] gives centerpulls a huge mechanical advantage
over sidepulls. Just like a bolt-cutter, where long arms and short jaws allow you to cut through
thick steel bolts." Later on, he adds that only with the advent of dual-pivot brakes did sidepulls
catch up to centerpulls in this respect.

That doesn't seem to jibe with Jobst's discussion in the FAQ, which seems to suggest that
single-pivot sidepulls and centerpulls had the same mechanical advantage.

He argues that only the part of the arm below the pivot needs to be beefy, because that's the only
place where flex matters. "As a result, the arms above the pivots can be incredibly skinny, as
they don't need to resist flex. That is where the weight savings of a centerpull originate." He
doesn't say what the weight savings is.

He writes, "And since the pivot on centerpull brakes is on the fork leg, adding reach does not
increase the flex or change the mechanical advantage. So there is not incentive to go to
short-reach brakes." I think he means that since the distance between the pad and the pivot point
always remains the same, adding lever length above the pivot doesn't effect the mechanical
advantage. That sounds dubious to me -- doesn't changing the length of a cantilever brake's
straddle cable effect mechanical advantage even if all else remains the same? Furthermore, from
the photos of old centerpulls in the Reader, it doesn't seem as if the distance from pivot to pad
was always kept constant (some long-reach brakes seem to achieve that reach in part by increasing
the lever below the pivot).

He notes that centerpulls don't stick out to the side like centerpulls. It seems to me that that
problem can be cured by using v-brakes, though that introduces problems of its own (e.g., mating
them to road levers).

He says that even the standard-reach (what would be called long-reach by most) dual-pivot brakes
offered by Shimano don't leave much room for fenders. Is that true? Just how big a tire would you
have to run before you would crowd out the dual pivots? (Elsewhere in the Reader, Grant says that
centerpulls will clear a 40mm tire or a 38mm tire with fenders; what's the limit with the current
long-reach dual pivots?) Furthermore, why not long-reach single-pivots, which are still available?

Probably his most controversial claim is that single-pivot sidepulls "offered inferior
performance" and became widespread mostly because of Campagnolo's advertising and gruppo
packaging. Contrast that with Jobst's claim in the FAQ that, "The centerpull brake of the 1950's,
was popular for nearly a decade, in spite of being entirely without merit, being worse in all
respects than the side pull brake with which it competed."

In the past, I've been pretty fed up with Grant's talk of reviving centerpulls, because it so
often appeared side-by-side with complaints about Rivendell's financial straits and lack of
popularity. I do notice a more refreshing tone in this issue: he mentions a bike planned for next
year that will have braze-on pivots for centerpulls and adds, "But we don't expect anybody to
follow that lead., and we fully expect to sell fewer bikes because of it. Centerpulls, for as much
sense as they make, will still scare off most customers."

There are also braze-on Mafac centerpulls - basically the predecessor of Shimano's long-dead
U-brake. These are supposed to be better than normal centerpulls, but the braze-ons are not the same
as for cantilever brakes so you'd have to have a frame modified or built specially.

Andrew Muzi said:

Quoted message said:

If you could dismount the brake and photograph it from the front an rear as well as into
(parallel to the center plate) the parts, I could make more of the picture. In any case the
return spring cannot center the brake because it is not anchored anywhere other than at its ends
on the two arms.

Quoted message said:

I know that brake. It's a sidepull with a single pivot. The return spring does not pass through a
slot in the center bolt but rather wraps around the bolt.

So what is the sub-frame doing in there and the bulge in the arm with a screw in it? How does it
center if the spring does not do it, as it can't since it is not anchored anywhere, bearing only on
the arms at the brake pads?

Jobst Brandt [email hidden]

Quoted message said:

Andrew Muzi said:

Quoted message said:

If you could dismount the brake and photograph it from the front an rear as well as into
(parallel to the center plate) the parts, I could make more of the picture. In any case the
return spring cannot center the brake because it is not anchored anywhere other than at its ends
on the two arms.

Quoted message said:

I know that brake. It's a sidepull with a single pivot. The return spring does not pass through a
slot in the center bolt but rather wraps around the bolt.

So what is the sub-frame doing in there and the bulge in the arm with a screw in it? How does it
center if the spring does not do it, as it can't since it is not anchored anywhere, bearing only
on the arms at the brake pads?

Jobst Brandt [email hidden]

It's almost off the bike More pictures shortly.

Marcus

Kinda sorta both.

Picture files are large. They will only be on the server until tomorrow.

ody.caPA180001.JPGOpen ↗ ody.caPA180002.JPGOpen ↗
ody.caPA180003.JPGOpen ↗ ody.caPA180004.JPGOpen ↗
ody.caPA180005.JPGOpen ↗ ody.caPA180006.JPGOpen ↗
ody.caPA180007.JPGOpen ↗ ody.caPA180008.JPGOpen ↗
ody.caPA180009.JPGOpen ↗ ody.caPA180010.JPGOpen ↗
ody.caPA180011.JPGOpen ↗ ody.caPA180012.JPGOpen ↗
ody.caPA180013.JPGOpen ↗ ody.caPA180014.JPGOpen ↗

Marcus

Marcus Coles said:

Kinda sorta both.

Quoted message said:

Picture files are large. They will only be on the server until tomorrow.

Quoted message said:

ody.caPA180001.JPGOpen ↗ ody.caPA180002.JPGOpen ↗
ody.caPA180003.JPGOpen ↗ ody.caPA180004.JPGOpen ↗
ody.caPA180005.JPGOpen ↗ ody.caPA180006.JPGOpen ↗
ody.caPA180007.JPGOpen ↗ ody.caPA180008.JPGOpen ↗
ody.caPA180009.JPGOpen ↗ ody.caPA180010.JPGOpen ↗
ody.caPA180011.JPGOpen ↗ ody.caPA180012.JPGOpen ↗
ody.caPA180013.JPGOpen ↗ ody.caPA180014.JPGOpen ↗

Thanks. That's what I needed to understand how these brakes centered themselves.

The clue is the "dog bone" swivel pin that has a sphere at each end and one in the middle.
Geometrically the swivel pin is positioned by its center sphere while its ends are engaged in the
brake arms that move in opposite directions, thereby tilting the pin and forcing each arm to mirror
the other's moves. As the pin tilts, its length is foreshortened between the two brake arms for
which it requires a cylindrical bore in which to "pump" in and out as it tilts. Because the arms
rotate about the center-bolt and the intermediate plate does not, the pin must be free to change its
radial position, requiring a slot in the intermediate plate.

The problem with this is that a cylindrical bore presents only a line contact for a sphere while a
slot (two parallel planes) present a point contact. Machining a curved surface slot is probably
prohibitively cumbersome. With normal grit contamination and poor lubrication, wear at these points
is rapid and with such a short pin, a small amount of wear will make centering poor. That is most
likely why we don't see this type of brake anymore.

...DEAD!

Jobst Brandt [email hidden]

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

Quoted message said:

Kinda sorta both.

Picture files are large. They will only be on the server until tomorrow.

ody.caPA180001.JPGOpen ↗ ody.caPA180002.JPGOpen ↗
ody.caPA180003.JPGOpen ↗ ody.caPA180004.JPGOpen ↗
ody.caPA180005.JPGOpen ↗ ody.caPA180006.JPGOpen ↗
ody.caPA180007.JPGOpen ↗ ody.caPA180008.JPGOpen ↗
ody.caPA180009.JPGOpen ↗ ody.caPA180010.JPGOpen ↗
ody.caPA180011.JPGOpen ↗ ody.caPA180012.JPGOpen ↗
ody.caPA180013.JPGOpen ↗ ody.caPA180014.JPGOpen ↗

Marcus

Dear Marcus,

Regrettably, some interested parties might not read your post in time, so your pictures are
available for Jobst, Andrew, and anyone else at the addresses below:

home.comcast.netPA180001.jpgOpen ↗
home.comcast.netPA180002.jpgOpen ↗
home.comcast.netPA180003.jpgOpen ↗
home.comcast.netPA180004.jpgOpen ↗
home.comcast.netPA180005.jpgOpen ↗
home.comcast.netPA180006.jpgOpen ↗
home.comcast.netPA180007.jpgOpen ↗
home.comcast.netPA180008.jpgOpen ↗
home.comcast.netPA180009.jpgOpen ↗
home.comcast.netPA180010.jpgOpen ↗
home.comcast.netPA180011.jpgOpen ↗
home.comcast.netPA180012.jpgOpen ↗
home.comcast.netPA180013.jpgOpen ↗
home.comcast.netPA180014.jpgOpen ↗

Naturally, I'll delete them if there are any objections.

Carl Fogel

Quoted message said:

Marcus Coles said:

Kinda sorta both.

Quoted message said:

Picture files are large. They will only be on the server until tomorrow.

Quoted message said:

ody.caPA180001.JPGOpen ↗ ody.caPA180002.JPGOpen ↗
ody.caPA180003.JPGOpen ↗ ody.caPA180004.JPGOpen ↗
ody.caPA180005.JPGOpen ↗ ody.caPA180006.JPGOpen ↗
ody.caPA180007.JPGOpen ↗ ody.caPA180008.JPGOpen ↗
ody.caPA180009.JPGOpen ↗ ody.caPA180010.JPGOpen ↗
ody.caPA180011.JPGOpen ↗ ody.caPA180012.JPGOpen ↗
ody.caPA180013.JPGOpen ↗ ody.caPA180014.JPGOpen ↗

Thanks. That's what I needed to understand how these brakes centered themselves.

The clue is the "dog bone" swivel pin that has a sphere at each end and one in the middle.
Geometrically the swivel pin is positioned by its center sphere while its ends are engaged in the
brake arms that move in opposite directions, thereby tilting the pin and forcing each arm to
mirror the other's moves. As the pin tilts, its length is foreshortened between the two brake arms
for which it requires a cylindrical bore in which to "pump" in and out as it tilts. Because the
arms rotate about the center-bolt and the intermediate plate does not, the pin must be free to
change its radial position, requiring a slot in the intermediate plate.

The problem with this is that a cylindrical bore presents only a line contact for a sphere while a
slot (two parallel planes) present a point contact. Machining a curved surface slot is probably
prohibitively cumbersome. With normal grit contamination and poor lubrication, wear at these
points is rapid and with such a short pin, a small amount of wear will make centering poor. That
is most likely why we don't see this type of brake anymore.

...DEAD!

Jobst Brandt [email hidden]

Thank you for inspiring me to dig further into the brake and for your analysis of the design. The
self centering function is no longer "magic".

The "dog bone" is made from a hard nylon like plastic and judging from the visible wear a
sacrificial item in the design.

The brake design was used by Bridgestone on their mid-range 400, 500, and 600 road bike models in
1984 and 1985 and then AFAIK disappeared.

This example has at least 10000km of mostly fair weather exposure, I imagine the front brake, which
has had most of the use in the three years I've owned the bike, will exhibit more wear, although it
does continue to center well at this time.

I was hoping the brakes would last until the lugged steel frame became too soft to ride ;^) Now I
will have to consider earlier replacement.

Marcus

Carl Fogel said:

Dear Marcus,

Regrettably, some interested parties might not read your post in time, so your pictures are
available for Jobst, Andrew, and anyone else at the addresses below:

home.comcast.netPA180001.jpgOpen ↗
home.comcast.netPA180002.jpgOpen ↗
home.comcast.netPA180003.jpgOpen ↗
home.comcast.netPA180004.jpgOpen ↗
home.comcast.netPA180005.jpgOpen ↗
home.comcast.netPA180006.jpgOpen ↗
home.comcast.netPA180007.jpgOpen ↗
home.comcast.netPA180008.jpgOpen ↗
home.comcast.netPA180009.jpgOpen ↗
home.comcast.netPA180010.jpgOpen ↗
home.comcast.netPA180011.jpgOpen ↗
home.comcast.netPA180012.jpgOpen ↗
home.comcast.netPA180013.jpgOpen ↗
home.comcast.netPA180014.jpgOpen ↗

Naturally, I'll delete them if there are any objections.

Carl Fogel

Carl,

No objections. Thanks for keeping them available for those interested.

Marcus

Marcus Coles said:

Thank you for inspiring me to dig further into the brake and for your analysis of the design. The
self centering function is no longer "magic".

Quoted message said:

The "dog bone" is made from a hard nylon like plastic and judging from the visible wear a
sacrificial item in the design.

After considering it further, it seems to me that getting someone to make a hardened steel "dog
bone" might be a worthwhile cause. The reason is that there are no significant forces on it, the
brake having no interest in staying centered or un-centered, its natural position when opening being
undefined and random. The only significant force transmitted would be from an off center mounted
brake or a laterally untrue wheel.

Quoted message said:

The brake design was used by Bridgestone on their mid-range 400, 500, and 600 road bike models in
1984 and 1985 and then AFAIK disappeared.

I find it a clever design, other than its wear susceptibility. It is far better than a dual pivot
for brake pad movement. It's mechanical advantage is optional in the design because it remains
centered and doesn't need large pad clearance to prevent pad drag.

Quoted message said:

This example has at least 10000km of mostly fair weather exposure, I imagine the front brake,
which has had most of the use in the three years I've owned the bike, will exhibit more wear,
although it does continue to center well at this time.

Unless it has large caliper flex, I would stay with it.

Quoted message said:

I was hoping the brakes would last until the lugged steel frame became too soft to ride ;^) Now I
will have to consider earlier replacement.

It's probably already softened up and doesn't climb hills well... junk it!

Jobst Brandt [email hidden]