From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Centering Side Pull Brakes
Date: 8 Aug 1997 18:09:46 GMT

Nick Carter writes:

> I'm trying to do some mechanicing and maintenance while I'm
> recuperating from a broken collar bone.  I'm wondering if there are
> any tips or tricks I can use to center my side pull brakes (Exage) on
> my road bike.  I can get the wheel centered but when I pull the levers
> they need to be centered again.  Will "opening" them up help any?

The reason single pivot sidepull brakes go off center is that the return
springs slide on the arms and with time increase their friction.  Since
this occurs randomly, one spring hauls back differently than the other.
The best solution is to oil these contacts.

Now you understand why there are dual pivot brakes.  With their higher
mechanical advantage, their pads must be adjusted closer to the rim so
that you won't run out of lever travel.  This requires accurate
centering that is only possible with two pivots at the expense of
having one arm sweep up into the tire as the pads wear.  Someone must
pay.  The higher ratio does not come without its cost.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: 2 Qs about brakes
Date: 27 Apr 1998 23:15:05 GMT

Robert L. Frazier writes:

> The questions are as follows.

> 1.  One of the brakes seems to work a bit oddly.  When I pull the
> lever, one side goes in faster than the other.  I took it apart,
> cleaned it, greased it, and pulled out the spring a bit, but its
> behavior hasn't changed. Is there something else that I can do, or
> should I ditch it for a newer one?

That's a perfectly good brake.  Don't bend around on the springs, just
rotate the spring anchor, the part that orients the spring.
Campagnolo was first to introduce brakes with wrench flats on the
spring anchor but there are ways around that.  Also, make sure that
there is grease or oil on the contact between the end of the spring
and the caliper.  You'll need to lift it off the notch to get lube in
there.

> 2.  I am using a back brake as a front brake.  Is this a mistake?
> (The brakes seem to be roughly the same shape, it is just that the
> front one has a longer mount bolt.)  Am I being excessively frugal?

The only difference between front and rear brake is the length of the
anchor bolt.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Spring Tension/ Road Bike Brakes?
Date: 28 Sep 1999 15:18:24 GMT

Lenny Wood writes:

> OK.  I worked on a nice road bike this weekend with Dura Ace
> components on it.  I tried to center the brake arms over the rim by
> bending the spring on the weaker side out a bit.  No luck though.
> And I didn't want to do it any more for fear of scratching the rest
> of the brake.

I'm not clear on what sort of brake you have, although I assume it is
a caliper brake.  The dual pivot brake was designed to give automatic
brake centering regardless of return spring condition.  These brakes
can be rotated to the centered position by loosening the anchor bolt
and for fine adjustment by the stop screw.

Single pivot calipers suffer from that which the dual pivot solves,
differential return spring friction.  For this reason some single
pivot calipers have plastic contact points while others rely on
grease between the spring and aluminum contact.  Both of these tend to
collect dirt and change their friction so that one arm retracts more
easily than the other.  Bending the springs is the worst way of
adjusting this, moving the caliper the second and lubricating the
contact points the correct one.  The spring does not change with time
nor does the geometry of the caliper.  Only the friction at the
springs does.

> So, I'm wondering, is there some secret to balancing the spring
> tension in road brakes?  Also, what is the purpose of those plastic
> blocks that the end of the spring goes through and then clips into
> the slot on the back of the brake arms?  Thanks in advance for any
> help you can give me.

The secret lies in understanding what is happening.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: tips on adjusting single pivots calipers...
Date: 15 Feb 2001 22:01:03 GMT

Jason Gersting writes:

> I am looking for some tips/hint son how to adjust the centering of
> the Modolo single pivot calipers I have.  The brakes are probably
> from the '80s and I have the problem that one of the arms is
> incessantly closer to the rim than the other.  I have loosened the
> bolt and re-centered, but after squeezing lever, it goes back to how
> it was.  I have tried adjusting the spring... but this seems to have
> no effect, perhaps I am not doing it properly?

The reason these brakes go off center is that friction between the
return spring ends and caliper arms increases unequally.  You can
rotate the return anchor base block and still have an offset brake
because one of the springs has high drag on its contact.

When initially adjusting the brake for center, make sure the spring
tabs are clean and lubricated.  Center the brake.  Tighten the anchor
bolt, and in the future, re-lubricate the spring contacts.  This
contact is a sliding interface that no one seems to have solved, the
spring having a pivot that is different from that of the caliper.  The
loop in the spring makes things worse because it makes the spring have
an even shorter radius than it naturally has, shorter than the
caliper, and increases sliding.  The loop is necessary for the spring
to work, however, and a shorter spring would yield at the required
force and stroke.

Always lube spring pivots on single pivot brakes.

Of course you should also do this on dual pivot brakes to keep the
return force constant, but even when barely sliding, they stay
centered.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jobst.brandt@stanfordalumni.org
Subject: Re: Centerpull brakes
Newsgroups: rec.bicycles.tech
Message-ID: <W2Fjb.32632$dk4.883354@typhoon.sonic.net>
Date: Thu, 16 Oct 2003 22:35:34 GMT

David L. Johnson writes:

> My memories of center-pull brakes were not all that pleasant.  I
> remember when most racing bikes had them, in the late '60s.  The
> competition, then, were usually poorly-designed sidepulls, which
> would tend to rotate so that one pad would drag on the rim.  Campy
> sidepulls changed all that, and everyone went for them despite the
> outrageous cost.  They could be properly centered fairly easily, and
> stayed where you put them.

I use those brakes today and find that they are as prone to going off
center as well as any sliding return spring brake.  That is 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.

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.
If you want to see how bad others are, note that Campagnolo and
Shimano have a special sleeve in which the spring slides like a
piston.  This gets full of fine grit and changes the return force, the
problem with non-dual pivot brakes.

> Centerpulls depended on the balance of their springs to center them.
> Use and dirt would make them one-sided as easily as old sidepulls.
> They also were mushy, so that it was easy to bottom out the lever in
> a hard stop, especially in wet weather.

That is the reason for having dual pivots.

Their reasonably good centering may be the only sliver of benefit of
the centerpull configuration.  It arises from its use of "torsion"
springs around the pivot bolts, and the short straddle cable that puts
a strong bias on returning to center almost as if it were made of a
rigid two bar linkage.  To go off center the straddle cable would need
to slide in the cable yoke.

> Modern dual-pivot sidepulls, and probably the Campy single-pivot
> rear brake, are much easier to adjust, stay where they are adjusted,
> and do an excellent job.  I also think that V brakes work quite
> well, and have the advantage of huge tire clearance.  The angle
> problem mentioned by Jobst has not been a difficulty for me, but
> would be if you were not attentive to pad wear.  I know that modern
> cantilever brakes also are easy to adjust, but they still have the
> variable-pull characteristic of old center-pulls.

No current single pivot brake centers well because they all use the
traditional "ram's horn" coil return spring.

>> dual-pivot sidepulls won't track an out-of-true rim the way all
>> other brake types will.

> Why would that be the case?  I don't see how the mechanism would be
> that different.  Of course, dual-pivot brakes have short reach and
> so small possible "tracking" distance, but they would have some
> capability.

It's not whether it should or not.  The brake pads cannot move
independently of the other.  That is the reason for these brakes...
to always remain centered.  If you doubt it, try pushing the wheel
sideways with the brake applied.  Open the QR so the wheel can move if
it wants to.

>> 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.

You'll notice that this ratio is 2:1 and that half the force goes to
each side resulting in a caliper ratio of 1:1.  I wrote an article
about that in the 1970's for Bike World.

> The problem with this is that any increased mechanical advantage
> means more cable pull for an equivalent motion of the pad, so the
> lever has to have less mechanical advantage as a result.  Net
> advantage, 0.  You have to be able to exert maximum force on the
> system before the lever bottoms out, and you need the same open pad
> clearance on any brake.  There isn't enough room to increase the
> mechanical advantage and still have a usable lever.  You can
> decrease, slightly, pad clearance when the brake is open, but that
> means the wheels have to be kept true.  A broken spoke makes the
> bike impossible to ride, not the best for a tour.

Today's road bicycles are dead in the water with a broken spoke, tire
clearance in the frame or fork being almost that of brake pad
clearance.  This plays to the short bike, fast cornering aficionados.
Everything has got to be as tightly packed as possible.

> The only other factor that comes into play is arm flex, which
> centerpulls had in spades.

.... to make up for all the positive features they didn't have.

Jobst Brandt
jobst.brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: Centerpull brakes
Newsgroups: rec.bicycles.tech
Message-ID: <gxGjb.32645$dk4.883749@typhoon.sonic.net>
Date: Fri, 17 Oct 2003 00:16:12 GMT

Donald Gillies writes:

>> You'll notice that this ratio is 2:1 and that half the force goes
>> to each side resulting in a caliper ratio of 1:1.  I wrote an
>> article about that in the 1970's for Bike World.

> This just shows that you can get anything published at all.  Such
> generalizations are clearly unwarranted, which is easily seen just
> by looking at the variation in calipers within one brand such as
> Weinmann.  The Weinmann 750 had more mechanical advantage than the
> Weinmann 610.  The earlier 610's had less mechanical advantage than
> the later 610's, by virtue of the shorter upper arms in the
> calipers, and a straddle wire and carrier that was designed to rest
> higher above the caliper than the later straddle carriers.

I think you'll find that they did not have different ratios because
their lever ratios were the same.  You'll need to make some
measurements as I did to verify what the MA is.  Often different pads
or pad clearance give the brake a different feel.  Typically, when a
brake lever is close to the end of its travel, one suddenly notices
how much sponge there is in the remaining stroke.

> The mechanical advantage varies _more_ over the stroke of the
> centerpull brake, and the ideal brake has a high mechanical
> advantage early in the stroke, and a low mechanical advantage late
> in the stroke.  This gets the pad to the rim quickly, then allows
> you to easily modulate stopping force once you touch the rim.  This
> is how centerpulls and cantilevers work, and its why they are easier
> to control and were the most popular brakes in the 1970's.

>   http://www.sheldonbrown.com/cantilever-geometry.html#vbrake

I think you'll find that the difference is insignificant unless you
run with excess pad clearance so that there is a large caliper motion,
an undesirable effect in any brake.  This is why brake centering is
more important than some brake people seem to think.  The dual pivot
is specifically aimed at that problem.

> In my opinion, Campy did everyone a disservice by popularizing the
> lightweight short reach sidepull caliper, and it has taken almost a
> generation to twist the centerpull caliper over to the side of the
> brake (e.g. dual pivot) and produce a brake with almost the same
> modulation in mechanical advantage and therefore to undo the damage
> that Campagnolo had wrought in the 1970's.

The whole short reach phenomenon was driven by frame builders and
their marketeers who played on the "close coupled", "tight", etc
spoiler on the family sedan gimmick.  Campagnolo only made the
equipment to go on these bicycles.  However, the short reach brake had
the same mechanical advantage of the standard reach brake.  You can
verify that by simple measurements with a ruler.

Jobst Brandt
jobst.brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: Centerpull brakes
Newsgroups: rec.bicycles.tech
Message-ID: <JnSjb.32771$dk4.885210@typhoon.sonic.net>
Date: Fri, 17 Oct 2003 13:45:13 GMT

Helmut Springer writes:

>> 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.

> 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.

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.

>> 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.

> How did they solve the problem?

The spring is a single smooth band that reaches from an anchor point
on the short arm to a "sliding point" at the long brake arm where it
doesn't slide due to its bending pint in the center over the anchor
bolt.  It is elegantly positioned covering the split between the two
arms of the brake.

Jobst Brandt
jobst.brandt@stanfordalumni.org