From: pardo@cs.washington.edu (David Keppel)
Subject: Re: dual-pivot brakes
Organization: Computer Science & Engineering, U. of Washington, Seattle
Date: Thu, 27 Aug 92 23:57:47 GMT

(Just when you thought this topic was dead...)

This is a summary of a conversation with Tom, at Gregg's Greenlake.
I'm writing this from memory and I'm not quoting him.  Mostly, I'm
trying to get across most of what I learned during the conversation. 

We measured my Dura-Ace sidepull brakes, vintage 1990, and a set of
Shimano Dual-Pivot 105SC 1992 brakes.  The D-A brakes are about 57mm
from the pivot to a perpendicular to the cable, and about 50mm from
the pivot to the center of the brake pad.  That's about 1.15:1
leverage.

The 105SC Dual-Pivot brakes have one arm shaped like a conventional
center-pull arm.  It is about 70mm from the pivot to a perpendicular
to the cable, about 38mm from the pivot to the center of the pad.
That's about 1.85:1 leverage.

The other arm on the dual-pivot brakes is cammed and the pad does
not move as far during braking as the normal arm.  How does that
affect leverage?  Here ismy analysis: the effective leverage cannot
be higher or lower than the leverage of the lowest-leverage arm, or
else the pads would exert unequal force on the rim and move it
side-to-side.  Put another way, if the rim was free to move laterally,
then the high-leverage arm would force the low-leverage arm to
retract, which would limit the available force to that provided by the
``weaker'' (lower-leverage) arm.

Cable travel is the same for both hand levers.

The short answer, then, is that the Shimano dual-pivot design has more
leverage then the side-pull brakes.  The trade-off is that more lever
travel is needed to actuate the brakes.  Thus, the pads must start off
near the rim, with the consequence that less pad wear is needed to
make the dual-pivot brakes `bottom' (have the brake lever hit the
handlebars).

Here's answers to some of my questions:

Q: Do you need the extra leverage?

A: In dry conditions, no.  When I'm racing in the wet, I can go faster
when I have extra leverage because the brakes are there when I need
them.  It's still not as good as dry brakes, but better than regular
side-pulls.

Q: Won't you get in to trouble because you need extra cable travel to
get the same pad movement?

A: You'll get in to trouble if you adjust your pads rarely.  If you
want high-performance braking, though, you'll adjust your brakes
frequently, whether you have dual-pivot, side-pulls, or anything else.
It might be a problem in theory, but in practice there's enough travel
and they do the job.

Q: The Shimano dual-pivot design won't track rim irregularities.  Is
that a problem?

A: I race my cycle-cross bike with them.  I can see that it could be a
problem but it hasn't bothered me yet.

Q: The dual-pivot design is complicated and has extra parts.  Why not
just build a side-pull with better leverage?  Sure, it will weigh
more, but dual-pivot brakes weigh more, too.

A: In theory, you can add some material and get better leverage.  You
can't just make the cable arms longer, though, you need to make the
whole arm stiffer.  So the weight you add has to be distributed on the
arm.  Once you add it all up, I don't know what's the lightest design
for a given leverage.  I think if you look at some of the Campy brakes
they have more material, and maybe that's because they do have better
leverage.  But all the beefy brakes are much more expensive than the
dual-pivot brakes.  Maybe you can get the leverage with another
design, but if you want to just go out and buy something with a lot of
leverage, this is what's available.

Q: What's the point of the extra pivots?

A: The leverage comes from the lengths of the arm from cable to pivot
and pivot to pad.  With a side-pull, the pivot is over the top of the
tire.  You can't get the pivot closer than the tire clearance.  With
the dual pivots -- like center-pull or U-brake designs -- you can make
that pivot-to-pad arm shorter than with a side-pull.  That means the
parts are small yet you can still get leverage.

Q: If it's basically a center-pull design, why not just use a
center-pull?

A: A center-pull needs a housing stop somewhere.  A lot of the old
housing stops have a lot of flex, so people stay away from them.
Newer ones are stiffer, but you've still gotta have one.  With the
dual-pivot design you can unbolt your sidepulls and bolt in the
dual-pivot brake.

Q: Cantilevers seem like the most powerful rim brakes.  Why not just
switch to them?

A: First, because lots of bikes don't have the braze-ons.  That's why
I use dual-pivot brakes on my cross bike.  Second, because cantilevers
have more wind drag.

Q: We've just been talking about changing the part at the wheel.  What
about hand levers with more leverage?

A: More cable tension means you might need fatter cables, and the
brakes are more sensitve to housing compression.  Maybe you could make
it work, though.


Summary:

They made design tradeoffs and it does well on the expense account.

 - they do need to be kept in close tune
 o if you want high-perf. rim brakes, you'll do that anyway
 - they weigh more
 + they stop well
 + they're cheaper than other high-perf. rim brakes
 - they use lots of nonstandard parts
 - they don't follow rim iregularities
 + they're easy to adjust
 + has more leverage than other things at your corner store
 o not sure how much leverage you can get w/ side-pulls
 - it's Shimano, it's the way the sell stuff

That's what I did out of school today...

		;-D on  ( A dual to the debt )  Pardo
Newsgroups: rec.bicycles.tech