From: kim39@scws8.harvard.edu (John Kim)
Newsgroups: rec.bicycles
Subject: Re: Re: Dual pivot calipers (?)
Date: 21 Aug 92 00:05:50 GMT
Organization: Harvard University Science Center

@hplomg.hpl.hp.com> BRANDT_JOBST/HP1900_01@HPLOMG.HPL.HP.COM writes:
>Tom Kunich writes:
>> How about because they are twice as powerful, need only a finger to
>
>have the same constraints of finger stroke to pad movement.  In this
>respect, hand levers have a ratio of 4:1 and "calipers" 1:1.  Of course,
>this is an average because the Delta brake, for instance, has a non
>linear ratio as do others like certain cantilevers.  However, the median
>ratio is 1:1.
>
>What is different is the amount of sponge.. how much the brake lever can
>be moved per unit of force after the pad has fully contacted the rim.
>This varies enormously from one model to the next.  It is this feature
>that makes great differences in brake response and effectiveness.
>So don't jump to conclusions on the basis of advertising claims.
>jobst_brandt@hplabs.hp.com

	Not that I want to risk contradicting Jobst, but I just tried
these brakes today (the test rides started one day early) with
the Shimano 105 dual-pivot brakes, and they *seem* to be more powerful.
My guess is that since, as Jobst said, all brakes face the same constraints
in design, these brakes may have sacrificed pad movement for braking 
force.  Thus if the brakes produce X% more force after contact for
Y kg of force on the levers than similar single-pivor brakes, then
they will move X% less per cm of brake lever pull than single-pivot
brakes.  THe disadvantages, besides the provlem of not being able
to track a wobbly rim, are that the pads must be set closer to the
rim than single-pivot brakes and have less tolerance for an untrue
rim (made worse by the inability to track).  Also, since these brake
calipers are usually set up with SLR levers, there may be a non-linear
response (though someone said his Shimano levers did not have a cam
like the SErvo-Wave levers do).  Servo-wave apperas to try to change
the mechanical advantage ratio at the lever, increasing the advantage as
the brake pads move closer to the rim on certain cantilever brakes.

	It is possible that the perceived improvement in stopping power
was really due to a combination of factors, such as better pads,
thinner pads, more rigid cliper arms, cleaner rims, less weight on
the bike (subtract pump, Kryptonite, fender, rack, bottle cages,
Vistalite, bell, shoulder bag, tools, extra tube, etc), and
a false perception of improved performance due to expectations.
I have the exact same levers on my heavy bike, but with canilever
brakes.

	As far as I can tell, the Force 40 feature on '92
Cannondale MTBs does sacrifice pad movement in favor of increased
braking force.  It's like using a two-pulley systme to lift
a load instead of one.  With one pulley you get no mechanical 
advantage except to change the direction of force.  With a two-pulley
system, you get twice the force but must pull the rope 10cm to
lift the load 5cm.  One bike mag warned that while Force 40
allows increased braking power, it required one to set the pads
very close to the rims.

	I think what Jobst said about the design constraints for
brake levers is not 100% true in that manufacturers COULD design
brakes with more lever pull, but these would only be usable
by people with large hands.  Since most manufacturers would like
to make one size for each model of brake lever, they try to 
design levers that the average cyclist can use without trouble.
People with certain Modolo-type anatomic bars may be able to
get more lever motion (and more cable pull) out of their levers, 
but these also put the levers further away from the outside edges
of the hands, making them harder to reach in the first place.

-Case

Tomorrow it's time to test the carbon frame and maybe the RB-1
or a trek.