This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <firstname.lastname@example.org>
Date: Fri, 11 Jun 1999 14:53:00 PDT
Bicycle brakes have changed greatly since the original wagon wheel
brake that pressed a skid pad against the tread, but they have also
stayed the same, the skid pad brake still being used. The single
pivot caliper brake, commonly called the side pull, came along about
100 years ago and is still the mainstay. This brake was displaced by
the centerpull, a derivative of a cantilever brake, to take a large
part of the sport market in the 1950s. Meanwhile the cantilever brake
with its large tire clearance existed only in a limited way until the
advent of the mountain bike that demanded this feature for its large
tires and the dirt that sticks to them. Recently, other forms have
emerged to meet changing demands of the sports bicycling market.
Until recently, most brakes had a hand lever ratio (mechanical
advantage) of 4:1, with a caliper ratio of 1:1, making most brakes and
levers interchangeable. The 4:1 ratio struck a convenient compromise
between the reach of the hand, its strength, and brake pad clearance
to the rim. At higher ratios too much hand movement is used to bring
the pads into contact with the rim, a clearance that is necessary to
prevent a dragging brake and to take up pad wear. An important
feature of the single pivot is that it has practically no position
error through its sweep, the pad remaining centered on the rim
throughout its wear life.
Its main weakness is poor centering (clearance), caused by sliding
contact of its return springs. Exposed to road dirt, the sliding
springs change their coefficient of friction unpredictably, causing
the pads to retract unequally from the rim. To prevent dragging,
liberal clearance is required, preventing the use of the higher
mechanical advantage desired by today's avocational bicyclists.
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. It had the same hand
levers and its caliper the same 1:1 mechanical advantage, but had
large position error, moving its pads upward into the tire with wear.
Its symmetry may have been its main appeal, an aesthetic that people
often admire without functional reason. Its acceptance might also
have been from dissatisfaction with flimsy sidepull calipers of the
time. It used a straddle cable on which the main cable pulled from a
flimsy cable anchor attached to the tab washer under the head bearing
locknut. Besides its two levers, it had a connecting bridge that
flexed in bending and torsion, making it spongy. Although Mafac was
one of the greatest proponents of this design it began to vanish on
sport bicycles with the introduction of the Campagnolo sidepull brake.
The cantilever brake offers clearance that fat tires and mud demand.
Its pads pivot from cantilever posts on the fork blades, giving it
large tire clearance and a fairly rigid action, there being no
significant bending elements in its mechanism. Nevertheless it has
its drawbacks. Its reaction force spreads and twists the fork blades,
something that became more apparent with suspension forks that require
a substantial bridge plate to restrain these forces. Its pads sweep
downward at about a 45 degree angle giving them such a large position
error that, as they wear, they easily pop under the rim, causing
unrecoverable brake failure. Its straddle cable is pulled by a main
cable that requires a cable anchor that is difficult to accommodate
with rear suspension, while the front straddle cable presents a hazard
in the event of a main cable failure, because it can fall onto a
knobby tire to cause wheel lockup.
The cantilever received a large resurgence in popularity on the
mountain bike, along with other innovative designs. One of these
concepts was the servo brake that had cantilever posts with a steep
helix that converted forward drag of its pads to contact force, a
dangerous servo effect that re-emerges from time to time.
Servo brakes, ones that use pad reaction force to reinforce braking
force, have been designed often and without success, mainly because a
small change in friction coefficient causes a large change in braking.
The servo effect makes the relationship between application force and
brake response unpredictable and difficult to control. The servo
effect inherent in drum brakes is what caused automobiles and
motorcycles to switch to disks. Brake application pressure being at
right angles to the rotating disk, prevents any interaction between
reaction and application force. For bicycles, that effectively
already have disk brakes, introduction of servo effect is illogical.
The V-brake is currently displacing the cantilever brake because it
offers the same advantages while solving two critical problems, those
of the brake hanger for suspension bicycles and brake pad dive. The
cable hanger seems to have been the main goal because early V-brakes
had rigidly mounted pads that traveled in the same arc as those of a
cantilever. Newer versions use a parallelogram link that keeps pad
motion perpendicular to the rim. As usual, these advantages are not
gained without drawbacks, such as brake chatter arising from more
complex linkage and clearance required for it to work in dirt, and
incompatibility with other brakes by its higher mechanical advantage
that requires different hand levers.
The difference in mechanical advantage has been bridged by third party
hardware, one of which is called the "travel agent", that uses a two
diameter wheel to change the mechanical advantage to that of common
road brake levers. The device can also be used in a 1:1 ratio to
replace the elbow tube of the V-brake to reduce sliding friction.
Greater leverage for the same hand motion requires smaller pad-to-rim
clearance, that the dual pivot brake achieves by using two pivot
points to define a line of action about which its two arms are
constrained to move equally and remain centered. Brake centering was
essential in reducing the pad-to-rim clearance needed for a mechanical
advantages of about 5.6:1.
Higher leverage also required compromise. The offset arm (the short
one) sweeps its pad upward into the tire so that this pad must be
adjusted as it wears. The brake cannot track a crooked wheel with,
for instance, a broken spoke, and because it has a high ratio, it does
not work at all when the quick release is accidentally left open. And
finally, it runs out of hand lever travel 40% faster with pad wear
than the former single pivot brake. Its low pad clearance and narrow
flange spacing of current wheels make the brake drag when climbing
hills standing, so that racers often ride with the rear quick release
Part of the light feel of the dual pivot brake arises from the lower
(reverse) ratio of the caliper, whose springs now no longer exert as
strong a return force on the cable and hand lever. Because this force
is lower, a return spring has been added to the hand lever, lowering
cable return force, that coincidentally reduces cable drag during free
motion of the brake (before making contact with the rim). This makes
the brake FEEL even more forceful than it is because it has such a
light action in neutral.
For lack of power brakes that motor vehicles have, brakes with
variable ratios have been designed for bicycles, one of which was a
major blunder for Campagnolo. Campagnolo introduced the Delta brake
(aka Modolo Kronos), whose mechanism is an equilateral parallelogram
in which a cable draws two opposite corners of a "diamond" together,
such that the other two corners expand. The motion can be visualized
by placing the tips of the thumbs and forefingers together to form a
diamond. Moving the tips of the diamond together at a constant rate
demonstrates the progressive nature of the mechanism and the resulting
braking action, the brake pads being connected by links to the
knuckles as it were.
The motion is a tangent function that goes from zero to infinity. An
example of this is the motion of the top of a ladder, leaning steeply
against a wall, as the foot of the ladder moves away from the wall at
a constant rate. At first the the top of the ladder moves
imperceptibly, gradually accelerating until, near the bottom, its
speed approaches infinity. Although the Delta does not use the
extremes of this range, it has this characteristic in contrast to a
sidepull brake that has a constant 1:1 ratio throughout its range.
Besides its adverse response curve, its pads moved in an upward arc
toward the tire similar to a centerpull, which it essentially is.
Hydraulic brakes have their own problems of complexity and reliability
that keep them in an almost invisible presence in general bicycling.
Their advocates insist that they are superior in all respects in spite
of their lack of acceptance by the bicycling public at large.