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Article #3: Bicycle brake systems

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Early bicycles such as the high wheeled conditions including water and mud. They
penny-farthing bikes had no brakes as we also avoid the problem that rim brakes
would recognize them today. As the have of wearing out the wheel rims,
machines were fixed gear bicycles a rider especially in muddy conditions, as well
could reduce speed by reversing the as the requirement that the rim be true.
thrust on the pedals. Otherwise, a rider Also, disc brakes offer better modulation
who wanted to stop quickly had to jump of braking power and generally require
off the bike as it was moving. less finger effort to achieve the same
Unsurprisingly there were many accidents, braking power. The disadvantages are that
some of them fatal, which limited the they are usually heavier and more
appeal of cycling mostly to young and expensive than rim brakes, and require a
adventurous men. hub built to accept the disc and a
The 1870s saw the development of the bicycle frame or fork built to accept the
"safety bicycle" which was roughly the caliper. Also, rigid forks on road bikes
bicycle we would recognize today, with and tandems that are made to handle the
two wheels of equal size, initially with forces of a front disc brake are heavier
solid rubber tires. These were generally and may not have the ride quality of a
equipped with a front spoon brake and no regular fork. Furthermore, a disc brake
rear brake, although with no free wheel, puts more stress on a wheel's spokes than
back-pedalling was an option. This was a rim brake, since the torque of braking
undoubtedly a big improvement on having occurs between the hub and the rim with
no brakes at all, but it was not very disc brakes, unlike with rim brakes. For
powerful and almost useless in wet this reason, cross-lacing of spokes is
weather. usually employed with disc brakes, while
With the introduction of the pneumatic rim brakes sometimes allow the option of
tire also came the next advance in lighter radial lacing.
bicycle braking around the 1890s, the There are two general categories of disc
invention of the rim brake. This is the brakes, mechanical and hydraulic.
type of brake most commonly used on Mechanical disc brakes (which are almost
bicycles today. Track bicycles, however, always less expensive than hydraulic) use
which are ridden at top speed regular brake cables and levers to
continuously, continue to be built with actuate the pads. Hydraulic disc brakes
no brakes; since they are fixed gear use fluid from a resevoir, pushed through
bicycles, braking on these bikes is still a hose, to actuate the pistons in the
achieved only by reversing the force on disc caliper, that then actuate the pads.
the pedals. This is not a disadvantage Hydraulic disc brakes occasionally
for the very specialized manner in which require bleeding of the brake lines to
these bicycles are ridden. remove air bubbles. There are two types
Types of bicycle brakes of brake fluid used in disc brakes today:
Rim brakes mineral oil and DOT fluid. Mineral oil is
There are several types of rim brakes. In generally inert and while DOT has a
all of these designs, the braking force higher boiling point, it is known to be
is applied by the rider squeezing a lever corrosive to frame paint. The two are
mounted on the handlebar; this causes generally interchangeable as the
friction pads (usually made of leather or different fluids may cause seals to swell
rubber) to contact the rim of the or be corroded
rotating wheel, thus slowing it and the Many disc brakes have their two pads
bicycle. actuated from both sides of the caliper,
Rod brakes use a series of rods and while some (less expensive in both
pivots (rather than Bowden cables) to hydraulic and mechanical varieties) have
pull the friction pads upwards onto the only one pad that moves. Many hydraulic
inner surface (facing the hub) of the disc brakes have a self-adjusting
wheel rim. They were often called mechanism so as the brake pad wears, the
"stirrup brakes" due to their shape. In pistons will keep the distance from the
order to fit the rear wheel as well as pad to the disc consistent in order to
the more directly actuated front wheel, maintain the same brake lever throw.
these brakes need a mechanism to transmit Mechanical discs have a manual adjuster
the force but still allow rotation where to dial in the pad to rotor distance.
the fork attaches to the frame. Although Calipers are now generally made in one
rod brakes are heavy and the linkage is piece to increase stiffness and reduce
complex, they are reliable and durable the threat of leaks, although the two
and can be repaired or adjusted with piece design still reduces heat builup
simple hand tools in areas where more effectively than the one piece, and
replacement Bowden cables are not most top end models still feature the 2
available or are too expensive. They are piece caliper.
still used on some bicycles, typically There are two standards for mounting disc
roadsters, particularly in the Far East. brake calipers - IS (International
More modern designs use similar friction Standard) and post mount. IS is the more
pads to squeeze the sides of the wheel common of the two. Manitou is the
rims, with the force transmitted to the inventor of post mount. The advantage of
brake from the lever by means of a Bowden post mounting is that it's easier to
cable. Designs include the scissor-action align the caliper to the rotor since
"side pull" and "centre pull" brakes, and there is some allowable side to side
the lever action "cantilever" and "V" adjustment. You must use spacers to
brakes. properly align IS calipers. The
Single pivot side-pull caliper brakes are disadvantage to post mount is that the
currently used on the most inexpensive bolt is threaded directly into the fork
bikes, before the introduction of lowers. If the threading was stripped or
dual-pivot caliper brakes they were used if the bolt was stuck, then it would
on all types of road bicycles. They require purchasing new fork lowers. Frame
consist of two curved arms that cross at manufacturers have standardized the IS
a pivot above the wheel and hold the mount for the rear disc brake mount.
brake pads on opposite sides of the rim. There are two standards for disc hubs -
These arms have extensions on one side; IS and centerlock. IS is a six-bolt mount
the cable housing is attached to one and is the industry standard. Centerlock
extension and the inner cable to the is patented by Shimano and uses a splined
other, so that when the brake lever is interface along with a lockring to secure
squeezed, the arms move together and the the disc. The advantages of centerlock
brake pads squeeze the rim. These brakes are that the splined interface is
are simple and effective when designed stiffer, removing the disc is quicker
for relatively narrow tires, but have because it only requires one lockring to
serious disadvantages if made big enough be removed, and the hubs and discs are
to fit wide tires. The low quality lighter than the 6 bolt ones. The
varieties also have a tendency to rotate disadvantage is that the design is
to one side during actuation and to stay patented requiring a licensing fee from
there, so that one brake pad continually Shimano. Advantages of IS six-bolt are
rubs the rim even when the brake is that you have more choices when it comes
released. to hubs and rotors. IS rotors use allen
Center-pull caliper brakes in the past bolts or Torx bolts to secure them to the
filled the price niche between the hub. This also makes IS rotors harder to
cheaper and the more expensive models of remove and the possiblity of stripping
side-pull brakes, being reasonably priced the bolts and requiring a new hub.
but without the tendency of the cheaper The rotors come in several different
sidepull brakes to have one brake pad sizes, generally six, seven, or eight in
continually rub the rim. In this type of diameter. These are the industry standard
brake the actuation of the two brake arms sizes, some prototype versions are coming
is designed to be symmetrical. The cable out with 9 in diameter rotors now. As the
housing is attached to a fixed cable stop rotor size becomes bigger, so does the
attached to the frame, and the inner torque on the wheel resulting in a more
cable attaches to a sliding piece or powerful brake. Also for every revolution
sometimes a small pulley, over which runs of the wheel more area of the disc passes
a straddle cable which connects the two through the caliper making the bigger
brake arms. Thus tension on the cable is rotors run cooler. If too small of a disc
evenly distributed to the two arms, is run for your specific type of riding,
preventing the brake from taking a "set" brake fade may be experienced because of
to one side or the other. the brake fluid boiling.
Dual-pivot caliper brakes are used on Drum brakes
most modern racing bicycles. The cable Drum brakes are used mainly on bicycles
housing is attached like that of a which are operated in wet or dirty
side-pull brake. Centering of side-pull conditions. Though heavier, more
brakes was simplified with the complicated, and frequently weaker than
mass-market adoption of dual-pivot rim brakes, they require much less
side-pulls (an old design re-discovered maintenance and are less affected by road
by Shimano in the early 1990s). These conditions. Both cable- and rod-operated
brakes also offer a higher mechanical drum brake systems have been widely
advantage. Dual-pivot brakes have produced. While most common on utility
slightly higher weight than conventional bicycles in certain countries, especially
side-pull calipers and cannot accurately the Netherlands, they are also frequently
track an out-of-true rim. One arm pivots found on freight bicycles.
at the center, like a side-pull; and the A bicycle drum brake's operation is
other pivots at the side, like a identical to that of a car's with the
center-pull. exception of the absence of a ratching
Cantilever, direct-pull, and V-brakes adjustment mechanism and hydraulic
have each arm attached to a separate actuation. Two pads are pressed outward
pivot point on one side of the seat stay against the braking surface on the inside
or fork just below the rim. The wider of the hub's shell, which is packed with
tires on mountain bikes present a problem grease. Shell diameters on a bicycle drum
with standard calipers because the long brake are typically 70 - 120 mm. Drum
distance from the pivot to the pad allows brakes have been used on front hubs and
the arms to flex, reducing braking hubs with both internal and external
effectiveness. Cantilever brakes are one freewheels.
solution to this problem. A common design of drum brake is the
The traditional cantilever has an Roller Brake, manufactured by Shimano.
L-shaped arm protruding outwards on each This is a modular cable-operated drum
side, with a cable stop on the frame or brake for use on specially splined front
fork to hold the cable housing and a and rear hubs. Unlike a normal drum
straddle cable between the arms similar brake, the Roller Brake can be removed
to the centerpull brake. The cable from entirely from a hub, allowing it to
the brake handle pulls upwards on the function as a regular freewheel. It also
straddle cable, causing the brake arms to contains a torque limiting device which
rotate up and inward and squeezing the reduces its effectiveness on bicycles
rim between the brake pads. with adult-sized wheels.
V-brakes (sometimes called linear- or Drag brake
direct-pull) mount similarly, but the In addition, drum brakes are also
arms extend straight up, and the outer installed on some tandems used in
housing is attached to one arm and the mountainous areas as a so called drag
inner housing to the other, similar to brake. In this application, the drum is
the cable attachment for sidepull brakes. not intended to stop the bike but,
They are generally more powerful and rather, to keep the speed down on long
easier to adjust than cantilever brakes, downhill sections where extended use of
and have the additional advantage of rim brakes can cause the tire to become
functioning well with the suspension hot enough to explode.
systems found on many mountain bikes The largest manufacturer of this type is
because they are self contained and do Arai, whose brakes are screwed onto hubs
not require a separate cable stop. with conventional freewheel threading on
However due to their higher mechanical the left side of the rear hub and
advantage, the brake pads must be operated via Bowden cables.
positioned very close to the rim. Also Coaster brakes
see V-brake adjustment. A coaster brake, also known as a back
Closely related is the U-brake; this type pedal brake or foot brake (or torpedo in
has the pivots for the arms mounted to some countries), is a drum brake
the frame or fork on each side above the integrated into hubs with an internal
rim. The arms cross over similarly to freewheel. Freewheeling functions as with
sidepull or centerpull brakes, and BMX other systems, but, when back pedalled,
bikes; its main advantage was that it did the brake engages after a fraction of a
not protrude sideways from the frame like revolution. It can frequently be found in
the early cantilevers. This advantage was both single-speed and geared hubs.
reduced by redesigned low-profile When such a hub is pedalled forwards, the
cantilevers, and nearly eliminated with V sprocket drives a screw which forces a
brakes. clutch to move along the axle, driving
One of the least common rim brakes is the the hub shell or gear assembly. When
hydraulic rim brake. These brakes are pedalling is reversed, the screw drives
generally able to be mounted on the same the clutch in the opposite direction,
pivot points used for cantilever and forcing it either between two brake pads
linear-pull brakes. They were available and pressing them against the shell, or
on some high-end mountain bikes in the into a split collar and expanding it
early 1990's, but declined in popularity against the shell. The braking surface is
with the rise of disk and linear-pull often steel, and the braking element
brakes. The moderate performance brass or phosphor-bronze, as in the UK
advantage (greater power and control) Birmingham made Perry Coaster Hub.
they offer over the latter is offset by Coaster brake bicycles are generally
their greater weight and complexity. The equipped with a single cog and chainwheel
only significant current use of these and use a 1" chain. However, there have
brakes is on bicycles used for trials been several models of coaster brake hubs
riding. with derailleurs in the past, most
Another design is the delta brake. In notably the Sachs 2x3. These use special
this design, the pivot points for the extra-short derailleurs which both can
arms are above the rim but the arms do stand up to the rigors of being
not cross over, and instead of a straddle straighted out frequently and don't
cable, the inner brake cable attaches to require an excessive amount of reverse
a wedge shaped piece between the brake pedal rotation before the brake engages.
arms; when the brakes are applied the Although coaster brakes have the
wedge forces the arms apart at the top, advantage of being protected from the
squeezing the rim between the pads. This elements and thus immune to ice or water,
has an advantage in that the shape of the because they are located in the rear
wedge can be varied other than wheel only limited braking force can be
straight-sided, to allow for a very high applied before the rear wheel locks up.
mechanical advantage at the point where This is due to the placement of the
the pads contact the rim to give high rider's weight ahead of the rear tyre's
braking power, but a lower mechanical contact with the ground, as well as the
advantage when the pads are not weight transfer forward proportional to
contacting the rim so that the pads move braking force, which further unloads the
well away from the rim when the brake is rear wheel. Additionally, although
not applied, preventing any rubbing. coaster brakes generally go years without
The advantages of rim brakes are that needing maintenance, they are more
they are inexpensive, lightweight, complicated than rim brakes to repair if
mechanically simple, and easy to it becomes necessary.
maintain, as well as very powerful. The Spoon brakes
main disadvantage of rim brakes is that The spoon brake was one of the first
their performance deteriorates in wet types of bicycle brakes and precedes the
weather when the rims are wet. This pneumatic tire. They were first used on
problem is less serious on more expensive penny farthings with solid rubber tires
bikes which use rims made of aluminum in the late 1800's and continued to be
alloys than on those with steel rims. used after the introduction of the
They are also prone to clogging with mud, pneumatic tired safety bicycle. It
particularly when mountain biking. Rim consists of a pad (often leather) which
brakes also need regular maintenance. is pressed onto the top of the front
Brake pads wear down quickly, and have to tire. These were almost always
be replaced. Over long periods of time rod-operated by a right-hand lever. In
and use, rims also become worn. Bowden developing countries, a foot-operated
cables can jam if not regularly form of the spoon brake is sometimes
lubricated or if water gets into the retrofitted to old rod brake roadsters.
housing, causing corrosion. The cables It consists of a spring-loaded flap
also wear, requiring frequent checking attached to the back of the fork crown.
and replacement. If the inner cables are This is depressed against the front tire
not replaced when they fray, they will by the rider's foot.
suddenly break when brakes are applied Perhaps more so than any other form of
strongly, causing brakes to be lost bicycle brake, the spoon brake is very
precisely when they are most needed. Rim sensitive to road conditions and
brakes also require that the rim be increases tire wear dramatically.
relatively true; if the rim has a Though made obsolete by the introduction
pronounced wobble, either side to side or of the coaster brake and rod brake, they
inwards and outwards, it will be continued to be used supplementally on
impossible to adjust the brake pads so adult bicycles until the 1930's and
that they can apply sufficient pressure children's bicycles until the 1950's, in
to the rim but do not rub when released the West. In the developing world, they
and do not contact the tire, which would were manufactured until much more
lead to a rapid blowout. recently.
Rim brakes also suffer from the Braking technique
disadvantage of rim heating, because the Effective use of a bicycle brake is
brake converts kinetic energy into heat, highly counter-intuitive. The casual
which increases the temperature of the rider will at first avoid using the front
rim (the brake caliper and frame do not brake, due to the unsettling feeling of
become hot because brake pads are "toppling up", or fear of being sent
excellent thermal insulators). In normal flying over the handlebars.
use and with lightweight bicycles this However, the most effective technique for
does not present a problem as the brakes powerful stopping is to use the front
are only applied with a limited force and brake almost exclusively. There are
for a short time, with the heat being several exceptions where the rear brake
quickly disspated to the surrounding air. is preferred; these are listed below. In
However, on heavily-laden touring bikes any stop, the rider should shift their
and tandems in mountainous regions the weight toward the rear and use their arms
heat build-up can be sufficient to to brace against the deceleration.
increase tire pressure to the extent that During braking (either with the front or
the tire blows off the rim. If this rear brake), the bike deceleration causes
happens on the front wheel a serious a transfer of weight to the front wheel.
accident is almost inevitable. The This means that there is more force
problem is worse when descending pressing the front wheel to the ground,
cautiously at slow speeds because the and the back wheel nearly none.
brakes are "always on" and the cooling Therefore, the front wheel can generate
airflow over the rim is insufficient. The more frictional braking force than the
risk can be minimised by not back wheel before locking up and
over-inflating tires and the adoption of skidding. In any conditions and
an aggressive riding style, only braking especially in wet conditions or going
for the corners, but the real solution is downhill, the rear brake can exert
a hub brake or a disc brake which allows relatively little braking force before
the rim to remain cool. the wheel locks and starts skidding. For
Brake pads (brake blocks) are of many a more-detailed analysis, see Bicycle and
designs. Most consist of a replaceable motorcycle dynamics.
rubber pad held in a metal channel (brake A skidding rear wheel can lead to
shoe), with a post or bolt protruding dangerous, uncontrollable bicycle
from the back to allow attachment to the movements eventually resulting in the
brake, but some are made as one piece cyclist falling on the ground. A key
with the attachment directly moulded in scenario for this is "light" braking on
the pad for lower production costs. The rapid alpine-type hairpin (serpentine)
rubber can be softer for more braking descents; further attenuation of speed to
force with less lever effort, or harder negotiate a decrease in curve radius
for longer life. The rubber can also (tightening of the bend in the road) can
contain abrasives for better braking, at be executed successfully while pulling
the expense of rim wear. Compounds vie through a tight corner regulating with
for better wet braking efficiency. the front brake only.
Typically pads are relatively short, but In an emergency stop, it is important to
longer varieties are also manufactured to grab the front brake and press it hard to
provide more surface area for braking; stop in the minimum possible distance.
these often must be curved to match the The rider should shift his or her weight
rim. It should be noted that a larger pad as far to the rear as possible to avoid
does not give more friction, but it does flipping over the handlebars. Maximum
wear more slowly and therefore a new pad deceleration is accomplished by
can be made thinner, simplifying wheel maintaining enough pressure on the front
removal with V-brakes in particular. In brake such that that the rear wheel is
general, a brake can be fitted with any barely touching the ground, just before
of these many varieties of pads, so long lifting up. In reality this is not
as the pad mounting method is compatible. practical for most cyclists. Instead, use
Carbon rims, as on some disc wheels, light pressure on the back wheel and hard
generally have to use non-abrasive cork pressure on the front. The back wheel is
pads. primarily useful as an indicator—when
Disc brakes it starts to skid, reduce the pressure to
Disc brakes consist of a metal disc both brakes to prevent the rear wheel
attached to the wheel hub that rotates from lifting, then increase pressure to
with the wheel. Calipers are attached to both again.
the frame or fork along with pads that Incidentally, on tandems, and
squeeze together on the disc. Although long-wheel-base recumbents with their
these brake types have been successfully long wheelbase and center of mass farther
used on, and have been the principal from the front wheel, it is virtually
choice for motorbikes for decades, impossible for heavy front braking to
numerous (partly successful) attempts at cause the machine to flip.
introducing disc brakes for bicycles over There are a few special situations where
the last decades [citation needed] may limited use of the front brake, and
now finally have reached fruition. Recent heavier involvement of the rear brake is
weight/costs/reliability material advisable:
advances have led to development and Slippery surfaces: It is difficult to
implementation of disc brake systems by recover from a front-wheel skid on a
several firms to the extent that they are slippery surface, especially when leaned
becoming a standard feature on many over, so on surfaces when skidding is
bicycles, and are almost certainly here likely (e.g. wet pavement, mud, snow, or
to stay. Disc brakes are most suitable ice), reduced speed and use of the rear
for and used mainly on mountain bikes brake may be preferred.
ridden off-road. They also are used on Bumpy surfaces: If the front wheel comes
hybrid bicycles and some road bicycles, off of the ground during braking, it will
although this is less common. Many tandem stop completely. Landing on a stopped
bicycles have a disc brake fitted on the front wheel with the brakes still applied
rear wheel in addition to rim brakes; the is likely to cause the front wheel to
disc brake can be set to provide a skid and, possibly, for the rider to flip
constant drag, so that during long over the front bars.
descents, the rim brakes are not Flat front tire: Braking the front wheel
overworked by the heavier when the tire is flat could cause the
machine.[citation needed] tire to come off of the rim, which is
The main advantage of disc brakes is that more likely to cause a crash.