Disc brakes Bicycle brake

1 disc brakes

1.1 advantages
1.2 disadvantages
1.3 hydraulic vs. mechanical
1.4 single vs. dual actuation
1.5 multiple pistons
1.6 caliper mounting standards
1.7 advantages , disadvantages of various types of mounts
1.8 disc mounting standards
1.9 disc sizes

disc brakes

a hydraulic front disc brake

a disc brake consists of metal disc, or rotor , attached wheel hub rotates wheel. calipers attached frame or fork along pads squeeze rotors braking. pads drag against rotor, wheel - , bicycle - slowed kinetic energy (motion) transformed thermal energy (heat). disc brakes may actuated mechanically cable, or hydraulically.

disc brakes common mountain bikes (including downhill bikes), , seen on hybrid bicycles , touring bicycles. disc brake employed drag brake.

many hydraulic disc brakes have self-adjusting mechanism brake pad wears, pistons keep distance pad disc consistent maintain same brake lever throw. hydraulic brakes, older ones, , mechanical discs have manual controls adjust pad-to-rotor gap. several adjustments required during life of pads.

advantages

disc brakes tend perform equally in conditions including water, mud, , snow due several factors:

the braking surface farther ground , possible contaminants mud can coat or freeze on rim , pads. rim brakes, first point mud builds on mountain bike ridden in thick mud brakes. mountain bicycle disc brakes less susceptible mud buildup provided rear frame , front fork yoke have sufficient clearance wheels.
disc brakes may made of materials dissipate heat better wheel rim, undersized sport sized discs small take advantage of fact
there holes in rotor, providing path water , debris out under pads.
wheel rims tend made of lightweight metal. brake discs , pads harder , can accept higher maximum loads.
it possible ride bicycle buckled wheel if has disk brakes, not possible rim brake because buckled wheel bind on brake pads.

other reasons include:

while types of brakes wear out braking surface, brake disc easier , cheaper replace wheel rim or drum.
the use of wide tyres favors disc brakes, rim brakes require ever-longer arms clear wider tyre. longer arms tend flex more, degrading braking. disc brakes unaffected tyre width.
unlike rarer rim brake designs, disc brakes compatible front , rear suspension.

disadvantages

disc brakes require hub built accept disc, , fork (for front brakes) or frame (for rear brakes) built accept caliper. front hubs designed discs move left hub s flange inward make room disc, causes wheel dished. dished wheel laterally weaker when forced non-disc side. other hubs use conventional flange spacing , provide wheel without dish, require less common wide-spaced fork.
a rim brake works directly on rim , attached tyre; disc brake applies potentially large torque moment @ hub. latter has 2 main disadvantages:



the heavier fork , wheels compound weight disadvantage of brake assembly itself.


heat build-up can lead failure disc brakes. disc brakes heat discs in same way rim brakes heat rims, discs provide inherently smaller surface dissipate heat. excessive heat leads boiling hydraulic fluid, resulting in brake fade or total failure. overheating more common in road cycling assuming brakes undersized reduce weight. recorded cases of disc brake failures involves several kilometers of downhill, combined small underweight discs.
the design , positioning of disc brakes can interfere pannier racks not designed them. reason, many manufacturers produce disc , non-disc versions.
there have been problems using disc brakes frame designs. under hard braking, front wheel can come out dropouts. problem has occurred when brake pads , dropouts aligned brake reaction force tends eject wheel dropout. under repeated hard braking, axle moves in dropout in way unscrews quick release. riders should make sure skewers tightened before riding. forks use different brake/dropout orientations or through-axles not subject problem.

hydraulic vs. mechanical

there 2 main types of disc brake: mechanical (cable-actuated) , hydraulic. advantages of cable-actuated disc brakes in lower cost, lower maintenance, , lighter system weight. hydraulic disc brakes, on other hand, offer more braking power , more control on braking power. cable-actuated traditionally type of disc brake used brake levers found on drop handlebars, no longer case.

single vs. dual actuation

many disc brakes have pads actuated both sides of caliper, while have 1 pad moves. dual actuation can move both pads relative caliper, or can move 1 pad relative caliper, move caliper , other pad relative rotor, called floating caliper design. single-actuation brakes use either multi-part rotor floats axially on hub, or bend rotor sideways needed. bending rotor theoretically inferior, in practice gives service, under high-force braking hot disc, , may yield more progressiveness.

multiple pistons

for disc brakes hydraulic system, high-performance calipers use 2 or 3 pistons per side; lower-cost , lower-performance calipers have 1 per side. using more pistons allows larger piston area , increased leverage given master cylinder. also, pistons may of several sizes pad force can controlled across face of pad, when pad long , narrow. long narrow pad may desired increase pad area , reduce frequency of pad changes. in contrast, single large piston may heavier.

caliper mounting standards

there many standards mounting disc brake calipers. i.s. (international standard) different 160mm , 203mm rotor , differs between forks qr , 20 mm through axle. post-mount standard differs disc size , axle type. many incompatible variants produced on years, fork manufacturers. mount used on rockshox boxxer typical of these specialty mounts, fork manufactures use either or post-mount standard current forks. point of reference, hayes sells no fewer 13 different adapters fit brakes various mounting patterns.

advantages , disadvantages of various types of mounts

a disadvantage of post mounts bolt threaded directly fork lowers. if threads stripped or if bolt stuck, threads need repaired, or seized bolt drilled out. frame manufacturers have standardized mount rear disc brake mount. in recent years post mount has gained ground , becoming more common. due decreased manufacturing , part cost brake calipers when using post mount. limitation of mount location of rotor more constrained: possible encounter incompatible hub/fork combinations, rotor out of range.

disc mounting standards

there many options rotor mounting. is six-bolt mount , industry standard. centerlock patented shimano , uses splined interface along lockring secure disc. advantages of centerlock splined interface theoretically stiffer, , removing disc quicker because requires 1 lockring removed. of disadvantages design patented requiring licensing fee shimano. shimano cassette lockring tool (or external bb tool in case of through-axle hub) needed remove rotor , more expensive , less common torx key. advantages of six-bolt there more choices when comes hubs , rotors.

examples of mounting standards shown here:

centerlock (shimano proprietary)
international standard (is) (in widespread use) 44mm bcd
hope technology s 3-bolt pattern (proprietary)
rohloff s 4-bolt pattern (proprietary)

disc sizes

rotors come in many different sizes, such 160 mm, 185 mm, , 203 mm diameter. other sizes available manufacturers make discs specific calipers — dimensions vary few millimeters. larger rotors provide greater braking force given pad pressure, virtue of longer moment arm caliper act on. smaller rotors provide less stopping power less weight , better protection knocks. larger rotors dissipate heat more , have larger amount of mass absorb heat, reducing brake fade or failure. downhill bikes have larger brakes handle greater braking loads. cross country bicycles use smaller rotors handle smaller loads offer considerable weight savings.

it common use larger diameter rotor on front wheel , smaller rotor on rear wheel since front wheel braking (up 90% of total).