Basic Guide to Disc Brakes
Disc brakes are different from brakes that work by squeezing the rim, instead they operate by applying pressure to a brake pad on either side of a steel disc (or rotor) mounted to the hub of the bicycle. Disc brakes have some advantages and some disadvantages over rim brakes. Some of the advantages include: consistent performance despite wet or muddy conditions, superior heat dissipation, the ability to work in spite of damaged or warped rims, and decreased wear to the rim surface. The few disadvantages include: increased cost, increased complexity, and increased weight.
Disc Brake Characteristics
Actuation- How does lever input = braking?
- Hydraulic - uses fluid (usually mineral oil or DOT brake fluid) in a hose to activate the caliper. This type can be lighter and smoother operating but is usually more expensive. Mineral oil brakes have the advantage of being more Eco-Friendly as any on trail leaks or mishaps (a rare occurrence but with off-road riding you never know) don't leave a trail of haz-mat in the wilderness. Many hydraulic disc sets come with the hoses pre-bled so installation is fast and easy. Bleeding (removing excess air from the system) your brakes only takes a few minutes, most hydraulic brakes have a bleed kit available that makes everything very easy.
- Mechanical - uses a standard cable type brake lever and special compression-less brake housing. This type is a good choice if your bike is currently set up with integrated brake/shift levers. Mechanical disc brakes have an unwarranted reputation for lacking power and modulation, this is mainly based on very inexpensive models found stock on entry level mountain bikes. Most aftermarket mechanical disc brakes such as the Avid BB7 discs stop just as well as comparably priced hydraulic brakes.
Action- How do the pads move?
- Single Sided Caliper - There is only one piston on one side of the rotor (usually the frame side), as the piston activates it moves only 1 brake pad to meet the rotor, the rotor is deflected to meet the other pad. Adjusting the fixed pad to the proper distance is important for good modulation and good pad life.
- Opposed Caliper - there are pistons on each side of the rotor and they both actively move both pads to meet the rotor. Opposed caliper brakes apply braking pressure evenly, reducing the chance of squeal and vibration.
Rotor Types - How the rotor mounts to the hub.
- IS standard 6-bolt - This rotor is secured to the hub by using 6 evenly spaced torx bolts on a 51mm diameter circle. Most aftermarket disc brake sets use this mounting pattern.
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- Shimano Center-Lock (XTR to Deore) - This rotor is secured using a lock ring similar to a cassette cog lock ring. This design makes rotor installation faster and easier, the hubs themselves are nearly as light as non-disc models so there is little drawback to upgrading your wheels to center-lock early, plus many riders have reported fewer warped and bent rotors.
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- Shimano Oversize Center-Lock - Found on Shimano Saint® hubs only. This center-lock design has a larger diameter to give more support to oversized rotors. It is not cross compatible with any other center-lock hub system or rotor.
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Disc Rotor Size - How big the disc is.
Max Rotor Size is determined by the frame/fork manufacturer. Frames and forks must have extra clearance designed into them to take oversized rotors. If you are interested in oversized rotors and unsure of the maximum rotor size for your equipment, contact your bike or fork manufacturer.
- 160mm (or 165mm) 6 inch standard for Cross-Country and most riding.
- 180mm (or 185mm) ideal for larger riders, riders who see a lot of tough conditions or steep down hill sections and Freeride. This size is very popular for the front disc paired with a 160mm rear.
- 203mm Suited for Downhill use and large riders who see tough conditions. Usually requires a frame or fork designed for oversized rotors.
- It is not uncommon to spec a larger diameter disc rotor for the front and a standard 6 inch rotor for the back. Most braking power is supplied by the front brake, so you can maximize performance and reduce expense/weight by running a more standard 6 inch rotor in the rear.
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Mount Types - How the caliper bolts to the frame or fork.
- IS or International Standard can be found on forks (top) or frames (bottom). This mount consists of two non-threaded holes 6 mm in diameter drilled 51mm apart perpendicular to the disc rotor. Many calipers require an adapter to fit IS forks, but most rear brakes come with the required IS adapter (double check the product listing to confirm). IS disc mounts are typically found on Rock Shox, Marzocchi 2005 and older, Fox and other suspension forks. Compatible front brakes: Shimano, Grimeca, Magura, Hayes and Avid (with included adapter).
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- 74mm Post Mount found on Manitou forks 2000 and newer, as well as Marzocchi forks 2006 and newer. (fig B1). This mount is two threaded 6mm holes drilled 74mm apart parallel to the disc rotor. Many front calipers bolt directly to this mount without any adapters. Sometimes called a Hayes Mount. Compatible brakes: Avid, Hayes, Magura, Shimano (certain models only)
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- 68mm Post Mount found on Manitou Forks 1999 and older. This mount is very similar to the 74mm post mount, but the holes are only 68mm apart. If you have an older fork you should confirm the spacing before purchasing a caliper. 68mm adapters are still around for certain brake sets so all hope is not lost.
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22mm Hayes direct mount this mount is becoming obsolete. This mount was typically found on rear chainstays only. It consists of 2 holes drilled vertically into a reinforced section of the chainstay, the holes are threaded and approximately 22mm apart. Even though there are adapters that mount a IS caliper to the 22mm direct mount, most current calipers will not fit due to lack of room under the seat stay.
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Disc Brakes FAQ
What if I have integrated brake/shift levers?
If you prefer to use your existing brake/shift lever you can use mechanical disc brakes. The cable pull for mechanical discs is the same as Linear Pull brakes. You simply need to upgrade your brake housing to compression-less disc brake housing. If you desire hydraulic discs you will have to also upgrade your shifters to separate pods, or get Shimano integrated Dual Control levers such as the XTR M965, XT M765, LX M585 or the Deore M535 and the compatible brakeset.
I have Center-lock hubs but the brakeset I want comes with 6 bolt ISO rotors, do I have to buy new rotors upfront?
No we have an adapter that allows you to install a 6 bolt rotor on a center-lock hub (item 50-3214).
My mechanical disc brakes came with gear cable housing, what's up with that?
If you look closely you will notice that the housing has an extra layer of reinforcement over the linear steel cable strands. This is what sets compression-less brake cable housing apart from compression-less (SIS compatible) gear cable housing. It is not recommended to use gear cable housing for mechanical disc brakes due to the lack of this layer.
I don't know which disc mount my fork has…
See the mount descriptions above, or use these guidelines:
IS 51mm= Rock Shox, Marzocchi 2005 and older, Fox, Pace and other suspension forks.
74mm Post Mount=Manitou model year 2000 and newer. RST, Marzocchi 2006 and newer. Some models of SR Suntour.
