Hydraulic disc brakes are now coming as standard on most mountain bikes, but what should you be looking for when it comes to shopping for your ideal stoppers? This guide explains the basics, and the Jargon Buster below will help you get to grips with the terminology.
Disc/rotor
Power varies with each calliper and its pad surface/leverage, but the biggest difference is in disc (or rotor in the US) size. The bigger the disc, the more leverage your brake has on the wheel and the faster it can stop it. Each 20mm increase in size roughly equates to a 20 percent increase in power.
140mm (5.5in) discs are the smallest and a bare minimum for braking on the back wheel. 160mm (6.3in) discs will slide a rear wheel easily, but only the most powerful brakes will really stand a bike on its nose easily with a 160mm disc up front. That’s why many brakes now use a 180mm disc up front. For ultimate downhill stopping power you want 200mm (8in) discs, although some 225mm (9in) discs are available in the aftermarket.
Big discs heat up less and cool faster than smaller ones, again making them better suited to downhill/heavy duty usage. They do weigh more though, and not all forks and frames are warrantied – or have space – for the largest discs. Removing metal from the disc to save weight makes it more likely to warp when hot, which is why some brakes use a pimp-looking alloy spider in the centre of a steel disc. Vented discs cool better, and wavy or toothed ones clear muck faster than round ones. F
Control
The brakes should apply their power smoothly and progressively, or you’ll just launch yourself over the bar or slide your tyres rather than making the most of the available traction.
Life
The brakes need to be easy to live with, easy to set up in the first place and easy to adjust to the feel you want. They also need to be as maintenance-free as possible in the long term, whatever the weather.
Caliper
The business end of the brake houses the cylinders that push the pads onto the rotor. These can be machined or forged in one piece or made from two halves bolted together. Some brakes use two pistons either side rather than the standard single one, but you can’t really tell any performance difference on the trail or dyno. A slotted post mount attachment on the calliper is almost universal now, although you’ll need different spacer mounts for different frames and rotor sizes
Hose
The hose joins the lever to the calliper and needs to be free of air bubbles to keep the brakes working consistently. It needs to resist massive hydraulic pressures without popping, and needs to resist squashing or splitting too. Braided hoses – used on some complete brakes and available aftermarket – use a steel sheath for a tighter, more accurate brake feel.
Pads
The pads create the friction that provides the stopping power of the brake and are therefore crucial to the overall performance. The basic division is between resin and sintered pads. Resin pads stop well in the dry and generate less heat on long descents but they wear very quickly in wet and gritty conditions. They’re the default fitment for most brakes. Sintered pads create more heat and noise but they last longer and stop better in the wet.
Lever/master cylinder
The lever end of the brake is arguably the most important part. The master cylinder contains the reservoir that automatically adjusts for pad wear and heat expansion of braking fluid. It also has the brake lever at one end and the bar clamp at the other. While it’s not essential, having a hinged or separate clamp plate and a symmetrical either side ‘flip-flop’ design make the brake a lot more versatile.
Lever blade shape, sweep geometry and material govern the feel under your fingers. Levers vary in shape from curved to crooked, narrow to broad, smooth or drilled, one finger, two finger, two-and-a-half and so on. Some of them wobble and some are rock-solid. If you’ve got the money you can save a few grams by opting for a carbon fibre lever blade on most brakes too. Make sure the levers fit your hands and preferred bar positions relative to the shifters.
Depending on the design, there are various adjustments you can make to the way the brake feels. Most brakes let you adjust the reach to the levers, but being able to adjust the bite point at which the brakes contact the pads is an increasingly common feature. Some brakes also let you adjust leverage and power. Be aware that the more adjustable mechanisms there are, the more potential problems there are too.
Bleeding hell
Don’t fret about bleeding brakes. If you have to trim the hoses to fit them, you might need to re-bleed the brakes, but most shops will fit them for free. Otherwise, they should be fine if you leave them alone – we can’t remember the last leaky set we had, and we can count the number of unplugged pipes we’ve seen after crashes on one hand. Changing pads is the only regular maintenance needed.
Jargon buster
Angle adjust – The ability to change theangle a hose exits the calliper.
Arm pump – Forearm numbness caused by excessive brake pull.
Bedding in – The process of wearing off surface glaze and contamination on new brake pads and rotors through repeated stop/go cycles to get them up to full power. A few sprints and sharp stops should normally do the trick.
Bite adjust – Mechanism that allows the contact point of pad and rotor to be adjusted independentlyof lever reach. Also known as‘contact adjust’.
Bite point – The point at which the pads make contact with the disc and the brake begins to work.
Braided hose – Hydraulic line using a woven metal layer for added protection against crushing and bursting. Using this can improve the response of the brake too.
Center Lock – Shimano’s spline and lock ring disc fitting method.
Cup-and-cone – Convex and concave washers used to allow angular movement between brake calliper and mount.
Clip fit – Pads that snap fit to the head.
Disc or Rotor – The round braking surface of the brake that mounts on to the hub and sits between the brake pads.
Flip-flop – Ambidextrous lever design.
Floating rotor – Rotor with steel braking surface on alloy spider to reduce heat build-up and allow for expansion.
Hayes mount – Now almost universal brake mount on forks. Uses two threaded posts running parallel to the disc and a slotted calliper for easy adjustment.
Heat fade – Loss of power when a brake overheats on long descents.
Heat sink – Draws heat away from the pads.
IS mount – ‘International Standard’ brake mounting using two drilled tabs parallel to the disc. Light but needs faffing with washers to get a rub-free brake fit.
Lever blade – The bit you pull.
Lever body – The block the blade pivots in.
Lever reach – The distance between the lever blade and the handlebar.
Modulation – The relationship between how hard you pull the lever and how hard the brakes stop.
Monoblock – Calliper forged or milled from a single piece of material, rather than two halves bolted together.
Organic pads – Softer, quieter but shorter-lived brake pads.
Post mount – See Hayes mount.
Pots – The pistons or cylinders of the brake. Single-pot brakes have one piston on one side and one fixed pad; twin-pots have a piston on each side; four-pots have two pistons on each side; six-pots have three…
Progressive: Power that increases in direct relation to how hard you pull the brake lever.
Pump out: Over-extension and eventual lock-out of lever caused by fluid overheating and expansion on long descents.
Reach adjust: Mechanism that adjusts the distance between the lever blade and the handlebar to suit different-sized hands.
Resin pads – See organic pads.
Rose – Hose end that can rotate.
Servo-Wave – Cam system that increases leverage through the lever pull.
Sintered pads – Pads with metal content for longer life. They tend to run hotter and noisier than resin pads.
Six bolt – Standard rotor mounting using six bolts threading into the hub.
Split clamp – Lever with closed single bolt bar clamp.
Two-bolt clamp – A lever that has a separate barclamp.
source: http://www.bikeradar.com/gear/article/buyers-guide-to-disc-brakes-45
