Recently, bike manufacturers have soared on the bicycle disc brakes trend, providing them not only with mountain bikes (as they have in the past), but also with many of their road and other offerings. Some purists remain staunchly anti-disc brakes, but the choice of brakes will undoubtedly be a major consideration when purchasing your next bike.
So, what's the distinction between rim and bicycle disc brakes?
All bicycle brakes work by pressing a brake pad against a wheel's surface. The location of the braking force is what distinguishes rim brakes from disc brakes. Rim brakes apply force to the outer edge or rim of the wheel, whereas disc brakes apply force to a rotor in the middle of the wheel.
Pistons in both types of disc brakes push brake pads onto a rotor. The distinction is in the manner in which the force applied to the brake levers is transferred to the brake rotor. Mechanical disc brakes work similarly to rim brakes in that the pistons are moved by a steel cable. Hydraulic disc brakes, on the other hand, use a fluid-filled system in which a plunger pushes fluid into the calliper and the brake pads are pushed onto the rotor to slow or stop the bike.
Better braking power - Disc brakes produce significantly more braking power than standard rim brakes. For the rider, this means using far less force on the brake levers, which reduces muscle fatigue, especially on long descents where you are constantly checking your speed.
Consistent braking - While depressing the lever of a rim brake does not always result in the same level of braking equal to the force applied, braking force is much more consistent with disc brakes. As a result, as a rider, you'll be able to judge how much force to apply to the brake lever to achieve the desired amount of braking.
Wet weather reliability - A disc brake performs much better in wet weather than a rim brake. A disc brake does not have the slight braking delay that a rim brake does because it must first displace water from the rim before biting on the surface. Disc brakes are generally protected from wet conditions due to their position on the bike and the fact that they are self-contained, so wet weather performance is largely unaffected.
A faster ride - It is thought that disc brake bikes can provide a faster ride. Because disc brakes provide more confidence and braking power, riders can brake fractionally later than rim brakes. As a result of spending less time on the brakes, they will spend more time travelling at higher speeds.
Reduced wheel wear - Rim brakes inevitably cause rim surface wear, especially when combined with grit and dirt in winter, meaning that wheels on a disc brake equipped bike will wear out much faster. Moving the braking point of contact away from the rim prevents heat buildup on the rim, reducing the risk of damage to the inner tube or tyre.
Even when the wheel is not true, the brakes remain unaffected - As anyone who has damaged a wheel's rim or whose wheel is out of true will know, this can be a problem because rim brakes must be slackened off (or the wheel straightened) to prevent them from catching on the rim. Disc brakes do not have this issue and continue to function effectively even if the wheel is slightly out of alignment.
Better clearance - By eliminating the need for a brake calliper on either side of the wheel rim, road bikes can now use much wider tyres. A wider tyre improves grip and comfort on the road. More clearance also allows for easier installation of winter mudguards, which would have been a much more difficult task with certain types of calliper brakes.
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Disc brakes add weight - When road cyclists are trying to keep their bike as light as possible for speed reasons, the addition of disc brakes will add weight to a bike. Disc brakes are generally heavier than rim brakes, especially hydraulic disc brake systems.
Costs more - Disc brakes are a more expensive option in terms of both initial purchase and maintenance. Although disc brake pads last longer than rim pads, they are more expensive to replace and can cost up to three times as much as equivalent rim pads.
Difficulty of maintenance - Because hydraulic disc brakes are sealed, they shouldn't require much in the way of day-to-day maintenance, but when something goes wrong, it can be more difficult to fix. If there is an air bubble trapped in the system, disc brakes may require 'bleeding,' which is not as simple as adjusting a calliper rim brake.
Not transferable - You couldn't simply swap disc brakes for rim brakes on your road bike that was previously set up for rim brakes, and vice versa. Both types would be incompatible with the frame and wheels. To use disc brakes, you'd need a different wheel hub than on standard rim brake wheels, as well as brake mount tabs on the fork.
Disc brakes can be dangerous - Issues can arise when riders in a group use a combination of disc and rim braking systems, allowing some to brake more effectively than others. Following race incidents, some riders have claimed to have been injured by disc brakes.
If you've checked your calliper and rotor but still hear a horrible squealing when braking, it could be because the braking surface (including pads and/or rotors) is contaminated and needs to be cleaned.
Remove the wheel and clean the rotor with isopropyl alcohol on a clean rag, then remove and clean the pads. Wear disposable latex gloves as well; you want to remove dirt and oil from the braking surface, not add more. Allow the pads to completely dry before .
When the total thickness of the backing plate and pad material is less than 3mm, replace the pads. Shimano recommends replacing the pad material when it is less than half a millimetre thick. If you change pad material (say, from resin to metallic), you should also change your rotors; pads and rotors pair by "bedding" small amounts of the pad material on the rotor, which means an old rotor won't perform optimally with a new pad material because it's already bedded to the other kind of pad material.
After putting in some serious time on your disc brake-equipped bike, you'll begin to gradually wear down the pads and rotors. If your brakes begin to feel less effective, but you're confident that the pads are still in good shape and that everything is in proper alignment, look to see if the rotors are glossy. If this is the case, grab some sandpaper and get to work.
Select at least 200-grit sandpaper. Remove your wheels, then sand each rotor gently until the glossy haze fades and you see a dull colour with a slightly textured finish. When you pull the brake levers, your bike's rotors will provide more friction against the brake pads, restoring the performance you expect from discs.
Personal preference ultimately governs brake selection. Some riders are content with rim brakes and are unlikely to switch. A rim brake is usually more than capable of providing the braking you require, and its lightweight nature means that the rider looking for those marginal gains is unlikely to want the extra weight disadvantage that disc brakes provide.
Bicycle Disc brakes, with their improved consistent braking, appeal to the average rider who rides for fun rather than racing. Disc brakes can provide extra security when riding in the rain for commuters or winter riders.
As bicycle disc brakes become more common on road bikes and among pros, improvements will inevitably follow with trickle-down technology from the sport's upper echelons, and costs will likely fall. The average rider will probably never go back to drum brakes. It is probably fair to say that the improved braking performance, especially in wet conditions, outweighs the risk of disc injury.