When I have a rest, I want to travel by bike. What kind of bike should I buy? What are the advantages and disadvantages of disc brakes and disc brakes?

Bicycle brakes have changed from drum type and rim type to disc type, keeping pace with the times. The use of disc brakes is becoming more and more popular, and the brake, a safe and lifesaving device, has no right to fail. Our intention is to understand disc brakes, not to judge and distinguish between good and bad. Which system to choose and use should be measured according to the user's riding mode, demand, budget and other aspects.

Mechanical or hydraulic (drive)

According to the braking mode, or the medium for actuating the brake disc, one is the brake line and the other is the oil.

Mechanical disc brake, relying on the tension of the brake line, makes the brake pad in the caliper rub and clamp the disc, resulting in braking effect. Oil pressure type, with oil as the medium, pulls the brake handle to actuate the piston inside to compress the oil, and the pressure is transmitted to the caliper through the oil pipe, thus pushing the piston inside the caliper and driving the brake pad to clamp the brake disc. Mechanical disc brake has its compatible convenience, and the general V-shaped brake handle can be used universally. The brake handle, oil pipe and caliper of hydraulic disc brake are a complete set of products, and naturally there is no intersection with traditional cable braking. Some disc brake manufacturers have designed semi-mechanical and semi-hydraulic disc brakes. This intermediate product keeps the hydraulic caliper, still relying on cable tension, but it is the compression piston on the caliper. The advantage of hydraulic disc brake is that the braking output force is several times that of a human hand, which belongs to the acute braking force of four or two kilograms. Good sealing performance and low maintenance cost. The application of hydraulic disc brakes on motorcycles has been very mature. When it comes to finding fault with bicycles, it involves many technical problems. A seemingly simple bicycle is actually difficult to operate. However, there is a reservoir on the brake handle of the hydraulic disc brake, so the volume is relatively large. Therefore, it is necessary to pay attention to the compatibility with STI-type shift handle and grip-shifting rotary shift handle in order to compete for land among many handlebars. Traditional rim brakes are also hydraulic. Guess what? Magula HS 1 1, 33, 66 is. However, the disc brake based on the mechanical principle of cable pulling may lose its braking force due to many factors such as the length and elasticity of the steel wire itself and the friction with the shell. On wet and rainy days, the brakes are often accompanied by screams, and some riders' backs will itch.

Tip: Some people are used to putting their cars upside down. If they are equipped with hydraulic disc brakes, the air and gas in the oil pot may run into the oil pipe, thus affecting the braking force. Get into the habit of trying to brake before riding a bike.

Open or closed system

There are many arguments about the internal structure of hydraulic disc brake. Starting from the open system and closed system, the biggest point is that oil will expand with heat and contract with cold!

The disc brake of hydraulic system will expand too much if the oil is heated, especially in hot weather. As long as the brake pad is pushed outward, it may grind, drag or even bite! In the so-called open (oil pressure) system, an oil valve is specially designed at the brake handle according to the characteristics that oil expands when heated and contracts when cooled, and a space is reserved in the oil pot to accommodate the increased volume when oil expands when heated, and at the same time, brake oil is allowed to flow back to compensate for the wear of brake shoes. In a closed system, the brake pad is isolated from the piston and the oil, so that the thermal expansion of the oil will not exaggerate the brake pad, so be especially careful with the oil (usually mineral oil). Brake pads will wear, so the disc brakes and oil pots in the closed system will have sensitive adjustment devices.

Tips: The oil expansion causes the caliper to bite the disc, which is definitely not the solution of "just put some oil". On the contrary, it may require the danger of brake failure! It is safer to send it back to the garage for a physical examination.

Mineral oil or DOT standard oil

Mineral oil or brake fluid approved by the Ministry of Communications

DOT-3 or DOT-4 is a special brake oil for automobile and locomotive disc brakes, and its composition is to overcome the characteristics of thermal expansion and cold contraction. When braking, the temperature generated by friction is very high, which can make the oil boil and evaporate, and then produce gas. The problem is that natural gas is easier to compress than oil. At this time, pulling the brake handle only compresses the air bubble, and the piston/brake pad can't be pushed at all, resulting in brake failure or great power loss! DOT-3 and DOT-4 have lower boiling points than mineral oil, are toxic and corrosive, and can dissolve paint. The advantage is that they can absorb water, even the steam in the oil pan can be eaten. If there is water in the oil, the boiling point will decrease. There is also Cannondale's Coda disc brake oil, which is said to be a special formula oil developed by NASA. It is not afraid of the high temperature when the rocket is launched, not to mention bicycle brakes! Magura first started using mineral oil. Its ingredients are very pure, although it is non-toxic, but it will not absorb water to wash off the car paint. The advantage is that the braking force will not be damaged when it is used for a long time or meets high temperature. Brake oil should be changed regularly. It is generally recommended to change the oil once a year because it absorbs moisture. Mineral oil doesn't need to be changed so soon, but once you feel a little weak, check it first. Maybe it's time to change the oil.

Tip: The oil used will generally be indicated in the manual or on the oil pot. When changing oil in DIY home, please don't mix these two oils. Igniting oil will corrode the oil seal of mineral oil system, and vice versa. DOT oil for Hayes hydraulic disc brake. Magura system uses mineral oil.

Floating calipers or fixed calipers Floating mounting or fixed calipers

There are two kinds of disc brake calipers, one is fixed tightly, and the other is movable after being mounted on the frame or front fork, generally sliding laterally. During the development of bicycle disc brakes, two theoretical schools have been formed. The design that the floating calipers can slide laterally is closely related to the negative influence brought by the unilateral action of the disc brake. But it has its advantages, such as the tolerance between the frame and the front fork, and even the coating thickness on the assembly seat, which will affect the accuracy between the disc and the brake pad, so it is often necessary to rely on washers/gaskets to remedy and correct it. Some floating calipers especially emphasize the effect of correcting the parallelism and adhesion of the disc and brake pad, without worrying about tolerance.

Tips: Generally, the paint of the disc brake seat of the frame and front fork will be scraped off when assembling the disc brake, because its thickness will also form the assembly tolerance. However, please handle it carefully, so as not to hurt the flatness of the metal, but make it self-defeating.

Unilateral drive or bilateral drive Unilateral or bilateral drive

Unidirectional actuation and bidirectional actuation and braking are not only found in hydraulic or mechanical disc brakes, but most mechanical disc brakes are unidirectionally actuated. Unilateral actuation, that is, only one piston pushes out the brake pad to press the friction disk, while the brake pad on the other side waits for the opposite brother to push the disk, so that both sides can have the running-in effect. As a result, there are two theoretical schools: floating calipers and fixed calipers. Bilateral drive naturally means that there are two opposite pistons in the caliper, so that the brake pads can clamp the brake discs together from both sides. At present, the number of pistons in disc brake calipers is 1 (single-side actuation), 2 (one pair) and 4 (two pairs), and the maximum number can be 6 (three pairs). The number of pistons is not necessarily equal to the number of brake pads, because some sides have two pistons, but they are all long brake pads; The two brake pads are not necessarily clamped on both sides, because they may be driven on one side. For example, Magura Gustav M is driven unilaterally. How many pistons and brake pads do you think it has? Most mechanical disc brakes with bidirectional braking are masterpieces of manufacturers in Taiwan Province, such as RST, Shenyi and Zhongying. The Winzip M 1 made by New Face Ying Ji Company is a "single drive bilateral clamp".

Tips: Simple test: assemble a disc brake or buy a car with a disc brake, stop, try to pull the brake handle by hand, keep your eyes on the brake disc and brake pad, and observe the clamping situation, especially the unilateral actuating system. Is the distance that the disc is pushed away too obvious and eccentric?

Metal or organic brake pads

The friction coefficient of brake pads with different compositions and materials is different.

Organic brake pads are made of Kevlar mixed glass fiber. There is no metal powder. It can have a strong braking force, yes, but under the condition of high temperature and high humidity, the strength loss is more severe than that of brake pads containing metal formula. The advantage is that this kind of brake pad does not conduct heat, and there is no need to worry about the thermal expansion effect of oil, so it is very ideal to be used in a closed oil pressure system. Metal brake pads, mostly containing copper or iron powder, are more wear-resistant and are not afraid of dust and soil affecting braking force. But the heat transfer effect is so good that it is naturally suitable for open oil pressure system. Between the disc and the inner and outer brake pads, it is generally emphasized that the distance should be parallel and equal, and it will be carefully calibrated during assembly, so as to achieve the ideal actuation mode of synchronous and equal force clamping on both sides. In this regard, emphasis is placed on the principle of bilateral drive calipers. However, in some floating caliper unilaterally actuated disc brakes, the disc is relatively close to the fixed and non-movable brake pads, or even slightly contacts, and the wheels do not rotate in still air, and there is a grinding sound, which is also accused of invisibly reducing the speed.

Tips: There must be a running-in period between the brake disc and the brake pad of the new disc brake. It's a bit dangerous to try the sharpness of the new disc brake.

Rotor disk

The main task of the disc is to accept torture, a full-fledged bitter character. Most of the materials are stainless steel. Some manufacturers make aluminum alloy dishes, saying that the heat dissipation effect is better, and some make titanium carbon. The disc brake must match the "disc brake hub". Generally, the traditional rim brake should be replaced by a disc brake, which can only be replaced in a "complete set" or a special adapter can be used. Perhaps we should also pay attention to the compatibility of flower drums and vegetables. The standard disk has six screw fixing holes, and there were four holes in the early days. Cannondale Coda is still in use now, and the Italian formula also exists. Britain's hope is 5-pass, but there are also general 6-pass. German bees are not fixed with screws, which are a bit like "plum blossom teeth" or ratchet wheels, so they can only use their own flower drums. The diameter of optical disc is not standardized, 160mm is the most common, and it is hoped to be as small as 130 and 145 mm ... In principle, the more intense the riding, the bigger the disc, because the greater the braking force, and it is usually large at the front and small at the back, or the same at the front.

Tip: just wipe off the dust on the dish at ordinary times, and pay attention to rubbing it with alcohol; During lubrication and maintenance, the disc should avoid becoming greasy and slippery, which is not what the brake wants.