Einstellen von Direktzugbremsen
Direct-pull brakes are a type of cantilever rim brake. In connection with this article, please read the lead article about rim brakes. It covers, among other things,
Why Rim Brakes? Rim Brake Types Centering Mechanical Advantage Noises/Squealing Shoe Types Shoe Adjustment
Most brake problems result from excessive friction or poor installation of the cables, not poor setup, or poor quality brakes. Also see the article on cables for information on cable selection and adjustment of brake cables and brake levers.
Direktzugbremsen im Vergleich zu klassischen Cantilever Bremsen
Traditional cantilever brakes used two cables, a main cable running down the centerline of the bike, and a second, "transverse" cable connecting the cantilever units on each side of the wheel. The main cable would pull upward on the middle of the transverse cable, causing the cantilever units to rotate inward.
The "direct pull" cantilever, also commonly known under its Shimano trademark name "V-Brake" is a simpler design, using only a single cable. The cable housing connects to one arm, and the inner cable runs across the top of the tire to the opposite arm. When the brake is applied, the housing pushes on one cantilever while the inner cable pulls the other.
Since the cable runs straight across the top of the tire, direct pull cantis need longer arms to get the cable high enough to clear the tire. This increases the mechanical advantage of the system, requiring the use of special matching brake levers. Quick Release DESCRIPTION Normal Position DESCRIPTION Boot Pulled Away DESCRIPTION Noodle Unhooked
For wheel removal, the noodle may be unhooked from the arm link. This will allow the brake shoes to open up wide enough to clear even a fat tire. First, pull the boot away from the end of the noodle. Then squeeze the brake arms together with one hand while unhooking the lower end of the noodle from the keyhole-shaped slot in the arm link.
Make sure to hook the brake back up immediately after you re-install the wheel!
Parallelogramm Koppelung
v-type brake Shimano's XTR and XT V-Brakes feature a special parallelogram linkage. This serves two purposes:
- It causes the brake shoes to remain at the same angle to the rim throughout the stroke, and throughout the service life of the pad.
- It causes the direction of motion of the brake shoes to be close to horizontal, rather than the usual slanted arc centered on the pivot boss. This is a major advantage for those who use very fat tires on narrow rims, because it prevents the shoe from rising up and damaging the sidewall of the tire on release, and also prevents having the brake shoes dive under the rim as they wear down.
Unfortunately, the extra pivots considerably complicate the mechanism, and this has caused maintenance problems and excessive squeal in practice.
Einstellen der Direktzugbremse
First, remove the arms from the studs, and make sure the studs are free of rust. Coat the studs liberally with grease (this is VERY important!)
Install the arms with them at their maximum spread and tighten the bolts that hold them to the frame. This is how you set the springs.
Shoe adjustment is covered in the article about rim brakes, but there are a few special things about direct-pull brakes:
- Most direct-pull brakes use smooth-stud type brake shoes, which allow adjustment of extension -- that is, how far the shoe is inboard of the brake arm.
- The brake shoes only need to be extended inward from the brake arms if the brake arms are too close together at the top, interfering with the tire or allowing too little cable travel.
- If your brakes use the Parallel-Push linkage, the pivots may need servicing. There are special kits for this.
Mittig ausrichten
When the brake is released, the brake shoes retract away from the rim. Ideally, the shoes on both sides should back off by the same amount. If they don't, the brake is not properly centered. In extreme cases, one of the shoes may not retract, and may rub on the rim even when the brake is not being applied.
If a brake appears off-center, check first that the wheel is installed straight in the frame/fork. If the wheel is crooked, and you maladjust the brake to compensate, you are creating two problems where there was only one before.
Einstellen der Vorspannung
If your wheels are centered, and your brakes are not, and, if the pivots are properly lubricated and free-moving, the brake shoes should be centered. If they are not, you probably need to adjust the spring tension on one or both of the cantilevers.
Most direct-pull cantilevers have adjustable spring tension. The adjustment will be a small screw with the head facing outward to the side of the bike. The screw is generally located near the bottom of the cantilever, below the pivot point. Sometimes it works with a Phillips screwdriver; other times, a small Allen wrench may be needed.
DESCRIPTION Tightening this screw tightens the spring, so you want to tighten the spring of whichever arm is too close to the rim. This will make it spring back farther.
Bremshebel für Direktzugbremsen
Direct-pull cantilevers have double the mechanical advantage compared with traditional brakes, so they require special brake levers. Direct-pull brake levers pull the cable twice as far, half as hard. The lower mechanical advantage of the lever compensates for the higher mechanical advantage. of the cantilever. It is not generally safe to mix and match levers/cables between direct-pull and other types for this reason.
- Conventional brake levers used with direct-pull cantilevers will usually not pull enough cable to stop in wet conditions without bottoming out against the handlebars. In dry conditions, they either won't work, or will grab too suddenly.
- Direct-pull brake levers used with any other type of brakes will feel nice and solid when you squeeze them, but due to their lower mechanical advantage you'll need to squeeze twice as hard to stop as you should, so unless you are a lightweight rider with gorilla-like paws, this combination isn't safe either.
Hebel für Dropbar Montaghe
To make a brake lever with low enough mechanical advantage for direct-pull cantilevers, the cable must run twice as far from the lever's pivot point. This is easy enough to do with levers for straight/upright handlebars. Levers for drop handlebars are harder to configure for direct-pull brakes, and currently, there are only a couple of drop-bar levers made that are compatible, the Dia Compe 287V and the Cane Creek SCR-5V "Æro" levers.
Most newer drop bar bikes are have Ergo or STI brake/shifter units as original equipment. Direct-pull cantis are not compatible with these. Other types of shifters may of course be used, along with the special brake levers. In any case many cyclists prefer bar-end shifters. These are simpler and less expensive than brake-lever shifters, and give an indication of what sprocket is in use: brake-lever shifters return to the same position after every shift.
Hilfskonstruktionen für Dropbars
If you want to use Ergo or STI brifters with direct-pull cantis, there are two workarounds available: QBP Travel Agent
- You can use a pulley-device, such as the QBP Travel Agent (Q.B.P. Photo). These are a bit of a hassle to set up, but do work. Any pulley of the small diameter commonly used on bicycles eventually leads to fatigue failure of the cable, so check the cable regularly for fraying.
- If you have a bike with skinny tires and no fenders, there are "shorty" direct-pull cantis that are more-or-less compatible with drop bar levers.
Neither of these systems is ideal, and neither is really any better than traditional center-pull cantilevers.
Kompatibilität
Note the differing levers and pivot stud positions. Cantilever Brake Compatibility/Interchangeability Cantilever Type Frame Pivot Studs Levers Cable Routing Direct Pull V-Brake ® Below the Rim Long Pull Low Tension Cable comes in from the side.
Lower housing stop is part of the cantilever Traditional Center Pull Standard Short Pull High Tension Cable runs down the bicycle's center line.
Lower rear housing stop on frame, either special braze-on , or mounted to the seatpost bolt.
Front housing stop on headset , fork or handlebar stem . U-Brake Cantilever Above the Rim Roller-Cam Cantilever
Quelle
Dieser Artikel basiert auf dem Artikel Adjusting Direct-pull Cantilever Bicycle Brakes ("V-Brakes ®") von der Website Sheldon Browns. Originalautor des Artikels ist Sheldon Brown.
