Cantilever Geometrie

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Dieser Artikel ist einer von mehreren, die sich mit der Cantileverbremse beschäftigen. Wenn Du nur auf der Suche nach einer Anleitung bist, wie Du Deine Cantilever Bremse oder V-Brake vernünftig eingestellt bekommst, lies die Artikel, die bei Die richtige Bremse für mein Fahrrad verlinkt sind oder lies den Artikel Das 1x1 der Felgenbremsen. Dieser Artikel hier beschäftigt sich eher mit theoretischen Aspekten der Cantilever Geometrie.

Aufbau der Cantilever Bremse

Vor den 1980er Jahren waren Cantileverbremsen unheimliche und exotische Ausrüstungen. Man fand sie zumeist nur an teuren Tandems oder sehr hochwertigen |Tourenfahrrädern Sie waren sogar noch seltener als Dreifachkurbeln. Die Zeiten haben sich geändert und, was früher nur finanzstarken wissenden Fanatikern vorbehalten war, wird heute sogar bei Baumarktfakrrädern eingesetzt.

Nichtsdestotrotz scheint ein wenig Mysterium bei Cantileverbremsen geblieben zu sein. Dieser Artikel will ein wenig das Mysterium lüften und helfen, dass das Einstellen der Cantileverbremsen leichter von der Hand geht, weil Du die wesentlichen Details der Geometrie verstehst, die der Funktion von Cantileverbermsen zugrunde liegen.

Im Speziellen wird die Frage angesprochen, wie lang der Querzug sein muss, bzw. anders ausgedrückt, wie niedrig der Querzugträger montiert werden kann. Bei den meisten Cantileverbremsen hat der Mechaniker relativ freie Wahl, wie das Zugsystem montiert wird. Das hat bedeutenden Einfluss darauf, wie gut die Bremse funktionieren wird. Das trifft insbesondere auf die Low Profile Bremse zu.

Hebelübersetzung

Man kann Fahrradbremsen nicht verstehen, ohne Hebelgesetzte zu verstehen.

"Hebelübersetzung" bzw. "Hebel" bezeichnet das Verhältnis zwischen der Kraft, die man aus dem einen Ende eines Angelpunkts erhält, wenn man eine bestimmte Kraft auf der anderen Seite ausübt. Man könnte vereinfacht sagen, dass die Hebelüberseztung das Kräfte- und Entfernungsverhältnis ist.

Stelle Dir einnen einfache Hebel mit einem Ankerpunkt (Gelenk) ungefähr bei 1/3 der Gesamtstrecke vor (s. Bild).

Seite B ist zweimal so lang wie A. Die Hebelübersetzung beträgt 2:1 (oder 1:2, je nach Sichtweise).

Seite B wird sich zweimal so weit bewegen wie Seite A, aber man benötigt auch auf Seite A zweimal so viel Kraft, um das Gewicht von Seite B anzuheben.

Des Pudels Kern erreicht man durch Ändern des Hebels (z.B. durch das Bewegen des Ankerpunkts). Dabei werden sowohl die aufzuwendende Kraft als auch Bewegungsspielraum gleichzeitig verändert. Sie sind sozusagen beide auf der gleichen Seite der Medaille. Man kann das Kräfteverhältnis nicht ohne das Entfernungsverhältnis ändern.

Bei Fahrradbremsen ist die Hebelübersetzung des Systems das Verhältnis der Kraft, die auf die Felge aufgebracht wird, und der aufzuwendenen Kraft, die die Finger dafür am Bremshebel ausüben müssen. Wenn also ein bestimmtes Bremssystem eine Hebelübersetzng von acht hätte, würde das drücken des Bremshebels mit fünf Kilogramm, eine Gewicht von rund 40 Kilogramm auf die Felge drücken. (Tatsächlich etwas weniger als 40kg, weil man noch Reibeungsverluste einrechnen müsste. Diese werden aber zur Vereinfachung in diesem Artikel ignoriert, weil sie für das Gesamtverständnis nicht wichtig sind.)

Die Hebelübersetzung kann auch aus der Perspektive von Wegeverhältnissen betrachtet werden. Eine Bremse mit hoher Hebelübersetzung bringt mit sehr wenig Handkraft am Hebel sehr viel Kraft über den Bremsschuh auf die Felge auf. Auf der anderen Seite muss man den Bremshebel sehr weit bewegen, um die Bremsschuhe das kurze Stück bis zur Felge zu bewegen. Bei zu hoher Hebelübersetzung trifft der Bremshebel schon den Lenker, bevor die Brenmsschuhe auf die Felge treffen. Wenn man die Bremsschuhe so eng einstellt, dass der Bremshebel nicht den Lenker berührt, kann es passieren, dass beim Loslassen des Bremshebels, die Bremsschuhe nicht mehr weit genug auseinanderbewegt werden, um sich von der Felge zu lösen.

Bei Zangenbremsen ist die Hebelübersertzung durch den Hersteller fest vorgegeben. Man kann sie nicht ändern, außer man tauscht Zangen, Hebel oder die Reifengröße aus. Wenn man kleinere Reifen montiert, muss man die Bremsschuhe weiter nach unten installieren, was die Entfernung der Bremsschuhe zum Ankerpunkt erhöht und die Hebelübersetzung verringert. Wenn man zum Beispiel 622mm (700C) Felgen auf einem Fahhrad benutzt, was für 630mm (27 Zoll) Reifen gebaut wurde, wird die Bremswirkung abnehmen.

Bei Direktzugbremsen wie der V-Brake von Shimano ist die Hebelübersetzung ebenfalls fest vorgegeben. Da sich die Ankerpunkte jedoch unterhalb der Felge befinden, erzeugen kleinere Reifen eine höhere Hebelübersezung - ganz im Gegensatz zu Zangenbremsen. Konventionelle Cantilever Bremsen erlauben es dem Fahrradnmechaniker innerhalb eines gewissen Rahmens, die Hebelübersetzung zu verändern, in dem die Länge des Querzugs und die Höhe des Querzugträgers verändert wird.

Gefühl und Funktion

With automobile brakes, a nice "hard" pedal feel is a sign that the brakes are in good condition. A soft, "spongy" feel at the brake pedal is a sign of trouble, perhaps air in the hydraulic lines. This is not the case with bicycle brakes. A hard, crisp feel to the brakes on a bicycle may be a sign that the brakes don't have much mechanical advantage. You squeeze them until the brake shoes hit the rim, then they stop. Brakes with a high mechanical advantage will feel "spongy" by comparison, because the large amount of force they deliver to the brake shoes will squash the shoes against the rim, deforming them temporarily under pressure. You can feel this deformation in your fingers. The brakes with the rock-hard feel may seem nice on the work stand or the showroom floor, but when it comes to making the bike actually stop, the spongy set-up will do the job better, with less finger pressure and greater margin for safety in wet conditions.

Definitionen

For purposes of this article, I have defined 1 distance, 2 arms, and 3 angles as shown in the illustration.

Pivot-Cable distance (PC)

The shortest distance from the center of the pivot to the line of the transverse cable. In the case of low-profile brakes, this is the shortest distance from the pivot to the imaginary line extending from the transverse cable.

Shoe arm (PS)

Runs from the center of the pivot to the part of the brake shoe that contacts the rim.

Anchor arm (PA)

Runs from the center of the pivot to the attachment point for the end of the transverse cable.

Yoke angle

The angle of the transverse cable from the horizontal.

Anchor angle

The angle between the end of the transverse cable and the anchor arm.

Cantilever angle

The angle between the shoe arm and the anchor arm.

Datei:Canti.gif

Cantilevertypisierung

Conventional cantilevers fall into three types, defined by their cantilever angle:

• Wide-profile cantilevers have a cantilever angle much greater than 90 degrees. The best example of this type is the old Mafac cantilevers, in which the anchor arm actually sloped downward from the boss in some installations. This design is now pretty much obsolete. Wide-profile cantilevers have rather low mechanical advantage, and work well only with levers with a high mechanical advantage.
• Medium-profile cantilevers have a cantilever angle of around 90 degrees. Most late-1980's cantilevers belong to this family. Medium-profile cantilevers are very forgiving and give excellent all-around performance with a wide range of set-ups.
• Low-profile cantilevers have a cantilever angle of less than 90 degrees. The principal advantage of narrow profile cantilevers is that they don't stick out so far from the frame or fork, very desirable, because protruding cantilever arms can cause a multitude of problems, particularly in the rear, where a rider's feet may hit them. Narrow-profile cantilevers are also capable of excellent performance, but only if properly set up. A carelessly set-up low-profile cantilever may have very low braking power, even though it feels great on the workstand.

Variant cantilevers have their own categories:

• Direct-pull cantilevers, such as Shimano's "V Brake" ®, which have not separate transverse cable.
• Roller-cam brakes, which mount on different, incompatible bosses.
• U-brakes, which use the same mounting bosses as roller cams.

Hebelübersetzungen von Cantilevern

Three separate factors determine the mechanical advantage of any particular cantilever braking system. The total mechanical advantage of the system is the product of all three multiplied together:

1. The first factor is the brake lever itself. The lever's mechanical advantage is determined by the distance from the lever's pivot to the cable end, and by the effective length of the brake lever from its pivot to where the rider's fingers grip it. Typical mountain-bike type brake levers give a mechanical advantage of around 3 1/2, old-style drop-bar levers around 4, and "æro" drop-bar levers around 4 1/2. Levers for direct-pull ("V-type") brakes are around 2.
   
   Shimano ("Servo-Wave" ®) and Odyssey both make mountain-bike type levers with a variable mechanical advantage that increases as the lever is pulled.

Two distinct aspects of the cantilever system determine its mechanical advantage

1. The individual cantilever's mechanical advantage is the ratio between the pivot-cable distance (PC) and the pivot-shoe distance (PS) . The pivot-cable distance (PC) is at its greatest when the anchor angle is 90 degrees, so that PC and PA are the same. Some authorities recommend adjusting the length of the transverse cable accordingly, but I believe that this is an over-simplification. With wide- and medium-profile cantilevers, the mechanical advantage of the cantilever unit increases as it travels inward, increasing as the brake shoes wear down. With narrow-profile cantilevers, the mechanical advantage tends to decrease as the cantilever travels inward. The mechanical advantage of a typical cantilever is generally between 1 and 2. Medium-profile cantis tend to have more of this type of mechanical advantage.
2. A larger contribution to the mechanical advantage of a well-adjusted cantilever brake, especially a low-profile one, comes from the transverse cable. The mechanical advantage is strictly determined by the "yoke angle ". The formula is:

   Mechanical Advantage = 1/sin yoke angle
   For readers without slide rules I have calculated a few examples: [How quaint :-) John Allen]

1. Yoke Angle

   (Degrees) 	Mechanical
   Advantage
   90° 	1
   80° 	1.015
   70° 	1.063
   60° 	1.15
   50° 	1.31
   40° 	1.55
   30° 	2
   20° 	2.92
   10° 	5.76
   5° 	11.47
   0° 	Infinity!

A 90 degree yoke angle would result from an infinitely long transverse cable, such that each side of the cable was running vertically down from the cable yoke.

A 0 degree yoke angle would represent the shortest possible transverse cable, running in a perfect straight line along the top of the cable yoke.

As you can see from the table, the shorter and straighter the transverse cable, the more difference it makes. This effect is what makes it possible to make a low-profile brake with good stopping power.

Spezielle Überlegungen zu Low-Profile Cantilevern

The small cantilever angle of narrow-profile brakes causes the anchor arm (PA) to be nearly vertical, especially on mountain bikes that have wide-set pivot bosses and narrow rims. Traditional good practice had been to slide the brake shoe holders all the way into the eyebolts, so that the back of the shoe butts up against the cantilever arm. This is not the case with the newer low-profile models.

With low-profile cantilevers,, the shoe needs to be extended inward from the arm, increasing the effective cantilever angle . The unsupported length of shaft connecting the brake shoe to the arm may cause an increased tendency to squeal, but that is one of the inherent trade-offs of low-profile brakes.

Many newer cantilevers replace the separate transverse cable and yoke with "link wire." This is a cable carrier that has a length of narrow housing running from the yoke to the anchor arm. The primary cable runs through this housing, and forms half of the transverse cable. There is a guide line printed on the round yoke, which is intended to be lined up with the exposed side of the transverse cable.

Link wires are commonly available in five lengths:

Kodierung	Länge	Verbindungszüge unterschiedlicher Längen
S	63 mm
A	73 mm
B	82 mm
C	106 mm
D	93 mm

If you substitute a conventional yoke and separate transverse cable, you may be able to increase the mechanical advantage slightly on a particular bicycle. In general, the stock set up works about as well as possible, but only if you use the Shimano guide.

Since the yoke angle is so critical to the mechanical advantage, the mechanical advantage gets less and less as the brake is engaged, and as the brake shoes wear down. The short transverse cables necessary to get high mechanical advantage from low-profile cantilevers exaggerate this effect, because the yoke angle gets larger for a given amount of upward travel of the yoke. Thus, low-profile cantilevers should be set up with minimum pad clearance if you want to get high mechanical advantage when the brake is actually engaged.

Direktzugbremsen (V-Brake)

The latest thing in cantilevers is the direct-pull cantilever, popularly known by Shimano's trademark "V-Brake". These resemble very tall, low-profile cantilevers, but they do not have a separate transverse cable. They are a side-pull, rather than center-pull design. One arm has the housing stop, and the inner cable runs from the top of that arm to an anchor bolt on the top of the opposite arm. Direct-pull cantilevers have a very high mechanical advantage, which makes them unsuitable for use with conventional levers. If you do use conventional levers with direct-pull cantilevers, braking may be too abrupt. The excessive mechanical advantage of this combination will either cause the brake shoes to rub on the rim when they are at rest, or the brake lever will bottom out against the handlebar, depending on the cable adjustment.

Also see my article about direct-pull cantilevers

There are a few new aftermarket gadgets that permit you to use conventional brake levers with direct-pull brakes. These generally use eccentric or doubled pulleys to cause them to pull farther (but less hard) than the incoming cable pulls. V-Daptor The unit illustrated above is a World Class "V-Daptor." In this case, it has been installed on a conventional Shimano LX cantilever, thus converting a traditional center-pull cantilever into a direct-pull unit. This is a handy way to improve braking on many touring bikes and tandems, but there is not usually enough clearance to let this modification work with fat mountain-bike tires--for them, you need a purpose-built direct-pull brake.

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.

Hebelübersetzung - Wieviel ist genug?

Generally, more mechanical advantage is better than less, but it is possible to overdo it.

There is a direct trade-off between how much force you get and how far the parts travel. Given a mechanical advantage of 8, pulling the brake lever in by 16 millimeters will only move the brake shoes 2 millimeters closer to the rim. The more mechanical advantage you have, the closer the brake shoes will be to the rim at their rest position. This is not a problem with a perfectly true wheel, but can cause the brake shoes to rub too easily on rims that have seen better days.

There is a case to be made for less than maximum mechanical advantage on the front brakes of bikes that are aimed at less experienced riders, lest they lock up the front wheel and hurt themselves. With a brake set up for maximum mechanical advantage, the shorter transverse cable has a shallower yoke angle. This may make it difficult or impossible to unhook the transverse cable for wheel removal. For some riders, it may be a worthwhile trade-off to give up some braking power for the sake of easier wheel removal.

On touring bikes with high-mechanical-advantage "æro" brake levers, excessive mechanical advantage may cause the brake to run out of lever travel, so that the lever hits against the handlebar. Shimano makes an extra-wide cable yoke for such applications, but you can achieve the same effect by lengthening the transverse cable, unless the bike has such a small frame that you run out of room.

Wide yoke

Das Potenzial der Cantilever Bremsen voll ausschöpfen

Aside from the issue of mechanical advantage, there are other ways to improve your cantilever braking.

If you reduce flex in the system, you can set the brake for more mechanical advantage without running out of lever travel. I would suggest the following:

• Cables should be as free of friction as possible, and the ends of the housing should be properly prepared so that they make a firm, solid connection with their stops. See my article on Cable Installation.
• Brake shoes for brakes with high mechanical advantage should be hard and rigid. I particularly recommend Kool-Stop Salmon brake shoes, which are among the hardest, and have a very high coefficient of friction.
  
  The shoes should be set up so that they make good firm contact with the rim. They should be slightly toed in, but not excessively. The vertical angle of the brake shoe should also be adjusted correctly so that it gets the largest possible contact area with the rim.
• A brake booster can also help considerably, particularly on the rear, where the bosses are mounted on relatively narrow seatstays, and can flex outward under hard braking.

Quelle

Dieser Artikel basiert auf dem Artikel The Geometry of Cantilever Brakes von der Website Sheldon Browns. Originalautor des Artikels ist Sheldon Brown.