Alles über Umwerfer

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Die meisten modernen Fahrräder haben eine Kettenschaltung. Gangwechsel werden durch das Führen der kette von einem Zahnrad zum nächsten durchgeführt.

Schaltwerke und Umwerfer sehen kompliziert aus, sind tatsächlich aber sehr einfache Mechaniken, die nach der Brecheisenmethode funktionieren. Das trifft insbesondere auf Umwerfer zu.

Sie bestehen einfach aus einem Käfig, der aus einem Stück Metall gebogen ist, die man hin und her von einer Seite zur anderen bewegen kann. Ältere Umwerfer hatten den Käfig an einem einfachen gleitenden Kolben montiert. Moderne Umwerfer haben ein Parallelogrammgelenk mit reduzierter Reibung und bessere Führung entlang der Kettenblätter.

Umwerfer funktionieren so, dass sie die Kette seitwärts drücken und diese dann in einem Winkel statt gerade auf dem Kettenblatt verläuft. Sobald der Winkel scharf genug wird, kann sich die Kette nicht mehr mit dem Kettenblatt verbinden und fällt herunter auf das Nächstkleinere.

Schalthilfen

Wenn die Kette von einem kleineren auf ein größ0eres Kettenblatt geschaltet werden soll, passiert etwas leicht anderes. Die Kette wird gegen die Außenseite des nächsten größeren Kettenblatts geschoben und wir so gegen die Seiten des größeren Kettenblatts gedrückt, dass es irgendwann mitgerissen wird und nach oben aufspringt.

Aktuelle Kettenblätter haben speziell geformte Schalthilfen an der Seite, die dabei helfen, die Kette einzufangen und einen weicheren Schaltvorgang auszulösen. das ist insbesondere bei den heutzutage üblichen schmalen ketten hilfreich, die keine nach Außen hervorstehenden Stifte, die noch bei Fünfgang-Freiläufen der 1970er Jahre üblich waren.

Der Umwerfer nutzt den oberen Kettentrumm für den Schaltvorgang, während das Schaltwerk den unteren Trumm nutzt. Der obere Trumm überträgt die Pedalkräfte auf die hinteren Ritzel. Wenn man also hart in die Pedale tritt, kann die Kette wegen der hohen Spannung weniger leicht gebogen werden. Daher ist es essentiell, dass man beim Schalten vorne, etwas nachlässt. Das ist beim Schaltvorgang hinten weniger ein Problem, weil hier der Schaltvorgang beim nicht unter Spannung stehenden Teil der Kette vorgenommen wird.

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Komplikationen bei der Wahl des richtigen Umwerfers

Montage

Es gibt fünf verschiedene Montagearten für Umwerfer, die unter anderem abhängen vom Sitzrohrdurchmesser des Rahmens:

  • 1 1/8 Zoll (28,6 mm) Klemme. Das ist der klassische Standard für die meisten Straßenräder und ältere Mountainbikes.
  • 1 1/4 Zoll (31,8 mm) Klemme für wenig oversized Sitzrohre
  • 1 3/8 Zoll (34,9 mm) Klemme für oversized Sitzrohre, die bei den meisten Aluminiumrahmen Standard sind.
  • Anlötteileinheiten werden an spezielle geschlitzte Aufhänger montiert, die entweder an den Rahmen angelötet, geschraubt oder geklebt sind.
    Diese Typen limitieren die Optionen, welche Kettenblattgröße man wählen kann. Sie erlauben nur eine sehr geringe Einstellmöglichkeit in vertikaler Richtung, das nicht immer ausreicht.
  • E-Typ Umwerfer von Shimano sitzen auf einer Platte, die vom Abschlussring des Innenlagers gehalten wird. Sie haben meist zusätzlich eine Vorrichtung, die den Umwerfer gegen das Sitzrohr stabilisiert.
    E-Typ Systeme haben die Möglichkeit, den Umwerfer im Kettenwinkel auszurichten aber man hat keine Wahl bei der Kettenblattgröße, so dass man im Allgemeinen genau die spezifizierte Kettenblattgröße verwenden muss.

Zugverlauf

  • Traditional front derailers are "bottom-pull", operated by a cable running up from the bottom bracket area to the derailer.
  • "Top-pull" derailers are operated by a cable running down from above. This design is mainly used on mountain bikes, with the intent of avoiding running the cable under the bottom bracket, where it is exposed to spray kicked up by the front tire.
  • Some older designs use a loop of cable housing running from a stop near the bottom of the down tube. This design hasn't been used since the mid-1970s. Fortunately, it isn't difficult to work around this. If you want to mount a modern derailer on a bike intended for this type of mount, just ignore the housing stop near the bottom of the down tube. Run the bare cable under the bottom bracket and up to the derailer's anchor bolt.
    If you are worried about protecting the paint under the bottom bracket, there are two easy ways to protect it.
    • Easiest is to use a short length of "floating" housing, a piece of housing perhaps 2-3 inches (50-75 mm) long under the bottom bracket. This does not need housing stops at either end. The tension on the cable will make it stay in place.
    • A more elegant solution is to install a modern plastic cable guide under the bottom bracket. Normally, such guides are secured by a 5 mm screw, so you would need to remove the bottom bracket mechanism, then drill and tap a 5 mm hole for the screw. This is easy to do.

Kettenneigung

The slope of the chain relative to the front derailer is affected by the seat tube angle, bottom-bracket height and size of the rear wheel. Derailers are specified for different angles. The issue is most critical with small-wheel bicycles, and can result in the chain's dragging on the bottom of the front derailer's chain cage. A braze-on or E-type front derailer can work around this problem.

Das größte Kettenblatt

Front derailers shift best when they are mounted as low down as is possible without having the cage foul the teeth of the chainrings. In particular, the bottom edge of the outer cage plate should be closely matched to the curvature of the largest chainring.

If you use a front derailer with a larger ring than it is meant for, the rear of the cage will get too close to the chainring before the front of the cage is close enough to provide crisp shifting.

If you use a front derailer with a smaller ring than it is meant for, the front of the cage will get close to the chainring while the rear of the cage is still far away from it. This won't hurt shifting, but will require excessive "trimming " of the front derailer as you change gears at the back.

Mismatched derailers and chainrings may be satisfactory with friction shifting, but will likely cause problems when used with indexed shifters, such as Shimano STI units.

If you have unusual gearing requirements, and are handy, you can reshape the outer cage plate with a grinding wheel to match a non-standard large chainring.

Zweifach/Dreifach

Zweifach- und Dreifachumwerfer

Modern derailers are optimized for either double or triple chainwheels . The shape of the shifting cage is tailored for the application intended.

  • A "double" front derailer has an inner cage plate that extends down only slightly farther than the outer plate.
  • A "triple" front derailer has an extended inner plate that goes down considerably lower than the outer plate. The extended plate greatly improves shifting from the small to the middle chainring, because it acts closer to the middle ring.

"Double" front derailers will work with triples, but if the middle ring is much smaller than the big ring, they will be balky shifting up from the small to the middle chainring. They do, however, work well with "half-step-plus-granny " setups, where the middle ring is only 3-5 teeth smaller than the outer ring.

Modern "triple" front derailers have interference problems in half-step-plus-granny setups between the extended inner plate and the inner chainring. "Triple" front derailers are generally optimized for a specific difference between middle and large chainring.

  • Shimano's "10-speed" triple front derailers are optimized for a 13 tooth difference, typically 52-39.
  • Most other Shimano front triple derailers are optimized for a 10 tooth difference between middle and large ring.

Surprisingly, the size of the "granny" chainring has little effect on derailer performance. Many cyclists customize their "road triple" cranksets by installing a more-useful smaller ring in place of the 30 that comes stock. The only limitation here is that the chain must not bottom out in the chain cage. This is generally not a problem if the large chainring's tooth count is in the mid 40s. It may be a problem if the large chainring is larger. A "compact" cassette whose smallest sprocket has 11 teeth can get around the problem by allowing the use of smaller chainring without sacrificing the top end of the gear range. With today's cassettes that have 8 or more sprockets, you can still have narrow steps between speeds, and a large sprocket with as many as 34 teeth for a low bottom gear. 7-speed, 8-speed, 9-speed, 10-speed...

Many front derailers are designated for some number of speeds in back. This is actually primarily a marketing distinction, not a functional one. Pretty much any front derailer will work with any modern chain system.

It is true that the cages have gotten slightly narrower to match the narrower chains used on systems with more rear sprockets, but this is rarely a problem in practice.

See also my Article on "Speeds."

Indexiertes Schalten Straße/Mountainbike

The ratio of cable movement to derailer movement differs with Shimano's "road " vs. "MTB " front derailers. This can cause indexing problems if you use a "road" front derailer with upright handlebars or a "MTB" front derailer with drop handlebars. This is a concern if you are running an indexed front shifter, such as Shimano STI or RapidFire shifters.

It is not a problem if you are using a friction system, such as bar-end shifters or downtube shifters. It is also not a problem with ratcheting systems such as Campagnolo Ergo.

Shimano does make one model of front derailer designed to work with "road" (52-42) chainrings and RapidFire upright-handlebar shifters, the FD-R440 model.

Also, JTek makes a Shiftmate adapter designed to convert between Shimano's "road" and "MTB" components, offering another solution.

Trimmen

As you shift the rear from side to side, the angle of the chain changes where it approaches the front. Sometimes, this results in the need to "trim" the front derailer.

For example, you may start out in the lowest gear, with the chain on the innermost chainring and innermost rear sprocket, and everything is running smoothly. As you build up speed, you typically shift the rear derailer outward, toward the smaller sprockets for successively higher gears. At some point, this may cause the outside of the chain to rub against the outer plate of the front derailer's cage. This is noisy and will wear a groove in the derailer cage if you persist.

With traditional systems, you would deal with this issue by "trimming" the front derailer, that is, you would pull back very slightly on the front derailer's shift lever, to nudge the cage a bit outward to eliminate the rubbing.

That works fine if your front derailer is operated by a friction shifter.

With the advent of indexed front shifters, such as Shimano's STI and RapidFire units, the possibility of trimming the front derailer is limited by the number and spacing of the indexing detents ("clicks.") Earlier indexed front shifters had little or no ability to trim, but most newer ones have extra detent positions to provide a limited trim capability.

In most cases, if you use the specific chainring sizes that the front derailer was designed for, and the front derailer is adjusted very carefully, trimming won't be necessary if you follow good shifting practices.

If you customize your chainring sizes, you are more likely to have a problem with indexed shifters, so you might want to use bar-end shifters, GripShifts , or Campagnolo Ergo shifters. These are indexed on the rear, but not in front. The Ergos and GripShifts are not strictly "friction" shifters, as they do "click" when you shift them, but they have many "micro clicks" permitting considerable "fine tuning" of front derailer position. (GripShifts sold in the aftermarket use non-indexed fronts, but O.E.M. GripShifts sold with new bikes frequently are indexed.)

The need to trim is largely dependent on the chainstay length...the shorter the chainstays, the greater the need for trimming, because the chain angle is greater for a given amount of sideways movement in back. This is one of the areas where recumbents have a real advantage, as their very long chains result in minimal angle variation in different gears -- the same with tandems that drive the rear wheel from the front crankset.

By contrast, the super-short chainstays found on many racing-type frames often require trimming of the front derailer. (There's an unfortunate near-superstitious belief that short chainstays somehow make a bike "faster" or more maneuverable.)


Quelle

Dieser Artikel basiert auf dem Artikel All About Front Derailers von der Website Sheldon Browns. Originalautor des Artikels ist Sheldon Brown.

Abgerufen von „https://wikipedalia.com/index.php?title=Alles_über_Umwerfer&oldid=14794“