Bremsen für Tandems

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Weil Tandems ungefähr das doppelte Gewicht gegenüber einem Solofahrrad tragen müssen und auch oft schneller unterwegs sind, sind Bremsen, die für ein Solo adäquat sind, nicht unbedingt für den Einsatz beim Tandem geeignet. Dieser Artikel soll ein wenig Klarheit in diese Gemengelage bringen.

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Felgenbremsen

Die meisten modernen Tandems haben mindestens ein Paar handbetriebene Felgenbremsen. Das sind entweder Zangenbremsen, Cantilever-Bremsen oder hydraulische Bremsen. Diese sind entweder spezielle Tandemvarianten oder Bremsen, die eigentlich für ein Solo gedacht sind.

Für die reine Bremsvermögen, sind gut eingestellte Felgenbremsen genauso geeignet wie für ein Solo.

Die größere kinetische Energie eines Tandems reduziert jedoch den Spielraum für Fehler in der Konfiguration der Bremsen. Wenn eine Bremse nicht ordentlich eingestellt ist, es Zugführungsprobleme gibt, die Bremsbeläge von geringer Qualität sind oder die Felgen zu rutschig sind kann das Bremsvermögen eingeschränkt sein.

Fahrradfahrer mit eingeschränkter Bremswirkung tendieren oft dazu, teurere Bremsen nachzurüsten. das ist jedoch überflüssig, wenn man ein qualitativ ordentliches Fahrrad als Basis hat. Bevor man die Bremsen tauscht, sollte man sicherstellen, dass die bereits verbauten mit all ihrem Potential funktionieren.

Hier findet man mehrere Artikel, die helfen, die Bremsen richtig einzustellen.

Zangenbremsen

Einige wenige Tandems werden auch heutzutage noch mit Zangenbremsen ausgestattet, weil man annimmt, dass Zangenbremsen nicht genügend Bremskraft aufbringen, um ein Tandem anzuhalten. Das ist ein Irrglaube. Renntandems haben perfekt adäquates Bremsverhalten, wenn sie mit hochqualitativen Zangenbremsen bestückt sind.

Der limitierende Faktor bei Zangenbremsen idt nicht die Bremskraft sondern die höchstmögliche Reifenbreite. Da die Bremsarme einer Zangenbremse sich oberhalb des Reifens kreuzen, sind hoch und breit bauende Reifen nur mit langen Bremsarmen möglich, insbesondere falls man noch Schutzbleche montieren möchte. Je länger die Arme der Bremse seind, desto niedriger wird die Hebelübersetzung der Bremse. Daher haben moderne Hochleistungszangenbremsen sehr kurze Arme und erlauben eine maximale Reifenbreite von 28 mm. 28 mm Reifen sind jedoch für den Tandemeinsatz eher am unteren Ende der Praktikabilität. Tandems mit Zangenbremsen sind daher ehr ungeeignet für lange Touren oder den Einsatz bei grobem Untergrund.

Während der 1970er Jahre waren Mittelzugbremsen sehr verbreitet, die eine vernünftige Reifenbreite zuließen. Sie sind jedoch aus der Mode gekommen und werden nicht an neuen Fahrrädern verbaut.

Konventionelle Cantilever

Da Cantilever-Bremsen die Gelenke unterhalb der Felgen haben, ist Reifenfreiheit kein Problem. Ein Tandem mit Cantilever-Bremsen kann soviel Reifenfreiheit und Platz für Schutzbleche haben wie nötig, ohne dass das nachteilige Effekte auf die Bremswirkung hat.

Die Hebelübersetzung einer konventionellen (Mittelzug-)Cantilever-Bremse kann durch die Variation der Zuglängen eingestellt werden. das kann man im Detail in den Artikeln Über Felgenbremsen an Anlötsockeln und Cantilever Geometrie nachlesen.

Ein Nachteil konventioneller Cantilever-Bremsen ist die abnehmende Hebelübersetzung der Bremse je näher sie der felge kommt. Das ist eigentlich umgekehrt wie man es sich wünscht.

Selbstverstärkende Cantilever

Wegen des Mythos, dass konventionelle Cantilever-Bremsen nicht genügend Bremswirkung für den Einsatz beim Tandem hätten, werden von manchen Leuten selbstverstärkende Cantilever-Bremsen bevorzugt. Diese haben kein einfaches Gelenk sondern jeder Arme bewegt sich um eine Schraube mit steilem mehrgängigem Gewinde, so dass der Bremsschuh sich bei der Bewegung zur Felge hin gleichzeitig vorwärts bewegt. Wenn der Bremsschuh auf die Felge trifft, erhöht die Vorwärtsbewegung den einwärts wirkenden Anpressdruck und man hat eine positive Rückkopplung.

Mit diesem Design kann man mit weniger Fingerkraft mehr Bremswirkung erzielen als bei einer konventionellen Cantilever-Bremse. Sie ist jedoch schwieriger zu modulieren. Die ursprünglichen Scott Pedersen SE Bremsen gab es für die Montage vorne und hinten (gegenläufiges Gewinde). Als SunTour das Design aufkaufte, hat ihre Rechtsabteilung ein Veto eingelegt und so gab in der Folge nur noch eine Variante für hinten. Sheldon Brown rät vom Einsatz selbstverstärkender Bremsen ab, weil es mit diesen zu schnell passieren kann, dass man das Rad blockiert.

Direktzugbremsen

Die neueste Entwicklung bei Cantilever-Bremsen sind (immer noch) Direkzugbremsen. Die bekannteste Version dieser Direktzugbremsen dürfte die V-Brake von Shimano sein. Diese haben einen eleganten und einfachen Aufbau. Die Hebelübersetzung ist konstant hoch und verändert sich im Bewegungsrahmen der Bremse nur unmerklich.

Weil die Hebelübersetzung der Direktzugbremsen so hoch ist, benötigen sie spezielle Bremshebel, die weniger Hebelübersetzung liefern (sie haben doppelt so langen Zugeinholweg wie Bremshebel für konventionelle Cantilever und nur die halbe Hebelübersetzung). Der Nachteil von speziellen Bremshebeln wird durch den glücklichen Umstand kompensiert, dass man nur halbe Zugspannung hat und der Effekt von Zuglängug weniger ausgeprägt ist. Das ist signifikant vorteilhaft beim sehr langen Zug zur hinteren Bremse eines Tandems.

Bei Tandems mit aufrechtem Lenker sind Direktzugbremsen defintiv die beste Wahl.

Unglücklicherweise gibt es für Dropbar-Lenker nur ein Modell, das mit Direkzugbremsen funktionert. das ist das Modell Dia Compe 287-V. Sie sind eine gute Wahl, wenn man Barcon Schalter hat, funktioniert aber nicht bei kombinierten Schalt-/Bremshebeln wie Campagnolo Ergo oder Shimano STI-Hebeln.

Auf dem Ersatzteilmarkt gibt es exzentrische Zugumlenkrollen, die die Hebelübersetzung an der Cantilever-Bremse ändern. Diese sind oft eine gute Lösung für Hinterradbremsen bei kleineren Rahmen, die nicht genügend Raum oberhalb der Cantilever-Bremse haben, um diese optimal einzustellen.

Manche konventionellen Cantilever-Bremsen können auf Direktzugbetrieb umgerüstet werden. Auch das ist eine gute Option für Hinterradbremsen.

Auch hier sei auf den Artikel Über Felgenbremsen an Anlötsockeln verwiesen.

Hydraulische Felgenbremsen

While most bicycle brakes are operated by Bowden cables, some high-end brakes, most notably Magura, use hydraulic lines instead. Hydraulic brakes are costly, but work very well indeed. In particular, hydraulics on the rear avoid the problems associated with the loooong rear cable.

To mechanics unfamiliar with them they appear a bit daunting at first, but they're actually quite easy to work on.

Nambenbremsen

Hub brakes operate on the hub of the wheel, rather than the rim. On most tandems, the hub brake is not intended to stop the bike! Common hub brakes do not have sufficient braking power to stop a tandem in a short distance.

Why have a brake that won't stop the bike? All brakes work by turning the energy of the bicycle's speed into heat. If a heavily-laden tandem is used in hilly conditions, especially for long, slow, descents, the rim brakes can heat up the rims and tires to the point that the tires will blow off the rims! Using a hub brake to help keep the bicycle's speed in check on long descents heats the hub, instead of the rim, reducing the risk of blowouts.

Rücktrittbremse

Old time bikes, and very cheap models used for resort rentals and the like, sometimes use a coaster (back-pedaling) brake as the primary brake.

This is not a good arrangement if the bike will be used at anything much above walking speed. Tandems of this sort commonly have idler-adjusted sych chains, and when the brake is applied, the idler is subjected to fairly considerable loads from the captain's braking action. This increases the risk of the chain falling off, making the coaster brake inoperable.

Tandems with coaster brakes often are set up with the assumption that the coaster brake will be the primary brake, and they commonly have rather pitiful caliper brakes in front. These calipers are generally inadequate to stop the bike in an emergency. The coaster brake, as with any rear brake on a tandem, becomes almost useless when the bike is ridden without a stoker, or with a very child stoker, because there's not enough weight on the back tire to provide adequate braking traction.

A further objection to coaster brakes for tandem use is the difficulty of getting the pedals set at a good position for getting started. This is a problem with coaster-brake bikes in general, but tandems are always harder to get started than solo bikes are, so the difficulty is compounded.

Scheibenbremsen

disc brakes have considerable potential for bicycle applications, but are not yet a mainstream product for tandem use. Some early ones, such as the 1970's Phil Wood could be quite dangerous, and fail completely without warning. some other disc brakes, while more than adequate for solo use, are at risk for overheating and fading in tandem service.

There is a lot of current work being done on disc brake design, and this may be the way of the future. Some new Santana models come with a Formula &tm; rear disc which is supposed to be powerful and reliable enough to eliminate the need for a rear rim brake. Here's a link to the service manual for the Formula &tm; brake.

I would not look to see disc brakes in the front of bikes that don't have suspension forks. A "rigid" fork is actually designed to flex a bit, to provide a small measure of natural suspension. A hub brake with a reaction arm pressing on one side of the fork causes unpleasant feelings. I speak from some experience, having run a front drum brake on a tandem for a while.

Suspension forks have very rigid lower blades, so a suitably-designed suspension fork can work well with a disc brake, if you don't mind the extra weight and the extra stresses on the spokes.

Trommelbremsen

Drum brakes, either built into the hub or screwed onto threads similar to freewheel threads have long been popular for heat dissipation on tandems used in hilly terrain. The standard of the industry is the very reliable Japanese Arai unit. The drum screws on to the left side of most tandem rear hubs, and the mechanism plate with the internal expanding shoes is secured to the axle by a locknut. There is a reaction arm which must be securely attached to the chain stay...all of the braking force is transferred through this arm, and if it comes loose, it creates a mess. Here's a tip for installing the Arai brake: After the drum unit is in place, slip the mechanism plate over the axle, then squeeze the activating arm by hand while tightening up the lock nut that holds the mechanism plate to the axle. Doing this presses the shoes against the inside of the drum, and helps ensure that the mechanism plate is properly centered. Here's a tip for removing an Arai drum: Have somebody sit on the stoker's saddle (for traction), apply the brake, and forcibly roll the tandem backward until the drum starts to unscrew from the hub. This is the easiest way to break it free.

Control Setup...3 X 2

So, for a heavy load in hilly terrain, you need 3 brakes: 2 on the rims, and 1 on the rear hub. How are you going to operate 3 brakes with 2 hands? Dual-cable levers One traditional solution has been to use a special brake lever for one hand that pulls two cables. I don't recommend this setup; here's why:

  • If the dual lever operates both the rear rim brake and the rear hub brake, the hub brake will not engage. This is because hub brakes generally require more cable pull than rim brakes.
  • If the dual lever operates both rim brakes, you have no way to operate the front brake independently of the rear. The ability to use the front brake alone is essential for safe riding, especially with a lighter stoker, or while riding stokerless.
  • (See my article on Braking & Turning for more on this.)

Stoker Brake

Some teams set up the hub brake so that it is operated by the stoker. This can be a good solution for some teams. The drum brake doesn't normally need to be applied very suddenly, so if the stoker operates it when the captain asks for it, everything is fine.

The danger of this setup is that a fearful stoker might apply the brake at an inappropriate time. For instance, suppose both the captain and stoker notice a car approaching on a collision course from a side street; the captain may try to put on a burst of speed to clear the intersection...if the stoker panics and grabs the brake, the results can be disastrous!

Another example...flying down a twisty mountain road, the stoker may decide that the bike is going too fast and apply the brake...but the captain may decide the bike is going too fast at the same time, and apply both brakes. The extra rear braking from the drum may be enough to break the rear tire loose, and cause the bike to skid out on the next switchback.

A stoker entrusted with a brake control must never use it unless the captain asks for it! Friction Control Drag Brake The most common, and most satisfactory system is to set up the rim brakes, one to each conventional brake lever, and operate the hub brake by a friction-type derailer shifter. This may be a "Barcon", a mountainbike-type thumb shifer, or a Sun Tour Command Shifter.

A friction-type shifter allows the captain to set the hub brake to apply the desired amount of drag, even once he or she has let go of the hub brake control. the captain then uses both hands on the rim brakes to modulate the bike's speed, and to stop if it becomes necessary. Front Brake/Rear Brake On a conventional solo bike, the maximum braking force is created when the front brake is being applied to the point that the rear wheel is just about to lift off the road. Since, the rear wheel has no traction in this condition, the rear brake contributes nothing to maximum braking.

Due to the greater length of a tandem, it is not possible to lift the rear wheel off the road by applying the front brake...the front tire will skid first. Consequently, maximum braking for a tandem involves the use of both brakes, at the point just short of what will skid both wheels.

For less than maximum braking, it is best to just use the front brake in most circumstances, as with a solo bike (if this strikes you as a bizarre suggestion, read my Braking & Turning Article!)

Using the rear brake can be particularly dangerous if you're riding with a light stoker or with no stoker, when the traction on the rear wheel is less than normal.

Bremshebel

Brake Levers for Upright Handlebars If your tandem has upright (straight) handlebars, you need to match your brake levers to the type of brake calipers/cantilevers you're using. This boils down to the fact that you either have direct-pull cantilevers, or you don't. If you have direct-pull ("V-type") cantilevers, you need to use levers made for that application, levers that pull a lot of cable.

If you don't have direct-pull cantilevers, conventional levers will work fine. Some, but not all, levers made for direct-pull cantilevers have a mechanical advantage adjustment feature which allows them to work with either system.

If you use levers made for direct-pull cantilevers with conventional cantilevers or calipers, the reduced mechanical advantage will not let you stop unless you squeeze very, very hard on the levers.

If you use conventional levers with direct-pull cantilevers, there will be too little cable travel, and too much mechanical advantage. The lack of travel can mean that the brake shoes will drag on the rims even when the brakes are released, or, it can mean that you can squeeze the levers all the way to the handlebars without having the brakes engage...it all depends on how they're adjusted. If you get this setup adjusted just right (wrong) the brakes will work but grab too abruptly, possibly causing skidding. Brake Levers for Drop Handlebars Brake levers for upright handlebars bend away from the handlebar; levers for drop handlebars bend toward the handlebar. They are not interchangeable. Drop-bar Levers and Direct-pull Cantilevers At present, thw Dia Compe is the only drop-bar brake lever designed to work with direct-pull cantilevers. Due to the nature of drop-bar levers, it is not so easy to make a suitable lever that can be operated both from the drops and from the hoods.

cantilever brakeIf you wish to use direct-pull cantilevers on a tandem with drop bars, there are accessory devices that change the mechanical advantage so that the mismatch is avoided. These are pulley devices that mount on one of the caliper arms. The most readily available of these is the QBC "Travel Agent." The "Travel Agent" uses a stepped pulley to change the mechanical advantage of the cable..

Since the mechanical advantage change occurs at the cantilever-end of the cable, the cable-stretch benefits of the direct-pull system are lost, but this can still be a very worthwhile system, especially for rear brakes on small frames.


Drop-bar Levers and Conventional Cantilevers Drop-bar levers fall into two families, traditional and "æro" styles.

Traditional drop-bar levers have exposed cables running straight up from the fronts of the hoods, and arching over in mid air as they run toward the brakes. This design is virtually extinct today.

"Æro" levers have concealed cables, running out the back of the hoods and underneath the handlebar tape toward the center of the handlebar. Combined brake/shifter units, such as Campagnolo's Ergopower and Shimano's STI incorporate æro-type brake levers.

Æro levers have a number of advantages besides the cleaner look of the concealed cables:

  • The cables are protected from damage...traditional cables often get kinked just where they exit the hood.
  • They provide generally reduced cable friction.
  • They typically provide a bit more mechanical advantage.
  • Due to the slightly different location of the pivot point, you can actually stop by applying the brakes from the hoods, as well as from the drops. This was not the case with the older style.

"Æro" levers also have two disadvantages: Too Much Mechanical Advantage Yes, you can have too much of a good thing. The greater mechanical advantage of æro levers comes at the cost of reduced cable travel, especially when they're used with newer "low-profile" cantilevers. If you wind up with insufficient cable travel, you have to choose between having the cables so tight that the brakes drag lightly on the rims even when released, or, having the cables so loose that you can squeeze the levers until they bottom out against the handlebars. Neither of these adjustments is satisfactory.

Since the mechanical advantage of conventional cantilevers can be adjusted, the usual cure is to raise up the cable yoke, as described in my Cantilever Adjustment article. This is generally not a serious problem with the all-important front brake, but the added stretch of a long tandem rear brake cable may preclude a satisfactory adjustment of the rear brake. If the suggestions in my Cantilever Adjustment article don't solve this problem for you, you may need to buy levers made specifically for cantilever use, such as the Dia Compe 287, or convert to a direct-pull system with a pulley. No Adjusting Barrels "Æro" levers have no place to install adjusting barrels for fine-tuning the cable length as the shoes wear. This is not a problem when they are used with caliper brakes, but cantilever brakes these days are designed more with MTB applications in mind. Since MTB-type brake levers always have built-in adjusting barrels, there may be no provision for cable adjustment when cantilevers are used with drop bars. This is un-acceptable. A bicycle that doesn't have brake adjusters is dangerous and defective, and no manufacturer or retailer should let such a bike go out of the store. This applies especially to bikes with drop-bar levers and cantilever brakes, because this combination is particlarly finicky about cable adjustment.

Fortunately, inexpensive adaptors are readily available to let you add a cable adjuster anywhere the cable runs through housing.


Siehe auch


Quelle

Dieser Artikel basiert auf dem Artikel Brakes for Tandem Bicycles von der Website Sheldon Browns. Originalautor des Artikels ist Sheldon Brown.


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