Derailleur gears

The derailleur is a power circuit used in bicycles with varying engagement of the bicycle chain provided with different numbers of teeth pinion and / or sprockets.

When switching the gear ratio between crank and rear wheel is changed. The derailleur is the most commonly used change gear on the bike. Other change gears are the gear hub and the rarer Tretlagerschaltung. For enlarging the ratio range respectively of " spreading " a chain circuit is also used together with a hub gear.

• 5.1 advantages
• 5.2 disadvantages

Basic structure and function

A derailleur with relatively many gear ratios has both a plurality of pinions which are arranged side by side on the rear wheel hub, and one to three sprockets side by side on the crank. Between the pinions and between the sprockets ( common term: Gears ) bicycle chain with an ever changing device is displaced transversely. The chain leaves over a larger portion of the wheel circumference, the previous gear and rises to the desired adjacent gear on. Only when a sufficient number of chain links there come into contact, that is, enough chain webs have fallen into the tooth gaps of the switching operation is completed. During the change between two gears, the transmission chain can not transmit drive torque, the cyclist may only pedaling with little force.

Except when the largest sprocket is coupled through the chain to the largest sprocket, the chain is too long, which is balanced with a chain tensioner in the current down draft-free part of the chain.

The changing device at the rear wheel, the chain tensioner is generally the combination is commonly referred to as a rear derailleur, the front derailleur as changing device. The pinion together form the sprocket set (also called a cassette). The located front sprockets are also called chainrings.

In a combination of chain and gear hub is not connected in the front usually, there's only a sprocket.

Details of the structure and function

Depending on the intended use of the bicycle chainrings (usually three chainrings ) or 30-55 teeth ( two chainrings 34-50, 39-53 or three chainrings 30-39-52 ) come with 20-44 teeth when Geländerad with road bikes for use. The sprocket set contains up to eleven ( typically 7 ... 10) pinion teeth in number from 11 to 36 teeth in racing bikes often only 21 or 25

On downhill bikes just a chainring is used. Very simple design and low -priced accordingly derailleurs with only one chainring came in earlier times, even at city wheels used. Circuits with four chainrings were briefly in the 1990s in the commercial, but could not prevail on the market.

The chain is guided both in switch of the rocker arm and tense. The rocker arm consists of two small gears, the so-called chain wheel, usually made ​​of plastic and are mutually perpendicular in a cage. The upper chain wheel is called Kettenleiträdchen or chain guide roller and serves to guide the chain under the desired sprocket. The lower chain wheel is called chain tensioning wheel or chain tensioner. The rocker arm is rotatable about an axis with the axis of either Kettenleiträdchens coincide (so-called two-point bearing ) or lies somewhat outside (so-called three-point mounting or pantograph ). It is biased by a torsion spring, so that the chain tensioning pulley, the chain moves to the rear, and thus tensioned. The rocker arm is also displaced laterally by a parallelogram mechanism to guide the chain using the chain guide wheel to the desired gear.

The change devices can move the chain side, just before she runs onto the gears (pinion, sprocket ). When the gears rotate, so the chain can be placed on different gears. From the ratio of the numbers of teeth of the chain ring and pinion on which the chain runs straight, gives the ratio of the chain drive.

Three front chainrings and rear sprockets eleven theoretical 33 gears can be shifted. Part of gears in which the chain is particularly inclined, but are not recommended due to the high wear, reduced efficiency and unpleasant noise. The number of mobile gears is three chainrings in front and nine rear sprockets at 23, as the two largest sprockets should not be combined with the small chainring not compatible with the large chainring and the two smallest sprocket. In addition, the translation of some passages is almost identical. The number of these almost identical translated passages depends mainly on the existing chainrings, to a lesser extent from the sprocket set and is usually smaller than equipped with three chainrings on mountain bikes and road bikes mountain equipped touring bikes. The overlap in the translations are often intended to simultaneously restrict Needful Turn the front and rear to a minimum.

The changing devices are operated via shift lever located within easy reach of the driver.

Historical development

The derailleur bicycle was invented around 1930.

The circuit " Vittoria Margherita " by the Brothers Nieddu the first one printed in practice solution. In 1938 the 5-speed model "Tour de France " on the market. In the 40s came the model Giuseppina duralumin. Models were invented the same time as the " Super Champion " in France. All these circuits had an idler pulley behind the chainring and were very susceptible to dirt. 1946 then came the Campagnolo Corsa on the market, their design has revolutionized the system of derailleurs. A lever was ( while driving) the axis relaxed, with a second lever was then connected with the axis a bit to the front or rear slid so that the chain remained tense. Then had to be again fixed to the first lever, the axis. This first practically useful circuit was fast spreading in racing bikes. Chain tensioner, which made the release of the axis unnecessarily were invented later.

Has on the simple principle of the derailleur to this day little changed, and yet the derailleurs have a burst of development behind it, which is reflected especially in improving the precision and speed of the switching process. He began in the 1980s with the first index circuits ( "Positron " by Shimano and "Commander" from Sachs). Up to that time, the derailleurs were almost exclusively reserved for racing bikes. The objective was to stepless shift lever soulful exactly to move so far, until the chain jumped round to the desired gear. Since the shape of the teeth was not adapted to the chain movement, even had to be switched initially slightly above the actual transition out to then easily take back the shift lever after contact of the chain on the sprocket.

A significant improvement came in 1984, when with the Shimano " Dura Ace " circuit the Schrägparallelogramm Suntour took over. With an approximately 25 degree slanted so-called Schrägparallelogramm the pivoting of the rocker arm extends not only inside, but also simultaneously down. The first corresponds to the climbing movement of the chain, on the other hand follows the rocker arm of the pinion embankment. This means that the further pivots the rocker arm to the inside, the further moves the chain guide wheel downwards and thus maintains the distance to the larger inward expectant pinions almost constant. A second return spring finally guaranteed that no matter what differences tooth pinion package is equipped, the distance from the chain guide wheel to the pinion gears is always between 1 ½ and 2 ½ chain links. Again, this is the optimal condition for the next switching step. Because the distance is larger, the gear change is hesitant because the lateral movable circuit chain can turn too far. If the distance is too small, the chain guide wheel can grind on the gears.

The next wave of innovation improved shift quality again significantly: with its " Hyperglide " tooth shape Shimano allowed in 1988, shifting even under load, which is very advantageous especially in off-road bikes, because there are always at sudden slopes situations where only a gear change protecting them from the rest of the wheel. The special feature of this tooth shape: The chain must now climb to change gear no longer have the tooth tops, over the next sprocket but it runs, dental tooth in cross, from one sprocket to the next round. To achieve this, the tooth height was for a drastically reduced, on the other hand, the space of teeth enlarged trough-shaped and ultimately " overflow points" created: At the points where the teeth of the adjacent pinion stand side by side (this situation is at pinions with one tooth difference once, when two teeth difference twice and three teeth three times, etc. applicable) of the tooth height is correspondingly reduced further ( they look almost like broken teeth, which frequently leads to astonishment of the uninitiated ). Moreover, the teeth are laterally flat, so that the chain may extend diagonally from one to the other pinion here.

Translation and deployment

The development also expiry length or space gain, is that distance traveled by the bicycle with a crank revolution. It depends on the ratio of the chain drive and the periphery of the rear wheel.

Identifies ZK the number of teeth of the rear sprocket and ZR the number of teeth of the pinion, the result is the translation of the chain drive to

And thus the sequence length L with the circumference U of the rear wheel to

Adjacent chainrings or sprockets may not have too great a difference in the number of teeth, otherwise the chain when shifting can not skip. This leads, in practice, that there are numerous overlaps related to localization, ie duplicate gear ratios. A clever choice of chainrings and sprockets to achieve three objectives: Fine gear ratios, large transmission range and little overlap of the translation. Because contrary to these goals in part, the solution is always a compromise.

When racing, the primary goal is to have fine gear ratios. Thus, the driver can always occur with the optimal cadence even for small velocity differences. Therefore sprocket sets are used, in which the pinion gradation in each case has a difference of one tooth and the translation range of the small chainring followed by neglecting the non- mobile translations with a small overlap area to that of the large sheet. For this reason, almost all the chainring combination 53/39 is used in road cycling today. With a cassette, for example, has the pinion with 11-12-13-14-15-16-17-18-19-21 teeth, 53/18 closes as the last mobile combination with a translation of (i) 2, 94 the combination of 39/14 with a ratio of 2.79 to - in the overlap region the translations 53/19 and 39/14 with 2,784 or 2,786, and 53/18 and 39/13 with 2.944 or 3.000 are almost identical. The remaining area of ​​overlap is quite intentional, as this twice the switching process ( chain ring and pinion change) can be avoided in certain situations.

By intentionally subtle gradations is for a given number of pinions of the translation field, ie the ratio of the largest to smallest translation is relatively small with road bikes. In the above example the field of translation by the smallest translation of 1.86 in the combination 39/21 and the highest ratio of 4.82 in the combination 53/11 is defined, the transmission range is 259 %.

In contrast, a large mountain bike gear range is important because especially on soft ground uphill very small passages are necessary. At the same time, it is important to be able to still occur in rapid descents. Are widespread sprocket Packages 11-13-15-17-20-23-26-30-34 teeth and chainrings with 22, 32 and 44 teeth. The smallest gear is then 0.65 ( "scan ") in the combination 22/34, the largest 4.00 in the combination 44/11. This results in a ratio range of 618 %. A comparison of the translation regions does, however, only an indication of the circuitry used; when comparing the overall transmission ratio of the drive ( ie, traveling distance per crank revolution) differently sized wheels have to be considered ( about 28 "at the mountain bike vs. 27" with road bikes ).

Half Step

Half Step (English for half-step ) refers to a design in which the two chainrings are similar in size. With the small of the two chainrings you drive the odd gears (first, third, fifth, etc.) and with the large sheet of the even ( second, fourth, sixth, etc.).

With correct interpretation is available in the version with two chainrings no duplicate courses. So you have for example a package with seven gears actually 14 different courses available. This automatically means that, despite a relatively large transmission range, the gear ratios are small and relatively uniform.

For the mountain suitability often a third, very small chainring is used. In this case it is called Half Step with Granny. ( Granny, Eng. Grandmother for as an allusion to small passages, how they use weak persons). In this case, there is some overlap, since the distance in the tooth number between the middle and the small sheet may not be arbitrarily large.

The main disadvantage of Half Step is that we need to understand the circuit diagram and that you have to press both shift lever for shifting into the next gear under certain circumstances. There are only a few suitable derailleur so. The chain runs obliquely in many passages; by the use of cassettes with fewer gears (often 7 ) and a bottom bracket can be as short as possible to reduce the slip.

New Half Step is: Up until the 1960s, was for this reason the so-called "step ring " is used. In this variant, " attacked " the translations of the small chainring in the gaps created by the 2- teeth - gradation of the sprocket set when using the large chainring.

Advantages and disadvantages of the derailleur

Benefits

• Relatively simple structure
• Low weight
• Large transmission range
• Individually adjustable transmission range by exchanging sprocket assembly or chainrings
• Very inexpensive to manufacture, particularly in simple design
• Faster and easier removal of the rear wheel - especially with snap closures

Disadvantages

• Chain is bad coverable
• Pollution of the pant legs, especially when everyday bike ( pollution can be prevented by attaching a substring protection that covers the top running chain and chainrings )
• Only be switched during pedaling (only without load, otherwise the circuit will wear prematurely )
• The usable gears and their complete order are not apparent at the two controls
• High wear and thus short lifetime of some parts (especially chain and sprocket frequently used )
• Poor efficiency in pollution and lack of lubrication
• Heavy load of dirt and moisture
• Chain can easily jump from inaccurate setting or off-road driving, caught and cause falls
• Chain and derailleur are very deep, especially in off-road bikes with large pinions and are thus abreißgefährdet
• Imprecise switching performance at minimum misalignment of the switching mechanism / derailleur hanger

When pollution poor use on paved roads in a regularly maintained chain can achieve a service life of 3000 km. The off-road use will reduce the life of 1000-1500 km. For long chains, which are common on the recumbent bike, the service life is extended accordingly. In recumbents with fully enclosed chain drive capacities are available up to 100,000 km.