Derailleur
The derailleur (also: external gear or external circuitry ) is an at bicycles used gears , rotating engagement of the bicycle chain in with different numbers of teeth provided sprocket and / or chain wheels .
When shifting the gear ratio between the crank and the rear wheel is changed. Derailleur gears are the most frequently used change gears on bicycles. Other change gears are the hub gears and the less common bottom bracket gears . A derailleur gear is also used together with a hub gear to increase the gear ratio range or the “spread”.
Basic information on structure and function
A derailleur with a relatively large number of gear ratios has several ring gears, which are arranged next to each other on the rear wheel hub, and up to three sprockets next to each other on the crank. The bicycle chain is moved transversely between the sprockets and the chainrings with the help of a changing device. The chain leaves the previous gear over a larger part of the wheel circumference and rises over to the desired adjacent gear. Only when a sufficient number of chain links have come into contact there, i.e. enough chain bars have fallen into the gaps between the teeth, the switching process is ended. During the change between two gear wheels, the chain transmission cannot transmit any drive torque , the cyclist may only pedal with little force.
Whenever the largest chainring is not paired with the largest sprocket, the chain is too long, which is compensated for with a chain tensioner in the tension-free part of the chain below.
The changing device on the rear wheel, with which the chain tensioner is usually combined, is usually referred to as the rear derailleur , the front changing device as the derailleur . The sprockets together form the sprocket package (also referred to as a cassette). The front sprockets are also called chain rings .
With a combination of chain and hub gears, there is usually no shift at the front, there is only one sprocket.
Details on structure and function
Depending on the intended use of the bicycle come chainrings with 20-44 teeth in Geländerad (usually three chainrings, since about 2014 model year are increased 2 and 1 times used chainrings) or 30-55 teeth in racing (two chainrings 34-50, 39 –53 or three chainrings 30–39–52) are used. The sprocket package contains up to twelve (typically 7 ... 10) sprockets with a number of teeth from 9 to just over 50 teeth, on racing bikes often only up to 21 or 25 due to the desired smaller gradation in high driving speeds.
Often only one chainring is used on downhill bikes. Very simply structured derailleurs with only one chainring in the low price segment were also used in city bikes in earlier times. In the 1990s, gears with four chainrings were briefly on the market, but they could not hold up on the market.
The chain is both guided and tensioned in the rear derailleur by the rocker arm. The rocker arm consists of two small gear wheels, the so-called chain wheels (also called switching rollers or switching mechanism wheels ), which are usually made of plastic and are arranged vertically one below the other in a cage. The upper chain wheel is called the chain guide wheel or chain guide roller and is used to guide the chain under the desired sprocket. The lower chain wheel is called the chain idler or chain idler . The rocker arm can be rotated around an axis that either coincides with the axis of the chain guide wheel (so-called two - point mounting ) or is slightly outside (so-called three - point mounting or pantograph ). It is tensioned by a torsion spring so that the chain tensioner pulls the chain backwards and thus tensions it. The rocker arm can also be moved laterally using a parallelogram mechanism in order to guide the chain onto the desired sprocket using the chain guide wheel.
The changing devices can move the chain sideways just before it runs onto the sprockets. When the gears rotate, the chain can be placed on different gears. The gear ratio of the chain drive results from the ratio of the number of teeth on the chainring and the sprocket on which the chain is currently running.
With three chainrings at the front and eleven cogs at the rear, 33 gears can theoretically be shifted. However, some of the gears in which the chain runs particularly obliquely are not recommended due to the high level of wear, the reduced efficiency and the unpleasant noise development. The number of sensible gears that can be driven is therefore - with three chainrings in the front and eleven sprockets at the back - 27, since the three largest rings should not be combined with the large chainring and the three smallest should not be combined with the small chainring. In addition, the translation of some gears is almost identical. The number of these almost identical gears depends on the combination of existing chainrings and sprocket pack. In the case of mountain bikes and touring bikes suitable for mountain use, this is usually smaller due to the larger bandwidth than in the case of racing bikes, which are equipped with two chainrings that are comparatively close together. The overlap in the gear ratios is not only a disadvantage, but intentional in order to limit the simultaneous shifting at the front and rear to a minimum.
The changing devices are operated using shift levers that are within easy reach of the driver. The frame shift levers on the down tube that used to be common on racing bikes have now been almost completely replaced by handlebar shift levers.
Historical development
The invention of the derailleur is u. a. Attributed to Paul de Vivie , who developed a derailleur around 1906. As early as 1889 he was using different chainrings. However, a prototype of a rear derailleur is said to have been presented at the Salon du vélocipède de Paris as early as 1869 . In 1895 Jean Loubeyre is said to have presented the «Polycelere», the first real rear derailleur, offered in the catalog of the Compagnie Générale des Cycle.
Gear shifting was obviously considered uncool at first. Henri Desgrange , the organizer of the Tour de France , wrote in L'Auto magazine that a gear shift would only be suitable for disabled people and women.
The “Vittoria Margherita” circuit by the Nieddu brothers was the first solution that could be used in practice. In 1938 their 5-speed “Tour de France” model came onto the market. In the 1940s, the Giuseppina model made of duralumin appeared. At the same time, models like the “Super Champion” were invented in France. All of these circuits had a tension pulley behind the chainring and were very susceptible to dirt. The Campagnolo Corsa came onto the market in 1946, and its design revolutionized the derailleur system. One lever was used to loosen the axle (while driving), then a second lever was used to switch, whereby the axle slid a bit forwards or backwards so that the chain remained taut. Then the axis had to be fixed again with the first lever. This first practically usable circuit quickly found widespread use in racing bikes. Chain tensioners, which made loosening the axle unnecessary, were only invented later.
The simple principle of derailleur gears has changed little to this day, and yet derailleur gears have undergone a leap in development, which is primarily reflected in the improvement in the precision and speed of the shifting process. He began with the first index circuits in the 1980s (“Positron” from Shimano and “Commander” from Sachs ). Up until that time, derailleur gears were almost exclusively reserved for racing bikes. The objective was to continuously variable shift lever sensitively exactly so to move far up the chain jumped round to the desired gear. Since the shape of the teeth was not adapted to the movement of the chain, it was even necessary to shift slightly beyond the actual gear at first, in order to then easily take the gear lever back again after the chain lay on the sprocket. In the GDR , in addition to racing bikes, some sports and touring models were equipped with the favorite derailleur from the ČSSR as standard - not least because no hub gears were available.
A considerable improvement was achieved in 1984 when Shimano took over the diagonal parallelogram from Suntour for the "Dura Ace" gearshift . With a so-called inclined parallelogram arranged at an angle of about 25 degrees, the pivoting path of the rocker arm runs not only inwards, but also downwards. On the one hand, this corresponds to the climbing movement of the chain; This means: the further the rocker arm swivels inwards, the further the chain guide wheel moves downwards and thus keeps the distance to the toothed rims, which become larger inward, almost constant. Finally, a second return spring guarantees that, regardless of the tooth differences in the sprocket set, the distance from the chain guide wheel to the sprockets is always between 1½ and 2½ chain links. This in turn is the ideal prerequisite for the next switching step. If this distance increases, the gear change takes place only hesitantly, as the shift chain that moves laterally can bend too far. If the distance is too small, the chain guide wheel can drag on the sprockets.
The next innovation push improved the shift quality again significantly: With their "Hyperglide" tooth shape, Shimano enabled shifting even under load in 1988, which is particularly advantageous for off-road bikes, because there are always situations on sudden inclines in which only one gear change is required can keep the bike from coming to a standstill. The special feature of this tooth shape: the chain no longer has to climb over the tooth tips to the next sprocket to change gears, but instead it runs, engaging tooth in tooth, from one sprocket to the next. In order to achieve this, on the one hand the tooth height was drastically shortened, on the other hand the space between tooth to tooth was enlarged in a trough shape and ultimately "overflow points" were created: At the points where the teeth of the neighboring tooth rings stand side by side (this situation is with tooth rings with one tooth difference, twice with two teeth and three times the case with three teeth, etc.) the tooth height is further reduced (they look almost like broken teeth, which often leads to astonishment among the inexperienced). In addition, the teeth are flatter on the sides, so that the chain can run diagonally from one sprocket to the other.
Translation and Development
Basic terms and relationships, selection of the translations used
The development , also the length of the run or the gain in space , is the distance that the bicycle covers with one turn of the crank. It depends on the translation of the chain drive and the size of the rear wheel.
If Z K denotes the number of teeth on the chainring and Z R denotes the number of teeth on the sprocket, the gear ratio of the chain gear results in
and thus the length L increases with the circumference U of the rear wheel
It is particularly important to note that in bicycle technology the translation has always been calculated differently than in general mechanical engineering (reciprocal value).
Chainrings or sprockets lying next to each other must not have too great a difference in the number of teeth, because otherwise the chain can no longer skip when shifting. In practice, this means that there are numerous overlaps with regard to the translation, i.e. multiple existing transmission ratios. A clever choice of chainrings and sprockets should achieve three goals: Fine gear increments, large gear range and little overlap in the gear ratio. Since these goals partially conflict, the solution is always a compromise.
The primary goal of a racing bike is to have fine gear steps. This means that the driver can always pedal with the optimal cadence even with small differences in speed. Therefore, sprocket packs are used in which the sprocket gradation has a difference of one tooth each and the transmission range of the small chainring, neglecting the non-drivable transmissions, follows that of the large one with a small overlap area. For this reason, the chainring combination 53/39 is used almost exclusively in road cycling today. With a cassette that has, for example, the sprockets with 11-12-13-14-15-16-17-18-19-21 teeth, the last mobile combination 53/18 with a gear ratio (i) of 2 follows , 94 the combination 39/14 with a gear ratio of 2.79 - in the overlap area the gear ratios 53/19 and 39/14 are almost identical with 2.784 and 2.786 and 53/18 and 39/13 with 2.944 and 3.000 respectively. The remaining overlap area is intentional because it avoids double shifting (chainring and sprocket change) in certain situations.
Due to the deliberately fine gradations, the gear ratio range , i.e. the ratio of the largest to the smallest gear ratio, is relatively small on a racing bike for a given number of sprockets . In the above example, the gear ratio range is defined by the smallest gear ratio of 1.86 in the combination 39/21 and the largest gear ratio of 4.82 in the combination 53/11, the gear ratio range is 259%.
With mountain bikes, on the other hand, a large gear ratio range is important, as very small gears are necessary uphill. At the same time, it is important to still be able to pedal on fast descents. Sprocket sets with 11-13-15-17-20-23-26-30-34 teeth and chainrings with 22, 32 and 44 teeth are common. The smallest gear ratio is then 0.65 ("reduction") in the combination 22/34, the largest 4.00 in the combination 44/11. This results in a translation range of 618%. A comparison of the translation ranges, however, only makes a statement about the circuits used; When comparing the overall transmission ratio of the drive (i.e. travel distance per crank turn), differently sized wheels must also be taken into account.
Step wreath
Stages rim ( engl. Half Step for half step ) indicates an embodiment in which the two sprockets are similar in size. With the small of the two chainrings you drive the odd gears (first, third, fifth, etc.) and with the large one the even gears (second, fourth, sixth, etc.). With this variant, the gear ratios of the small chainring engage in the gaps that are created by the 2-tooth gradation of the sprocket set when using the large chainring.
If correctly designed, there are no double gears in the variant with two chainrings. In a package with seven sprockets, for example, you actually have 14 different gears available. This automatically leads to the fact that the gear steps are small and comparatively uniform, despite a relatively large gear ratio range.
For the mountain suitability a third, very small chainring (often English. Halfstep plus Granny ) is used. In this case, there is some overlap, as the difference in diameter between the medium and small sheet cannot be arbitrarily large.
The main disadvantage of the step ring is that you have to understand the shift pattern and that you may have to use both shift levers to shift into the next gear. The chain runs diagonally in many gears; By using sprockets with fewer gears (often 7) and the shortest possible bottom bracket axle, skew can be reduced.
The step ring is not new: It was used until the 1960s.
Advantages and disadvantages of derailleur gears
advantages
- relatively simple structure
- low weight
- high efficiency in good condition
- large translation range
- Ratio range can be individually adapted by exchanging the sprocket package or chainrings
- Can be produced very cheaply, especially in a simple design
- quick and easy removal of the rear wheel - especially with quick release fasteners
disadvantage
- The chain is difficult to cover
- Soiling of the trouser legs, especially on everyday bikes (soiling can be prevented by adding a chain guard that covers the chain and chain rings)
- Can only be switched while pedaling (only without load, otherwise the circuit will wear out prematurely)
- the usable gears and their complete sequence cannot be seen on the two control elements
- high wear and thus short service life of some parts (especially chain and frequently used sprockets)
- high maintenance effort
- poor efficiency in the event of pollution and insufficient lubrication
- heavy exposure to dirt and moisture
- The chain can easily jump off, jam and cause falls if it is incorrectly adjusted or driven off-road
- The chain and rear derailleur are very deep, especially on off-road bikes with large sprockets, and are therefore at risk of tearing off
- imprecise switching capacity with the slightest misalignment of the rear derailleur / derailleur hanger
When used on asphalt roads with little pollution, a chain that is maintained regularly can achieve a service life of 7000 km and more. Off-road use reduces the service life to 1000 to 1500 km. In the case of long chains, as are common on recumbent bikes, the service life is extended accordingly. For recumbents with a completely encapsulated chain or for bicycles without derailleur gears, mileages of up to 100,000 km are possible.
literature
- Michael Gressmann, Franz Beck, Rüdiger Bellersheim: specialist knowledge of bicycle technology. 1st edition, Verlag Europa Lehrmittel, Haan-Gruiten, 2006, ISBN 3-8085-2291-7
- Fritz Winkler, Siegfried Rauch: Bicycle technology repair, construction, production. 10th edition, BVA Bielefelder Verlagsanstalt GmbH & Co. KG, Bielefeld, 1999, ISBN 3-87073-131-1
See also
Web links
- Overview of bicycle circuits test.de, October 25, 2012, accessed on February 1, 2013
- Adjust derailleur
- Pictures of circuits from the 1930s (nl.)
- Construction of a half-step circuit with granny
Individual evidence
- ↑ Buy a bike in St. Pölten. In: redplates.at. Retrieved May 25, 2017 .
- ↑ Electric bicycles for the population of the region Description: Siga e-Urban women's / men's bike. In: eggenburg.gv.at. Retrieved May 25, 2017 .
- ↑ Virtual tour. In: radwelt.cc. Retrieved May 25, 2017 .
- ↑ NEWS from HEBIE. (No longer available online.) In: bikespirit.at. Archived from the original on November 23, 2009 ; Retrieved May 25, 2017 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Christian Smolik: Online Glossary Velotechnik - Schaltschwinge .
- ↑ Exploded view of the Shimano XTR RD-M971 rear derailleur ( Memento of the original from September 29, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 261 kB).
- ^ Clifford Graves: Velocio, Grand Seigneur . Retrieved March 17, 2007.
- ↑ http://ddr-fahrradwiki.de
- ↑ Chain maintenance on the bicycle - myths, legends and nonsense
- ↑ http://www.rennrad-news.de/forum/threads/lebenszeit-rennrad-kette.31751/
