Shimano does make one model of front derailer designed to work with "road" chainrings and RapidFire upright-handlebar shifters, the FD-R model. 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.
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. GripShifts sold with new bikes frequently are indexed. The need to trim is largely dependent on the chainstay length 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. New electrical shifting systems as of include automatic trimming of the front derailer, if needed. Reports of the demise of this Web site are greatly exaggerated! We at sheldonbrown. Harris Cyclery has closed, but we keep going. Keep visiting the site for new and updated articles, and news about possible new affilations.
Translations of this article: Russian. This is the traditional, standard type for most road bikes and older MTBs. These also use a special braze-on to stabilize the derailer. Cable Pull Traditional front derailers are "bottom-pull", operated by a cable running up from the bottom bracket area to the derailer.
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 mids. 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. Easiest is to use a short length of "floating" housing , a piece of housing perhaps inches 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. It consists of a metal cage and a spring. The spring moves derailleur to one side when the shifter cable is released, while the shifter cable, when pulled, moves it to the opposite side.
If there is no rear derailleur installed only one sprocket at the back , a chain tensioner will be necessary to gather chain slack when shifting from bigger to smaller chainrings up front. Front derailleurs are mounted to the frame via a built in clamp, or on a braze on frame holder. Derailleurs designed for braze on holder can be put on frames without braze ons, using a clamp with braze on. Which one of these derailleur mount types is used, depends on the frame and derailleur type that is available.
Following sub chapters will explain those standards. FD is attached directly to the frame with one bolt, and has a groove on the back side to help hold it in place.
The frame must be made with an appropriate mount with a matching groove for this kind of FD to fit. First versions pre-date the DM system and these FDs came with a back plate mount that was fixed by threading in the threaded bottom bracket. Openings cutouts for mounting screwin in are cut at a The FD itself when mounted sits a lot furhter outward towards the chainrings, away from the frame than the S3 E2 type.
With FD-s, number of sprockets at the back they are made for is more or less a marketing trick. Important thing, though, is the number of front chainrings. There are FD-s for two double and those for three triple chainrings. Mixing is not advised. Lower inner cage plate of triple FD comes closer to the chain when it is on the smallest chainring, helping with shifting. Double FD will shift poorly with a triple chainring setup.
So FD was pulled from the bottom. With popularization of MTB-s and off road cycling, this design caused some problems. Cables were easily covered in mud and even damaged on rocks. Cable routing was then transfered to the top part of the bicycle along the top tube.
That is when the first top pull FD-s emerged. Nowadays, MTB derailleurs are often double pull — designed so they can work with both bottom and top pull cable routing. Road FD-s are usually designed with bottom pull, the traditional way.
Shimano Side-Swing FD-s are explained below chapter 5. Older FD swing standard still used on road bikes is the high clamp: the cage swings at the bottom of the parallelogram, fixed to the body of the derailleur.
Newer standard, introduced for mountain bikes is the low clamp. The cage swings above the FD body. That way the body is closer to the BB and leaves more clearance for rear suspension linkage on the seat tube — for example. It is made to provide more clearance for ever wider tyres and shorter chainstays, so that a proper chainline can more easily be maintained. Side-swing FDs can be attached to the frame via low, high clamp, or direct mount.
For example, a high clamp FD used to always be bottom swing, but now it can also be a side-swing one. You will need a front mech that corresponds with the number of gears on your current groupset for it to work best ie.
The majority of road groupsets have two front chainrings double but if you have three you triple you will need a different front mech.
Triple mechs three front chainrings have a larger rear plate than a standard mech as this helps the chain cope with the bigger change in chainring sizes from a very small front ring to a very large front ring.
The bottom of the rear plate is also lower as the chain will have to drop further down to go on the smallest ring. Campagnolo Compact chainset-compatible mechs also have a larger rear plate to help cope with the large difference in chainring sizes, usually Band : this as the name implies is a band which clamps around the seat tube of the frame.
There are two band sizes Tip: clamp size is usually written on the inside of the band.
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