Front derailleurs explained

Explanation of all the important facts related to bicycle front derailleurs. A separate article explains the front derailleur compatibility.

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Table Of Contents (T.O.C.):

1. How does the front derailleur work?
2. FD mounting to the bicycle frame
   2.1. Direct mount system known as H0, or DM
   2.2. E-type (also known as low direct mount, E2-type, S3, Spec 3)
   2.3. SRAM S2 (Spec 2)
   2.4. SRAM S1 (Spec 1)
3. Double vs triple
4. Cable pull: bottom vs top pull
5. Swing (pivot) position and direction
   5.1. Bottom swing (High clamp)
   5.2. Top swing (Low clamp)
   5.3. Side-Swing
6. Number of speeds, compatibility

1. How does the front derailleur work?

Front derailleurs (FD in the remainder of the text) are used on bicycles with external gearing and (usually) multiple sprockets at the rear hub. They work on a simple, brute force principle: moving the chain sideways, until it slips off a sprocket onto the adjacent one.

This derailleur movement is usually operated via shifters (shifter levers), that are located on the bars (or somewhere at hand). Shifter cable is attached to shifters at one and and at the derailleur at the other end.

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.

– T.O.C. –

2. FD mounting to the bicycle frame

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. Frame with braze ons will have problems accepting clamp type FD-s, because braze on holder will get in the way (did someone just say “angle grinder”?  🙂  ). For mountain bikes, a few new direct mount systems have been introduced, that differ from the classic “direct” (braze on) systems (still used on road bikes). Following (sub) chapters will explain those standards.

Common clamp mount diameter standards are 28.6 mm (common on old, steel frames), 31.8 mm and 34.9 mm (common on modern aluminium or carbon fibre frames).

The number of teeth on the largest chainring, and the chainstay & seatstay angles are also important specifications. For more details, see the 3rd Chapter in the article about front derailleur compatibility.

– T.O.C. –

2.1. Direct mount system known as H0, or DM

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.

– T.O.C. –

2.2. E-type (also known as low direct mount, E2-type, S3, Spec 3)

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.

This is what the mounted E-type FDs look like:

Openings (cutouts) for mounting (screwin in) are cut at a 22.1 mm horizontal distance on the FD, while the front opening is 5 mm towards the outside (away from the frame) than the rear one.

SRAM marks their E-type FDs as S3 (Spec 3), while Shimano’s mark is E-2.

Shimano’s E-2 FDs allow for height and angle adjustment (mounting holes on the FD are cut with a longer vertical opening and are a bit wider than the mounting bolts), while with SRAM’s S3, one needs to pay more attention to the size of the front chainrings that the FD is made for, since they don’t allow any wiggle.

Apart from this E-type, SRAM offers two more (very similar) mount standards: S2 and S1.

– T.O.C. –

2.3. SRAM S2 (Spec 2)

Easily confused with SRAM S3 (and Shimano E-2), since the mounting holes are made at the same distance (22.1 mm), but the front and the rear one are at the same height. The FD itself when mounted sits a lot furhter outward (towards the chainrings, away from the frame) than the S3 (E2) type.

– T.O.C. –

2.4. SRAM S1 (Spec 1)

S2 FDs have mounting holes at a 42.7 mm distance.

– T.O.C. –

3. Double vs triple

With FD-s, number of sprockets at the back they are made for is more or less a marketing trick. E.g. an 8, or a 10 speed FD will function more or less the same. Cage width will differ, but if one “size” is missed (9 speed with an 8 speed chain), it will not cause any problems.

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. Triple FD will work decently on a double chainring setup though, as long as the difference between the big and the small ring isn’t greater than 14 teeth.

Double FDs can easily handle up to 16 teeth differences in chainring size.

– T.O.C. –

4. Cable pull: bottom vs top pull

First FD-s were made so that cable is routed under the BB then up to the FD. 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.), but here it suffices to say that they use a different cable routing (cable comes from the front) and the frame needs to be built to accommodate such cable routing.

– T.O.C. –

5. Swing (pivot) position and direction

FD cage rotation and position of mounting clamp relative to the cage are closely related, so this chapter could be placed under the chapter 2 (“FD mounting to the bicycle frame“).

5.1. Bottom swing (High clamp)

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. This system is called both “bottom swing” and “high clamp” – but note that the new(er) side-swing FDs also come with both high, and low clamps.

– T.O.C. –

5.2. Top swing (Low clamp)

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.

– T.O.C. –

5.3. Side-Swing

Newest (Shimano’s) system where the FD moves sideways. 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. Before the introduction of this system, the clamp position was enough to determine the FD cage rotation (top, or bottom swing), but side-swing has “messed that up” a bit. For example, a high clamp FD used to always be bottom swing, but now it can also be a side-swing one.

– T.O.C. –

6. Number of speeds, compatibility

Detailed explanation of which FD-s can be combine with which shifters, chains etc, can be found in this series of articles: Compatibility.

Like it was mentioned before, speeds of FD-s are a marketing trick. Even FD-s and FD levers of various makers can be mixed (e.g. Shimano and SRAM). Still, attention should be paid to the following two factors:

1. MTB and road FD-s, and shifters (as well as the cranksets) have different cable pull ratios (and chainring spacing) so they are not compatible and shouldn’t be mixed.
2. Double and triple shouldn’t be mixed.

– T.O.C. –

Author

Relja Novović

Relja is an avid cyclist, a pretty good bike mechanic, and a professional computer systems administrator – with decades of experience in both those fields (computers and bikes 🙂 ). Relja Novović’s credentials and a short biography

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https://www.bikegremlin.net/threads/front-derailleurs-explained-article-comments.140/