Front derailleurs can be one of the most confusing parts of a bicycle, this is in part because they come in a vast variety options. Hopefully this article will take some of the myth out of the front derailleur and help you on your next repair or bike build.
Cable pull is the orientation of the cable as it interacts with the derailleur and is determined by the routing on the frame. Road bikes/frames traditionally use a bottom pull system, therefore the road derailleurs typically will be bottom pull as well. A large percentage of cyclocross frame use top pull routing despite the need for a road derailleur to match most drivetrains. This may seem counter productive but serves a valuable purpose in that it keeps the cable and guides away from a lot of the mud, grime, and debris a bottom pull system would see. To make everything mesh, most cross frames will incorporate a guide wheel to redirect cable appropriately. Mountain front derailleurs come in bottom, top, or dual pull (derailleur will work on top and bottom pull frames) options as mountain bikes vary largely in how the frames are designed.
As the number of gears on the rear cassette increase the chain gets thinner, and so does the cage on the front derailleur. You can sometimes get a derailleur of a higher speed to work with a lower speed (9 speed derailleur with 8 speed drive train) but because of the thinner cage there will be more chain rub especially in extreme gear ranges.
One additional thing to note is that when installing a cable, make sure that you’ve routed the cable correctly at the derailleurs pinch bolt. If routed incorrectly, you’ll effectively change the pull ratio making it impossible to reach one or more of the chainrings successfully.
While you may think to yourself that they do it to confuse you or so that your local shop has to buy more derailleurs (and always be out of the one you need the night before the race), it’s not. The reasons why:
This saves weight because making a derailleur to route the cable a certain way takes less material.
It creates more positive shifts because the derailleur cable is designed to interact with the derailleur in a certain way. Dual pull derailleurs tend to have a little play in either the cable or part of the derailleur because of the way the cable attaches. This could result in slower shifts and is why you will see pull specific versions of SRAM’s XX and X.0 groups.
A front derailleur’s capacity is the total difference in teeth between the lowest possible gear and the highest possible gear.
For modern road double front derailleurs the capacity is typically 16t which will cover both conventional standard gearing (39/53t) and compact gearing (34/50t). Some of the older double front derailleurs were set up for the then standard gearing and have a capacity of 14t. What this means is that most current or modern double derailleurs will be cross compatible between compact and traditional double cranksets. Not all older derailleurs designed for traditional doubles will be compatible with compact doubles.
For modern road triple front derailleurs the capacity is 20-22t. The additional capacity allows there to be a greater range between the lowest (smallest) gear and the highest (largest gear). There generally also is a minimum of a 9t difference between the gears. Road triples can be tricky because mixing different series in Shimano for example could give you a system that doesn’t work.
- If you were to set a bike up with a Shimano Ultegra FD-6703 front derailleur and a Shimano 105 FC-5703 crankset, it would work fine because the 105 cranks have a gear set-up of 50-39-30t or a difference of 20t and the Ultegra derailleur has a capacity of 22t.
- If you were to take the same bike but use the Shimano Ultegra FC-6703 crankset with the Shimano 105 FD-5703 you would run into shifting issues because the Ultegra triple uses 52-39-30 or a difference of 22t and the 105 front derailleur has a capacity of 20t.
For mountain bikes, the triple setup has been the standard since about day one, with only a few short lived attempts towards a mountain double. This was true up till recent years when the SRAM XX group came out in late 2009 bring a double to the masses.
Triple mountain front derailleurs will typically have a capacity of 22t to match up with 44-32-22t cranks that are commonly available. Most mountain triple front derailleurs also need a minimum of 10t difference between each gear.
SRAM lead the way in late 2009 with the introduction of the XX group which unlike the triple setups of the past uses a double front along with a 10 speed rear cassette. For 2011 the trend continues with SRAM offering 2×10 in XX, X.0, X.9 and X.7 levels. Shimano is offering the new XTR M980, XT and SLX groups in a 2x option. Because these groups are new, the standards haven’t really been set and could change in the near future. Currently the capacity for both groups is 15t difference.
Maximum Chainring Size
Each derailleur has a limit of how large of a chainring can be used. These are listed by the manufacturers for each particular derailleur. Road doubles will range from 53-56t and triples up to 52t. Mountain bike triples are normally 44t with Shimano making a set up for touring that tops out at 46t. Mountain double is going to be more specific at either 39t or 42t.
Seat Tube Angle
The seat tube angle when in reference to front derailleurs is different than when being referred to in terms of frame geometry. When speaking of front derailleurs, the seat tube angle is measured as the angle between the seat tube and the chainstays. Most derailleurs and bikes will fall into the 66-69 degree range for mountain and 61-66 for road.
This is the measurement on the front derailleurs that use a band to clamp the derailleur to the frame. Common band sizes are 28.6/31.8/34.9, while 38.2 is used on some oversized carbon and aluminum frames. Shimano mountain derailleurs in recent years come with a series of shims so that one derailleur works for the 3 major frame sizes. For this measurement, make sure you measure the outer diameter of your seat tube. A common mistake that is made would be the assumption that the seatpost diameter is the same as the clamp size. This is not the case as the seatpost is measured on the inside of the seat tube while the clamp is measured on the outside of the seat tube. Additional note: 34.9 is sometimes listed as 35mm.
Derailleur Mounting Types
The derailleur type or swing style can be super confusing. Front derailleurs come in Traditional, Top Swing, E-type, Braze-On and Direct Mount. Generally the frame you have will determine which style is going to work best for you, and most frame manufacturers will list the type required on their website. The most common style of derailleur is going to be traditional.
Traditional as its name applies is the oldest style of front derailleur currently available and the most common. Traditional derailleurs are also known as Bottom Swing because the cage for moving the chain is below the clamp band. Road front derailleurs will all be either traditional or braze-on, which is a form of a traditional front derailleur.
Top swing derailleurs are also known as low clamp. They have a lower profile than traditional derailleurs because the clamp is located slightly below or in line with the derailleur cage. These are used exclusively on mountain bikes and are designed to be used on frames where there may be a pivot point, water bottle boss, or some other obstruction that would interfere with the clamp of a traditional derailleur. The low design which helps with clearance from above can be a hindrance on frames with thick or sharply angled down tubes because you may not be able to get the derailleur low enough to shift properly.
Braze-On front derailleurs are a form of a traditional front derailleur but do not use a band to clamp to the frame, instead uses a mount that on steel frames was “brazed onto the frame”. On carbon bikes this mount is either bonded to the frame or welded to the metal in the bottom bracket shell. With the mount built onto the frame the risk of crimping carbon or thin walled aluminum is next to nothing and because the derailleur doesn’t have a clamp it’s going to be lighter. A braze-on can also be converted to a standard traditional front derailleur with the use of a braze-on clamp adapter. Braze-on derailleurs do have a little more flex than their banded cousins because of how the body of the derailleur interfaces with the mount.
E-Type or bottom bracket mount derailleurs are used exclusively on mountain bikes. Most E-type derailleurs require a threaded boss on the seat tube to prevent the derailleur from spinning although some have a bracket that wraps halfway around the seat tube to prevent spinning. Because of the way e-type derailleurs mount there is very little adjustment and if a frame’s chain line is not spot on rubbing can be an issue. E-types require an English bottom bracket and some square taper and ISIS bottom brackets need an e-type specific version while others require the removal of a spacer. Reasons E-Type derailleurs are used:
- Frames that have seat tube angles outside of what the derailleur manufacturers normally spec.
- Frames that have odd or oversized seat tubes
- Full Suspension frames that have low pivot points
- Frames like the Surly Pugesly that have a 100mm wide bottom bracket to make up for the wide snow tires.
Direct mount derailleurs are mountain bike versions of a braze-on – only way more complicated. All braze-on derailleurs from Campagnolo, Shimano, SRAM or FSA will fit all braze-on frames because they use the same standard. Direct mount derailleurs however come in S1, S2, S3 (E2) and H0. They are replacing e-type derailleurs on a lot of modern bikes because they are lighter weight and with press fit bottom brackets such as BB30, e-types will simply not work. The popularity of carbon mountain frames have also increased the need for direct mount derailleurs because like the braze-on derailleurs for road they eliminate the band clamp which can be a point of stress and failure (if over tightened) on the seat tube. Direct mount derailleurs are chainring specific because they can’t be raised or lowered and must be installed before you install the crankset on most frames. Direct mount by style:
- S1: frames are the easiest to determine because at 42.7mm they have the widest spacing between the bolt holes. Example of a frame that uses this mount is the 2009 Specialized Epic.
- S2: frames can be confused with S3 frames because they both use 22.1mm spacing but the mounting surface for the S2 is flush.
- S3/E2: is the standard that Shimano currently uses exclusivly. The S3/E2 will be referred to E-Type without backplate. Most of the current Shimano E-Type can be converted to direct mount by removing the 2 screws that hold the backplate onto the derailleur; these holes will become the mount point to the frame. S3/E2 derailleurs have 22.1mm spacing like the S2 but the forward bolt has an offset of 5mm toward the drive side. Currently some of the companies that use this mount are Pivot, Cannondale, Specialized (with an adapter) and Yeti.
- H0: or High Direct Mount derailleur is designed to fit frames with the older E1 E-type single bolt, because of the location of this mount the H0 uses a traditional style derailleur body instead of the topswing design found on the other mount styles. Rotation is eliminated by the curved back of the derailleur in most cases although some versions use a second bolt.