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The Derailleur is regularly referred to as ‘Mech’, short for ‘Derailleur Mechanism’.

Derailleurs are the part of the drive train responsible for moving the chain between different sprockets to change the gear.

Rear Mechs

The rear mech controls the gear shifts at the rear of the bike, between the sprockets of the cassette/freewheel. It also provides tension to the chain, preventing it slipping when under pressure.

It is important to realize that although they perform a similar task, road and off road mechs are different, so when purchasing a rear mech it must be appropriate for the intended use. Road mechs usually have a shorter cage and are lighter than off road mechs. This is because a longer cage takes up more chain slack, which in turn allows the chain to work across a wider range of gears needed for off road riding. An exception is when a triple set up is used on a road bike, as this too requires a long cage rear mech, for the same reason. Specific road mechs are made for this purpose. Off road mechs are built a little tougher, as they have to be quite resilient being situated in such a vulnerable position.

The following points contain the key information you will need to make sure you purchase the correct rear mech.

What Gears Shifters do you have?

Because the different manufacturers use different amounts of cable pull in their shifters, it is not usually possible to mix and match gear components. For best performance, choose the mech to match your gear shifters. The most likely choices will be Shimano, Campagnolo or SRAM on the road, and Shimano or SRAM on a mountain bike.

How many sprockets does the rear mech need to shift across?

Count the number of sprockets on your cassette/freewheel. As of recently road bikes can have anything from 5 right up to 11 sprockets. On a mountain bike it could be up to 9, and up to 10 on a hybrid bike.

With lower numbers of gears, there are more compatibility options, so your purchasing decision will be more open. As technology has introduced more gears, so the equipment used needs to be more precise, and mechs need to be specific to the number of speeds, in order to shift accurately.

One rule that you can generally follow is that a mech capable of shifting more gears than you require, will usually work. i.e. a 10 speed mech will work on 9 speeds, and 8 and so on. However it doesn’t work the other way, so if you have a 10 speed road bike, it requires a specific 10 speed mech, anything else will not shift properly. Currently 11 speed is only available for road use, from Campagnolo.

What is the size (no. of teeth) of the largest cassette sprocket?

Count how many teeth the largest sprocket on the cassette has. Note this number and check that you are not going to be shifting the mech to a size beyond its capacity. For example, the maximum sprocket size recommended for many road mechs is 27 teeth.

What cage length do you need?

The ‘cage’ refers to the part of the mech that hangs down, with the jockey wheels held within it. Its length varies according to the set up that it’s designed to be used with. A long cage mech is required when using a triple chainset, as the mech needs to take up more chain slack in the smaller gears. Long cage mechs are used almost all the time on mountain bikes. A double road set up uses a short cage mech. You will occasionally come across ‘medium cage’ as an option in some products and this is recommended for use with larger cassette ratios, e.g. 12- 29 teeth, or to keep tension if you are using compact gears on the chainset, where the inner ring at the front is smaller.

Front Mechs

The front mech is responsible for shifting the chain across the chainrings at the front, on the chainset. Because a shift here is usually only between 2 or 3 rings, the shift is more dramatic and will make a much larger difference to the gear ratio.

It may only perform a relatively simple task, but the front mech is perhaps the most complicated purchasing decision of all components, as there are so many varieties and sizes.

When selecting a front mech for your bicycle you need to know the following information:

What type of fitting is required?

  1. 'Braze-On': Where the front mech bolts directly to a slotted mounting plate which extends from the seat tube of the bike. There are not different sizes for this type of mech which makes it easy to purchase the correct product. Also, adapters are available to convert a braze-on mech into a band fitting as below.
  2. 'Band-On': Where the mech has a clamp that closes around the seat tube. The size required depends on the diameter of the seat tube on the bike. You must measure this accurately to get the size, or best is to have the old mech as the size is usually stamped on it. There are three common sizes, 28.6mm 31.6mm and 34.9mm although manufacturers now make multi-fit options that just use shims, included, to adjust the band size.

  1. 'Band On': As per the road bike above, available int he same three sizes.
  2. 'E-Type': Where the mech is part of a plate that fits onto the bottom bracket shell. These mechs are most commonly used on full suspension designs where the shock position, frame shape or pivots, mean there is no space for a front mech to attach to the frame tubes.

What type of Chainset are you using?

Mountain bikes almost always use a triple front chainring set up, so this is not usually in question, but on a road bike it is possible that you might have one of three common set-ups.
  1. Standard Road 'Double', i.e. two chainrings, with sizes 42/52 or 39/53 being the most likely.
  2. Road Compact, where there are two rings but both smaller in size, i.e. 34/50 or similar.
  3. Road Triple, with, as its name suggests, three chainrings.

It’s important to get a front mech suited to the ring size and number of chainrings that you are using. A triple ring front mech has a longer cage to cope with the range of gears.

Does the mech swing from the top or the bottom?

This is really only applicable to mountain bikes, but some frame designs require the clamp to be positioned low down, below the mech. This is called a ‘Top Swing’ mech. Previously the clamp was much higher, positioned above the body of the mech, and this is called ‘Conventional’ swing.

Where does the gear cable come from?

If the cable comes from above, then this is classified as a 'top-pull' front mech. If the cable comes from below, it is classified as a bottom-pull. Mechs termed as "multi-pull" will work from above or below.

This Buying Guide gives our customers general advice on Derailleurs & Mechs. It is a guide only and we always recommend visiting one of our stores or contacting one of the experts in our sales team on +44(0)1293 574 905 if in doubt about your needs.
This story was last updated on 17/01/2012