The front derailleur, or front mech as it is more commonly known, is the device just above the chainset which moves the chain on to the large or small chainring at the front, ie changes your front gears. It is operated by a cable which pulls a cage a certain distance away from or towards the frame moving the chain at the same time. Most mechs tend to be indexed in that they move the chain a certain distance without sending the chain over the big chainring or sending it too far in and off the smaller chainring. There is a certain amount of leeway built into the changing process which allows riders to move the mech slightly to avoid chain rub on the mech itself without changing gear, a process called ‘trimming’, depending on which gear you are in on the back and the angle of the chain from front to back cogs.
Front mechs are mostly attached to the frame via a clamp which goes around the seat tube of the frame. The diameter of the seat tube determines the size of the clamp you need but most modern frames are 34.9mm wide and supplied with a shim for thinner diameters. The number of chainrings the mech has to move across determines whether you need a double, two chainrings, or triple, three chainrings, as the more chainrings the more the mech has to move.
Front mechs all perform the same job but although they may look similar they can operate very differently so care must be taken when buying as a number of factors determine which type you need. Here are the different versions of front mechs:
Conventional – traditional type where the cage hangs down from the clamp around the seat tube
Top swing – the cage sits above the seat tube clamp, better for full suspension bikes where clearance may be an issue on curved tubes or pivot points
E-Type – mounts via the bottom bracket shell rather than a clamp round the seat tube
Shimano Direct Fit – bolts directly on to a special tab on the seat tube
Top pull – the cable comes in from above pulling the mech over from the top
Dual pull – cable can enter from above or below to pull the mech over
It is possible to mix front mechs from the same manufacturer as long as they are designed for the same number of gears, ie you can use a 10 speed XTR mech with an XT 10 speed groupset as they are both 10 speed compatible. Some front mechs will work across different numbers of gears but it can be tricky to set up so it’s always easier to run a matching mech with the specified amount of gears. Some front mechs also work with other manufacturer’s systems so check individual product specifications for more details.
Shimano are the world’s biggest cycle groupset manufacturer making front mechs for all types of bikes and riders. From professional riders to shopping bikes, Shimano make a front mech for it.
The hierarchy of Shimano off-road front mechs from professional to entry level is as follows
Cross country -
SRAM are the newest groupset manufacturer and have come up with a series of ingenious inventions which have quickly gained a wide fan base from professional to recreational with their alternative view on operating systems.
The hierarchy of SRAM off-road front mechs from professional to entry level is as follows: