Rear mechs

The rear mech or rear derailleur is basically a spring mechanism which moves your chain from one sprocket to the next and also provides the tension needed in the chain to stop it slipping when under pressure.

The first and most obvious thing you need to know when choosing your rear mech is whether you need a road or an off-road mech (hybrid bikes can be either).

Road Mechs

Road mechs are normally lighter than off road mechs but the lack the strength. Please follow the steps below to get the right derailleur for you...

What Gear Shifters do you have?

If you are looking for a rear mech you need to base your choice on which shifters you have. Shimano and Campagnolo work on different ratios meaning that you need to run a Shimano mech with Shimano shifters and a Campagnolo mech with Campagnolo shifters.

How many Sprockets do you have on your Cassette/Freewheel?

The next stage to getting your rear mech is simply counting the number of sprockets on your cassette/freewheel. These will generally vary from 5 speed up to 10 speed, 5 - 8 speed will work with a 8 or a 9 speed derailleur where as 9 speed requires a specific 9 speed mech as 10 speed does with a 10 speed rear mech.

Which Cage length do you need?

Every rear mech has a cage underneath it to guide the chain with using the jockey wheels. The length of this cage is important and it is generally best to stick with original length your bike came with.

You get the choice of a short or medium cage; a short cage will normally be more effective however will only work if you are running a double chainring setup. The reason for using a medium cage is that the chain needs to be able to move a lot more with a triple chainring setup, because of the larger range of gears.

Please note that you don't need to worry about thread which screws the mech to the hanger as this is a universal fit.

Off-Road Mechs

Off road rear mechs can take a lot of abuse as they are in a vulnerable position on the bike. They gradually getting lighter but must do so without compromising on strength. Please follow the steps below to get the right derailleur for you...

What Gear Shifters do you have?

If you are looking for a rear mech you need to base your choice on which shifters you have. Shimano and SRAM work on different ratios meaning that you need to run a Shimano mech with Shimano shifters and a SRAM mech with SRAM shifters.

How many Sprockets do you have on your Cassette/Freewheel?

The next stage to getting your rear mech is simply counting the number of sprockets on your cassette/freewheel. These will generally vary from 5 speed up to 9 speed, 5 - 8 speed will work with any speed of derailleur where as 9 speed requires a specific 9 speed mech.

Which Cage length do you need?

Every rear mech has a cage underneath it to guide the chain with using the jockey wheels. The length of this cage is important and it is generally best to stick with original length your bike came with.

You get the choice of a medium or long cage; a medium cage doesn't hang down as low and therefore is less lightly to get damaged, however will only work if you are running a double chainring setup, which is unusual unless you have a bash ring. The reason for using a long cage is that the chain needs to be able to move a lot more with a triple chainring setup, because of the larger range of gears. The vast majority of mountain bikes will use a long cage.

Please note that you don't need to worry about thread which screws the mech to the hanger as this is a universal fit.