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Chainset Type - the number of chainrings the derailleur was designed to work with. ‘Double’ derailleurs are actually capable of working on ‘triple’ chainsets but they may struggle when shifting, especially if there’s a big step down from the middle to inner ring, so it’s better to match these correctly.

Frame Fixing - the way the derailleur fixes to the frame, if you have a tab on your frame to mount the derailleur to you need braze-on. If there’s no tab then you require a clamp-on type (also referred to as band-on or clip-on), these come in different sizes to match the diameter of your seat tube.

Pull Type - the direction of which the cable needs to ‘pull’ the mech in order to shift. If your cable runs up to the derailleur from the bottom bracket area then you need bottom pull, if your cable comes down from above you need top pull. Some later derailleurs can be used for either setups, these are referred to as dual pull.

Designated Speed (9 Speed & Above) - the speed of the chain that the mech was designed to be used with. This only becomes relevant for mechs marketed as 9 speed or above when chains became narrower, as the derailleur cage plates became marginally narrower too. We refer to it as ‘designated’ speed as technically you can use (almost) any derailleur with any chain, however if you want optimum performance it’s best to stay close to the manufacturers recommendations.

Indexing System (Shimano Only) - this is only applicable when using Shimano shifters that have an indexed left shifter such as STI or Rapidfire. For the indexing to work correctly when using these shifters the derailleur and shifters must be of the same type, so Shimano drop bar shifters need to be matched with a Shimano ‘road’ derailleur, and Shimano flat bar shifters must be used with Shimano ‘MTB’ derailleurs.

Shifting system - friction means your gear levers don't click when you change gear, when they do click it's referred to as indexed shifting. If you have indexed gear levers they must use the same system as the rear derailleur, if you're using friction levers you can mix and match.

*Speed - the number of cogs at the back the derailleur was designed to be used with. This is important with indexed shifting, but with friction shifters you can in theory run any rear mech with any number of cogs, but you are more likely to have issues running high numbers of cogs with mechs from earlier eras.

Max cog size - the amount of teeth on the largest cog of your freewheel/cassette can be no greater than this to work with this derailleur.

Chain wrap - subtract the size of your smallest rear cog from your biggest, then do the same with your chainrings, add the 2 numbers together and you have your chain wrap.

Chain width - generally speaking, derailleurs designed for more than 3 speed use will fit a standard 3/32'' chain, the exception being some Cyclo & Simplex models from around the mid 1900s.

Spoke Drilling - the number of holes for the spokes, your wheel rim needs to match the hub exactly.

Spacing (O.L.D.) - the width of the hub measured at its widest point, from the outside of the locking end nuts (O.L.D. = Over Locknut Dimension), not including the hub axle. This measurement needs to match the inside of your frame's dropouts. Steel frames can be stretched or squeezed a few mm but it's always best to avoid this if possible, aluminium or carbon frames should never be stretched.

Speed - the number of cogs the hub was designed to be used with.

Sprocket Fitting - for cassette hubs this refers to the spline pattern (the shape of the cogs where they slide on to the hub) and this must match the cassette you intend to use. With freewheel hubs it refers to the thread, ISO is the modern standard and is almost exactly the same as older English/British and Italian thread, allowing you to mix and match between these thread types as long as it's not going to be under extreme force, in which case it's best to stick to an exact match. Freewheel hubs with a French thread must be used with a French threaded freewheel.

Speed - the number of cogs on the cassette.

Spline Pattern - the shape of the cogs where they slide on to the freehub body, this must match the spline pattern of your hub to able to fit. In the 1980s Shimano introduced Uniglide cassettes that share the same spline pattern as their now standard Hyperglide, but the smallest sprocket threads on to the hub to secure it rather than using a conventional lockring, these will only fit hubs with a threaded section on the outside of the freehub body.

Range - the number of teeth on the smalles and largest cogs, this gives you an indication of how hard or easy the gearing will be, the lower the number the harder the gears. The tooth count of the cogs inbetween are usually evenly spaced, ie: a 6 speed 14-24 freewheel would likely have cog sizes 14, 16, 18, 20, 22 & 24.

Speed - the number of cogs on the cassette.

Spline Pattern - the shape of the cogs where they slide on to the freehub body, this must match the spline pattern of your hub to able to fit. In the 1980s Shimano introduced Uniglide cassettes that share the same spline pattern as their now standard Hyperglide, but the smallest sprocket threads on to the hub to secure it rather than using a conventional lockring, these will only fit hubs with a threaded section on the outside of the freehub body.

Range - the number of teeth on the smalles and largest cogs, this gives you an indication of how hard or easy the gearing will be, the lower the number the harder the gears. The tooth count of the cogs inbetween are usually evenly spaced, ie: a 6 speed 14-24 freewheel would likely have cog sizes 14, 16, 18, 20, 22 & 24.

Optimal Speed - the ideal number of rear cogs, based on the outer width of the chain and the cog width it was designed for. For 6 Speed Shimano Uniglide a 5/6 spd chain will perform best.

Shifting System(s) - indexed shifters 'click' when you change gear. For 9 speed upwards (or Suntour Accushift), to guarantee the best performance we recommend using the dedicated chain. Otherwise, for 8 speed and below, or any speed when using friction shifting (non-index), you just need the correct outer width.

Outer width - measured across the rivets at the widest point, these are the approximate widths for different speed setups (up to 4 speed is also dependent on cog width):

Cog Width - the gap between the inner plates allows for different rear cog/sprocket widths. 3/32" is the standard for 5 to 8 speed, above that it’s 11/128”. For 4 speed or less, 3/32” and 1/8” were both used, for the latter you will need a 1/8” chain. A wider 3/16" moped chain can be used as a heavy duty opton for single cog setups if required).

Chain pitch - the distance between the rivets/pins. All chains will be 1/2" with two exceptions; 3/16" (“inch pitch”) for very early track bikes, or Shimano's short lived 10 mm pitch system from the 70s/80s.

Number of links - if you're unsure how many you need, measure your old chain (each link has two rivets) or one with a similar setup to yours.

Chainset Type - the number of chainrings the derailleur was designed to work with. ‘Double’ derailleurs are actually capable of working on ‘triple’ chainsets but they may struggle when shifting, especially if there’s a big step down from the middle to inner ring, so it’s better to match these correctly.

Frame Fixing - the way the derailleur fixes to the frame, if you have a tab on your frame to mount the derailleur to you need braze-on. If there’s no tab then you require a clamp-on type (also referred to as band-on or clip-on), these come in different sizes to match the diameter of your seat tube.

Pull Type - the direction of which the cable needs to ‘pull’ the mech in order to shift. If your cable runs up to the derailleur from the bottom bracket area then you need bottom pull, if your cable comes down from above you need top pull. Some later derailleurs can be used for either setups, these are referred to as dual pull.

Designated Speed (9 Speed & Above) - the speed of the chain that the mech was designed to be used with. This only becomes relevant for mechs marketed as 9 speed or above when chains became narrower, as the derailleur cage plates became marginally narrower too. We refer to it as ‘designated’ speed as technically you can use (almost) any derailleur with any chain, however if you want optimum performance it’s best to stay close to the manufacturers recommendations.

Indexing System (Shimano Only) - this is only applicable when using Shimano shifters that have an indexed left shifter such as STI or Rapidfire. For the indexing to work correctly when using these shifters the derailleur and shifters must be of the same type, so Shimano drop bar shifters need to be matched with a Shimano ‘road’ derailleur, and Shimano flat bar shifters must be used with Shimano ‘MTB’ derailleurs.