Shimano

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 fork's dropouts. Steel forks can be stretched or squeezed a few mm but it's always best to avoid this if possible, aluminium or carbon forks should never be stretched.

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.

Teeth - the tooth count has increased with the introduction of more gears, but this happened very gradually so 10 tooth jockey wheels were standard for a very long time.

*Compatible Speed - the number of cogs the jockey wheels were designed to be used with. When using derailleurs for indexed shifting it's important to stick to this, but friction shifting is much more forgiving so you're unlikely to have any problems using the next speed up or down, providing all the measurements are correct.

Diameter (OD) - the diameter of the jockey wheels, from the outside edge of the teeth. It’s important to stay close to the ones you're replacing to ensure they fit.

Thickness - measured at the widest point, by the centre hole. The spacing has generally become narrower over the years to accommodate more gears.

Centre Hole (ID) - the inside diameter (usually inside the bushing/spacer if present). This rarely presents a problem, but some older jockey wheels have a different diameter so it's worth double checking.

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.

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.

Cleat System - the cleat the pedals were designed to fit. The brand of cleats doesn't have to match, as long as they were designed for the same system.

Pedal Threads - this must match the thread on your cranks. Sometimes it's stamped on the end of the crank arm, but most bikes use a standard thread of 9/16” x 20 tpi. The exceptions being older French bikes, bikes with cheaper one-piece type chainsets and Shimano’s Dyna Drive system (easily identifiable as the pedal holes are about an inch wide!).

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.

Bike Type - this is just a guide, any brake levers can technically be used on any bike as long as everything's compatible.

Brake Type - if you’re running direct pull cantilevers (V-Brakes), you’ll need compatible brake levers. For calipers, cantilevers and mini V-brakes you’ll need levers designed to be used with these as the cable pull is different.

Bar Clamp Size - this should be close to the outside diameter of your handlebars at the point the brake levers will be fitted. 23.8 mm is standard for drop bars and 22.2 mm is standard for 'flat' bars (including risers & swept-back types), but they can differ so this should be checked.