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Yesterday's 'screw-on' freewheels have been superseded by various (mutually-incompatible) designs of 'freehub' and 'cassette'. There are two main groups: Shimano types with 9 splines and Campag ones with 8 splines - but there are also variations within these groups. The most common type is the Shimano 8/9 speed pattern and replacement sprockets are readily available for this.

Alternate sprockets and spacers slide onto freehubs and are usually retained by a lockring. Most sprockets are interchangeable, making it easy to customise your gear ratios and replace worn sprockets.

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Modern freehubs measure 130 or 135mm between their locknuts but, with care and the right tools, most older bicycle frames can be widened to accept them. You can often adjust the hubs themselves by adding or removing spacers from under their locknuts, and the spacers can be moved from the left to the right end to increase chain clearance if your chain rubs your frame in top gear.

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The following table identifies some modern freehubs:

 

	freehub	splines	body length	sprocket compatibility
	Shimano 10 sp (alloy)	9 deep	35 mm	x
	Shimano 10 sp (steel)	9 shallow	35 mm	Shimano 8/9 sp
	Shimano 8/9 sp	9 shallow	35 mm	Shimano 8/9 sp
	Shimano 7 sp	9 shallow	31 mm	Shimano 8/9 sp (*)
	Campag 9/10/11 sp	8 deep	38 mm	x
	Campag 8 sp	8 shallow	36 mm	x
	Mavic 10 sp (M10)	9 shallow	38 mm	Shimano 8/9 sp
	Profile 6 sp	9 shallow	24 mm	Shimano 8/9 sp
	Profile 3 sp	9 shallow	11 mm	Shimano 8/9 sp

( x )  incompatible with anything else
( * )  Shim 8/9 sp sprockets can be modified to fit

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Cassettes

Most shops only sell ready-assembled 'cassettes' of sprockets. These work perfectly so long as everything else in your setup matches as the manufacturer intended, but they are only available in certain combinations of sprocket sizes.

If you can't find exactly what you want, you can make your own by breaking down cassettes and mixing their sprockets as required (see below).

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How many sprockets can I fit?

The precise number of sprockets you can fit on your hub depends on your rear derailleur and gear shifter. You may have to sacrifice a sprocket or two if you mix different types of hub and gears, as described under Indexing. The following table shows how many correctly-spaced sprockets you can fit on your hub:

 

				Shim 7sp gears	Shim 8sp gears	Shim 9sp gears	Shim 10sp gears		Cam 8sp gears	Cam 9sp gears	Cam 10sp gears	Cam 11sp gears
	Shim 8/9/10 sp hub Shim 7 sp hub			7+ 7	8 7	9 8	10 9		8 7	8+ 8	9 8	10 9
	Cam 9/10/11 sp hub Cam 8 sp hub			7+   7+	8+   8+	9+ 9	10+ 10		8+ 8	9 9	10   9+	11 10
	Mavic 10 sp hub Profile 6 sp hub Profile 3 sp hub			7+   5+ 3	8+ 6 3	9+   6+   3+	10+ 7 4		8+   5+ 3	9 6   3+	10 7   3+	11 7 4

( '+' means you must add some packing spacers before the largest sprocket )

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Don't space your sprockets evenly!

The same principle applies for all cassettes, but consider a 6 speed one starting with 12 teeth and ending with 32 teeth:

12-16-20-24-28-32 is evenly spaced (each sprocket adds 4 teeth).

Shifting into bottom gear to climb a hill (28t to 32t) would only help a little, whereas shifting into top gear on the flat (16t to 12t) would be almost insurmountable. This is because the percentage jumps between these sprockets are 14% and 33% respectively.

12-14-17-21-26-32 would be a much better cassette.

This has percentage jumps of 17%, 21%, 24%, 24% and 23% which is as even as can be achieved - unless, that is, you find a way to replace the 12 tooth sprocket with an 11½ tooth one!

In mathematical terms, a geometric progression is better than an arithmetic one.

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Step-by-step instructions

NOTE: I no longer make Cassette Conversion Kits - sorry!

First of all - if you have an old-fashioned screwed hub that takes a screw-on freewheel, then you can't change its spacing and must use gears that match your freewheel. And no, I'm afraid there's no such thing as a screw-on cassette adaptor for your hub!

If you have a splined freehub, then you can use it with any gears you like - for example, you may have a Campag 8sp hub and use Shimano 9sp gears...

1. Buy a cassette which fits your hub. It doesn't matter whether it's a 7,8,9 or 10 speed cassette, but make sure it consists of individual sprockets and spacers (not an expensive cassette with some of the sprockets rivetted onto an alloy carrier). You can even buy several cassettes and mix the sprockets in order to achieve your ideal set of gears.
    
2. Dismantle the cassette to separate the sprockets:
         Some cassettes are clipped together - remove the clips.
         Some cassettes are bolted together - unscrew the bolts.
         Some cassettes are rivetted together - file the heads off the rivets and punch them out.
   Keep the lockring, but you can discard the spacers, clips, bolts and rivets as they will not be used again. Note that the smallest sprocket has a built-in spacer - it doesn't matter that this can't be changed.
    
3. Order a Cassette Conversion Kit (if someone else makes them today). Select the one that corresponds to your sprockets and gears.
    
4. Replace the sprockets and your new spacers onto your hub, one at a time.
    
5. Add the lockring and do it up tight (if you have a torque-wrench, set it to 40 Nm).
    
6. If the sprockets are loose, add some Packing Spacers behind the largest sprocket. If the lockring isn't long enough, remove a sprocket and use Packing Spacers to take up the slack.
         A couple of thin Packing Spacers are included with every Cassette Conversion Kit.
         One of your old spacers can be used if the gap is very large.
         Or you could purchase a Packing Spacer Kit.

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