Subject: RE: Bike Tech Question If you mean you are replacing an old cassette with a new one with the same number of cogs, it is a good idea to replace your chain as well. Its the wearing of the chain that causes the 'hooks' in the teeth of the cogs. If you replace your cassette but not your chain, it will wear much quicker than if you replace both. If you replace your chain on a regular basis, your cogs can last for years. You can get a tool for this at your LBS which measures the distance between links and gives a quick go/no-go on the chain. Alternatively you can measure with a ruler. 12 complete links will measure 12 inches under tension on a new chain. If 12 links measure more than 12.25 inches, replace the chain.
If, on the other hand, you are changing from an 8 cog to a 9 cog, you still need to replace your chain. 7/8 speed systems can use the same chain, but a 9 speed chain has narrower links.
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