Building Cassettes

[TehYoyo]

Can you add and remove cogs of a cassette as needed, provided that they go from large --> small in order?

That is, if I have a 9-speed cassette that goes 11-13-15-16-18-20-22-25-27, but I want a 10-speed cassette that goes 11-12-13-15-16-18-20-22-25-27, can I just add the cog in, provided I switch out my derailleur and shifter as well?

[UrbanPrimitive]

I would assume the spacing between cogs will be different. Otherwise adding another cog and spacer would result in the need for a new cassette body for your hub. Since we're talking about a fixed amount of space simply adding a cog would result in a cassette that's too long. It's why ten speed is not compatible with eleven speed (different cable draw is needed to account for the closer spacing of the cogs within the limited space).

This is off the top of my head, without any recent research. Others may correct me if I'm wrong, but my hunch is it won't work without replacing the spacers. Even then the cogs may not fit the ten speed chain.

[old_fuji]

Nope, spacing will be off.

I couldn't get a similar setup to work on my MTB.

[ratherBclimbing]

You might be able to get 10-speed spacers, but even then the shifting action isn’t going to be as smooth as it was designed to be. The grooves in the sprockets that help the chain smoothly transition on and off are designed to work with specific sized sprockets on each side. You’d also need a new chain, btw. If you’re going to pay all the money to change derailleur you might as well buy a 10-speed cassette too.

[cxwrench]

you're getting mostly good advice from the first couple of posters. i'm assuming you have Shimano components.
1) for the reasons stated above you can't just 'add' a cog to a 9spd cassette and have a 10spd cassette. they both occupy the same amount of space on the freehub, so the 10spd cogs are thinner and the space between them is smaller.
2)you don't need to change the derailleur to switch to 10spd, just the shifter, chain, and cassette.

[TehYoyo]

Oh OK. Thanks, cxwrench (and everyone else, of course).

You would, however, need to adjust the screws on the derailleur, no?

[ratherBclimbing]

Oh OK. Thanks, cxwrench (and everyone else, of course).

You would, however, need to adjust the screws on the derailleur, no?

You're going to have to tune your derailleur of course. Technically I suppose there's a chance that the limit screws wouldn't really need to be adjusted since the cassette is still the same width. Same with the b-screw since you're not getting a larger top cog. Theoretically the new 10-speed shifters should work within the constraints of the derailleur once you get the tension set properly - that said, if I changed out new shifters I'd expect to just have it up on a stand and tune it up proper.