Change Cassette
Need Expert Advice on changing sprockets of a 7 speed cassette body of a Road bike?
My current 7 speed cassette sprocket configuration ratio is as follows: 14-16-18-20-22-24-28. What i need is this type of ratio: 12-14-16-18-21-24-28. So basically i want to buy a 12 and 21 tooth sprocket and remove the 20,22. I have never delt with bike repairs all that much. I need to know if this kind of change is possible? Or do I need to buy an entirely new cassette? Are there any other precautions I should consider?
It might be much simpler to look for a new cassette in the cheapie variety. Nashbar has a big sale right now and you might be able to pick up one of their house brand cassettes that has what you want. You can drive out the rivets and separate cogs....you may or may not need to pick up some spacers from the bike shop....depends on how the cassette is put together.
I'm not sure if Sram still offers these parts, but as recently as two years ago they still listed individual cogs for replacement in just about every tooth. Most of Sram's cassettes have the upper cogs held together with three teeny screws rather than the rivets everyone else uses, so it makes it easy. You might check with a shop to see if they can still get them, or pick up a cheap Sram cassette on sale, or even ask shops if they have any in the scrap bin (I've seen a number of them go out for recycling along with all the broken parts).
Since most mfr's use the standard Shimano spline pattern, as long as you can get the cogs you want, you can assemble your gearing the way you like it. The only thing you'll need to be concerned about is using the same size spacers if you need to pick up extra ones.
I'd respectfully disagree That You should replace other parts in your drive train right now...unless they're verifyably worn out and need it. In my experience, cogs last a looong time. It's the chain and rings that need replacement more frequently. I've never kept track, but it seems like I'll go through 4 or 5 chains, 2 rings, and one cassette before everything is new at the same time. The smartest thing to do is actually measure them for wear...you can get the tools or you can have a shop do it for you (usually for free).
Good luck with the search! 
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Changing your bicycle's cassette / gears
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