9 speed cassette and 7 speed shifter So I bought a new (used) wheel set for my mountain bike and it came with a 9-speed cassette. I currently have 3x7 rapid shifters, which presents an incompatibility. I'm wondering how to fix this problem. The way I see it my options are


Replace my shifter with a 9 speed shifter
Replace my cassette with a 7 speed cassette and an extra spacer.
Replace the spacers with 7 speed cassette spacers and remove some sprockets.
Get a 9 speed chain and roll with it.


I'm not sure what the best approach is but I'm wondering if anyone has tried number 4. I know that there will probably be issues wit shifting because the stops on the shifter are bigger than the travel between sprockets, by I'd like to know if it is manageable or not. Any advice? Failing that what is my best approach?
Option #4 won't work - the spacing between the cogs is different for 7,8,9 speed (4.34 mm for a Shimano compatible cassette for 9 speed, 5 mm for 7 speed Shimano). An index shifter is designed to move the chain over by a certain amount based on this spacing, which is why you have to typically match the number of speeds on the shifter with the cassette. Note that a 7 speed, 8 speed, 9 speed chain can usually be used on any of the 7/8/9 speed setups though (not necessarily optimally, but it usually works out with a bit of adjustment). 

The cheapest solution is to just buy a 7 speed cassette and put it on, and is likely the best (#2). 7 speeds chosen appropriately is more than enough for almost everyone. 

Disassembling a cassette is a hassle and its likely you won't get the spacing correct anyway, so #3 is out. 

Option #1 is a decent option given 9 speed alivio or whatever shifters are not too expensive now, and you get a fresh set of cables if you buy a set of Alivio shifters. But, if you have brake-shift combined units, you may also have to buy new brake levers. Its also more work, but if your shifters aren't working too well, this is a good choice - there isn't much on the market for good quality 7 speed shifters anymore, so if you're going to be using the bike a lot, this may be a good choice (I just upgraded my 7 speed commuter to 8 speed because the shifters went). 

To conclude, option 2 (installing a 7 speed cassette) or option 1 (installing 9 speed shifters) are the only ones which would work. Option 2 is cheaper (and I'd likely go for this if the shifters are in good working condition), but option 1 is a decently reasonable one as well (and is recommended if it looks like the 7 speed shifters will go).