I am looking at the 2009 Rockhopper SS. Down the line, if I wanted to make this a 1x9, is that possible? Or is a singlespeed bike frame more narrow in the back and can't accept a 9 gear cassette for example? In other words, is a stock SS, SS for life?
Rear spacing on modern SS bikes is the same as geared. You may need a new hanger as C Dunlop mentions, but that should be easy to get. Depending on the rear wheel you have, you may or may not need a new one of those too. If it's a standard cassette hub with spacers and an SS cog, pull the SS cog off and slide a cassette on. Done. However, if it's a freewheel type hub, or an SS specific freehub, you'll probably have to replace the hub/wheel to go geared.
Cable routing for gears helps, but isn't necessary. Full length housing and tape will work in a pinch. You can also get stick-on housing stops that make it a bit easier.
Of course, you'll also need to buy all the gear bits.
You could also consider an internally geared hub. If you have to replace the rear wheel anyways, costs probably wouldn't be much different, and you'd preserve the looks and low maintenance of an SS, albeit with a weight penalty
Just as pretty well any geared bike can be converted to SS, pretty well any SS bike can get gears on it, although the former conversion is certainly cheaper than the latter.
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