I currently have a 1x7 setup on my mtb, originally converted from a 3x7, as you can imagine. I'd like to replace my 7 gear cluster to an 8 or 9 gear cluster. Will this be nothing more than a simple swap of clusters? Or will I run into trouble putting a larger cluster on a 7 gear hub (if there is such a thing), and will I have enough width between the stay arms at the back to accommodate the larger cluster?
Imagine you have a splined cassette hub, common these days on newer wheels. Most newer Shimano standard hubs will take at least 7-10 even 11 sprockets these days, give or take inexpensive plastic spacers to define the precise position of a cassette on a hub. Meaning you could slap a 9-speed cassette on your freehub that formerly carried your 7-speed cassette and have your machine converted from an n x 7 to an n x 9.
None of such. There are several reasons why this is just a nice dream.
- the 7-s chain is wider than the 9-s chain and will rub against and wedge between the two neighbouring sprockets, although quite often 7/8/9-s chains mostly fit either,
- but to fit the 9 sprockets in the same space as the previously 7, the sprockets are more tightly packed and thinner, 3 meaning that the motion of the RD is smaller, the steps are shorter, implying that both your shifter lever and your RD must be from the same speed-group to work with a 7-s, 8-s or 9-s system.
Conclusion: The three components belong to a family, shifter, cassette and derailleur. The FD is a component of a lesser level, fitting 7, 8, 9 speeds. You can't mix groups in speeds. Shimano does have a data-sheet on a page that informs on (in)compatibilities. It's not the letter of the law, some things may work but others surely won't.