Sheldon Brown’s article “Shimano Cassettes & Freehubs” mentions the below, but doesn’t elaborate too much further.
7-speed hubs only accept 7-speed cassettes –though 8 or 9 sprockets can be installed on a 7-speed hub, using 9- or 10-speed spacing. It is also possible to transplant an 8- 9- 10-speed cassette body onto most 7-speed Freehubs.
This left me wondering:
- What derailleur and shifter should I use? The number of true speeds, or matching the cassette?
- Where should the ‘missing sprocket’ be indexed on the shifter?
Searching exactly how to do this yielded questions from others, though without conclusive results.
- r/bikewrench: “9,10 and 11 speed cassette on 7 speed hub”
- r/bikewrench: “Sheldon’s 10 of 10 on a 7-speed body”
To answer these
Q: What derailleur and shifter should I use?
The derailleur should match the number of sprockets the cassette was designed with. For example, if 9 out of the 10 sprockets on an originally 10-speed cassette are used, a 10-speed derailleur should be used. This is because the shifter’s indexing should match the spacing on the cassette. You might be able to get away with a nine speed derailleur, but if you’re buying new parts I wouldn’t try.
Q: Where should the ‘missing sprocket’ be indexed on the shifter?
I would index the smallest (existing) sprocket to the second-to-lowest position on the shifter. To do this, set the low limit screw to the smallest sprocket, to prevent the chain from being dropped onto the axle.
This prevents the possibilty of shifting the derailleur past the largest sprocket into the wheel. Note that it is still necessary to carefully set limit screws.
As a bonus, if the limit isn’t set well and the chain falls onto the axle, then you can shift to the empty position, then back up to the smallest or second-smallest gear to put the chain back on, without needing to dismount.
If I got anything wrong, or you have any questions, feel free to send an email to hello[at]ben-p.dev