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I could run 8speed cassette with 9 speed XT mech, and for example an ultegra compact, with an 8spd chain and it would work fine?
Very unlikely to cause trouble. In theory, a wide chain can be unhappy going through a narrow RD pulley cage but you'd probably have to be running 7-speed chain in an 11-speed RD for it actually to happen.
gbj_tester-
Okay cheers. I mean I'm running an 8spd chain through a 9spd rear mech at the moment and it's fine so it's more about the 10 speed up front that I was worried about, I've heard running a compact 10speed with appropriate front mech and 8speed in the back can make it easy for chain to rub on the front mech due to wider chain and narrower front mech cage.
I don't like the bigger jumps between sprockets on the 11-34 8spd cassette, but could always have a touring cassette and a normal cassette.
zooeyzooey
I imagine running the wider chain would be fine though? So I could run 8speed cassette with 9 speed XT mech, and for example an ultegra compact, with an 8spd chain and it would work fine? Front mech is friction, rear is indexed or friction, but mostly run it indexed on current set up.
Could always go 10speed all round and just set bar ends to friction, but I'm a bit scared of thin, expensive chains.