Bolo Grubb

ok let me see if I can explain this.

For 1 bike I have 2 wheel sets. Should I expect to be able to swap the rear wheels with no adjustment to the derailuer?

Both wheels have a 9 speed cogset. One cog set is a shimano 12-25 the other a SRAM 12-26.


When I put the shimano cog wheelset on the bike for trainer use, I got some cog skipping in the largest 2 cogs.

what is causing this? and How can I fix it? Or is this normal when switching wheels.

Al1943

It depends on what kind of skipping you've got. If it acts like it's trying to shift from one cog to another then you can correct this with a cable adjustment by turning the barrel adjustor. Counter-clockwise moves the derailleur toward the frame, clock-wise moves it out. When you change wheels again you may need to re-adjust the derailleur.
Or it's possible the the offending cogs are worn out.
And check your chain for stretch. Replace if a 24 pin length measures as much or more than 12 1/16th inches.

Al

well biked

Unfortunately, even with the small difference in a 25t and 26t large cog, you'll probably have to do a little tweaking of the B screw on the rear derailleur (the b screw adjusts the angle of the derailluer body, giving the desired small gap between the upper pulley and the large cog) when you switch back and forth between wheelsets. Since the two cassettes are so close to being identical (the SRAM cassette is "Shimano compatible", so that aspect makes no difference), I'd determine which one you like best and buy another one just like it, that way you should have complete compatibility between the two without having to make any adjustments when you swap wheels. I've got two identical wheelsets for a mtb I have, with identical nine-speed casettes on them (I use one for road tires, one for knobbies), and I can switch between the two with no problems-

Bolo Grubb

thanks, it is possible that the shimano cassette is somewhat worn out, but I doubt it. The chain is fairly new.

DMF 

Or the chainline isn't the same for the two cassettes, in which case you could shim one.

lennyk

I have similar setups with my indoor trainer wheel,
shimano hg 12-25 and sram 12-25 they both work fine
so I suspect could be wear since the cog spacings should be the same and not affect the derailleur once the high and low work

Rowan

It sounds like cogset wear if the chain is new, and the Shimano cassette is more worn than the SRAM one.

tomacropod

I've fixed this problem in the past by respacing the axle slightly on the two hubs. Often the freehub to locknut distance will be (slightly) different with different hubs. I just use thin washers to do the respacing. The total change is typically less than 1mm.

- Joel

tonyt

+1

Also, are the hubs identical? If not this may be causing slightly different alingment. Even with identical hubs the exact alingmernt will depend on axle spacing. Shim as described above and/or respace slightly to get equal distance from small cog to outer of driveside locknut.

Since Sram and Shimano index the same I doubt this is the root of the problem (UNLESS the cogsets sit slightly deeper/higher on the freehub body). If it was then you would have a real problem like you get when mixing shimano/campy cassttees/shifters