Swapping cassettes
 

On some thread I heard someone say about switching from road to mountain cassettes a spacer was neccessary for proper alignment of the other cassette.
I'm buying a Trek 920 that comes with a Sram PG1030 11-36 cassette. I also bought a Sram PG 1070 12-28 so I would have tighter gearing on pavement. So can I switch these back and forth as I choose, or is some sort of spacer needed for proper alignment of the other casssette?
Thanks

do they have the same number of gears? if so, then you probably don't need a spacer, although you might have to adjust the b-tension screw on the derailleur for optimal operation. I would say you need a long cage derailleur, but if the bike comes with the 11-36 then it most likely already has one. so you should be able to just drop in the new cassette and go, although again, some tension or position adjustments might be necessary to make it work perfectly

If there is significant mileage on the existing chain, you may want to consider getting a new chain for the new cassette and keeping the cassette/chain pairs together. This will allow you to run an optimum-length chain for each cassette (the 28 tooth can run a shorter chain) and eliminate a possible issue with a somewhat-worn chain not playing well with the new cassette. You will still likely need to make minor indexing, limit- and B-screw tweaks when you swap chains/cassettes; this is true even when switching like for like.

On my Cannondale touring bike with 9 speed Shimano I interchange between 13-25, 12-27 and 11-24 cassettes. Never change chains, adjust deraileurs or use any spacers and it all works fine.

Shimano 10 speed road cassettes come with a 1 mm shim. As far as I know SRAM does not.

Shimano 11 speed MTB cassettes are supposed to fit on a 10s freehub.
Shimano 11 speed Road cassettes are supposed to require a 11s freehub.

There is also apparently a difference in cog spacing between the two which could cause headaches with indexed shifters.

Yes, you can swap those back and forth with no other changes. The cassettes are the same length.

There is no such thing as a "mountain" or "road" cassette. All 10 speed cassettes are the same length. All 11 speed cassettes are the same length. There is no difference in cog spacing.

However, as [MENTION=409340]ypsetihw[/MENTION] says, a b-tension adjustment may improve shifting.

Yes but no. Shimano 10 speed road cassettes come with a 1 mm shim - MTB cassettes don't.
Shimano 11 speed road cassettes require a 11 speed freehub - MTB cassettes will fit on a 10 speed freehub - at least that's what I've read.

Usually a road cassette will only go to 28 or 30 T. Mountain cassettes will go from 30 to 46 now. More than 30 or so teeth will require a mountain or long cage derailleur.

Correct, but the distinction between an "MTB" and "Road" 10s cassette is in the marketing. SRAM doesn't differentiate, for example. All 10s Shimano/SRAM cassettes are cross-compatible.

Of course, pay attention to the chain capacity of your RD.
Shimano makes this difficult with different cable pull ratios for 10s MTB and Road. You can solve this with a long cage MTB RD (9-speed or earlier) paired with 10-speed road shifters.
With SRAM, their 10-speed road and MTB RDs use the same cable pull, so you can use a 10-speed long cage MTB RD with their 10-speed road shifters.

Everything got screwy again with 11-speed.

Yes but no.

All Shimano/SRAM 10 speed cassettes are the same cog pitch and overall width. They all come with a 1 mm spacer.

I was incorrect on 11-speed though (I have no 11-speed bikes, so I should've kept my mouth shut).
Shimano and SRAM went with different cog pitches for their MTB and Road product lines. Their shifters and cassettes are not cross-compatible between MTB and Road for 11 speed.

11-speed MTB hubs overhang the end of the freehub body, that's how they fit on the smaller 10-speed freehubs.

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