I have a compact double with a 12-25 cassette on a CX bike that is mostly ridden on the road. There are some long and very steep rides that I'd normally do on my MTB, but would like to try on the CX bike with knobbies. I sit and spin on the MTB and there is no way I want to grunt the CX bike up the same ride with the gearing it has.

I've bought a new 10spd XT cassette and new Wippermann chain. I'll swap the Ultegra RD on the bike for an XT from my 9spd MTB's.

The existing 10spd drivetrain on the CX bike only has ~200mi on it. I was thinking I'd size the new Wippermann chain for the new cassette and swap cassette/chain/RD together on the infrequent occasions I want the low gearing. But would it make sense/not-be-a-worse-idea to just splice in the extra few links I need into the existing chain when I need it? A few thousand miles down the road that might mean a stretched chain that has a few unstretched links in it, but will that be noticeable/cause extra wear? Cassette and chainrings are a couple year old Shimano Ultegra.

I don't expect to use the large cassette much so thought the major benefit of splicing in would be to not have to maintain a dedicated chain. That dedicated chain could always be resized down to the 12-25 cassette when it needs to be replaced though.

I would save the existing chain for the existing cassette and use a new chain with the new cassette.

Unless your new cassette exceeds the RD's wrap capacity, there is only ONE chain length that will work with both cassettes. In most cases, there is no reason to use a shorter chain with the 25T large cog.

Using the little/little method suggest the longest chain within the RD's wrap capacity. If the big/big method, with the larger cassette suggests an inch longer chain, then you have exceeded the RD's wrap capacity and the chain would hang loose in the small chainring and one or more of the smallest cogs.

Thanks for both responses.

I'm not averse to leaving the XT derailleur on it permanently, though I did have a spare set of derailleur cable quick-disconnects (Ritchey BreakAway CX frame) that would make swapping back and forth simple and fast. I'll try the Ultegra RD first though and see if it can handle the excess links though. Regardless, I'll skip the idea of temporarily splicing in links.