Alternating from 11-25 to 11-29 cassette - Chain length ?

[Westbank]

I'm currently using a Campagnolo 11-25 cassette with a 53-39 crank. I'm planning to order a 11-29 cassette and to alternate between the 11-25 and 11-29 cassette (depending on terrain). My current chain has been sized for the 11-25 cassette.

I wanted to know if I should be ok switching from the 11-25 / 11-29 while keeping my current chain length ?

[2lo8]

The answer is maybe. Sorry that wasn't a useful answer. If you sized the chain with small-small, then probably. If you sized with big-big then maybe not. Test before imploding your drivetrain.

[Calnago]

@Westbank: as long as you sized your chain using small/small for the longest chain possible, then you’re totally fine. The one exception may arise if you’re using the very first iteration (2009) 11sp rear derailleur. Depending on your hanger length, you may need a slight modification to the rear derailleur ratchet wheel, as they revised it a bit to allow for the addition of the 29tooth cassette, which didn’t happen until the following year. It’s a pretty easy mod, you just need the updated ratchet wheel.
Regardless, when you put in a larger cassette you may need to adjust the “H” screw a bit if you find the upper pulley is too close to the 29tooth cog when you’re in the small ring. Little twist of a screw driver is all that should take.