Think I need some more chain?
 

I got a brand new, shiny XT derailleur and 11-34 cassette in the mail today, and couldn't help putting them on.

https://lh6.googleusercontent.com/-9.../bikechain.jpg

But, how much chain is enough chain?

Edit: That's a Surly CC with stock crank and chain.

Apart from the gear combo big/big not being a good one to be in, did you measure the chain before installing? from the look of it, the chain is probably long enough, just

Nope, just took off the old 12-28 cassette and derailleur, and put these on. It seems to be shifting ok, and I only put it on the big/big combo for the picture. Worst case type of situation.

As you have replaced the cassette, and it's not a new drivetrain, would suggest replacing the chain anyway. Keep the current (fitted) one for use with the 12-28 as it will have worn into that cassette. and a new one for the 11-34.

That chain length looks right to me. What happens when you shift into the little/little?

I was mistaken, the original cassette is a 12-25. So far it's shifting just fine, apart from a few tiny adjustments here and there. Just looks a little funny on the big/big combination.

The chain is plenty long enough, and with that long cage RD it's probably short enough also. The acid test of minimum chain length is the amount of room the RD cage has to swing in the big/big combination. Yours still has room to spare. (test by pulling the lower loop forward a bit).

worst case situation is correct one for testing, i agree.

looks like it barely fits (without damaging anything), as is.

if i had all the time in the world... i would take one link out and see if it would even span that large cog. if not then i would add two more and try again. set it to small/small and see if it was too loose (either falls off or chain rubs on chainstay, or clatters on it with the slightest provocation). if it does then i would set it back to where you have it now and consider it job well done.

No need to take the chain apart. Simply lift the lower loop away from the chainring & pull it forward.

But it's obviously OK since the OP was able to shift into the big/big combination without bending the RD or rear axle, which are the issues of concern with minimul chain length.

To me it looks like you could probably stand to add a bit.

It looks like the derailleur is just barely clearing the big cog. If the pulley and cog are interfering you may need to adjust the 'B' screw. But if it's running smoothly, it's ok.

Disclaimer: I am not a professional mechanic.

Shift it to little/little. If the lower part of the chain almost touches the leading edge if the derailleur but doesn't, you're good. You "might" be able to add one link(actually two segments) but then the chain might drag.,,,,BD

That's what I use when installing chains, and I can't really think of a time it failed. Maybe it just never happened yet?

You might also shift the chain to the big chainring and the littlest sprocket. In this configuration the center of the guide pulley should be directly above the center of the tensioner pulley. That's Shimano's recommendation for determining correct chain length.

Regards,
Bob P.

If you can shift into the big/big combo then the chain is long enough. Don't shift into that while riding though, as you probably know. But if you ever do accidentally get into that gear you know it will work. If the chain were too short then the drivetrain would jam or rip itself apart when you tried to go into the big/big. Since it goes in you are good to go.

As mentioned above, if you have done any significant mileage on the old chain consider replacing it with a new one for use with the new cassette.

FB is right. See if the derailluer cage still has room to travel farther forward before hitting it's limit. If so you are good to go. Don't break the chain apart if you don't need to.