Bicycle Mechanics - chain length on cassette change

Madone 5.2 2011......have a sram 11-26 cassette on bike now w/ a ultegra 6701 chain...I have a new 11-28 Ultegra 6700 cassette,and dura ace 7901 chain that I want to install,does the new chain need to be longer with the 11-28 cassette? I can always try out the 6701chain w/ the new cassette....any help appreciated. I am a good Shimano mechanic and tuner.......too bad it's fishing reels

The simple math is you add 1/2" (1 link) of chain for every 2 teeth added to the sprocket size, either chainring or cassette. That accounts accounts for the added distance wrapping 1/2 way around.

Unless you measured the chain super close to the minimum size, odds favor the chain being long enough. But don't need to and shouldn't take anything like this for granted. Shift the bike to the big/big combination with the existing gearing, and gently pull the lower loop forward 1/2". If you can, then your chain is long enough.

Yes, test the new cassette with the old chain. Shift into the big chainring and big cassette cog to be sure that the rear derailleur does not "bottom out". With the chain in the big to big combination you should be able to push the upper half of the rear derailleur a good amount before it reaches its forward limit. Shift the chain into the small chainring and small cassette cog. The rear derailleur should not bottom out to the rear. There should be some tension on the chain and no sag in the chain. If this is confirmed then lay the chains next to each other and cut the new chain with the same number of links as the old chain.

I'll take 2 THIN screw drivers and insert through the links about 3" (or so) apart on the bottom "run" of chain.
I'll then squeeze them together with one hand above & one below the chain.
IF I could pull 3 half links slack, I'd call it good.

To correctly size a chain (according to Shimano's instructions) shift the front derailleur into the big ring. Then, with the chain on the largest chainring and largest sprocket, add one whole chainlink (1") from the first place that the chain will connect.

That's it, really. Also, make sure that you break the chain on the rearward side of an outside plate. Apparently it's stronger that way.

See: http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Dura-Ace/SI_09G0B/SI-09G0B-003-ENG_v1_m56577569830682232.pdf

What torqueismygod said is correct. With the Shimano 11-28 cassette, you add one whole chainlink from where the chain will connect. I believe this is in the instructions that come with the 11-28 cassette and even in the chain instructions.

To correctly size a chain (according to Shimano's instructions) shift the front derailleur into the big ring. Then, with the chain on the largest chainring and largest sprocket, add one whole chainlink (1") from the first place that the chain will connect.

That's it, really. Also, make sure that you break the chain on the rearward side of an outside plate. Apparently it's stronger that way.

See: http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Dura-Ace/SI_09G0B/SI-09G0B-003-ENG_v1_m56577569830682232.pdf

This is the standard method for determining minimum length, but doesn't answer the OPs question as to whether adding 2 teeth to the largest sprocket will require a new chain.

Unfortunately there's no definite answer to the question because it's like asking "If I move the cornflakes to a higher shelf in my kitchen, will I still be able to reach them?" Obviously the answer depends on information not given, namely whether the person is barely able to reach now, or how much higher he can reach.

To the OP, the question is the same. If his chain is already at the minimum length, he'll need to lengthen it. But if it now has length to spare, he won't. He can use the method I described in my earlier post to determine whether he has the needed 1/2" to spare and use that info to decide if the new cassette will be an issue.

To the OP, As a rule I always use a new chain on a new cassette because worn chains will very rapidly wear sprockets to match their worn condition costing potenrial cassette life. Unless your drivetrain is fairly new, you should consider replacing the chain also regardless of whether the current one is long enough. If you're replacing anyway, then your question is moot, since you'll properly measure the chain to be 1" longer than the big/big wrap.

seems pretty tight in the big,big combo,will go down about 1 1/2 inch if you press in middle....chain has a KMC link, and I also plan on joing new chain with a KMC link,so I definitely won't cut the new chain the same size until I try the old chain....or should I just add a link to new chain and try it? can always take another off.....Thanks for your help

FBINNY: what do you mean by lower loop?....bottom jockey wheel?.......Thanks

seems pretty tight in the big,big combo,will go down about 1 1/2 inch if you press in middle....chain has a KMC link, and I also plan on joing new chain with a KMC link,so I definitely won't cut the new chain the same size until I try the old chain....or should I just add a link to new chain and try it? can always take another off.....Thanks for your help

Pressing in the middle is meaningless for sizing a chain. A bike with longer chain stays would result in greater deflection.

Wrap the chain around the Big:Big combinination.
You want at least ONE complete link SET but not more than TWO Complete link SETS slack.

A link set being an inner & outer link or 1" of chain.
It has to be an integer because you can't use 1/2 sets. (inner link only connects to outer link & vice versa)

Has one extra link now.....so I need to add one more?

You won't need to add any links to the new chain. Just take off the old chain and throw it away -- that'll simplify the process.

Has one extra link now.....so I need to add one more?

One extra link
or
link SET
or
Link SET + link?