derekt1975

I'm borrowing my friend's Kestrel Evoke on a long term basis. I'm not as strong as him, and the 39t/23t gearing wasn't enough for me to go up hills, so I changed the cassette to a 12-27. It wouldn't shift to the big gear, so I lengthened the chain by two links. I now have two problems
1) When on the biggest gear, the derailleur gear and the cassette gear interfere.
2) When on the small chain ring, the small gears have a slack chain.
I read that the "B" screw on the derailleur is supposed to move the derailleur farther from the casette, but on the bigger gears, it doesn't seem to do anything. On the smaller gears, it changes the angle of the derailleur, and on the small/small gearing, can make the chain rub against the chain [the cage tilts up instead of down], or provide some clearance.

It seems like both problems could be solved if I can convince the top gear of the derailleur to move back/down. Then it moves away from the big gear, and also picks up the slack on the small gears. Unfortunately, I can't find an adjustment for this.

Is there one, or do I need to buy a new derailleur? If so, how do I know the new one supports the 27t gear?

Components are Shimano Ultegra.

thanks for your help,
Derek

Al1943

1) Try re-wording this problem. I'm not uderstanding what you are saying. Maybe it's just me.
2) Sounds like the chain is too long.

You'll get the right help here if you can help us understand the problems.

Al

curbtender

My only thought is the chain still isn't long enough.
That combo shouldn't be a problem.

Al1943

??? Slack chain surely doesn't sound like it needs to be longer.

zonatandem

Longer cage rear derailleur.

operator

Let's stop guessing and start doing things thoroughly and comprehensively.

First stop guessing the chain length. There's a several ways of determining proper chain length choose one from the links below. Then do the FULL rear derailleur adjustment then front derailleur.

https://www.parktool.com/repair/readhowto.asp?id=26
https://www.parktool.com/repair/readhowto.asp?id=64
https://www.parktool.com/repair/readhowto.asp?id=75

This assumes that

a) Your chain/cogs aren't significantly worn out
b) derailleurs are not bent/significantly worn out
c) hangers are not bent

Al1943

Not needed for a double crankset.

derekt1975

I went from a 23T to a 27T and added two full links, which is probably one full link too many. but that's not the real problem.

The real problem is the top derailleur pulley touching the 27T gear. The "B" screw doesn't move the pulley away even when I tighten it all the way down (or loosen it all the way). Is there something I can change in the way that the derailleur is mounted to get that top pulley farther away from the casette?

BCRider

Does the frame have semi horizontal dropouts by any chance? If so it sounds like the axle needs to be allowed to seat a little further back. Also ensure that when you're on the big rear cog that you're on the smaller front ring to give the chain tension a chance to let the derrailleur sit as "slack" as it can on that combo.

derekt1975

If I remember correctly, the dropouts are somewhat vertical, but it seems that moving the axle back would move the gear closer to the pulley, no? I have the same problem on both chain rings. On the small chain ring, there's very little tension. With my two extra links, it actually falls slack on the smallest few gears, and again, if the pulley were farther back, it would help with that.

Al1943

It's possible to replace the "b" screw with a longer one or some people say you can take it out and put it back in from the other side for more clearance. But this should not be needed. The derailleur is rated for a 27 and many folks have reported using up to a 30 cog. A long cage road derailleur has the same large cog rating.

Be sure that the axle is fully seated in the dropouts. Set the bike on the ground, open and close the quick release to make sure it's fully seated.

derekt1975

I wonder if the derailleur was somehow mounted incorrectly by my friend, then. The "B" screw really doesn't seem to work the way I think it's supposed to. On the small/small setting, I can get the chain to rub against the derailleur, which seems odd. Lastly, is it normal to need to push the derailleur out of the way to get the axle into the dropout? That's the way it is on this bike.

Al1943

The chain should not rub the rear derailleur when on the small chainring and small cassette cog. Of course you should not be riding the bike in this combination.

Not normal for Shimano derailleurs.

Can you post a picture or two?

Al

curbtender

https://bicycletutor.com/calculate-chain-length/

derekt1975

I took out the two links that I added, and things work a lot better. The extra tension rotates the derailleur cage, and rotating it actually helps to pull the pulley away from the gear. With the B screw all the way in, the pulley and large cassette gear no longer generally interfere when the chain is moving. and of course, things are much happier on the small/small configuration. The chain isn't really long enough for the big/big setting, but there's really no reason why I'd ever need that one, anyway, so I think I'm done.

Thanks for your help,
Derek

DannoXYZ 

Yes, never use big-big. Heck with the gearing you have, you shouldn't ever use the two biggest sprockets with the big-ring or the two smallest ones with the small chainring. Those gears have duplicate gear-ratios available on the other ring.