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Old 12-27-07, 06:26 PM   #1
Donegal
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B-screw length

Is a b-screw something to be changed? Or is each one specific for each derailleur? I am a pretty fair common sense wrench, but I am having a little problem.

For the first time in a long time, I bought a new bike from an online dealer. It was last year's model and had been on the rack for awhile and therefore I got a great deal.

Facts: Shimano Dura-Ace throughout.
12/23 cassette
Medium cage derailleur


I switched to a 12/27 for a heavy climbing century and the fun begins. I couldn't figure out why I had so much resistance in the lowest gear. Usually, something as low as a 27 would give me tree climbing capability. I have some pretty fair sized quads and tend to mash a bit. I like the close ratios of the 12/23 for riding around where I live, and the 23 is low enough for the riding I do. I live north of atlanta on pretty hilly tererain. I checked the rear end, the derailleur pulley was almost dragging on the 27 gear. I checked the b-screw, it was useless. The derailleur was not mounted properly on it's detents, it looked like it was installed while the wheel and chain were under pressure. The b-screw didn't even line up with the frame stop.

I took the wheel off, put the derailleur on the frame properly and adjusted the b-screw all the way out. It seems just about 1/4" short of getting the derailleur really clear of the low gear. It is not touching, but it is definitely too close and causes some pretty extreme chain angles. I would feel better with a bit more clearance. Would it be counterproductive to get a slightly longer b-screw, or should I be experimenting w/chain length, screw or spring tension, etc? New ideas would be welcome. I don't really have a lot of faith in most of the shops around me. It's a good thing manufacturers only rely on them to put seats, wheels and handlebars on the bikes they build. I have seen some of you guys have some real experience, any old hand to help me out here?
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Old 12-27-07, 06:57 PM   #2
Ex Pres
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Try "reversing" the b-screw in its hole to gain some additional length.

Does your chain appear to be the right length or does it need to be longer?
Changing from a 23 to a 27 big cog may be an issue with chain length.
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Old 12-28-07, 06:42 PM   #3
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B-Screw

Thanks for the help. I haven't tried the flipping of the b-screw yet, will tomorrow. The chain length is big enough for me to run the big ring up front w/the biggest rear cog. I played w/the derailleur and chain tension, no prolems there. The tensioning spring just seems too loose. The top jockey pulley is not hitting the cog, but it is in my opinion, tempting fate. I will keep you posted.
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