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Old 03-10-10, 04:04 PM   #1
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Shifting problem not sure from B-screw?

I am a newbie, but don't want to go to the shop for every little thing... On my road bike I have 105 shifting on a standard double, and the FD seemed much too high compared to the usual recommendations of 2-5mm clearance from the big ring -- I wasn't happy with the shifting. So after a ton of fiddling, I finally got it lowered and the cable tension figured out (I think) so it shifts noticably better/quicker up front.

However, in the course of trying to get everything going, I messed with the B-screw for RD adjustment, don't even remember why! It looks like the RD gets bumped down when the chain is on the largest sized cog about twice through a revolution when freewheeling backwards; it is also noisy when it does that bouncing up and down. When I took the bike out on the road to test, the front shifted great but now my rear shifting is sluggish and clunks very loudly. There is a very noticeable delay in rear shifting versus before. Was this from messing with the B-screw? The rear shifting was just fine 3 days ago the last time I rode it. Now it is sluggish and seems to clunk loudly shifting in either up or down the cassette and across all the cogs.

I don't think I turned the B-screw more than one or two revolutions either way but... Not sure of the proper procedure and how to tell whether the B-screw is turned in enough. Do I loosen it as much as possible then tighten it until the top jockey wheel is not seen to move up and down as I freewheel? Or should I pedal forwards instead of backwards to check this (I don't have a repair stand, but I could figure out someway to check it). Thanks for the help!!!
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Old 03-10-10, 04:41 PM   #2
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Sounds to me like the upper jockey pulley is bumping the big cog. If that's the case, screw your "B" screw in (clockwise) until it stops.
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Old 03-10-10, 04:42 PM   #3
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The B screw is there so that you can adjust the angle of the RD's body. You want a small gap between the upper pulley wheel and the largest cog. Try for as small a gap as you can without having the pulley hit the cog. Unscrewing brings the pulley closer to the cog, screwing it in moves it further away.

As for tuning the rear shifing, it sounds like you need to make cable tension adjustments. Use the barrel adjuster for fine tuning cable tension. There's plenty of info at Park Tool's website or at to walk you through this.
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