Bicycle Mechanics - "B" screw tension adjust?
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10-12-11, 10:33 PM
Was having some funky shifting (indexing really) after cleaning the chain and rear casette this week. Read Sheldon Brown's tips on adjusting the rear derailleur and got everything shifting fine again with just an adjustment to the cable tension (barrel).
However, when in low gear it would make a clunking noise. Turns out the jockey pulley was hitting the low gear. Turned the B screw in about 2 turns and it fixed it.
My question is, why would this suddenly go out of adjustment? Is something else out of wack? The alignment looks perfect. Or is it possible it was never right to start with? I have about 1200 miles on the bike since new last year...
This is a Shimano 105 set on a Specialized Sectuer Comp.
Curious more than anything. There is not much left on the B screw for threads (almost all the way in), but the low gear is only a 27 tooth, so that is not excessively low.
Nothing you did cleaning should have affected any of the adjustments Since you had multiple adjustments to make, I suspect that something moved. My first guess is that the rear wheel wasn't replaced in the identical position. Either it's not fully in the dropouts now, or wasn't before.
Since it seems to be OK you might leave it alone until you have occasion to remove the rear wheel, or if you're curious you might stand the bike on the floor, open the QR, tap on the wheel to see if it settles and close the QR (after centering in the chainstays if not vertical dropouts).
It's important that wheels are tightened with the bike on the floor, so gravity ensures that the axle pockets fully to the top of the dropout. This ensures an identical position every time.
You should have 5-6mm clearance between the tips of the jockey wheel and the teeth of the cassette. The reason is the chain needs to pass between the two when shifting. With the rear wheel off the ground rotate the crank slowly by hand and shift slowly to the largest cog. The chain should move to the large cog without getting jammed between it and the jockey wheel.
10-13-11, 11:01 AM
There is not much left on the B screw for threads (almost all the way in), but the low gear is only a 27 tooth, so that is not excessively low.
The 105 derailler, depending on which version you have, is only rated to 27 or 28 teeth max large cog size. That you are almost out of threads at 27 teeth demonstrates an efficient design in my opinion :) If you it makes you feel any better, I've had to wind the screw all the way in to clear a 27T cog with an Ultegra 6600 derailler on one of my bikes and had enough adjustment to get a 28T cog working quietly with a Sora derailler on another.
10-13-11, 12:43 PM
If you had bent your RD or the hanger, that would account for these needing to be set. I assume the chainline is vertical when viewed from the rear.
10-13-11, 06:19 PM
Chainline is vertical, alignment looks good. Axle is in the dropouts. Perhaps the axle was not fully seated before?
At any rate, I am chalking up the indexing being off to normal cable stretch and the B screw to never being right?
Thanks for all the advice, I appreciate it!
10-14-11, 09:27 AM
My Sram 7 is adj for 6mm between the jockey wheel and the largest sprocket. Check with the manuf of your derailer.
The other 2 things are derailers do have specs on how large a cassette cog they can handle, and if you bought your bike used, the former owner might have installed cogs of a size that dont work well.
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