What is the b screw on the rear derailleur and how to adjust the b screw: Most derailleurs have some form of adjustment for the spacing of the guide pulley to the cassette cogs. This spacing is controlled by adjusting the b screw on the rear derailleur. This adjustment is check when the chain is on the smallest sprocket up front, and the largest cog in back. The majority of road bike and mountain bike derailleurs require a gap between 5 and 6 millimeters between the guide pulley wheel and the largest cog on the rear sprocket. You can use a hex wrench to gauge this distance. We'll demonstrate the b screw adjustment on a Shimano XT derailleur. Rarely do you need to adjust the B screw. If you’re changing cassette to a smaller or larger size or replacing your derailleur with a different length or manufacturer’s make, adjustment may be necessary*. A gap that is too small will result in “pinching” of the chain and a rumbling sound (or bicycle strange noise) while pedaling. And if there is contact between the teeth of the large chainring and the guide pulley wheel, damage to the plastic guide pulley teeth may occur. Check when performing bike maintenance.
If the gap is too large, poor shifting may result and you may note the chain skip 2 or 3 cogs as you shift out of the largest sprocket.
If the bike is shifting well, the B screw is probably within acceptable adjustment range. To increase the gap, tighten the B screw. To decrease the gap, loosen the B screw. If you do make significant changes to the B screw, double check the indexing adjustment. This may also be important in normal bicycle maintenance and repair on older bicycles.
*The B screw adjustment seems to be the most crucial when you are running at the limits of the derailleur capability (see references below). Most derailleurs have certain limitations. These includes the maximum difference in chain rings and the drivetrain capacity, and maximum cassette size (included with the manufactures specifications). On my compact (34/48 front chain rings, 11/34 cassette, and Shimano Deore XT long cage derailleur), the maximum difference in chainrings is 14T, the largest sprocket that can be handled is a 34T. To find the drivetrain capacity, take the difference between the large and small chainring and add it to the difference between the large and small cassette cog. On my compact that’s 14 + 23 = 37. I’m within all the specs: max difference in chainrings: 22T, largest cog: 34T and a capacity up to 45T. Note that the closer you are to the manufacture’s derailleur limits, the more important the B screw difference become, at least in my experience.
REFERENCES:
Derailleur Capacity: What is the max my derailleur can handle:
Rear Derailleur Buying Guide:
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