Shimano XT B-screw adjustment

mar_k

can anyone tell me what gap I should have between the cassette and the rear mech?

supersonic

6mm on the top cog (cog to jockey wheel)

mar_k

Thanks, I havent adjusted it. it was set when I bought the bike however I have noticed quite a large gap.
Certainly much bigger than 6mm.

supersonic

Often are, not many know what it does!

The other way is to adjust until you hear rubbing, then back off a bit until goes.

mar_k

just adjusted it now, Thanks

keni

supersonic wrote:

Often are, not many know what it does!

The other way is to adjust until you hear rubbing, then back off a bit until goes.

What does it do? I don't remember ever having to adjust one

supersonic

Sets the derailer angle: optimises performance.

cooldad

Changes the distance between the mech (jockey wheel) and cassette.

cooldad

Sorry opened too many pages again.

bikaholic

The B-screw controls chain wrap around the cogs and the ease at which the RD cage moves up and down the cassette.

More chain wrap means faster shifting.

So, you want maximum chain wrap at the point where the cage moves freely (ie where the teeth of the jockey wheel does not touch the teeth of the cogs). This will be the optimum point.

You adjust the RD by shifting onto the largest sprocket, keeping the cranks spinning, and then adjusting the B-screw until the RD jockey wheel runs smoothly over the sprocket teeth (the RD starts off in a jerky washboard motion and will smoothen out as the B-screw is screwed inwards).

Repeat for each of the chainrings to find the optimum point. It may be 6mm or it may be not, but this method will find it.