Rear derailleur and the B-tension screw

[Fnaar]

(Shimano Sora on road bike)
Hi folks
On rear derailleur, the jockey wheel appears to be rubbing against the sprocket when in lowest gear: have adjusted B-tension screw, but even when it's fully in, there's still rubbing... any ideas oh fountains of knowledge?
All other gears and changing no problems whatsoever.

 

[Tollers]

Chain too long? How many teeth on the sprocket and are you using a short/lond cage derailleur?

 

[Fnaar]

Chain too long? How many teeth on the sprocket and are you using a short/lond cage derailleur?

I was thinking that about the chain... it's been on bike for 2yrs (and about 3,000 miles, prob)... maybe stretched a bit? Would have to go count teeth etc (I'm a numpty really) but I'm meant to be cooking dinner...

 

[fossyant]

Chain a bit short...?

 

[Fnaar]

too long, too short... not sure! Been fine till just last time I was out, no bumps or accidents, just not riding right on biggest sprocket at back. As I said above, all others fine, and changing is smooth.

 

[Tollers]

Give us a pic Fnar!

 

[fossyant]

I popped a 24T on my best bike with old 7400 DA. Had to adjust the B screw as the top jockey didn't like the 24T much - did the job, but the 24 is just shy of the 26 max of the old 7400 Mechs. Did think an extra link would slightly release tension in the spring though, but didn't touch it as it's using a 9 speed DA chain that needs the special links.

 

[rich p]

Why would it start to do this after being okay before?

 

[Theseus]

Bent hanger?

 

[Fnaar]

Why would it start to do this after being okay before?

Good question....

Bent hanger?

I'm pretty certain not

 

[Fnaar]

Give us a pic Fnar!

TBH, a pic would just show you my rear mech (which is a bit dirty )
I took one, but it doesn't show anything of note

 

[bike_the_planet]

(Shimano Sora on road bike)
Hi folks
On rear derailleur, the jockey wheel appears to be rubbing against the sprocket when in lowest gear: have adjusted B-tension screw, but even when it's fully in, there's still rubbing... any ideas oh fountains of knowledge?
All other gears and changing no problems whatsoever.

The B-tension screw adjusts the angle of the body of the derailleur in relation to the sprockets.

This is most critical when you are in the lowest gear on the rear cluster (ie the largest sprocket).

If the chain is too long then the angle of the derailleur's jockey arm is at a more acute angle forcing the jockey pulley upwards towards the sprocket.

The reason that it has got more noticeable is most likely that the chain has worn and therefore lengthened further.

If your derailleur or hanger were bent, you'd almost certainly either a) have difficulty with all gear changes and possible have the lower end of the jockey arm (the tension pulley) bent towards the spokes.

When you're on the middle chainring and the middle sprocket at the rear your jockey arm should not be more than about 45 degree to the vertical. If it more horizontal than that, your chain may be too long.

Check the angle of the jockey arm for the chain on the middle chainring and the largest sprocket - the jockey arm should be near vertical.

Cheers

 

[Fnaar]

Thanks for all your answers folks, and especially BTP, just above this ^^
As it happens, I took it out on Saturday, and it rides fine, with just a hint of noise when in lowest gear (which I only ever use on the steepest of local climbs anyway). All other changes/gears absolutely fine. Suspect chain has stretched a bit (mentioned before it's done about 3,000 miles). Will measure, and change if nec.
Thanks all.