B Tension screw - what's it for?

[CopperCyclist]

Ok, I can see what it does - move the derailleur backwards and forwards - but I can't see what actual effect this has. How should it be set, and what problems can it cause if it's set incorrectly?

 

[ianrauk]

In a nutshell It pulls the deraillieur closer to/away from from the cassette.

 

[mickle]

It sets the clearance between bottom of sprocket and top of jockey wheel. Too close and the chain will be all that separates them - causing noise and drag and all kinds of shennanigans, too far away and the derailleur will struggle to control the chain.

 

[Edge705]

Its a good question and its something thats often overlooked Ive been biking for 20 years and didnt even bother with it until I recently upgraded to a 28 tooth cassette and noticed the jockey wheel was almost touching the 28 tooth As they say you learn something new every day or in my case every 20 years

 

[CopperCyclist]

Thanks all - so there's no magic setting for it, and it is just a case of setting it 'by eye' as it were. I've pretty much done the same anyway. Ta again.

 

[benb]

So what's the procedure?
Get the chain in the biggest sprocket, then adjust the B screw until it just starts making some noise, and then back it off a bit?

 

[Davidc]

Don't know if it's right but mine are set to give about 3mm minimum clearance. I only found out what it does a couple of years ago when I was looking for the cause of chain noise. Before that I'd gone at least 40 years without knowing what it was for or adjusting one for myself.

 

[tyred]

Not all mechs have them in my experience.

 

[Globalti]

...and they can get in the way when you replace the derailleur on the hanger, so that if you don't realise what's happening and you carry on tightening the bolt you bend them and cause damage to the hanger. Don't ask me how I know this.

 

[HLaB]

I could be wrong but I thought setting the jockey cage to far from the cassette was more likely to cause derailment but too close will cause grinding, So I usually settle for 3-4mm back

 

[gwhite]

"B" screws can cater for slight differences in the configuration of different frames. Best to adjust them after lifting the RD up away from the resting position in case of distortion.
If these are bent then better to replace them with an appropriate size of Allen screw.

 

[rusky]

Have a read of this...

http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur