I am trying to set up my new rear mech and I am unsure how to set up one of the screws. What is the purpose of the small screw that pushes against the mech hanger and how do I adjust it so that the rear mech is set up correctly. (I’m not talking about the high and low limit screws just to be clear).

That’ll be the B-tension screw.

When you’re on the largest cog on the back, adjust that screw so that the top jockey wheel on the mech is just, like a teensy-tiny bit, below the cog.

Look on the Park Tools website for a more technical explanation

That’s the B-screw….

Parktool

wot they sed

tis the angle of the dangle.

Well, dunno about “teeny” as when i built up my current bike I had to put that jockey wheel out further than I thought I’d have to. Figured that out riding up and down the car park with my multitool clutched in one hand swearing profusely.

Yup, that’s the B-screw. Bolt on the mech without the cable attached. Push the mech over to line up with the largest cog – the gap should be 6mm (at least for SRAM X9). Adjust the B-screw so the gap is correct, using a 6mm allen key to measure it. Align mech with smallest cog and set high limit screw. Attach cable, run mech to largest cog and set low limit screw. Job done