What does it do? MBR have a feature this month showing how to adjust it, they even recommend a specific screwdriver for the job (Yeah I know – MBR), but I've been building bikes since the year dot and have never touched the screw, ever. So what exactly does it do? MBR don't say, they just tell us how to adjust it, but why would anyone want to adjust it?

Anyone know what this is for?

needed to add some to the b screw when i fitted a 11- 34 cassette

It stops the top floating jockey wheel from sitting on the sprockets. Screwing the screw moves the mech further away from the sprockets, unscrewing it moves it closer. With mech in biggest sprocket the top jockey wheel should be close, but not touching it.

Bit numpty this, but is this the ideal B limit set-up: to get the guide jockey as close to the sprockets as possible without touching? I was faffing with mine yesterday and it ran much smoother with the guide jockey miles away from the cassette (stupid 1×10 🙂 ).

Generally you want to set the 'B tension' screw so the upper jockey wheel is close (but not touching) the cogs on the rear cassette. This gives you the best shifting (that way, when the mech moves to change gear, it makes a sharper angle) and also wraps the chain as far around the cassette as possible (to reduce risk of skipping).

Essentially the screw adjusts the balance between the spring that keeps the chain pulled taut (the 'A Tension') and the spring that pulls the mech back around the mounting pivot (the 'B tension').

If, with a properly set B tension, the mech chatters in the highest gear but not in the 2nd highest, check the H limit screw is set properly.