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Last couple of properly rough rides (local rocky descent and 50 Shades at BPW) I have had my B screw break off, seemingly just through the bouncing of the rear mech. This is a Sram X0 10 speed mech that I have been using for the past 4 years with a 42 tooth expander. It has a couple of Alpine trips, Snowdon trips, Antor Stiniog, BPWs, etc, i.e. 100's of thousands of metres of rough descending under it's belt without problem, and now on the last 2 rough rides the B screw has snapped (it did do a Cwmcarn loop in between with out breaking, but then that isn't that rough). I recently fitted a new chain but can't remember exactly when. Maybe I fitted it one link longer but I don't think so, I always measure them against the old one.
Anyone else had this happen or care to postulate what may be happening. Mech works perfectly in operation, is straight and the B screw lines up with the stop correctly.
The only thing that I can think of that there's less tension in the chain. Chain slightly longer than before and/or that the mech spring is loosing tension allowing things to flap around resulting in the B screw impacting on the stop.
Not broken but bent and slipping over the top of the stop which has caused the hole to elongate!!! Only fitted new with large cassette last Aug for a trip to Spain. For various reasons I’ve not ridden much since then and I doubt it has more than a couple of hundred miles on it, if that...
i suspect davesport may be correct, to get gears to work a long cage Mach was required as opposed to the med cage fitted as per original therefore the ranges of gears from the largest to small means a huge difference on tension and the screw is close to the top/end of the stop.
Perfect excuse for a new bike.
Happened when you were biking in a fast gear I guess. The only thing to keep the "chain mass" and the "rear mech" mass in place is the spring. Shorter chain means higher spring force and less chance that the whole rear mech will start bouncing.
Going fast and rough downhill: total rear mech failure isn't rare...
Never had this problem: but is it possible that you need a new spring for the rear mech?
These are the two things to check maybe: chain lengths and spring-stiffness of the rear mech (move the mech without chain / without shifter wire. Feels "soggy" ? Or like new?).
This sounds odd. Can't believe it's cos you've added a link or we'd all be regularly breaking them. Sorry, nothing helpful to add
Cheers for all the replies. I think I can definitely remove one link set and still be withing range. I will also check the spring tension. I have a feeling the X0 mech can have the spring tension increased (thinking back to one I have taken apart in the past).
yes. sounds strange.
But right now: quite some problems with 1x12. Lots of discussions about it. Many cases right now where the rear mech falls apart when going downhill in a "fast gear". Sounds like chain has to be as short as possible for 1x12. Critical so: the suspension movement needs "stroke" as well....
Maybe beef up the b screw if space allows it what I mean is add a couple of nuts on to it to make the bendable bit shorter it's not like you need to adjust on the fly
It's a pretty old design fixture based on I'm woods pootling rather than maxed out dh with oversize cassettes
Ps b screw plus nuts is b boost 🔩😎
^^^tried to edit but got a coffee cup and page not found
Edit may look like a gaffer tape solution