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A mechanically challenged mate brought his bike round yesterday becuase it was no longer shifting up into the big ring in the front and the chain had started rubbing on the mech when lower down the cassette in the granny ring.
So I loosend off the cable tension and started to check the limit screws were adjusted correctly. The lower limit screw appeared to be way to low so the mech was sitting too near to the frame, hence the rubbing when in the granny ring. So I tried to check the upper limit screw and as I was pushing the cage across it snapped in my hand. It snapped where the outer plate of the cage joins the bit that goes under the chain. It didn't take much force at all.
So my diagnosis was a rock/front mech interface issue which had bent the cage inwards causing the initial issues with alllignment (limit screws don't just move afterall). This must have weakend the cage and I just finished the job. There are lots of rocks here in the Peaks.
So am I right?
It was a Sram X7 direct mount mech, are these just made of cheese and I broke it with my bare hands? I've only ever had new Shimano road front mechs and the one on my mountain bike was a 1998 STX-RC model that never missed a beat till I removed it to go 1x10 so maybe my lack of familiary with Sram and front mechs in general meant I was getting it all wrong.
I feel bad if I've just cost him £25 for a new mech, he even paid for the spannering in the correct currency, a lovely bottle of Bradfield Blonde.
It's also possible that chain rub has gradually worn away that part of the mech.
Not many rocks strike above the chain ring though as the bike would need to effectively flat to do so. Rock strike wouldn't move the screws either.
You were pushing against the outer plate but adjusting the upper limit screw? Normally you push down on the lever mechanism which extends the arm out and adjust the screw to stop excess travel. The lower limit screw is set without cable tension to above the inner chain ring. The cable is then tensioned to take up the slack and adjusted to provide crisp shifts. Some rubbing will still occur at the extremes of the cassette.
It's also possible that chain rub has gradually worn away that part of the mech.
Seems unlikely, the bike is fairly new, <800 miles, and the chain had only just started rubbing. I'm seeing the fact that the shifting/rubbing issues came on fairly suddenly, that the bike was fine before, wasn't brand new but also wasn't old and worn as validation of my theory that it must have been wacked by something.
Not many rocks strike above the chain ring though as the bike would need to effectively flat to do so.Rock strike wouldn't move the screws either.
Yep - That is a flaw in my theory.
Although I think I'm OK on the limit screws though. I'm assuming they hadn't moved but the cage had been bent so was now in a new position relative to the chainrings despite the limit screws not moving.
Maybe not a rock but a misplaced foot or something in his garage? Am I clutching at straws?