Derailleur lines up with half of cassette only

primalcarl

I had a mechanical recently where the chain jammed and it ended up ripping the rear derailleur and hanger off the frame. I've bought a new hanger, rear mech (secondhand) and cassette/chain but am having problems.

I've set all the limit screws properly and the derailleur is perfectly lined up with the chain in the highest gear (11t). It shifts up the first 6 sprockets perfectly, but when it gets to the lowest 4 it keeps catching the gear above and tries to climb up to the next one. If I back the tension off enough to stop it trying to climb, when I then go back down the cassette to the high gears the derailleur is now out of alignment with the corresponding sprocket and jumps. Whatever I do, I can only get one half of the cassette to shift properly.

I've checked the jockey wheels are the correct way round. The shifters are brand new also, with new cables.

The only thing I can't vouch for is that the secondhand rear mech I bought is actually straight, or if the hanger isn't straight.

I have very basic bike shops including a Halfords so not convinced they would have a hanger alignment tool

Smokin Joe

You've connected the cable to the wrong side of the clamp on the rear mech. It should clamp on the wheelside of the bolt, not the outside.

primalcarl

Smokin Joe wrote:

You've connected the cable to the wrong side of the clamp on the rear mech. It should clamp on the wheelside of the bolt, not the outside.

Already checked this, cable is clamped on the wheel side already

dj58

Could be that the frame is out of alignment even though you have replaced the hanger. Maybe the s/hand RD is the problem, but you wont know for sure unless you swop it out with a new part. Did you use new inner and outer cables?

mamba80

RD top pulley isnt catching on sprocket? - b screw adjustment, lifeline alignment tool is £30 from wiggle and they work really well for occasional use, i have one.
No guarantee its what s wrong with your set up but it ll rule out one thing and would leave only the RD - you d be v unlucky to have a bent frame from a broken hanger, frame isnt snapped anywhere is it? though i had this after a crash, replaced hanger, only to find that seat stay was cracked and alignment out.

You ve got the correct shifters and rear mech for the speeds your running i ll assume?

primalcarl

Nope top jockey is clear. Cables are new. The shifters, derailleur and cassette are all 5700 105.

It looks like I may have to borrow the mech from best bike, or gamble on the tool.

Thanks for the feedback. It helps to check you haven't missed anything obvious!

ElectronShepherd

Being able to only index one half of a cassette is a classic symptom for a bend frame / hanger / mech.

Your LBS will have a hanger alignment check tool - maybe ask them to see if it's the hanger or the frame?

The other thing to check is that the hanger is mounted cleanly. I once spent far too long trying to index the MTB after replacing a bent hanger, only eventually to find that the new hanger hadn't "bedded down" cleanly into the recess on the frame due to a small bit of swarf left over from the manufacture, so it wasn't lined up properly. Once I made sure the hanger was cleaned up and installed properly, everything was fine.