Cycling UK Forum

simonineaston wrote:Love Calvin's demenour and accent - he's a star!

There is surely no finer moustache out there.

The bike is the MTB Comp that came out in 2016 and the chainset and cassette are original. I can screw the H screw out enough to get clearance, but by then the chain starts hunting to try to jump off towards the chainstay. I can get it nearly there, but not quite and I'm coming to the conclusion that the answer is to shim out the 11T as it is closer spaced than the rest. I can't figure out why this would be the case unless there is a shim missing there. I think 0.5mm would easily fix the problem if the RD would tolerate this and still shift OK. The chainline looks fine, otherwise though I've just used a straightedge to check. It is a SRAM rear mech so I'll make some further checks to see if it gets skewed.

It's certainly the rear that's rubbing. I just took another very detailed look and can see the chain catching on the adjacent 13T. At the rear of the cog the chain is nicely centred, but the chain angle is just a little but too acute to clear the 13T. If I release the skewer and push the axle over towards the opposite side slightly (to effectively shift the chainline) the problem completely disappears. I'll try a temporary shim on the 11T and if that works I'll machine a shim out of some stainless sheet as a longer-term fix.

Some experiments with shimming fixed the problem, but because the shim is so narrow and it has to sit in a machined recess in the 13T cog I decided it may not be reliable. So this morning I went and bought a new SRAM PG-1050 cassette to replace the original PG-1020. Same x10 11-36 gearing. Immediately the original problem was resolved and the bike no longer rubs on the 11T and shifts really smoothly. There's a difference in the machining on the face of the 13T - it doesn't have a recess and the spacing is equal right the way across the cassette.

However, there's now insufficient travel in the RD to shift onto the largest rear cog (with the 24T front chain ring). I eliminated the cable from the equation and even with the L screw right out and with considerable hand pressure it's only just able to go into bottom gear. This has never previously been an issue and the L screw is usually about halfway out. Because the RD is mechanically at the end of travel no amount of cable tension can pull it over that last bit. The cassette is torqued up to 40N/m as per specification. There are no spacers or shims between the free hub and cassette, just like the PG-1020. Any spacer there would throw out the 11T and leave me back where I was.

Any ideas?

Before getting into options... what's the new alignment of the front and rear gears?

Jonathan

Chainline is 46mm front, 44.5 rear.

is the gear hanger/RD straight?

cheers

I did some checks with a straightedge this morning to re-check. Whilst it looks OK, it does angle outwards slightly by 1mm throughout the range, except that at the extreme of travel (largest rear cog) this is just a little under 2mm. Measurements are taken comparing top and bottom of side plates compared with vertical. The cassette sits closer in than it did previously and now the RD is at its extreme of travel the outward angle looks to be the problem because if I apply gentle pressure to align it truly vertical it goes into gear (though the L screw still has to be backed right out).

Gregoryoftours mentioned this distortion in an earlier post with SRAM RDs. Mine is an X5. Very low mileage, never dropped and no pivot wear. Just looking at it again and the main hanger pivot looks as though it may be at a slight angle, which would account for the overall misalignment.

IIRC X5 mechs, when knocked, can bend in the parallelogram and/or the cage plates, so can sit at a jaunty angle even on a perfectly straight hangar. This causes all kinds of problems.

If you have an old M10 threaded axle with a cone on it, you can secure that to the RD mounting bolt (using the cone as a 'joining nut') and use that as a tool to help straighten the RD (if it is bent in the parallelogram). Simply hauling on the mech when it is in the frame will bend the gear hanger on most modern frames, which just stores up trouble in the long run. If the mech is bent in the cage plates, they are best straightened individually, i.e. with the pulleys out and the mech in bits.

It is possible to work minor miracles by straightening mechs this way; needless to say it is also possible to wreck them, so it is good to have a 'plan B' in case it doesn't work out.

cheers

Thanks. I removed the RD and did what you suggested. There were two issues - the hangar was slightly bent outwards, and the parallelogram slightly out as well. I've just had a session of slightly tweaking and reassembling and the second attempt has got it working perfectly. The L screw is now in the centre of the range and the RD maintains its alignment. The next step is to make a proper alignment tool, but for the time being the bike has never changed and run as smoothly as it does now.