Help Please! Front mech shifting help…

First of all, I don’t generally have issues getting my shifting set-up correctly, but the front mech on my road bike seems to give me nothing but trouble.

I replaced the sq taper crankset with a Sora compact and it shifted OK, but always feels a bit like it’s only just working.

I bought a 48t chainring recently, fitted it tonight & went for a spin up & down the road to check the shifting. The mech won’t shift from large to small ring, so I’ve tried to sort it but can’t.

If I push the chain onto the inner ring and then shift back to the outer ring it always works. But it won’t drop back down again.
When the shifter is actuated, the mech moves but not enough to shove the chain far enough inwards. The cable is just slack and the limit screw has no more room to allow the mech to move in; it looks like the mech is on the limit of it’s movement and it’s still not enough. Even with the cable bolt loosened and the limit screw wound fully out, the chain won’t shift.

I also noticed that since changing to the 48t ring (from 50t) when the chain is on the smaller ring and I shift to the smaller sprockets on the cassette, the chain rubs the outer ring when any of the smallest 3 sprockets are used. It didn’t do this with the 50t ring.

It is almost like it needs a spacer in the BB on the drive side to push the rings outboard slightly. This would straighten the chainline slightly to stop this weird rubbing and also allow the front mech limit screw to actually be required for setting the inner limit position.
But, I am pretty sure that with a 68mm BB shell, the HTII BBs shouldn’t have any spacers fitted and the available axle length on the NDS does seem to concur with this.
There is a Shimano tech doc that mentions a 3mm spacer on the NDS of some Sora cranks, but this wouldn’t help with altering how far out the rings sit.

Thinking of just taking a 2.5mm spacer off my Inbred BB tomorrow and trying it to see if it helps, but don’t want to end up with not enough axle length on the NDS.

Any ideas much appreciated!

Is the 48T ring Shimano? Is it fitted the correct way around?
Is the mech moving freely across its full range?

What is the chainline?

Measure from the centre of the seat tube to the mid point between the chain rings, it should be around 43.5mm.

Timba, no it’s a Stronglight chainring not a Shimano one. I couldn’t find a Shimano 48t ring with the right bcd.

I think it is fitted the right way round but will check.

With the cable removed, the mech appears to move freely across its full range. It just looks like it can’t come any further inwards and the movement isn’t enough.

Neil, good idea. I will check the chainline as best I can (once I get out of bed!)

Hmmm. Well, i struggled to get any info online about which way round the chainring should go. I put it so the name and teeth details are facing outwards. Apart from shifting pins that stick out further on one side than the other, it appears virtually symmetrical.

I tried getting it set-up properly again and failed, so put the original ring back on to see if it was me being a moron. I had the original Shimano ring shifting fine within about 10 mins.
The original ring is dished slightly (on a flat surface you can see it’s not flat), whereas the Stronglight one is flat. Wonder if that has anything to do with it.

Not sure i understand why the outer ring affects shifting onto the inner though. It still looks to me like the mech isn’t moving far enough inwards. But it moves enough to shift with the original Shimano ring.

Arse.

Have you move the deraileur?

Would you not need to drop it to get tbe correct section of the cage to hit the chain?

It’s probably the 48t chainring’s fault. Try dropping the mech down a bit, but it’s probably just the fact that the ring doesn’t have very good shifting ramps.

Joshvegas i dropped it down a bit, but the diameter variation is pretty minimal between the 50t and the 48t rings. I also tried adjusting the angle slightly, although it seemed fine to start with.

Bikewhisperer, i am starting to wonder if it is the ring too given how quickly i managed to set the original ring up when i put it back on.
Seem to get good reviews though so I’m surprised that it might be no good.