On my road bike I’m having trouble reliably shifting onto the big ring. If the chain is on the large half of the cassette it’s fine, but if the chain is on the smaller half of the cassette the chain doesn’t sit down in the big ring. It sits on top, sort of folded over and at the completion of the pedal stroke goes completely over the top of the big ring and off. It’s not being pushed over by the mech.

I’ve never seen this with any of my MTBs. The variables are, all my mtbs are 9 speed whereas the road bike is 10 speed. I’ve replaced the chain and the big ring, and checked that the rings are aligned and the chain tension. It’s a shimano 105 compact crank set 50/39.

Any ideas?

you say you replaced the big-ring, what with?

i’ve found that the shifting performance of some chainrings is terrible.

Front mech at the right height?

The new big ring was the same as the old one, Shimano FC-5650. A couple of teeth had been dinged and I was hoping that it was that.

How would I check the mech height?

Is the chain clean and well-lubed and if you take it off, does it hang in a straight line? Chains do get bent and distorted.

How would I check the mech height?

The cage at it’s closest point should clear the big ring by 2-3 mm only when you shift down. Take the chain off and move the front mech back and forth and see how much clearance you have.

Is it also parallel with the big ring?

If you read and follow the Shimano tech doc to the letter then you should get there.

I’ll check the chain, but it’s only got about 100 dry miles on it so far.

Sounds like the limit screw needs winding in a touch to stop the mech moving so far outwards.

FWIW, I have never had trouble setting up mechs on my mountain bikes, but the front mech on my road bike is a PITA; always seems to be on the cusp of not working.