Sram X7 Front Mech Problems

[Ajax Bay]

I had put (and have left in situ) a chunky ziptie round the right hand chainstay, pushed right up against the bottom bracket shell. With the chain in the small ring, the ziptie gets in nothing's way, but if the chain tries to suck I hope that the ziptie will provide a tiny nudge 'down' to the rising chain and stop it (from sucking up into the FD cage).

Chain suck happens when you change down also.

I think that this happens because there is a momentary reduction in tension the return leg of the chain as the RD cage spring takes up the slack (generated by a drop of 16 teeth (say) at the front).
This seems a good article: https://bikemagic.com/how-to/mountain-bike-maintenance/chain-suck.html
"Tooth-Wear, and Low-Stretch (New) Chains, and High Friction (eg Mud), when present in various combinations, cause the chain-ring’s bottom teeth to become loaded or overloaded by the chain’s pedalling tension, and also cause resistance to disengagement of the chain. The rear-derailleur spring cannot provide enough tension, and the chain does not have enough weight to disengage itself from the bottom teeth of the chain-ring. The links are carried up the rear of the chain-ring under continuing load …the chain “sucks” !!
Wear on the tooth’s pressure-faces (not burrs on tooth sides or thickened teeth) is the MAIN CAUSE of [ . . . ] Suck"
Article suggests (with rationale) that a new chain is more likely to suffer from this.

 

[autolycus]

Thank you all for the input. I had noticed a couple of days ago on the bike stand that there seemed to be a slight 'slap' following each FD change, but i just assumed it was something i'd just failed to notice before. Maybe that was an indication of chain suck. There certainly is a lot to this particular problem which seems all too common.

I found some very good prices for chainwheels on amazon so i've ordered the large 50t and the small 34t. It's probably more than necessary but with them, the new FD and a brand new chain that i already have, it should be possible to set the bike up for the medium term at least!

I gave the current chainwheels and chain a serious clean today, but i could not get it to change gear satisfactorily despite spending an hour or more setting it up. I've decided to put up with it while i wait for the new bits and then if i can't sort it out i'll take the bike to evans for them to set up.

 

[Yellow Saddle]

You can diagnose and predict impending chainsuck quite easily but you need two people. Find a nice big flat area, like a wide street or such. Have your helper rider the bike around you in a big circle, whilst on the middle chainring (that's usually the one that poses the problem unless you use the small very often). Now watch the rear derailer cage. If it oscillates a bit when the bike is pedalled steadily, that's impending chainsuck. That indicates that the chain is just-just sucking and with a bit of dirt thrown in and a downshift, you will be in trouble.

 

[autolycus]

Thanks, that makes sense. For 'middle' read 'small' on a double chainring, i presume?

 

[Yellow Saddle]

Thanks, that makes sense. For 'middle' read 'small' on a double chainring, i presume?

Yes