UKC

Having replaced the damaged rear derailleur hanger on my full suspension eMTB (see this thread https://www.ukclimbing.com/forums/biking/help__countersunk_head_screw_stuck...), and replaced the chain because I checked it once the drive train was back together and it turned out to be .5% worn, I now need to check/adjust the chain gap, as adjusted using the B screw on the derailleur.

The SRAM manual says: "For full suspension bicycles, measure the chain gap while the bicycle is in the sag position."  If that means with the suspension compressed, that's going to be a bit difficult because I can't sit on the bike and adjust the B screw at the same time!  Is there any other way to do it, or do I have to try and recruit the missus to assist with the process?  (I'm not hopeful that that would be agreed to, or successful!)

You are right, sag is the position the bike is in when you're sitting on it in normal riding position and gear, but not going anywhere. This should be around 15-20% of the travel of the bike. The chain shouldn't change that much between unweighted and sagged. 

I've very rarely ever had to adjust a B-screw across dozens of bikes, odds are it will be fine where it is, and if in doubt, a bit further out (further from the cassette) is better than a bit too much in. Basically you just don't want the top pulley on the derailleur crashing into the biggest ring on the cassette. 

Though it will vary a bit between bikes depending on the suspension geometry. 

If Martin's really keen to check with it in the right position, he could let air out of his suspension until it sags about the right amount when he's not sat on it. But only if he's got a shock pump to get it back to the right pressure again afterwards!

Air suspension I have.  Shock pump I have.  Thanks, this sounds like the way to get the job done without aggravating the Mrs (who has just spent most of today doing fairly heavy gardening, so is entitled to some down time!)

Update: the trick with the air shock worked just fine, and revealed that the upper jockey wheel was far too close to the biggest ring.  B screw now adjusted to give the 15mm gap indicated in the SRAM manual, and gears working fine.  Except that...

I now have another problem: the drive train is 'ticking' when under load.  It seems to do it in all gears, but only when [even minor] effort is being put through the pedals.  If I let the bike freewheel and turn the pedals without applying any effort, no noise.  There was no noise when I had it on the work stand, either, but it's there when I ride it, and it's annoying as well as worrying..

The fact that it only happens when putting effort through the pedals suggests to me that the tension in the chain is causing it to rub against something adjacent to the upper part of the chain run, but I'm blowed if I know what.  I can't pinpoint where it's coming from while I'm riding, which makes it more annoying that it doesn't do it on the workstand.

Anyone got any ideas?

I don't know if it's your problem but I recently had a 'ticking' sound on my road bike and replacing the lower jog wheel, which was worn, fixed it.

Not sure if this is helpful, I ride a hardtail mtb. Apologies if you have already done the following - check for stiff or bent links, if you have a new chain, perhaps try that and see if the noise persists.

I had a ticking noise that only occurred when the chain was under load, much like what you describe. I couldn't figure it out at all - jockey wheels not worn or loose, no obvious sign of damage to derailleur, cassette or chainring, nothing appeared to be touching the drivetrain ... so ignored for ages until on a fairly steep uphill, the chain snapped.

Close inspection of the snap indicated that it had fractured at the end of the link where the pin passes through the outer plate. Whipped a new quick link on, no more annoying ticking. I hadn't spotted the damaged link on the work stand, so my vague theory is that it was maybe a hairline crack that just opened up a little when it was loaded before eventually giving up the ghost.

I should have known better, I had a similar noise and experience on my road bike several years ago.

Good luck, bike noises are a pain to diagnose! 

If the ticking has appeared after a new chain I would guess that the new chain is now slightly mismatched to either slightly worn chainrings or jockey wheels. If it's not skipping and the teeth don't appear overly worn it could settle down over the course of a few rides. 

Alternatively ticking could be bottom bracket, suspension pivots, headset, stem... Any number of possibilities! Not helpful I know.

That is my current thinking.  From the fairly cursory inspection I've given the cassette so far it doesn't look particularly worn - and, as I say, I got a new chain as soon as the Park Tool chain checker said it was up to 0.5% wear.  I have swapped out the derailleur* - so new jockey wheels - but there's still the ticking under any kind of load.

True, but I'd rather not risk accelerating the wear on a brand new chain.  My current plan is to swap the old chain back in and see if the ticking goes away.  If so then I would suspect the cassette, or maybe the chainring.  The latter I have a spare of, the former I have ordered a spare of just now.  (I don't mind keeping the new cassette to hand as a spare if it turns out not to be that; the last time I ran a cassette into the ground the bike ended up off the road for several weeks 😢).

If none of the previously mentioned actions fix the problem, I think I'll give it to my LBS to sort it!

* Longish story, but the original one did take quite a whack some time back, which is probably what caused the damage to the derailleur hanger that I only noticed recently.  I suspect that the slightly bent derailleur and the damaged hanger conspired between them to work more or less OK, but as soon as I replaced the damaged hanger the damage to the derailleur itself started to make itself known, hence I swapped in the brand new one that I had on hand as a spare.  So new chain, new derailleur and new hanger but still not right, which suggests to me the other parts of the drivetrain ie cassette and/or chainwheel are now the most likely suspects.

And the conclusion of the story is: it was the chainring which was much more badly worn than I could easily discern without removing it.  It's a 16T chainring (the bike is a mid-drive eMTB) and was starting to show signs of "hooking" on the teeth.  On reflection I guess that a small chainring like that is likely to wear quicker than a more normal sized one, since it will do more revolutions per turn of the rear wheel than a larger chainring.

I had held off swapping the chainring partly because the last time I attempted that job I ended up with an ISIS crank immovably stuck on the motor axle and had to take the motor with the much-butchered but still not loosened crank to the LBS where they managed to get it off (I didn't ask how).  I bought a pair of self-extracting cranks after that, but this was the first time in a long time that I'd attempted to remove one.  Fortunately it came off just fine, as did the old chainring.  All I had to do then was to fit the new chainring, which required tightening a lock ring to 43Nm on a left-handed thread using a rather less than positive 'spider' tool, while holding the chainring in place with a chain whip.  One slip and it could have ended up with anything from barked knuckles to an irredeemably trashed lock ring, but I got it done without too much drama in the end.

So with new chainring, new chain, new cassette and new derailleur (mounted on a new derailleur hanger) I set off for a shakedown run and...nothing but a blissful metallic purr from the drivetrain.  Much relief all round.

Correction, the torque for the lock ring was 25kN.  So, not so much effort, but still a bit of a contortionist act.