clunk of the folder - what have I missed

[mzjo]

I have a pedal rotation related clunk on the folder. Definitely right side, a bit left side as well. On mid-stroke in all gears and goes off a bit  if pedalling very lightly - so definitely linked to crank rotation, not further back in the drive train.
So far : the cranks are tight on the bb, I have changed the pedals with no change to the noise (not new pedals but better than what came off and the noise is no worse and no better), the saddle clamp is tight, I have greased my seat pillar extension to damp out any noise there, with no change.
Today I have tightened up the frame clamp (an involved story with loctite and heat that has no place here!), not yet tested the bike since. I can't see anything obvious with the chainring and protectors, although tooth wear might lead to a problem with the plastic protectors, which wouldn't be obvious. I'll have to try taking the protectors off and seeing if riding without them makes a difference (but I don't want to do without them permanently, they are what the bike sits on when I'm folding it).
The bb seems to turn very sweetly with no play, but that's without any load and it does have nasty plastic cups. I am coming to the conclusion this may be the source of the problem.
The problem could still be with the interface between frame and seat pillar or bars and stem (it's a folder so the bars/stem extension telescopes into the stem hinge/clamp) but it seems unlikely from the nature of the noise.
Can anyone think of anything I've missed?

[Kim]

Chainring bolts?

[mzjo]

Chainring bolts?

That's easy, there aren't any! It's a one piece ring and crank with plastic chain protectors bolted through it. But, given the rate at which the transmission has worn, wear on the ring causing trouble with the protector fixings is not out of the question.
I do wonder about a problem with the freewheel giving a clunk every pedal stroke due to the uneven cadence (effect of a replacement knee joint). I have a spare freewheel body somewhere  I might swop them over.

[ElyDave]

you say you've looked at the chainring - so no odd-looking teeth, chips etc?
Stiff chain link or quick link on the wrong way round? I've come across some that are VERY directional 

[mzjo]

you say you've looked at the chainring - so no odd-looking teeth, chips etc?
Stiff chain link or quick link on the wrong way round? I've come across some that are VERY directional 

Unlikely to be chain when it's every pedal stroke regardless of gear selection (and mathematically quite difficult to do - even if you wanted to). The chain is quite low mileage, changed a couple of months ago. I don't use quick links, hate them! But chainring is still a possibility. Inspecting it properly involves taking the protectors off, which means taking the crank off. At that point I will probably take the bb out (if it will co-operate), measure it and order a new one. Changing the chainring might involve a bit of faff finding a new one since it's one piece with the crank, I want to keep the protectors and I want to keep the same ring size. No decent folder specialists around me unfortunately.

[BFC]

With plastic cupped BB - I had two frames with these in (plastic NDS, steel DS), the frames had never had the paint cleared from the threads, plastic cups were rather tight when extracted and the threads had lost their tops. I cleaned the threads before fitting ally cupped BBs.

[mzjo]

With plastic cupped BB - I had two frames with these in (plastic NDS, steel DS), the frames had never had the paint cleared from the threads, plastic cups were rather tight when extracted and the threads had lost their tops. I cleaned the threads before fitting ally cupped BBs.

I approach the job wiith some trepidation. Plastic both sides, cheap alu frame put together by an Italian, what could go wrong? At least it would appear to be ISO, not italian, threads.

[BFC]

Where (roughly) are you, either find a tooled up YACF participant or LBS that has BB tools, purging the frames threads of paint, stripped plastic, aluminium corrosion before fitting any replacement BB will make life a lot easier

I have a 111mm UN72 BB in the obsolete spares box (Mid Essex), previously used in my now scrapped 1990's Claud Butler frames that cracked.

[mzjo]

Where (roughly) are you, either find a tooled up YACF participant or LBS that has BB tools, purging the frames threads of paint, stripped plastic, aluminium corrosion before fitting any replacement BB will make life a lot easier

I have a 111mm UN72 BB in the obsolete spares box (Mid Essex), previously used in my now scrapped 1990's Claud Butler frames that cracked.

I'm in Limoges (bit more than halfway down France) I think that between me, clubmates and the LBS we should be able to see it off. I'm in UK at LEL time but Cambridge is the limit of my ambitions, Essex is outside my orbit.

[ElyDave]

Cambridge is practically next door to Essex

[mzjo]

Cambridge is practically next door to Essex

Yeah when you measure in light years!

I had the cranks off and the bb out this evening. No problem; I just had to use a clamp to hold the tool onto the ds cup to start it. It's a Thun, 118mm, cartridge bearings, clean, well sealed and turns sweet as a nut. I don't think this is my problem!

I then looked at the chainring, took off the protectors and examined in depth. It's a steel ring, pressed onto an alloy crank. It isn't true that steel rings don't wear, no matter how much one would like to think they were immortal. This ring has worn ever so slightly and it would appear possible that the bottoms of the teeth have worn to the extent that the chain must be very close to the shoulder on the bottom of the protectors. Tomorrow I will put the bike back together without the protectors and do a test ride. That might tell me something!

[Kim]

IME steel rings (which I'm generally in favour of for the small ring of a triple) are perfectly capable of being The Wrong Sort Of Steel, vis the formation of burrs that snag the chain where similarly-deformed alloy would just flake off.

[mzjo]

Just an update to say that a very brief test ride round the block (which would normally have produced a or some clunks) has been clear of unexplained noises. The freewheel is however not of the smoothest, will have to look out the spare body.
I may take the folder to Montmorillon on sunday to give it a longer run.
So far so good!