FD/ Chainset Problem

[autolycus]

I've just given my BB and other drive-chain components a long-overdue overhaul. I noticed a rather chewed large chainring so i replaced it but the small chainring looked fine so i didn't bother with that. I gave the chain and RD a thorough clean and regreased the jockey wheels and was feeling pretty smug as it all seemed to go well. However.... i've noticed when changing down to the smaller chainring, there is sometimes an initial slipping. In fact once or twice i was pedalling air for a full rotation. Given this scenario, is there anything obvious that might be causing the problem?

Many thanks.

 

[I like Skol]

Suggests the gap between big and small chainring is perhaps too wide? Is the new chainring a like for like replacement or aftermarket item? Also, while highly unlikely, have you checked to make sure none of the chainrings have been fitted back to front?

 

[iluvmybike]

If you felt you were 'pedalling air' that may be an issue with the rear freehub losing rather than front chainring

 

[Ajax Bay]

Is this the same (?SRAM) chainset / FD that had the chain suck bitd? 50-34t.
When in small/large, what is the gap between the inner plate of the FD cage and the chain?
Same question in large/small.

 

[autolycus]

Suggests the gap between big and small chainring is perhaps too wide? Is the new chainring a like for like replacement or aftermarket item? Also, while highly unlikely, have you checked to make sure none of the chainrings have been fitted back to front?

I did spot that the big ring had a rivet that goes on the outside behind the crank, but i didn't see any difference on the small ring. That could be the problem i guess; how to tell which way round?
I'm pretty sure the problem is at the front end.
Ajax Bay - well remembered, i did have a chain suck problem that may have been down to the oil i used. In fact i bought the new chainring back then but never needed to use it. Actually, I think the problem was mostly due to a badly stretched chain, which i eventually determined and rectified. In any event all has been going well since. I'll have a look at those measurements tomorrow, but meanwhile i'm interested in the response to the 'sidedness' of the small chainwheel.

 

[Ajax Bay]

The inner ring is plain: no pins/rivets (the one on the large ring is there to stop the chain getting wedged between ring and crank). The chain should drop off the large ring straight onto the inner ring and engage the teeth. The gap between rings is less than the outside width of the chain. Writing on the inner ring should face outwards.

 

[autolycus]

The inner ring is plain: no pins/rivets (the one on the large ring is there to stop the chain getting wedged between ring and crank). The chain should drop off the large ring straight onto the inner ring and engage the teeth. The gap between rings is less than the outside width of the chain. Writing on the inner ring should face outwards.

Hi again - i'm glad you said that because it makes me feel a bit better!

I did fit the inner ring with the writing facing outwards, but i think that may be wrong in this case. (It's not so much writing as engraved part no. etc). I say that because there is a recess around the bolt holes on the "writing side" of the inner ring, presumably to accommodate the outer 'rim' of the female half of the bolt - in which case that side must surely face inwards?

There's also a single nib (circled) along one inner edge; i don't suppose that means the ring has to go in any particular position? Behind the crank?

Unfortunately i can't find anything of google because as usual the stuff there is doesn't quite address my actual problem :-(

 

[Twilkes]

Hi again - i'm glad you said that because it makes me feel a bit better!

I did fit the inner ring with the writing facing outwards, but i think that may be wrong in this case. (It's not so much writing as engraved part no. etc). I say that because there is a recess around the bolt holes on the "writing side" of the inner ring, presumably to accommodate the outer 'rim' of the female half of the bolt - in which case that side must surely face inwards?

There's also a single nib (circled) along one inner edge; i don't suppose that means the ring has to go in any particular position? Behind the crank?

View attachment 633618

Unfortunately i can't find anything of google because as usual the stuff there is doesn't quite address my actual problem :-(

I recently looked up how to refit a couple of chanrings, and am pretty sure that nib goes behind the crank, not sure how much difference it would make though as the ring seemed pretty much rotationally symmetrical.

 

[Ajax Bay]

Ref 'nib': agree with @Twilkes (rotationally symmetrical). I wonder if this is like the direction arrow some tyre manufacturers put on road tyres: saves people asking the question.
On orientation (in/out) I have pulled out a Shimano inner ring and the writing is designed to face outwards. I don't know what make the ring you are fitting is (assume SRAM). I agree it looks as if the indent should face inwards to accommodate the nut. If the plane of an imaginary circle passing through the tooth tips is laterally displaced from the plane of the chainring itself (the Shimano one is), choice of orientation will shift the teeth closer to or further away from the large ring (I observe this in the context of this thread).

 

[autolycus]

Damn - i thought i had it sussed. But if i get into the small cogs on the cassette on the small chainring, the chain actually fouls the large chain ring. So the rings are now too close together and i think i had it right the first time. Maybe those recesses are for if you just use the small chainwheel as a single or something.

And i'm getting really confused now. Are the female (6mm allan key) halves of the chainwheel bolts supposed to go inside or outside of the assembly? Perhaps it doesn't matter?

 

[Twilkes]

On all the chainsets I've dismantled and rebuilt, the female part of the bolt goes on the inside, then the small chainring, then the spider, then the large chainring, then the male part of the bolt.

However, I have only dismantled and rebuilt one chainsets.

 

[Smokin Joe]

Damn - i thought i had it sussed. But if i get into the small cogs on the cassette on the small chainring, the chain actually fouls the large chain ring.

They will do on all chainsets if there is a big enough difference in size between chainrings, 53/38 for example.

 

[Ajax Bay]

The nut goes on the inside and the bolt (with hex slot) comes in from the outside. If the chain is fouling small/small you may have the inner the wrong way round (confirm 50-34?) or this may be (unlikely) a chain line issue.

 

[autolycus]

Thanks for all the input. My instincts tell me that the correct way on this sram chainset is to have the 'writing' on the inside. This etching is essentially the same sort of info as on the inside of the big ring (the catchy white writing is on the outside). There's also an extremely shallow concentric rebate on the small ring (in contrast to the proud ring on the side which can be seen in the picture) that seems designed to seat neatly into the spider.

I fixed the bolts female/nut outside; it has a 6mm hex whereas the bolt has a 5mm hex. If no-one says it will cause a problem, i think i'll leave it that way - i'm a bit sick of undoing/doing them up and i'm running out of thread lock!

I seem to have very small tolerances which might be explained by this not being the original 48-34 but a carbon 50-34, which i bought ages ago when i was feeling flash and amazon had it at a silly price. Small front, small rear and vice versa have always shown reluctance, but the good folk here long ago made it clear that it was not good practice anyway. I'll try to have a bit of a ride tomorrow and report back if there are any further issues.