Is my chainwheel warped?

[autolycus]

A follow up to my last thread, which rambled a bit because i didn't really have a proper handle on the problem. The situation - after adjusting front and rear derailleurs is that i cannot prevent the front outer chain/chainring/cage from rubbing.

If i adjust the h screw to provide the smallest possible clearance between between the OUTER rail of the cage and the outer edge of the chain, 4 spots on the inner side of the chainwheel catch on the INNER rail of the cage. This is very clear on inspection after removing the chain. Furthermore, these 4 spots rub through half a revolution of the crank; the other half of the revolution there is no rubbing. If i loosen the h screw just enough to stop this rubbing the chain scrapes in places against the outer rail, but this seems the best compromise at present.

(However, in this position roadtesting under load, i get what sounds uncannily like a large frog croaking - in EVERY gear on the large chainwheel. I could understand it in the highest couple of gears, but lower down there seems plenty of clearance to the ouside rail of the cage.)

Taking a point on the inner rail of the cage as a datum and measuring the distance between it and the chainwheel, i get a difference of about 1mm between the closest and farthest gap over one revolution - if you see what i mean. I can take pics if it would help, just say what of!

So it seems the outer chainwheel is 1mm (or is it technically 2mm?) 'out of true'. I don't have the experience to know if this is acceptable, but I can't think of another reason why adjusting the h screw leaves me rubbing on one side or the other. Can anyone help please, or must it be a trip to the bike shop?

Thanks.

 

[raleighnut]

TBH it sounds more like the BB than the crank.

 

[autolycus]

Thanks i appreciate your input. I had thought about that, but the bike runs so well off the smaller chainring i rather discounted the possibility. Would that not likely affect the smaller ring?

 

[Ajax Bay]

it sounds more like the BB than the crank

But the inner chainwheel is fine.

 

[raleighnut]

But the inner chainwheel is fine.

The chain is further out (along the BB axle) so the pressure the BB has to cope with is greater plus any play is magnified.
If it was mine I'd have had the lot apart degreased/cleaned it thoroughly, inspected it and re-assembled everything (replacing anything that looked dodgy)
It was the 'croaking frog' sound that made me think BB, it could be the bolts that hold the outer ring on, could even be a pedal.

 

[Crackle]

it could be the bolts that hold the outer ring on

That would be one of my guesses too. Loosen and re-tighten to be sure. I've had one creak that I didn't re-tighten properly.

 

[accountantpete]

I had a chainset with one arm slightly out of position and ended putting a washer as a spacer between the ring and the arm to straighten it out.

 

[autolycus]

If it was mine I'd have had the lot apart degreased/cleaned it thoroughly, inspected it and re-assembled everything (replacing anything that looked dodgy)
It was the 'croaking frog' sound that made me think BB, it could be the bolts that hold the outer ring on, could even be a pedal.

I think i'll follow this advice tomorrow, with particular attention to the chainring bolts. I mentioned in the other thread that the bike had supposedly not been used much and it was quite dusty when i bought it, so perhaps it might have 'dried out'. The idea that the outer chainwheel puts a greater strain on the BB makes sense.

Thanks to all respondents. I'll let people know how i got on.

 

[alecstilleyedye]

i had exactly the same problem and it was play in the bb that caused it…

 

[autolycus]

Well, i got the BB out and i think this may be the problem. It looks as if when it was installed the label was forced partly off and rolled up. And where the label should be, the metal is badly gouged. I've cleaned and greased the threads and put it back, but kermit is still croaking under load. Would it be a good idea to replace on the basis that the bearings are probably knackered? It looks like chainreaction cycles have one for 12.99 at the moment, so not too extravagant.

And can someone please tell me: is this a BB30 or GXP - can't find any simple answers amongst the mountains of info on google.

Thanks

 

[raleighnut]

Looks like the Truvativ 'powerspline' to me, what make is the crank.

http://www.googleadservices.com/pag...93FCU-B0OHp1xFNkHa5AIB9YEpS-G4vyNT9hnl0B4~AE_

 

[autolycus]

Yep, i'm sure that's the one (Sram crank). Apparently they don't have a great reputation for longevity.

So is it BB30 or GXP :-) ?

 

[raleighnut]

Yep, i'm sure that's the one (Sram crank). Apparently they don't have a great reputation for longevity.

So is it BB30 or GXP :-) ?

Neither, best bet is now it's out nip into a bike shop and say "Can I have one of these please".
I'm sure there are 2 that are very similar but fractionally different (and non-compatible)

 

[autolycus]

It was really just a general question of interest. I shall probably order the one from CRC because that's clearly the one i need. But i noticed chainrings were available for either option and i just wondered which one i had and what the difference is. I found a website called 'what'sthedifference' or something like that, and all they did was list the similarities between the two so i was none the wiser :-(

 

[raleighnut]

It was really just a general question of interest. I shall probably order the one from CRC because that's clearly the one i need. But i noticed chainrings were available for either option and i just wondered which one i had and what the difference is. I found a website called 'what'sthedifference' or something like that, and all they did was list the similarities between the two so i was none the wiser :-(

Like I said there are 2 types 'Powerspline' and 'ISIS drive' ( I had to go out to the shed to see what I'd had fitted to my Saracen 'Blitz' when I changed the crank) and I'm sure that there is a difference between the 2 although they look very similar.

http://www.googleadservices.com/pag...Z9u-fQjo9-Dl4Zxwm9sgaj-hn1kSlLwbT2iwQ1g4EwvcP