UPDATE Bent Chainring(s)

[migrantwing]

I posted here recently about chainrub problems I was having. No matter how I set up my front DR, there was chainrub when in small front/large rear and large front/small rear. NOTHING I did to tweak the FD position would prevent the rubbing. After trying everything, even widening the FD cage slightly, I still got chainrub. It could only be one other thing...a less than perfect chainring.

I know that the bike hasn't fallen over or been in any kind of accident that would bend the chainring, so it must have been bent whilst in transit at some point or other. I have succeeded in sorting this problem out with square taper BB's with this method (video below) but is it possible with a Hollowtech II BB? The setup on HTII has it so the little 'pin' on the large chainring sits directly behind the crankarm and the little arrow on the smaller chainring lines up with the crankarm too. Would the front shift be as smooth if I rotated the chainrings to a different position to try and alleviate the run-out in the chainring , or does it require that the pin and arrow are ALWAYS set up to be behind the crankarm? It is not that there are a few bent teeth on the chainring, it's more like the chainring is not perfectly flat when placed on a flat surface making it run out for the distance of about a third of the chainrings circumference, hence the rub. It's more warped than bent per se.

Click on link underneath the video NOT the video itself, it will jump a few minutes to where the interesting stuff is


View: http://www.youtube.com/watch?v=-mU6Dyt4SQg&t=2m2s

 

[raleighnut]

The pin is to stop the chain becoming jammed behind the crank arm if it comes off, you may have a faulty crank or a bent ring. I'd return it to the shop under warranty.

 

[Brommyboy]

The large ring will bend under tension: if you drive big gears hard, expect the chain to pull the chain ring out of line. The bend remains in the down stroke position.

 

[migrantwing]

The pin is to stop the chain becoming jammed behind the crank arm if it comes off, you may have a faulty crank or a bent ring. I'd return it to the shop under warranty.

Hi again @raleighnut I've had the bike a few years now, sadly.

 

[migrantwing]

The large ring will bend under tension: if you drive big gears hard, expect the chain to pull the chain ring out of line. The bend remains in the down stroke position.

I don't really grind my gears, Brommy. Maybe it's just one of those things that's happened along the way, a poor shift uphill or something that's pulled the rings out of true.

 

[raleighnut]

Hi again @raleighnut I've had the bike a few years now, sadly.

Looks like you may have to change the ring, I've never had any luck flattening things out but maybe a metalworker could do it, dunno.

 

[fossyant]

Could be cheap chain rings. More expensive ones tend to be dead straight

 

[migrantwing]

Looks like you may have to change the ring, I've never had any luck flattening things out but maybe a metalworker could do it, dunno.

Yeah. I may give em a bash with a dead blow hammer or try and straighten them on the bike with a wrench. If not, I'll get some new ones.

 

[migrantwing]

Could be cheap chain rings. More expensive ones tend to be dead straight

105 chainrings, Fossy, so would have thought they'd be of decent quality.

 

[Profpointy]

Not saying it's the case for you, but worth checking / eliminating: I had a bent bottom bracket axle giving the symptoms you mention. It was an old style square taper, likely not great quality, but still; worth a look

 

[Brommyboy]

You can just rotate the ringforwards by one or two holes. If there is a peg that should go under the crank arm, just remove it. Rings will bend under tension and the more expensive ones are lighter in weight, so really no stronger than the economical ones. Resetting the LH cable could remove any rubbing on the cage.

 

[migrantwing]

UPDATE:

Well, I took the bike outside yesterday for a bit of maintenance and I found that two of the crank spider legs were off by quite a few mm. I rested a small pencil on the seat-tube, held on with an elastic band as a visual indicator and turned the cranks. I could see by eye that a couple of the legs were further away from the pencil than the other three. I used a crescent wrench to bend the legs so that they were straight. I expected a little more force to be used, but they bent easier than I expected. I would have thought, being Shimano 5700 cranks, that they'd be a bit more durable. Obviously not. Still getting intermittent rubbing on the FD, so I don't know if it's still the chainrings, crank spider or just a stiff chain. I've tried everything now. Will just make do with it.