Bottom Bracket Axle Bends. Normal?

[grhm]

I've got a question for those wiser and more knowledgable than me.

On my bike, I've got an irritating rubbing noise coming from the largest chainring/front derailieur.

When using the smallest two cogs at the rear, the chain rubs on the outside edge of the front derailieur mech. However, it only does this when I'm peddling with some force, and only for half of the chainring rotation.

It doesn't rub when I turn the bike upside down and have a look.

It looks like the bottom bracket axle is bending as I push down on the right pedal. If I stand the bike upright, stick the right pedal forwards and stand/lean on it and I can see the front cogs all lean out a few mm's and the chain then touched the front mech.

Is it normal for the cogs to move like this? My LBS seemed to think there was nothing wrong, but I'm not sure. (It's been over a decade since I owned a bike with more than one front cog, so I can't remember if any of my previous bikes did this.)

I've adjusted the end stop screw and the front mech won't move any further out. If I adjust the cable so it can move far enough out, I can't use the inner ring (of three). It can't possibly be right.

Its really irritating that on some downhill bits I can zoom along a 30+mph but the effect of effortless speed is spoilt by the loud errk...errrk...errrk noise.

 

[Smokin Joe]

It isn't the axle that it is bending, but the chainring flexing under load. Why not have a look on the Sheldon Brown site where there are instructions on how to set your front mech up from scratch and go from there.

 

[gbb]

I had this on a budget roadbike too....i put it down to flex in the chainrings (as Smokin says) or maybe flex in the frame. It only happened when i got out of the seat and give it some
Is frame flex common anyone ?

 

[Globalti]

A combination of frame and axle, yes.

 

[grhm]

I was fairly certain it wasn't the chainrings bending, but I'll check...

I see see a bit the the axle to the left of the inner chainriong and it definitely moves as I lean on the pedal. The chain rings all move together, if it was the chainring bending under load (from the chain?) surely the outer ring would bend more than the two inner rings?

The bike is a Giant CRS 3.0 if that helps.

I've had a go at ajusted the front mech and I've fixed the rub on the outer ring ... but now get rubbing on the middle ring if in the largest 3-4 cogs at the back (8 speed). The inner ring rubs if I'm on cog 7-8 but that's ok as I don't (and shouldn't) use those combinations.

It's all just so damn fiddly!!

 

[MartinC]

If you want to see how much a frame flexes hold the bike leant slightly to one side and push the bottom bracket sideways with your foot. You'll probably be surprised how much flex there is.

Things will rub on the road that don't in the workshop because of flex in the frame and components.