Bent chainset?

[pjray82]

Hi there,
I’m new to bicycle repair. I’ve just been tinkering trying to master adjusting all gears. I think I’m getting there. I understand that with some/most bikes that some gears will always get chain rub. However, I’m getting it on too many... I think. In trying to work out why, I notice the gap between the chain and the derailleur is getting bigger and smaller as the pedal rotates. There doesn’t seem to be any play in the bottom bracket. I think, when looking from above, I can see that the biggest gear (maybe the two others but not noticeable) are moving left and right. I think the whole thing is bent for warped.
So I have two things I would really appreciate some advice on:
1. Do people agree with my diagnosis?
2. How do I go about replacing this part? (I also don’t know exactly what it’s called - is it the chainset?)

Thank you very much

 

[pjray82]

I’m not sure I have been clear enough with number 2. At this stage I’m not looking for advice on how to physically replace it. I need to know how to order the part. Do I have to replace like for like? Where should I search for it?
Thanks

 

[Ajax Bay]

You are describing a bent large chainring [ETA: or loose chainring bolts]. To give you a decent answer/advice, we need to know the make/model of triple chainset, the exact distance between two adjacent chainring bolts (of the larger two rings, assume 5 bolts), and the speed of the drivetrain (eg 8 speed - number of sprockets at rear).
Edited to add: First check that all your chainring bolts are tight.
Front derailleurs can be correctly adjusted so there's no chain rub, provided the rider sticks to 'normal combinations of chainring and sprocket (ie avoids cross-chaining).
Spa Cycles will be my first recommendation, but please provide the data requested.

 

[midlife]

Biopace type chainring ?

 

[Ajax Bay]

Would a Biopace track left and right? Seems unlikely. And anyway I'll suggest the chances of a new poster aged 38 having procured and fitted a Biopace ring and asking a question in this way is . . . Nil.

 

[midlife]

Sort of read it as the chain was going up and down rather than side to side......

 

[figbat]

I suppose a bent axle is also possible, if all of the chainrings are doing it; are the crank arms and pedals following a true circle? Not sure how much force is needed to bend a shaft like that (although the front wheel spindle of an old enduro motorbike I had was banana-shaped somehow - that was a hefty axle too!).

 

[roley poley]

hope its just a case of simply truing with an adjustable spanner

 

[Deleted member 26715]

A few images may help

 

[rogerzilla]

Bent chainrings are fairly easy to true up. I've had to straighten a few new ones before.

 

[pjray82]

The biggest cog moves this way left and right slightly as I turn the peddle.

 

[pjray82]

It’s only fractionally by a mm or so but this is enough to move the chain and cause a bit of chain rub when there wouldn’t otherwise be.

 

[pjray82]

I’m not sure where to find the exact name of the triple chainset. Clearly it’s Shimano and it says Shimano 50-0 in small writing on it. Also says Octalink on a sticker.

 

[pjray82]

75mm between centres of the bolts on the chain-set. 5 bolts. 9 sprockets on the back set. Those 5 bolts on the front at very tight. I can tighten them or loosen them by hand for that matter. The bottom brackets bolt wasn’t as tight loosened this and tightened it firmly. Still same movement.

 

[fossyant]

You have a choice of replacing the big ring (if that's the 'wobbly one') or, as it looks like it's steel, try and gently straighten it with an adjustable spanner - and tease it straight.