Wobbly crankset

[mikey P 500]

Need opinions from some bike experts. Issue with crank wobble. All the front chain rings on my crankset seam to have a wobble to them like they are buckled, they cause the chain to hit both sides of the front mech as I cycle. The chain visaully wobbles about 5mm off centre either side under a complete rotation. I have changed the bottom bracket for a new one assuming this was fault (maybe slightly better but not much), I also checked the crank and chain rings and all look fine (lay flat on worktop), could the fault be with the frame in some way. (Hard tail mountain bike that I do use for jumps etc). How can I see if frame as fault, can't see any cracks etc. If frame is at fault I guess it's not worth trying to repair.

[Bobley]

Sounds like the crank is bent. Can only be replaced really.

[jesusbuiltmycar]

Does it only affect the big ring or both? Could it be one or more bent teeth on the chain ring?

[idiotgap]

My crankset on my £50 commuter bike seemed to have warped chainrings. Possibly from being carelessly lain without wheels at various points. I googled and youtubed and discovered people do bend them back, there's even a tool for it!

https://simplybearings.co.uk/shop/product_info.php...


I had a play with mine one weekend with some mole grips, stilsons, anything else I had lying around and made a meaningful improvement.

I found it's more scientific if you use a clothes peg to gauge the unwanted deflection



-
Sorry I got carried away there before actually reading the bit that says the chainrings lie flat on the bench. A bit of man-handling still might fix this for you though I reckon, not sure what you have to lose at this point.

Edited by idiotgap on Thursday 29th June 12:50

[mikey P 500]

Thanks for reply (wobble is there on all 3 rings), and have removed the crank again tonight and the wobble is still there on the axel sticking out of bottom bracket. It's a new bottom bracket and the problem was there on old one too, so assume it must be it's not in the frame correctly some how but can't see how else it could go. (The specs of bottom bracket match old one and this came with bike brand new)?

[idiotgap]

mikey P 500 said:

Thanks for reply (wobble is there on all 3 rings), and have removed the crank again tonight and the wobble is still there on the axel sticking out of bottom bracket. It's a new bottom bracket and the problem was there on old one too, so assume it must be it's not in the frame correctly some how but can't see how else it could go. (The specs of bottom bracket match old one and this came with bike brand new)?

Get the BB shell on the frame faced?

[james7]

When you say wobble do you mean there is play in the cranks? Or no play but all chain rings running off centre the same amount?

If there is play the likely one is the bottom bracket.
If not then it sounds like the crankset is bent and running out of true.

What crank set and bottom bracket is it?

[mikey P 500]

The bottom bracket is a standard shimano Bb-un55 I think it's 68mm and 122.5 long. It's has some play under load but not much, I think it's not seated in frame properly and now thinking a thread less bottom bracket might be way to go, assuming the threads in frame are no good, but don't know how they got damaged given the problem was there on old BB which was in bike from new. Bike is a giant revel 0, about 2014,so nothing special or worth spending alot of money on).

[james7]

What makes you think its not seated properly? Dodgy threads etc could be tapped or reclaimed with the right tools, most likely at a bike shop.
If there is no play or very minor then it will be the crank set bent.
Any pics of theads or the not seated properly bit?

[james7]

Just thinking as its the square taper type could you swap cranks frommone side to the other and see if the wibble moves? If it does its the cranks but if not its the spindle in the bb.

[mikey P 500]

Thanks for help both sides of crank are moving off centre I'm sure the BB can't be true to frame for this reason alone, both BBs tried are the same (threads looked ok to me visaully it only required what I would consider normal force to do up and both sides are fully in. Think I will let bike a bike shop have a look with the plan of a thread less BB as a possible fix if they can't sort the frame out.

[gazza285]

Can't see how it can be the frame, if the chainring are moving out of true as you turn them, if the BB isn't square in the frame the rings wouldn't wobble, they would just be out of line. Either the BB spindle is bent, which is unlikely as they are very strong, or the crank is bent, or the taper has worn, so the crank isn't sat square. What make of crank is it?

Edit:
Google says it's an FSA Dyna Drive crank, a cheap and cheerful square taper. You could try moving the crank relative to the spindle, at least then you will be able to tell if it is a bent spindle or a crap crank, as the wobble will either follow one or the other

Edited by gazza285 on Thursday 29th June 21:32