Crank arm loose

White Boy not in Charge

White Boy not in Charge

Member
I have a Giant Fathom 27.5. After riding it for a few months I've managed to loosen the crank arm and subsequently the sprocket (not sure if that's the correct term) on the drive side loose. There's a significant amount of wobble that makes the chain hit the derailer.

I looked at it and have absolutely no clue how to fix this issue, any help would be appreciated.
 
extremedave

extremedave

Well-Known Member
Team MTBNJ Halter's
I looked up specs briefly on the Fathom, if it has Shimano cranks they are pretty easy to work on. Can you grab the non-drive side arm and wiggle it (crank arm vertical and push/pull towards the seat post seems to be the best way)? I would make sure it is indeed the cranks that are loose not the sprockets (chainring is the term for bikes) or the spider/carrier they mount on. Brand of cranks and some pics wouldn't hurt.

The little spline drive tool is specific to Shimano but you can get them at any LBS probably, the plug isn't usually tight.

Your issue could be:
Loose chainrings.
Loose cranks.
Missing or broken shim/washer under the crank where it touches the bottom bracket.
Worn bottom bracket.

Video showing how they come off. You will need a torque wrench to set the allen heads properly, but I have just snugged them up for a quick test ride.



The Fathom also might have a FSA crank, which is self-extracting and works like this terrible video.
 
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qclabrat

qclabrat

Well-Known Member
yeah your non drive side got loosened
you'll need to tighten the crank arm on that side
most fsa cranks will need a super sized Allen key to tighten i
though not sure for your crank
 
E

ekim

Member
If you can tighten it then you can also take it off. It may be a good idea to pull it out and inspect the bearings to to make sure that riding it loose didn't mess them up. At least make sure that they feel tight and move smoothly. You can remove the crank and then stick your finger in the bearing where the crank rides to feel it.
 
pooriggy

pooriggy

Well-Known Member
Team MTBNJ Halter's
White Boy not in Charge said:
Any idea what tool I'm looking for?
Click to expand...
319V2YRA1RL.jpg
 
E

ekim

Member
Did you try to stick a 10mm allen (or maybe 8?) through the outer hole? You're not removing the cap on the crank. You're removing the bolt behind that cap. The bolt will push on the cap and push the arm off.

BFH is a bad idea in this case. Especially if the inner bolt hasn't been loosened up yet!
 
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qclabrat

qclabrat

Well-Known Member
Fathom 2s come with the FSA Comet crank, you'll need a 10mm Allen key. you may be thinking that it's a star pattern due to the machining at the ends. If you shine light into it, you'll see it just a plain hexagon shape
 
Mr.Moto

Mr.Moto

Well-Known Member
Wait, what's with the shifting back and forth? A loose crank arm on one side wouldn't let the bottom bracket shaft move from side to side. I am assuming this is a standard bottom bracket cartridge which shouldn't have that kind of movement. Might be bad bottom bracket?
 
blackburn1973

blackburn1973

Well-Known Member
Mr.Moto said:
Wait, what's with the shifting back and forth? A loose crank arm on one side wouldn't let the bottom bracket shaft move from side to side. I am assuming this is a standard bottom bracket cartridge which shouldn't have that kind of movement. Might be bad bottom bracket?
Click to expand...

I'm thinking it's a two-piece crank where the spindle is attached to one of the crank arms and slides through the bottom bracket, right?
 
Paul H

Paul H

Fearless OOS Poser
I would recommend not to.
The preload cap itself is pretty cheap... It's just a matter of getting the correct one. I would suggest that you take it to you LBS so that they can get you the correct cap and torque down the pinch bolts to spec. From what I have read, FSA kinda has a rep for this issue if not torqued to spec.
 
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