Bicycle Mechanics - Crank/BB question (possibly dumb) - TruVativ
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04-14-11, 03:03 PM
Picked up about half (frame, drivetrain, fork) of an old GT Avalanche 1.0, and have been putting it together for a friend.
The bottom bracket seems solid, but is unbalanced -- which is to say, with the chain off, it will strongly prefer a specific side down. I was looking at it to try and figure out why, and noticed that the cranks do not go straight across the bearing. This is to say, where I would expect them to be exactly opposite of each other, or 180° across, there is about a 3-5° deviation.
Hopefully I have explained this clearly...
So is it normal? And if so, why?
If not, what could be causing this and how would it be fixed? Nothing seems to be damaged or bent.
TruVativ FiveD crankset (assuming it's a Powerspline BB)
A little reading tells me this is generally regarded as a lower-grade product, but still no explanation of the offset.
Shouldn't be an offset. Something is disturbed or bent. Either the spindle is bent, or one of the crank arms is wallowed out (or installed incorrectly?)
Edit: I wonder if it's possible to install one of the cranks off by one tooth on the BB?? I never tried. lol..
04-14-11, 09:36 PM
Or the cranks were never installed with the proper 180 deg orientation to begin with. It is possible to do this with the powerspline BB/crankset the OP has.
04-14-11, 10:00 PM
I remember having a power spline crank, but I remember it had something like 12 splines or something? I would think if they were installed incorrectly it would be off by 10-15 degrees or so. You could take one of the crank arms off and see if you can mount them 180 as it should, but I think the spindle has been twisted by some serious force. I remember those power spline setups were always considered terrible and hard to get rid of. I sold mine for $15 crank and BB together.
04-15-11, 03:19 AM
Thanks for the info. Sounds like it's down to the LBS to figure out what's wrong with it.
In case I have to replace some or all of it, what is a good BB & Crankset in the same quality/value category as deore components? Not a real weight nazi so durability and function is as important as the ounces.
I'm sorry, I have never played with powerspline cranks very much. But you may only need an allen wrench to remove the crank arm on that. If so, just take off the crank arm on the non-drive side and see if anything looks bent, then try and put the arm back on 180 degrees from the other one. If it works, fine. If not, LBS.
Old guys like me need square taper cranks. That way if I'm off by 90 degrees, hopefully I notice real quick.. :D
04-21-11, 12:39 PM
So here's how it plays out. Turns out this was not a powerspline crankset, but a square taper (bad assumption on my part-- awesome though, I don't feel so motivated to scrap the whole thing).
When I took off the left side crank the retaining fastener was barely finger tight, and the problem became immediately clear. Pictures:
Left crank, back side
Pretty much ruined...
Left crank, front side
The little ridge visible inside the bore is pushed up metal, not grease or dirt.
The bolt seems to be unaffected.
Right side crank/rings
The right side appears to be in good condition, though I wasn't able to remove it, and didn't want to hit it too hard. Is there a tool for this?
So basically, sometime in the life of the bike, the crank was either poorly installed or poorly maintained, came loose, got b0rked, and needs to be replaced.
Thanks for all your help and comments. A newbie appreciates.
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