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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 10-16-10, 11:31 AM   #1
bigshuse
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bottom bracket spindle/crank offset

Ok this is a noob question for sure but I just noticed that there is a slight offset in the line of my crank arms. If you place the cranks horizontally and look at them from the rear at their level one clearly bends up slightly so that one pedal is 2-3mm above the other. I have a square taper bb so I know its not a spline issue. Is there any offset designed into this type of bb spindle or have I twisted the spindle out of shape. Also, could I have bent the cranks, they are aluminum but cheap?
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Old 10-16-10, 12:19 PM   #2
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Wich cranks are u talking about, what BB are u talking about?

No u should not have pedals off set from each other. Maybe the cranks are not right from factory, since u said u have a non expensive crankset, so this could be possible i guess.

BB are never off, but there was a guy who bought a set of cranks and one clearly was off like +8 mms, he sent a picture and was not even funny to look at it. Instead of having a 3:45 position he was like 3:50 position hehehe Just nutz
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Old 10-16-10, 12:32 PM   #3
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FSA Vero Crank. I don't know what brand my bb is. My cranks are less than a month old so maybe they just built them slightly crooked since it only cost like $50. I just happened to notice when I was laying down behind my bike. It is definitely a noticeable amount of offset. I just did not know if this was intentional, but now I do thanks!
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Old 10-16-10, 12:34 PM   #4
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Theoretically, the crank arms should be parallel (exactly 180 degrees from one another).

Just thinking out loud, other possibilities:

1) spindle flats are not correctly aligned to one another (manufacturing error).
2) crank arm could have slightly ovalized holes, and allowed misalignment

(Realize, a very small misalignment at the spindle is magnified at 175mm away (location of the pedal). Mathematically, arc length = (radius) (angle in radians) )
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Old 10-16-10, 12:34 PM   #5
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If you hit something with the pedal like the pavement, the pedal spindle will bend and the crank arm will bend also

even the best of them, with enough force, will bend.. like it picks up the bike rear wheel a one extreme .
pedal hits ground at 5:00 and the momentum picks up the bike as it is pushed over BDC, by forward motion..

Spindle bent?there will be a feel of the pedals rotating up as it goes around.


Note some crank designs require the spindle on the right to be longer
than the spindle on the left... if that is what you define as offset.


splined crank arms can be put on the wrong spline. and misaligned .

Last edited by fietsbob; 10-16-10 at 05:36 PM.
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Old 10-16-10, 12:36 PM   #6
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Also make sure the arms are on tight usually 30 to 35 foot pounds max.
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Old 10-16-10, 12:51 PM   #7
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honestly. just go to a bike shop and have them look it over. we can only guess what the problem is without inspecting and removing the crank/bb.
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Old 10-16-10, 01:38 PM   #8
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Quote:
Originally Posted by nwbikeman View Post
Also make sure the arms are on tight usually 30 to 35 foot pounds max.
That's what I was thinking too.

It's fairly common for the left crank arm to loosen and even to completely fall off. When that happens it means that the square hole in the aluminum crank arm has been boogered and the arm will never stay tight after that. The solution is to replace the crankarm and the problem is common enough that better bike shops keep generic left crank arms in stock.

Based on the information that you provided, I suspect that's what's happening to your bike.
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Old 10-16-10, 01:52 PM   #9
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Retro, I had a issue where my left crack came a bit loose during a long ride and I didn't tighten it until I got home. I bet that is the problem. Thanks!
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