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flex - crankset or frame?

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flex - crankset or frame?

Old 11-04-08, 12:29 AM
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flex - crankset or frame?

I have a FSA energy crankset on one of my bikes (titanium) with an unknown BB. I don't particularly know too much about this crankset since I've always used shimano ultegra.

When I mash, I can feel (and see) swaying at the bottom. I guess the best way for me to describe is that I notice the crankarm deviate "inwards" at the bottom of my pedal stroke.

I want to know: is a new crankset/BB going to reduce flex? or is my frame just flexy? I want to know so that i don't unnecessarily spend money on a new crankset if my frame is doomed to be flexy!
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Old 11-04-08, 12:47 AM
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Frame almost certainly. Crank change won't help or BB for that matter.
All frames have side flex under torque at the BB shell, varies from frame
to frame. One reason why newer bikes have such big honking downtubes.
Litespeed down tubes in bi oval actually rotated the long axis of the oval
from vertical at the head tube to horizontal at the BB to stiffen up the BB.
Bike Tech flyer long ago had a machine they devised to measure stiffness
and one was side flex at the BB with a given load - variation was from 1-2"
depending on the frame (steel or Al in that era).

Last edited by sch; 11-04-08 at 12:07 PM.
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Old 11-04-08, 01:00 AM
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ack! that's bad news.. I guess I've been spoiled by riding cervelos, kleins, and cannondales.
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Old 11-04-08, 02:02 AM
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FWIW some people think that this springiness in steel or ti frames can be a good thing. The pedal stroke is modified so your power foot moves more directly underneath you (or something like that); they call it "planing." I tend to suspect it's not really good or bad but a matter of what you're used to.
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