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Old 05-22-09, 09:00 PM   #1
canaxer
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dead spot in pedal stroke..

I find that near the top of the revolution on one side (right), my crank seems to skip, i.e. lose torque, for a cm or two when pushing relatively hard. It`s most noticeable when pushing up a moderate hill at about 85 rpm. Its an FSA MEGA EXO BB (2.5k km) with an FSA crank on a Caad8. I tried searching, but maybe i`m not using the right words...

thx for your input...
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Old 05-22-09, 10:05 PM   #2
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A "dead-spot" in a pedal revolution is well know. Guess it took a heavy foot to find it. This was the idea behind Biopace Chainrings and similar - to keep an even pressure going full-circle. It has to do with power-transfer and human physiology. You bike isn't broken - either you are, or your human.
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Old 05-23-09, 07:15 AM   #3
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A "dead-spot" in a pedal revolution is well know. Guess it took a heavy foot to find it. This was the idea behind Biopace Chainrings and similar - to keep an even pressure going full-circle. It has to do with power-transfer and human physiology. You bike isn't broken - either you are, or your human.
yah, i understand the physiology of the pedal stroke. it is not that, and much much more pronounced.
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Old 05-23-09, 07:26 AM   #4
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Do one legged drills on a trainer to correct that. Start with 30 seconds on each leg and increase to at least one minute. Also do a range of cadences including grinding our tough gears where you only go 45 - 50 rpm.
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Old 05-23-09, 07:58 AM   #5
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Are you trying to looking for a mechanical problem?
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Old 05-23-09, 10:04 AM   #6
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Are you trying to looking for a mechanical problem?
yes...maybe my title could be better worded, but the descrpition of my problem is apt.

i don`t have a problem with my pedal stroke, i`ve got multiple bikes, just have an issue with this one. The gap or looseness in the right side is much more than a dead area in my pedal stroke, something of which i would also notice on my other bikes, or during the thousands of km i`ve ridden. Also, i`ve had this bike for 4 years, so it`s not a new bike phenomenon.
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Old 05-23-09, 11:05 AM   #7
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Loose crank arm, maybe?
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Old 05-23-09, 11:12 PM   #8
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examine every link in the chain. i had a problem that i would describe tobe very similar to yours although it wasn't happening at the same point of the pedal stroke, but with every revolution or two. turned out a few rollers in the chain had partially disintegrated and were causing the chain to sit really poorly on those teeth. I replaced the chain and all was well again.

If it's at the same place in a pedal revolution definitely check the crank arms - and the pedals and chainrings!
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Old 05-24-09, 12:31 AM   #9
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Sounds like a loose crankarm to me.
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Old 05-24-09, 08:39 AM   #10
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I know that is what the old square taper BB would do when the crank was loose. I suppose the newer EXO's would as well, just have never experienced it on those.
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Old 05-24-09, 10:37 AM   #11
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loose crank arm or worn bearings in the bb
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Old 05-24-09, 10:46 AM   #12
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The reason this dead-spot wobble occurs at the top of the pedal-stroke is that you are reversing the torque on the arm. On the way up, the crankarm is actually pushed backwards by the dead-leg. That is, the arm is "trailing" the spindle since torque through the spindle is "pulling" the crankarm up to lift the left. Then at the top, you sequence reverses and your leg "pushes" on the crankarm and moves it into a "leading" position where it's ahead of the spindle.

The "dead spot" you feel in your foot is due to the clearance between the crankarm and spindle (loose crankarm). It's like a woodruff key that's going into too wide of a slot, depending upon the direction of load, it will slide back and forth inside the slot until it bumps up against the stop at the other side of the slot.

Last edited by DannoXYZ; 05-24-09 at 06:28 PM.
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