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I have an 1980ies steel road bike which I used to ride clipless pedals on. I have recently switched to platform pedals for commuting and found out that when I'm coasting and move my feet left and right, I can flex the frame a little bit. The flex is so strong that I can tell only by looking at it that the BB is not aligned centrally under the top tube / seat tube. When I quit the pressure, everything goes back to normal again. Steel frames are known to have some flex, but is this normal?

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    I don´t know if this is normal. But I used to ride an old bike with a steel frame and Reynolds 531 tubing on a daily basis when I was in my twenties, and could definitely see the flexibility of the frame when I put pressure on the pedals. It was indeed very obvious similar to your description but never caused any problems during the approx. 10 years I used the bike. – Snijderfrey Dec 7 '19 at 20:36
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    youtu.be/73inseR2zwI – Lamar Latrell Dec 8 '19 at 0:09
  • One way to measure frame flex is to rest the leading edge of the bike's front wheel into a wall, then hold the brake levers closed with your hands. With the crank level, stand on the front pedal. Bike won't move (though take care to not fall over!) and visually look at the BB in relation to a ruler lying sideways on the ground underneath the BB. My old steel bike has over a centimetre of flex. – Criggie Dec 8 '19 at 10:48
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It's hard to tell how much flexing is happening when you're on the bike due to your movement when pedaling, particularly if you're not smooth.

The answer is "some" (all frames will flex a bit when you ride), but putting a number or picture to it would depend to some extent on how much weight and force you have on the bike and the frame type and stuff too. For example, a 300 pound rider can probably get the same frame to flex a lot more easily than a 100 pound rider (assuming they fit, etc.)

I'd also check is if the frame has any damage (e.g. cracks, dents, etc.) that is causing it to flex more than normal.

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    If the frame is built with lugs check the edges of the lugs for cracked paint where the tubes have moved in the lugs. – Carel Dec 7 '19 at 17:00
  • Are you experiencing “ghost shifts” when pedaling hard? That’s when the frame flexes so much that the rear derailleur changes alignment with the freewheel. If not, what’s the problem with it flexing? Also, do you know what kind of tubing it’s made of? The really lightweight stuff flexes more – Andrew Dec 7 '19 at 22:15
  • No, there are no gost shifts. I don't know what the exact material is. The problem is that I feel a lot less safe occasionally hopping off a curb with this in mind. – Equi Dec 8 '19 at 3:23
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It sounds to me like your BB may be loose.

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  • No, the BB is perfectly fine. It's the actual frame that is bending. – Equi Dec 8 '19 at 3:19

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