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flex?

Old 03-07-08, 10:58 AM
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flex?

So I've heard a lot of talk about bottom bracket flex. Could someone explain what this means? As I understand it (which isn't well, obviously), steel frames have more bb flex than aluminum, etc. I've only ridden steel and I've never noticed the bb bouncing around. Maybe if I were to ride a different frame I would notice it but I read somewhere that a flexy bb is old school Euro, which appeals to my natural snobishness. Anyway, I'm just curious.
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Old 03-07-08, 11:10 AM
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Generally if you have flex anywhere on the bike, it is energy that is loss and not going directly into the energy needed to propel you forward. Although in many cases a bike that is too stiff might be a ride that is not very comfortable. Some flexibility may provide a smoother ride. However Flex in the BB area has very little to do with comfort and much more to do with loss of power. The numbers are really small but a frame with little or no flex in the bb is a good thing. Many designs have move flex to other areas of the bike which will help with comfort, especially on long rides.

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Old 03-07-08, 11:35 AM
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This has been argued in the past, but some will say that a flexy bb doesn't lose energy, but rather stores it to be released on the return. In that reference, it doesn't matter until it gets so flexy that you'r chainring starts rubbing the derailleur. I'm no engineer, just repeating previous arguments, so take it as you will.
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Old 03-07-08, 01:21 PM
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Ok, I get that but can you actually feel it? Nearly every bike review mentions it. Maybe I need to ride lots of different bikes to notice it. Is it something you notice more when, say, climbing or sprinting?
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Old 03-07-08, 01:35 PM
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Some aluminum frames have a lot of BB flex. Some steel frames have very little. Depends on how the frame was designed, especially the tubing diameter and wall thickness. And yes, if a frame is very flexy, a strong rider can really feel it. The flex can disrupt your cadence and even cause your dérailleurs to shift unexpectedly.
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Old 03-07-08, 01:48 PM
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A prerequisite for any discussion on flex vs. efficiency.
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Old 03-07-08, 05:24 PM
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I have a Cannondale CAAD3 that doesn't flex much at all. I thought it was snobbery myself till I picked up an aluminum bike with small diameter tubing. It even flexed while climbing seated, enough to hear the chain rub the derailleur. I didn't think much of it till this happened. Trek replaced the frame with a much stiffer model. I can tell a bit difference in responsiveness, but I'm not a snob!


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Old 03-07-08, 09:07 PM
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Originally Posted by antimatt_er
Ok, I get that but can you actually feel it? Nearly every bike review mentions it. Maybe I need to ride lots of different bikes to notice it. Is it something you notice more when, say, climbing or sprinting?
I felt it a lot when I had a first generation Trek OCLV frame. I think I felt it a little on my ti frame, and I didn't seem to notice it on my steel, aluminum, or scandium frames. I'm sure they all flex, but it's a matter of how much. I know plenty of heavier riders who will complain that they can flex a bb enough to actually shift gears! That's why many tall people don't like ti.
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Old 03-07-08, 09:15 PM
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Originally Posted by urbanknight
I know plenty of heavier riders who will complain that they can flex a bb enough to actually shift gears! That's why many tall people don't like ti.
I had that problem with an old steel Bianchi but never with aluminum or ti. Hopefully never will with carbon either.
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Old 03-07-08, 09:25 PM
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Originally Posted by urbanknight
I felt it a lot when I had a first generation Trek OCLV frame. I think I felt it a little on my ti frame, and I didn't seem to notice it on my steel, aluminum, or scandium frames. I'm sure they all flex, but it's a matter of how much. I know plenty of heavier riders who will complain that they can flex a bb enough to actually shift gears! That's why many tall people don't like ti.
Better frame builders will butted and/or thicker tubing around the BB on bigger frames to avoid the flex problem. All lightweight frames are susceptible to this problem, not just Ti, though perhaps it is more expensive to work around with Ti.
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Old 03-07-08, 09:27 PM
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Makes sense. Tall people just complain a lot because the longer tubes allow more flex. I imagine compact geometry has remedied this a little?
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Old 03-07-08, 09:39 PM
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Originally Posted by Mr. Beanz
I have a Cannondale CAAD3 that doesn't flex much at all. I thought it was snobbery myself till I picked up an aluminum bike with small diameter tubing. It even flexed while climbing seated, enough to hear the chain rub the derailleur. I didn't think much of it till this happened. Trek replaced the frame with a much stiffer model. I can tell a bit difference in responsiveness, but I'm not a snob!


jayzus. were you sitting when you busted that mofo? did it make a loud CRACK! sound? looks like it might've.
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Old 03-07-08, 11:51 PM
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Hey Scotch, no no loud glorious crack sound. I was climbing a steep hill when I felt a lite snap. Looked down to notice my legs pedaling around and sort of sideways. Thought it was a chainring bolt at first but turned out to be the frame.
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Old 03-07-08, 11:56 PM
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Originally Posted by vic32amg
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