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Flexible Frames(9 posts)

Flexible FramesDCP
Aug 16, 2002 1:30 PM
With the model year ending, I am thinking about getting a new bike lighter than the steel, not particularly high end, one I have now. I have seen light bikes which are not well engineered described as too flexible and I understand that to rob power and, I assume, adversely affect handling. Having only owned one road bike, I do not have much to compare to and am not sure what to look for in test ride. What are the characteristics of a frame with excessive flex?
Its just a matter of test riding the bikePODIUMBOUNDdotCA
Aug 16, 2002 5:32 PM
Its just a matter of test riding the bike. However, test rides can only tell you so much about the characteristics of the bike since you rarely are able to take it very far. So another test you can do is stand beside the bike, hold the seat and push on the bottom bracket with your foot and see how much flex their is. When done properly this is a good indication of the flex in the frame.

In my personal opinion if its a lighter stiff frame your after you should go for an aluminum. I really like Cannondales but beware of their factory wheels. They are made overly light and have broken spokes on me after about 1000 km.

Cheers,
Nick
PodiumBound.ca
That's a good test.hayaku
Aug 16, 2002 7:19 PM
I will give you a pretty good idea (comparitively) about the bike's BB stiffness. If you have the opportunity, do this test with a Fuji "Team" and a Cannondale, the difference is amazing.

I want a test that I can do on a bare frame to get an idea of the stiffness, anybody know one???
M.
BB testcyclopathic
Aug 17, 2002 6:37 AM
you can shift to smallest gear, set cranks parallel, apply rear brake and step on pedal. Depending on how flexy frame is and on how low gear bike has you'd see BB deflect 1/2 to 1".
BB testdivve
Aug 17, 2002 7:30 AM
Is it normal that the frame seems to flex more when you push down on the drive train side? I don't quite understand the physics, but it seems to happen on both my bikes.
longer levercyclopathic
Aug 17, 2002 10:34 AM
there're 2 forces applied one is the chain pulling back and another is weight applied to pedal. Since left pedal offset more from chainring torque will be higher and frame should twist more to drive side.

Your case seems to contradict maybe your right leg is stronger?

Also bikes with triples will twist ~40% more (30% smaller chainring and 8% wider BB).
longer leverdivve
Aug 17, 2002 12:46 PM
Nah...my legs are pretty well matched...I'm sure one is stronger than the other but nothing as obvious as that. The only explanation I could come up with was that the chain actually promotes the flex on the drive side, because the BB shell tends to twist downward and slightly to the rear on the side that you're applying force on. Meaning, that on the non-drive side the chain torque will counter act some of those forces and preventing the BB from twisting in as far....that's about my take on it. I have no idea whether that's accurate or not. I'm not an ME by trade.
not sure its so simpleJekyll
Aug 17, 2002 7:39 AM
If you perform the tests above on bikes with different wheels, forks and tires (plus factor in difference in inflation of the tire) and try to ascertain an objective opinion of the frame's actual stiffness at the BB you will most likely fail. Pushing a BB on a built bike will pretty much show you how flexible the wheels and tires are and almost nothing about the frame. A better test would be to put the bike on a trainer (one that holds the fork and rear axle) and watch deflection as you pedal. Probably rather inaccurate as well. Sheldon Brown performed a controlled test on frames which made sense but the data produced is less than useful to your current question because of the age of the test (I don't remember the URL - its on his web site).
Flexible frames don't really rob power (the amount of power robbed by flex is minimal - most is returned as the BB flexed back). Geometry and stiffness will play heavily into handling. Lack of flex at the BB does give you a feeling of instant acceleration.
The best thing you can do comes down to pretty much the same advice as almost all fit and feel questions: Ride the bikes..
The TarantulaSpoke Wrench
Aug 17, 2002 8:24 AM
Way back in the olden days, probably about the mid-70's Bicycling magazine comissioned a jig to measure frame stiffness that I believe was designed and built by Gary Klein. It was called the Tarantula. As I remember, the bike frame was clamped by the dropouts and a measured load was suspended from one pedal at the 3 o'clock position. Dial indicators measured bottom bracked lateral deflection and forward fork movement. Back then, when Bicycling tested a bike, they put it on the Tarantula and published the results compared with other bikes they had tested.

I don't recall ever reading an article explaining why they stopped using the Tarantula. I do vaguely remember there was some discussion about the validity of the data and that the data seemed to correlate poorly with their testers subjective opinions of the bikes.