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Carbon frame compressed.

Old 05-05-18, 02:28 AM
  #1  
rublow
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Carbon frame compressed.

While trying to locate a noise from a fork/headset I push a top tube with my hand. I felt how it bent and returned to a previous shape. There are no obvious visual damage, a paint seems to be uncracked, however I am wondering how my unwise behavior could influence a frame performance?
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Old 05-05-18, 03:07 AM
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Would be very suspect that the carbon 'bent', as the material doesn't do that (in the frame application), thinking you wrist bent naturally, and you took this to be the frame.
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Old 05-05-18, 03:20 AM
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Originally Posted by jimc101 View Post
Would be very suspect that the carbon 'bent', as the material doesn't do that (in the frame application), thinking you wrist bent naturally, and you took this to be the frame.
Of course it is possible, however I pushed it very strong with a body weight and felt this 'point bent'.
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Old 05-05-18, 10:01 AM
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where do you clamp the bike on a workstand, top tube? mightta crushed it
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Old 05-05-18, 02:29 PM
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An engineering decision was made on how many layers of carbon fiber cloth was enough
for the stresses a part of the frame would see, then using minimal material where it is sufficient to save weight.

you wish your carbon bike weighed more?
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Old 05-05-18, 03:35 PM
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I’d throw that frame in the trash. Any frame that I can compress the tubes by hand is no frame for me.

Under load, that tube will buckle and then... bye bye.
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Old 05-05-18, 04:51 PM
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Double butted steel tubes at thicker tube wall on the ends than the middle for basically the same design goal..

But remember the Pros have a truck full of spares.. an so they are disposable..
And every season the truck gets restocked with new stuff..

Now with Carbon fiber racing yachts they are going for light too
and they are patching them together between many races..



...

Last edited by fietsbob; 05-05-18 at 05:02 PM.
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Old 05-05-18, 05:56 PM
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Originally Posted by fietsbob View Post
Double butted steel tubes at thicker tube wall on the ends than the middle for basically the same design goal.
Sorry, but it’s never ok to be able to compress a tube by hand. I’ve had many double butted steel frames and still do. cannot compress any tube on any of them. Not my titanium, not my aluminum.

the only frame I ever felt was as fragile as the OP’s was an old performance (forte’) aluminum mtb hardtail I bought maybe 15 years ago. Those tubes were FRAGILE. Bottom tube felt like a beer can. Frame failed at the bottom tube/ headtube joint after a pretty mild cross country ride. Didn’t have 5 rides on it.
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Old 05-05-18, 08:30 PM
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Jeeze... Frame materials are not infinitely stiff. Any tube can be compressed and rebound if done within it's "strength" limit. We use to do this with lightweight steel tubes decades ago and can still do this with carbon. The question, or trick, is to use enough pressure for some deflection but not deformation. One way to see the deflection is to watch the light reflection around one's finger/thumb. As the tube's surface deflects the light's reflection distorts. When the pressure is released the light reflection shows no distortion so no surface deformation remains.

One could use a controlled clamping device to really test this, oh that's right a repair stand jaw is such. measure the tube diameter at 90* to the clamp's compression before, during and after and report back.

there seems to be a miss guided, "on a pedestal", impression (yes, a pun that's fitting) of what constitutes tube damage and what happened before said damage is real. Andy
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Old 05-06-18, 05:02 AM
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Originally Posted by Andrew R Stewart View Post
Jeeze... Frame materials are not infinitely stiff. Any tube can be compressed and rebound if done within it's "strength" limit. We use to do this with lightweight steel tubes decades ago and can still do this with carbon. The question, or trick, is to use enough pressure for some deflection but not deformation. One way to see the deflection is to watch the light reflection around one's finger/thumb. As the tube's surface deflects the light's reflection distorts. When the pressure is released the light reflection shows no distortion so no surface deformation remains.

One could use a controlled clamping device to really test this, oh that's right a repair stand jaw is such. measure the tube diameter at 90* to the clamp's compression before, during and after and report back.

there seems to be a miss guided, "on a pedestal", impression (yes, a pun that's fitting) of what constitutes tube damage and what happened before said damage is real. Andy
I think I we can agreee on your statements. However, just leaning on a top tube shouldn’t invoke a ‘materials testing lab’ conversation. I’m sure he didn’t damage it; I just wouldn't ride a frame with main tubes that flexible.

jeez, if it visibly moves when casually leaned on, I’d rather not imagine what it might be doing as I’m doing sprint intervals or riding a rough gravel road.

But alas, my guess is it didn’t really flex, just felt like it.

Last edited by RobotGuy; 05-06-18 at 05:43 AM.
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Old 05-06-18, 05:52 AM
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Yes, you can flex the top tube on most carbon frames by hand. I showed this to a guy I saw sitting on his top tube. It was his first carbon bike and he doesn't sit on the top tube anymore.
​​​​​​
You can easily see the frame deflection if you angle the lighting just right.
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Old 05-06-18, 08:57 AM
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The frame tubes of any material have not been optimized for forces running radially into the tube's surface to any extent. As a bike sees far greater stress along the tube torsionally and bending/beam like the tubes are shaped (or with carbon the plies are oriented and numbered) to best handle the riding forces. Denting tubes (beer canning as example of a failure mode most here are familiar with) is quite easy, mere handle bar swings can do this. But as long as the forces don't result is the tube's skin failing (carbon cracking, delaminations-- metal denting/crimping) there's no problem but what's between the rider's ears. Andy
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Old 05-06-18, 02:15 PM
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Make, model, location of pliable carbon?
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Old 05-06-18, 07:38 PM
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Originally Posted by Darth_Firebolt View Post
Yes, you can flex the top tube on most carbon frames by hand. I showed this to a guy I saw sitting on his top tube. It was his first carbon bike and he doesn't sit on the top tube anymore.
​​​​​​
You can easily see the frame deflection if you angle the lighting just right.
i can see that being the case. I guess it just struck me that the op had deflected the wall of the top tube with his hand. That beer canned mtb frame still irritates me these 15 or so years later.
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