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Tonkabaydog
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Discussion Starter · #1 ·
I was looking at a new CF frame today and wanted to see how rigid its tubing walls were. So, I grabbed the frame top tube and squeezed, then I did the same on the down tube. Both tubes are substantial in diameter, but both tubes demonstrated some "pliability". In other words, they were not solid feeling, they actually sqeezed in slightly.

Is this normal or something to be concerned about?
 

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It's normal. Same with carbon rims. Seatpost and handlebar tubes are much thicker than frame tubes and don't squish... in my experience.
 

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Tonkabaydog
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Discussion Starter · #3 ·
padawan716 said:
It's normal. Same with carbon rims. Seatpost and handlebar tubes are much thicker than frame tubes and don't squish... in my experience.
Not to be flippant, but normal or the "norm"? I don't recall this being the case with any name brands, i.e. Giant, Specialized, Look, Scott or others.

Is it more the norm for lighter CF frames?
 

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tonkabaydog said:
Not to be flippant, but normal or the "norm"? I don't recall this being the case with any name brands, i.e. Giant, Specialized, Look, Scott or others.
If you look at this review of the Scott Addict they mention exactly the same top-tube sqeezability:

http://www.roadbikeaction.com/Bike-Tests/content/66/2299/Scott-Addict-R1.html

It's more pronounced on some frames than others, but it's always localized - no way will you be able to squeeze the headtube! And of course, just because you can squeeze a tube doesn't mean it's not strong in the direction that it has to resist force. An egg shell is strong if you squeeze it from end to end, but not from the sides.

Maybe it's actually a sign of a high quality, well designed frame, where the strength is concentrated where its needed to reduce weight. If you are making a frame cheaply it is easier to overbuild it in order the achieve the required strength.
 

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The larger the tube diameter the thinner the tube wall. If you try that on a lugged carbon frame (Look, Time, Colnago) there will no pliability.

One of my concerns with carbon framesis that I would puncture a thin walled frame on a sharp object. I tend to be less than careful with them especially when transporting them so I have avoided those frames with exagerated tube shapes.
 

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Sucker for carbon
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smoo and ewitz are correct. The frame is designed to handle stresses in the directions and locations where the largest loads are applied under riding conditions. Carbon is an anisotropic material, meaning its strength depends on the direction of the load. It'll do great under most tensile and torsional loads, but as ewitz mentioned, it can fail pretty easily under shear.
 
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