Carbon seattube strength

[cribbit]

Has anyone had experience with determining the minimum wall thickness for a carbon seattube vs rider weight?

[Kontact]

Kind of weird question, unless you are building your own frame.

And even if you are, it is still meaningless:

http://3.bp.blogspot.com/-gOclg2-ojpg/Uw-xgcnHSNI/AAAAAAAAAck/qqdYS3y-yGY/s1600/kestrel.jpg

[cribbit]

Trying to make a custom diameter seat tube, figure if I'm doing that I might as well go carbon.

I measured some seat tubes I have, they seem to mostly be 2-2.5mm wall thickness. Wonder how overkill that is or if it's really needed.

[dustyrider]

Not sure, but I used an old carbon Easton seatpost as a breaker bar/wrench extension for years. I’m talking like standing and bouncing on it in order to break loose rusty what have you’s. I think I might have swapped that post onto a bike I flipped because I can’t find it anymore...I bet it’s still being ridden by someone!

[Kontact]

Trying to make a custom diameter seat tube, figure if I'm doing that I might as well go carbon.

I measured some seat tubes I have, they seem to mostly be 2-2.5mm wall thickness. Wonder how overkill that is or if it's really needed.

Wall thickness isn't just supporting vertical load but preventing wall collapse. You can only go so thin before there isn't enough matrix to work as a structural member.

But there is no minimum strength for the seat tube since it is not necessary to construct a bike. Its strength is dependent on the rest of the tubes. How strong are the stays and top tube? (Not a question that has an answer.)

[wangster]

Not sure, but I used an old carbon Easton seatpost as a breaker bar/wrench extension for years. I’m talking like standing and bouncing on it in order to break loose rusty what have you’s. I think I might have swapped that post onto a bike I flipped because I can’t find it anymore...I bet it’s still being ridden by someone!

reminds me of this video.

https://www.pinkbike.com/news/santa-cruz-bicycles-test-lab.html

[tumbler]

Trying to make a custom diameter seat tube, figure if I'm doing that I might as well go carbon.

I measured some seat tubes I have, they seem to mostly be 2-2.5mm wall thickness. Wonder how overkill that is or if it's really needed.

I'm no engineer and have no experience in this whatsoever, but I would probably try to match the existing seat tubes that you measured (assuming they were also carbon and from a reputable manufacturer) and wouldn't assume that the average seat tube is overbuilt for no reason. There isn't much to gain from skimming a few grams off a carbon tube that is located directly below your... lower regions.

[Drmojo]

Kind of weird question, unless you are building your own frame.

And even if you are, it is still meaningless:

http://3.bp.blogspot.com/-gOclg2-ojpg/Uw-xgcnHSNI/AAAAAAAAAck/qqdYS3y-yGY/s1600/kestrel.jpg

In Black--HED 3 spokes
(when I was a Tri-geek, and rode carbon ****e)

[wallymann]

Wall thickness isn't just supporting vertical load but preventing wall collapse. You can only go so thin before there isn't enough matrix to work as a structural member.

But there is no minimum strength for the seat tube since it is not necessary to construct a bike. Its strength is dependent on the rest of the tubes. How strong are the stays and top tube? (Not a question that has an answer.)

building on your 1st point, and contrary to your 2nd point -- would not the seat-tube need some sort of minimum wall thickness to effectively support the seatpin, to withstand big fore-aft torque loads and vertical compression loads to a lesser extent?

taken to extreme, a seat-tube with wall-thickness equivalent to a soda-can would not be usable at all!

this would be more of an issue for MTBs and roadies with a bunch of seatpin exposed. basically working in conjunction with the seatpins minimum insertion length.

[Kontact]

building on your 1st point, and contrary to your 2nd point -- would not the seat-tube need some sort of minimum wall thickness to effectively support the seatpin, to withstand big fore-aft torque loads and vertical compression loads to a lesser extent?

taken to extreme, a seat-tube with wall-thickness equivalent to a soda-can would not be usable at all!

this would be more of an issue for MTBs and roadies with a bunch of seatpin exposed. basically working in conjunction with the seatpins minimum insertion length.

Not really, because most bikes with oversized seat tubes have 2" long inserts to old the seat pin. The rest of the tube is too large for the seat pin to contact.

And you could make a bike with beer can thin tubing - and it would work until the first chip, dent, bump caused the wall to collapse. This is actually known as "the beer can problem", and it is part of what sets the lower limit on frame weight for different materials.

[lhuerta]

To OP, your question is premised on the assumption that wall thickness determines the "strength" of a a seattube or any carbon tube. The strength of a carbon tube will be more dependent on the type of carbon that is used, the layup characteristics, the epoxy/resin used to impregnate the fibers, the process with which voids are eliminated, etc.... My point is that wall thickness of a carbon tube tells you nothing about its strength without knowing other important characteristics of the tube in question. The same can be said about other materials....steel, Ti, aluminum...