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PostPosted: Fri Apr 14, 2017 7:09 pm 
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Posts: 279
gewichtweenie wrote:
Seen an old magazine comparing steel makers who crafted 7 variants of a model using different tubing. differences were minimal.

Here's a similar one by Josh Poertner

http://www.slowtwitch.com/Tech/Thoughts ... _4571.html

That is Steel, metal, which is Isotropic material. So their stiffness of material are equal in any direction (but tubing make it different, however)

Carbon is Anisotropic material. Each layer of sheet is only strong in a direction it design for. So it would give very differrent result if we change the test from tube shaping of steel to tube shaping and lay-up of carbon.

a video of the subject with real carbon example
www.youtube.com/watch?v=_4ALUyrTe90


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Posted: Fri Apr 14, 2017 7:09 pm 


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PostPosted: Sat Apr 15, 2017 1:50 pm 
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Joined: Fri Sep 25, 2015 3:12 pm
Posts: 136
Hexsense wrote:
gewichtweenie wrote:
Seen an old magazine comparing steel makers who crafted 7 variants of a model using different tubing. differences were minimal.

Here's a similar one by Josh Poertner

http://www.slowtwitch.com/Tech/Thoughts ... _4571.html

That is Steel, metal, which is Isotropic material. So their stiffness of material are equal in any direction (but tubing make it different, however)

Carbon is Anisotropic material. Each layer of sheet is only strong in a direction it design for. So it would give very differrent result if we change the test from tube shaping of steel to tube shaping and lay-up of carbon.

a video of the subject with real carbon example
http://www.youtube.com/watch?v=_4ALUyrTe90


But bikes aren't made from single ply (=sheet) of carbon and the forces aren't aligned to the principle axes , meaning the anisotropy isn't fully exploited. The primary control in characteristic still being general tubing dimension and shape.

The real point being that unless a manufacturer truly and fully gimps 1 variant of their bike from the same mould (like, say, putting jello suspension, 14" tires, and 150hp motor on a porsche,), there is way more marketing value in saying "we used the best carbon in the best orientation in the best places" than there is in real appreciable differences.


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PostPosted: Sun Apr 16, 2017 6:19 am 
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Posts: 279
gewichtweenie wrote:
But bikes aren't made from single ply (=sheet) of carbon and the forces aren't aligned to the principle axes , meaning the anisotropy isn't fully exploited. The primary control in characteristic still being general tubing dimension and shape.

The real point being that unless a manufacturer truly and fully gimps 1 variant of their bike from the same mould (like, say, putting jello suspension, 14" tires, and 150hp motor on a porsche,), there is way more marketing value in saying "we used the best carbon in the best orientation in the best places" than there is in real appreciable differences.

what?
did you watch the video i posted?
for fork as an example, if i use purely 45 degree carbon sheet in the video it would be very tortional stiff (won't rotate) but can flex forward and backward quite a lot.
In another extreme, if i use purely straight 90 degree going down carbon. it will not flex forward and backward but it can twist which is obviously bad.

Real fork will not make from single type of sheet of carbon, but how much of each type and where would it place will dictate how it perform even if it has the same shape.

Then stem is another very good example in other way.
Stem require stiffness in all direction so making a carbon stem will require quite a lot of carbon sheets in different directions that it will not be any lighter than aluminium stem. Unlike other place of the bike where designer can tune stiffness in only some way and leave it to flex in other and also save weight by doing so, Carbon stem only improve aesthetic as it can be made to look real cool easier.


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PostPosted: Sun Apr 16, 2017 5:33 pm 
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Joined: Fri Sep 25, 2015 3:12 pm
Posts: 136
Hexsense wrote:
[
what?
did you watch the video i posted?
for fork as an example, if i use purely 45 degree carbon sheet in the video it would be very tortional stiff (won't rotate) but can flex forward and backward quite a lot.
In another extreme, if i use purely straight 90 degree going down carbon. it will not flex forward and backward but it can twist which is obviously bad.

Real fork will not make from single type of sheet of carbon, but how much of each type and where would it place will dictate how it perform even if it has the same shape.

.




Respectfully, acknowledgement of elementary material science should already have been shelved in this discussion.

As said before, a bike isn't built from a single strand of fiber or a plane of plies. It is a large'ish cylinder of fair thickness from many plies, with diverse stress vectors. Bending a simple bar of carbon as a demonstration serves only to exaggerate.

For example if you had to fill a mould of a beefy zipp stem with random sheets of commercial CF like t700 vs t800 vs t1000.... The end result wouldn't be much different than a layup resulting from 1000s of runs of FEA.

So back to the point, of a frame of same dimension, where both share the same conditions and target characteristics -- say a supersix vs supersix hm.....Unless the manufacturer willfully makes major sacrifices to that one weighs half or double the other , then their result will be very much the same.


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PostPosted: Sun Apr 16, 2017 7:58 pm 
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gewichtweenie wrote:

For example if you had to fill a mould of a beefy zipp stem with random sheets of commercial CF like t700 vs t800 vs t1000.... The end result wouldn't be much different than a layup resulting from 1000s of runs of FEA.

So back to the point, of a frame of same dimension, where both share the same conditions and target characteristics -- say a supersix vs supersix hm.....Unless the manufacturer willfully makes major sacrifices to that one weighs half or double the other , then their result will be very much the same.

Yeap, making a stem with carbon is like creating a tube material that require Isotropic property with an Anisotropic material. It need a lot of random direction to make it stiff in any direction. Grade of carbon doesn't have anything to do with this (lower stiffness carbon are heavier and thicker for the same stiffness of course). It's pretty much only shaping that matter, just like steel.

But other parts of the bike is not like that though, Tube shaping can be the same but internally it can use different mix of carbon layup (not necessary grade of carbon, just mixture ratio of different angle carbon sheet is enough, seat tube doesn't need much of twisting stiffness, chain stay can be made to completely stiff in all direction or only torsional stiff but can flex up and down for comfort etc. ). And ride different, as i give an exaggerated fork example above. My point is only about that you talking about different in ride is minimal between tube shaping of steel so assume the same for carbon, which is not that simple and safe to assume. It has tube shaping and also lay-up as another factor to make difference in ride quality. But whatever, go back to thread 8)


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