# Carbon Fiber Seat Stays????



## Hellion Darklord (Feb 10, 2009)

I have what is probably a simple question for you frame builders but nobody seems to know for sure at the LBS's around here.

Why do manufacturers use composite materials in the seat stays? I thought that the compressive nature of carbon fiber composite materials was not something you wanted in a bicycle frame. Doesnt' carbon work best under loads that stretch the material rather than compress it? Is there more to it that just vibration control, or is there something to the carbon fiber "implants" in an aluminum frame that produces good vertical compliance and retains decent horizontal stiffness? 


This is a puzzle to me because it looks to me like the manufacturers are just going. 

"look, carbon fiber, you should buy this bike cause it has carbon fiber right here so you can see it really well." 

Wouldn't it make more sense to use a good butted aluminum curvy seatstay that is a lot more resistant to things like compression and cracking and still very vertically compliant?


Just Wondering

HD


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## unterhausen (Sep 28, 2008)

I think you've broken the code. I like carbon forks a lot, anything else seems silly to me.


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## jay_ntwr (Feb 15, 2008)

Call Boeing and alert them to this shortcoming with carbon fiber immediately! The 777s have carbon wings in compression (at least on top of the wing). Clearly, they should have the software guys designing this stuff and not the mechanical guys as they've got no idea what they are doing apparently.


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## Schmitty (Sep 7, 2008)

You've got it HD. Marketing. Carbon may, or may not be 'appropriate' in that app. One thing for sure, a well designed (fit/function) frame trumps whether the stays are carbon or the same material as the rest of the frame.

I don't think a carbon plug in stay bike necessarily rides bad, but it kinda begs the question, why not go full carbon. 'uh because I have an oxy setup outside and don't know how to make carbon'. Or some folks simply aren't into the material. Being stuck in the middle seems a bit odd, as do the hard core old guard lug heads who pooh pooh everything new, and then proceed to deck their lug frames with every hokey carbon component known to mankind. But 1 1/8 headsets and carbon forks... no way.

-Schmitty-


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## dbohemian (Mar 25, 2007)

jay_ntwr said:


> Call Boeing and alert them to this shortcoming with carbon fiber immediately! The 777s have carbon wings in compression (at least on top of the wing). Clearly, they should have the software guys designing this stuff and not the mechanical guys as they've got no idea what they are doing apparently.


I could be wrong here but I do believe that the Boeing 777 has CFRP only on the wing trailing edge, tailfin, elevators and a few other things like engine nacelles and landing gear doors. The top of the wing is not carbon and most of the structures are not in heavy compression. Boeing 787 dreamliner has carbon fiber wings. Then again, they haven't delivered 787 yet. Some experts are still very sceptical of the durability of the carbon structures. Guess we will have to wait and see. I am looking to flying on one someday.

Dave B
Bohemian


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## unterhausen (Sep 28, 2008)

jay_ntwr said:


> Call Boeing and alert them to this shortcoming with carbon fiber immediately! The 777s have carbon wings in compression (at least on top of the wing).


Obviously, carbon stays are successfully used on top of the line bicycles, so there is nothing wrong with them at all. Carbon rear triangles make sense on a full carbon bike. I don't really see the purpose of a hybrid carbon/steel frame. Why not a carbon main triangle and steel/alu rear triangle?


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## pvd (Jan 4, 2006)

I'd take carbon seatstays over anything else in a second. I don't know what anyone else is thinking. What's good for forks is good for seatstays.


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## buckoW (Feb 7, 2007)

The seatstays on my dh bike are carbon and are much stiffer and 50g lighter than the aluminum ones. They are holding up very well and have withstood plenty of impacts.


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## jay_ntwr (Feb 15, 2008)

dbohemian said:


> I could be wrong here but I do believe that the Boeing 777 has CFRP only on the wing trailing edge, tailfin, elevators and a few other things like engine nacelles and landing gear doors. The top of the wing is not carbon and most of the structures are not in heavy compression. Boeing 787 dreamliner has carbon fiber wings. Then again, they haven't delivered 787 yet. Some experts are still very sceptical of the durability of the carbon structures. Guess we will have to wait and see. I am looking to flying on one someday.
> 
> Dave B
> Bohemian


Dave, I stand corrected. I've got no idea where I got that from I suppose as I can't find anything to reference it. It must have been hearsay that I repeated.


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## compositepro (Jun 21, 2007)

at last it begins to dawn on people carbon aint so bad afterall


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## unterhausen (Sep 28, 2008)

compositepro said:


> at last it begins to dawn on people carbon aint so bad afterall


Carbon is great. I realized after that last picture that I know nothing about mountain bikes so I should STFU


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## compositepro (Jun 21, 2007)

no fella when youve been around carbon you hear all the horror this and that the naysayers and then you know the actual truth its far better than many imagine and to be honest sometimes even better.

I suppose it comes down to acceptence and whilst its my preferred tool of choice sometimes its not the best ...but its been a long time coming for people to accept it


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## MrCookie (Apr 24, 2005)

Carpet fibre is good stuffs.
Here's where I work and do my composites stuff.

http://www.cessna.com/single-engine/cessna-400/cessna-400-features.html


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## compositepro (Jun 21, 2007)

which bit of the cessnas do you work on 

jets or the prop driven???

always interested to find out what peopl do


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## JOURNEYC6 (Jun 7, 2008)

Here we go again, the old carbon contest. I'm pro carbon so here we go......Airbus has more carbon than Boeing on the biggest passanger plane to fly....F1 cars are 90% carbon, the suspension on those cars have comprersion, extension forces that equal the G forces oln a fighter plane. The problem to me is that on the bike industry CF.is designed to imitated the desing of Ti, steel or aluminum couterparts. To achive the real eficiency of CF. it has to be designed different. The "tube" is the norm for a steel or aluminum on a triangle frame, with carbon we can make any shape that we want, but again we still see triangle tube CF frames. A good example of CF is the Lahar bike.


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## byknuts (Aug 9, 2008)

carbon fiber as a seatstay unit can be easily designed through fiber orientation and different epoxies to offer either vertical compliance or an ultra-rigid incompressible structure.

there are few places where i think the ease of manipulation of the material and its characteristics would be more appropriate...


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## Hellion Darklord (Feb 10, 2009)

*What about a trailer?*

I want to build a carbon fiber single wheeled trailer. It would need to be able to carry a full 7 gal. propane tank through the woods from my Fifth Wheel Trailer to the nearest gas station with LPG. (Liquid Propane Gas) And depending on location that could be 16 miles or more.

Would anyone know how to do that kind of thing? Would Fiberglass be cheaper, but just as good if not a little heavier? Hills are fine and I have used a BOB Yak 14, but need to carry a heavier load. I have a Gary Fisher Bitter that I have done about everything on. (I upgraded to a 140mm Dirt Jumper III fork recently)

Thanks for any Infos


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## RoyDean (Jul 2, 2007)

Seriously, you need to buy a book on carbon fiber wet layup and and make a few practice parts before you attempt it. And even then, you'll need to figure out the mounting hardware for the trailer to the bike and the wheel to the bike, and the tank to the trailer.

Building a carbon fiber assembly (not just a single part like a number plate or a helmet visor) is serious work best left to seasoned vets.


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## Frank Tuesday (Jan 12, 2004)

Carbon fibers do only have strength in tension. The structure you see in the bike is not only carbon fibers. It is much like reinforced concrete in which a material that is very strong in tension (rebar and carbon fiber) is surrounded by another material which is relatively strong in compression (concrete and resin). Without the other, neither is sufficient to create an adequate structure. The other thing to remember is that when a member is compressed, there cannot be pure compression stress. The forces will not be uniaxial and this will cause a bending moment in the member. This will force one half of the the member to be in compression and the other half to be in tension. I guess EE and CSE don't include classes in solids, eh.


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## ATBScott (Jun 4, 2006)

If I read the OP's original question correctly, the answer may be more along the lines of:
Manufacturers use CF in the seat stays to allow them to tailor the ride of a rigid frame (usually Aluminum) to be more comfortable without losing strength in the structure as would happen if you were to thin the walls and decrease the diameter of the aluminum tubing to get the same compliance. Large diameter aluminum tube frames can be light and rigid, but not very good at dampening shock and vibrations. CF can be easily tuned by layup and shape to absorb some of the impact and vibration in the rear of the bike, yet still retain the lateral rigidity that makes the bike perform well. CF may also lower the weight of the frame, but in a lot of cases I don't think that is the goal as much as ride tuning. Looking cool, etc... is a side benefit.


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## JOURNEYC6 (Jun 7, 2008)

for the trailer start with some foam, resin and fiber glass. if you like it, do it with CF.


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## rocwandrer (Oct 19, 2008)

Frank Tuesday said:


> Carbon fibers do only have strength in tension. The structure you see in the bike is not only carbon fibers. It is much like reinforced concrete in which a material that is very strong in tension (rebar and carbon fiber) is surrounded by another material which is relatively strong in compression (concrete and resin). Without the other, neither is sufficient to create an adequate structure.


Nice internet forum analysis. Plausible, easy to understand, wrong (sorry).

The carbon fiber still does most of the work. It is way more complicated than this, but think of the resin matrix as more of an anti-buckling restraint for a slender column.

The carbon is less strong (in some cases 50% less strong) in compression mostly due to material properties, but as you said, partly also because of the "can't push on a rope" bucking issue.



Frank Tuesday said:


> The other thing to remember is that when a member is compressed, there cannot be pure compression stress. The forces will not be uniaxial and this will cause a bending moment in the member. This will force one half of the the member to be in compression and the other half to be in tension. I guess EE and CSE don't include classes in solids, eh.


That depends on geometry. Usually the tension due to the induced moment on a compression member is exceeded by the compression force, meaning the structure is in imbalanced compression, not bending. That is obviously the case in the seat stays pictured above, for example.


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