Should i buy a carbon frame

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bungis
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Joined: Sun Oct 12, 2014 6:59 pm

by bungis

I guess the key question is if you prefer riding aluminum vs carbon frame. Not one is superior in terms of ride quality because of varied preferences.

If you enjoy aluminum, I'd say stick with that.

Raineman
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by Raineman

IIRC an FSA rep said it takes more to damage CF handlebars than Al ones, so wouldn't surprised it the same was true with frames. The difference is that cracked carbon makes for better pictures.

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treetrees
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by treetrees

If that's the case with carbon being stronger how come over tightening bars stem and seatpost crushes and damages it so easily and Ali doesn't

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ITTY
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by ITTY

From Wikipedia:
The physical properties of composite materials are generally not isotropic (independent of direction of applied force) in nature, but rather are typically anisotropic (different depending on the direction of the applied force or load). For instance, the stiffness of a composite panel will often depend upon the orientation of the applied forces and/or moments. Panel stiffness is also dependent on the design of the panel. For instance, the fibre reinforcement and matrix used, the method of panel build, thermoset versus thermoplastic, type of weave, and orientation of fibre axis to the primary force.

In contrast, isotropic materials (for example, aluminium or steel), in standard wrought forms, typically have the same stiffness regardless of the directional orientation of the applied forces and/or moments.
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Samuel Sanchez Gonzalez
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by Samuel Sanchez Gonzalez

Cleaner wrote:I chose a CAAD10 frameset over the carbon offerings from Cannodale, Specialized, Trek, Giant, Cervelo and a few others. The geometry was the same as the SuperSix and I was not able to discern a difference in the ride or handling between the aluminum and carbon frame with the same geometry and both having a full carbon fork.

I bought the frame only and built it up with SRAM Red and Hollowgram SL crank and DT Swiss 240 hubs laced to HED C2 aluminum rims with Sapim X-ray. With these rims and a larger volume tire (23-25 mm) at 80-90 psi much of the road buzz is not transmitted. It is very noticable improvement over my first Cannondale, a Criterium 3.0 with a steel fork which would really beat on you over anything bumpy.


Sounds like a no nonsense, great race bike

Raineman
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by Raineman

treetrees wrote:If that's the case with carbon being stronger how come over tightening bars stem and seatpost crushes and damages it so easily and Ali doesn't

The word stronger is very vague. Composites are notoriously bad in compression/crushing due to their structure as the fibres are effectively useless in this loading. This why clamping them is a problem as you are clamping something not much stiffer than plastic.

In impacts you are effectively applying a loclaised bending. Here the stiff nature in tension of composites come through allowing it to take a bigger force for the same deflection. The composites also have a higher yield stress which means the can take a much bigger punch than Al.

treetrees
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by treetrees

I disagree Iam no stranger to carbon as I use it for fishing for the last 30 years and I did a test , ok it's not the same as what they use in bikes but bear with me so this test was a length of carbon pole which I held between my fingers at 1ft long and I applied pressure as I thought it was bending how it was supposed to but when I moved my fingers in to half way and applied pressure it snapped ,so my thinking is that it does not like localized pressure .

treetrees
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by treetrees

Also with stone chips how does that effect carbon as the material would surely be compromised if the strands have been damaged

treetrees
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by treetrees

Sorry for all the questions I'm just trying to get a feel for what it can and can't do . Thanks everyone for the response it's an absolute pleasure chatting

Raineman
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by Raineman

treetrees wrote:I disagree Iam no stranger to carbon as I use it for fishing for the last 30 years and I did a test , ok it's not the same as what they use in bikes but bear with me so this test was a length of carbon pole which I held between my fingers at 1ft long and I applied pressure as I thought it was bending how it was supposed to but when I moved my fingers in to half way and applied pressure it snapped ,so my thinking is that it does not like localized pressure .

This is effectively clamping so the same argument applies, just uniaxial rather than biaxial compression, but that doesn't change the maths for tubes really.
treetrees wrote:Also with stone chips how does that effect carbon as the material would surely be compromised if the strands have been damaged

There is a clearcoat/paint over the carbon to protect it from this. The same damage will be done to Al in this case

treetrees
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by treetrees

Cool

Svetty
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by Svetty

I treat my carbon bikes the same as my metal framed ones - just ride the hell out of them. If I stopped to worry about spontaneous failure I'd never cycle again. What does worry me is random events - eg animals running out in front of me at speed.

davidalone
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by davidalone

Mechanical engineer here, I'll chime in:

We will define 2 things here: strength and toughness.
strength: ability to withstand stress without failing. (i.e. up to the point of PERMANENT deformation.)
toughness: ability to plastically deform without fracture. ( i.e. it will bend, permanently, but wont break)
stiffness- the resistance to deformation.
catastrophic failure: a sudden, total breaking of the part.
fatigue failure: a slow failure, usually occurs when you get a crack. the crack grows slowl under repeated loads until it reaches a critical size, after which catastrophic failure occurs.

CF is a 'cloth' of fibers glued together with resin. essentially, thing cloth which has been hardened with some sort of concretre, superglue. etc.

this makes it anisotropic- it is stronger in tension than other modes. try to pull a T shirt apart. easy? no. theres the fiber giving the material it's strength. it doesn't do so well in shear . try to tear the same T shirt from one of the seams. easier, right? it gets especially easier if there's a prior crack/tear in the t shirt. CF doesn't do that well in compression, either. there are various ways of weaving- you can weave at 45 degrees, etc, so different varieites of CF will have slightly different properties as well.

CF has MUCH, MUCH higher strength than aluminium or steel. but it is not a tough material. thats why metals dent but CF cracks. which is also why catastrophic failure is usually what you see for CF. it makes for better pictures and scary stories. but that is BS. any force that causes catastrophic failure on a CF part is going to cause catastrophic failure on an equivalent metal part. as for fatigue failure, CF is actually MORE resistant to fatigue (subject to direction) thanks to its low toughness than metals. metals bend, so any crack is likely to grow larger over time. the only difference- you're likely to have noticed the crack/dent occuring and stop using the metal part.

so- your aluminium frame develops a crack at astress level of , say, 1000MPa ( just a rnado number.) over the next year or so you keep riding it. the crack grows bigger, you eventually notice and replace it. so it seems 'safe'. of course, if you didnt notice it, it could grow until it reaches a critical size, and then it just breaks one day when you're just riding along.
your CF frame, however, nothing happens at 1000 MPa. but when it gets hit by a 2000 MPa force, it breaks catastrophically. ( BUT a 2000 MPa force into an aluminium frame is almost for sure to have destroyed the same alu frame)
alternatively, a small crack appears at 1500 MPa in your carbon frame. it grows over time, but slowly as well. lesss slowly than the metal crack. same thing applies- you either notice and replace it, or you dont and one day it fails.

not all CF frames are equal, either. CF strength, stiffness depends largely on the layup- the direction in which the fibers are oriented. this is the secret sauce every company has their own- they will NEVER EVER share layup direction ( especially of forks) and resin. bike engineers will tune the bike properties based on layup direction. fibers along the BB area you want as stiff as possible, so they are longitudinal to the BB. you want some flex in the seat stays and seat tube, so most likely orient them at an angle.

your fishing pole experiment has nothing to do with carbon fiber as a material. it has to do with geometry and dimensions. CF fishing rods are designed to flex- flexing disspates the energy. by moving closer you restricted the flex and concentrated the bending stress- that energy has nowhere to go. go look at 'polar moment of area' if you want to know more. NO material likes concentrated stress, which is why engineers try to avoid it always. . over tighten an alu bar and you dent it- its the toughness that keeps it intact

You are right to say that there may be variances in layups based on human error. I wouldn't be concerned about this from any of the big asian factories- giant, merida. the way their operation is run minimizes this risk. OEMs, I have no idea. maybe, maybe not.
Alot of past carbon fiber accidents you have heard about are plain bad luck. if you hit hard enough CF at the wrong angle, it will fail. thats why alot of those hard luck stories you hear are weird things- derailleur breaks and slams into chainstay, guy dropped a pot onto his frame, guy crashed into a kerb... etc. it's unlikely to happen, but engineers can't take into consideration everything. but a disclaimer- morden composites are so good that any hit that causes a CF part to catastroophicaly fail - i.e. to break completely- , is guaranteed to have destroyed a similar aluminium, steel, or titanium part, and at a lower stress. metal

If you are buying a CAAD 10 frame because aluminium is 'tougher', well, I'm sorry to say you've been had. the mordern CAAD frames- caad9 and 10 are superb bikes, cheap, stiff, and light, but compromises have to be made somewhere- they are more disposable than your average CF frame. to get that low weight, the tubes on the caad 10 are VERY thin. so thin that they don't do compression very well, either. it is very easy to dent a caad 10 frame. a friend dented one just by dropping a spanner on it. with denting comes cracks. cracks grow and cause things to fail. with thin walls come low yield strength as well, unless cannondale has found some secret aluminium ( I doubt so.) the CAAD 10 is a very nice, cheap, and disposable ace bike. but don't kid ourself that it's a hardy frame. older CAADs and other aluminium frames, yes. not the caad 10. the caad is precisely that- a very nice aluminium bike that doesnt cost too much, but is ultimately disposable as well.

I see no reason not to go with a CF bike if you want to. CF is a very strong material, stronger than people give it credit for. it resists cracks and fatigue well- I well let you in on a secret that it is rather well known that CF wing panels on large commercial jets are flying with cracks in them- but with a careful eye on them of course. it is immune to wet weather. all you really have to do is be vigilant with your bike. tighten things properly. inspect regularly, checking for cracks. even with cracsk, doesnt mean it's dead. ( I ride a cracked carbon seatpost - 3 years and going strong. but with regular inspection. ) composite manufacturing has come a long way in the past 10 years and the CF bikes we have now are very, very good. I have crashed several times on mine, with no ill effects. a local racer here raced ( and won) a 3 day tour on a specialized frame with the chainstay sheared through.

hope this helps.

efeballi
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by efeballi

^ :')
This should be recited on all alu vs. carbon threads.
I'd like to add that the type of carbon used also plays a role in the fragility/crashworthiness of the frame. For example an RCA has ultra extra hyper super high modulus fibers in it. This is the elastic modulus and determines how much you have to pull each fiber to get a unit amount of deflection. However, these fibers have the same (sometimes even less) ultimate tensile strength with lower-end fibers.
Having a very high elastic modulus allows the engineers to use less material to achieve the same stiffness (remember: amount of force to get deflection) in the frame. This has 2 consequences:
-it reduces the wall thickness. Think of it like a gas pipe versus a soda can, even if they were scaled to have the same compressive or torsional strength, it is much easier to dent a soda can.
-it reduces the ability of the material to tolerate local stresses. As all fibers have a very high modulus, none of them will be willing to bend(by lengthening) to adapt to the shape. And thus, snap. And create local defects, which grow with repeated cycles, and catastrophically fail.
So, lower-end carbon frames are less fragile than higher end ones. Cervelo R3 is hardier than RCA. Emonda S is sturdier than Emonda SLR. Scott Addict 20 is more bombproof than Addict Team Issue.

My two cents...


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kgt
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by kgt

Very nice posts from davidalone and efeballi.
@ treetrees. Unless a carbon frame is from a no-name chinese manufacturer you don't need to hesitate. Go for it.

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