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Carbon vs. Composite

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Carbon vs. Composite

Old 07-08-06, 04:12 PM
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not2fast
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Carbon vs. Composite

The guy at the Trek store tells me, in his sales shpeal, to avoid any bike that says COMPOSITE, only get a true carbon.

So, what is "composite?" He said usually a fiberglass and resin mix?

Any one have any ideas on this? I figure just his way of keeping me out of other bike shops, but is there any truth to it? Is Composite as strong as Carbon?
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Old 07-08-06, 04:19 PM
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This reminds me of the accepted terminology of "alloy". Technically steel is an alloy as is titanium but in the cycling world, most people are referring to an aluminum alloy when they say "alloy". Carbon fibre is a composite. However, not all composites are carbon fibre based. Glass and kevlar fibres amongst others can also be used in composites. A composite is simply a material made using dissimilar categories of other materials. In the case of carbon fibre, it would be a plastic resin combined with carbon filiments. The term composite in the cycling world has throughout history meant different things. Sometimes the term refers to the construction methods rather than just the materials used. For instance, a while back aluminum tubes glued to steel lugs were considered composite bikes. Later on carbon fibre tubes glued to aluminum lugs were called composites.
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Old 07-08-06, 10:03 PM
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I think he might be taking a veiled shot at Giant, they refer to their CF bikes as Composite, but if you read the website info on the process the composite is created with carbon threading.
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Old 07-09-06, 01:02 AM
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Originally Posted by vampares
Perhaps he is referring to thicker, less dense carbon tubes that are sometimes employed. There is a certain advantage to the thicker less dense material as seen w/ aluminum which is a weaker metal than steal.
I've touched a little on this in a previous thread concerning weaves.

Originally Posted by khuon
The weave is made up of fibres. Each fibre is actually composed of a certain number of strands. A 1K weave will have 1,000 strands per fibre while a 3K weave will be composed of 3,000 strands. Thus this number can sort of be considered a "density" of carbon strands in a fibre.

For a given modulus of CF, a 3K weave will be three times as heavy as a 1K weave but a structure with a 3K weave will be stiffer than one made with a 1K weave. Note that this does not mean that all 3K structures will be stiffer than all 1K structures as manufacturers will use high modulus CF to achieve in a 1K weave structure what others do using a 3K weave structure.

When considering the material of CF to be used in a structure, an engineer or designer will look at three basic attributes. These are tensile strength, modulus (stiffness) and density. So why would someone choose a lower modulus CF when they can use a higher modulus one, use a lower strand count and still achieve the same stiffness at a lower weight? One word: strength. In actuality, intermediate modulus CF has a much higher tensile strength than high or ultrahigh modulus CF.

Now of course the weave and grade of CF prepreg is just one small part of the equation. The ride quality and strength are chiefly determined by design. A CF structure may not be a homogenous one either. Some layups may have a few layers of high-modulus combined with layers of intermediate modulus. And mixing of portions of the structure of one grade of CF with another section of the same structure with another grade of CF is not uncommon. This process along with directional alignment can be used to tune the properties of the structure.
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Old 07-09-06, 08:40 AM
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Originally Posted by vampares
As you may know, a hollow cylinder is stronger than a solid tube of the same material.
Many people dont understand this concept completely. The caveat is, "of the same weight!"

If I have an aluminum tube, outside diameter 1", inside diameter 7/8", and compare it to a solid aluminum bar, 7/8" outside diameter, the solid bar is stronger than the tube. Some people automatically think the tube is stronger than the solid piece.

Some people think that any tube is stronger than solid stock of the same outside diameter. This is absolutely wrong. That would imply that if I had a 1" bar, I could make it stronger by drilling 3/4" out of the center, and therefore making it a hollow tube. It is impossible to remove material, and make it stronger.
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Old 07-09-06, 09:16 AM
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Originally Posted by not2fast
The guy at the Trek store tells me, in his sales shpeal, to avoid any bike that says COMPOSITE, only get a true carbon.

So, what is "composite?" He said usually a fiberglass and resin mix?

Any one have any ideas on this? I figure just his way of keeping me out of other bike shops, but is there any truth to it? Is Composite as strong as Carbon?
What bikes say only composite? Carbon fibers frames are Advanced composites. as stated before they are weaves, prepreg and wetted making an extremely strong and light material that is not dependent on tube radius to increase tensil and torsional strengh. Early composite bikes were made of carbon and the method of forming tubes was thermoplastics. these were not stable and reacted to reheating, being left in a hot car , etc. most bikes today are newer advanced composite CF, aluminum, titanium or steel, oh yeah don't forget bamboo. comparing the old composites, that are obsolete, to the new advanced stuff is just sales mumbo jumbo, apples to oranges if you will.
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Old 07-09-06, 11:00 AM
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Valid comments above. My suggestion is to find a different lbs to buy from; one where the sales staff is more interested in helping you vice scaring. Bob
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Old 07-09-06, 12:02 PM
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Originally Posted by onRoffR
Early composite bikes were made of carbon and the method of forming tubes was thermoplastics. these were not stable and reacted to reheating, being left in a hot car , etc. most bikes today are newer advanced composite CF
I would not necessarily call thermosets more advanced than thermoplastics. Thermoplastics have their place and actually have some advantages over thermosets. Thermoplastics are different from thermosets. Thermosets use a two-step process whereby the initial chemical reaction (aided by heat and pressure) produces the linear chains and is then stopped. The final molding process (heat and pressure) continues the polymerisation and sets up the actual crosslinking. Typically this is done in two seperate phases of construction whereby the prepreg mats are delivered to the molder who will then lay up the mats to produce the shape of the structure and then finalise with the molding process. It's kind of like a par-baked pizza or bread.

Thermoplastics on the other hand use only heat and pressure to create the carbon-carbon crosslinking. Also when compared to thermosets, they have a lower strength to weight ratio and they have a lower compactness meaning the resulting structure will be comparitively heavier than a thermoset constructed frame. They however are less succeptable to abrasion and notch-failures than thermosets. They are also more ductile and can theoretically be "recycled", patched and repaired because their C-C chains are not formed chemically but rather only through heat and pressure. Thus heat and pressure can be used to undo and reform the crosslinking. But it's not an easy task by any means.

Thermoplastics is actually well suited for MTB applications where weight is not an overriding concern but durability and the ability to deviate from traditional shape constraints are more important. Richard Cunningham (Mantis) teamed together with GT back in the mid-1990s to produce a thermoplastic bike. Not all of the bike was CF however... only the main triangle. GT later went on to make a full thermoplastic frame. Mantis also produced a full thermoplastic frame called the Screaming V (a very rare bike that I still lust over). A year later, K2/Pro-Flex introduced the 4500C/5500C/Oz frames which were also full thermoplastic. Cannondale also produced the thermoplastic Raven. The Screaming V, K2 Oz and Cannondale Raven shared some common features... primarily in construction where the main frame was produced using two shell halves bonded to a central spine. In the Mantis and K2 bikes, this spine was made of CF. In the Cannondale bike, this was initially made of aluminum and then later on magnesium. Both K2 and Cannondale had been playing with thermoplastics before releasing a full frame. In the case of K2, they used thermoplastic construction (hot-coat outer layer) for some of their forks (Girvin).



Now if you want to talk advanced CF then look at multiwall and better yet singlewall nanotube CF.
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