# Aluminum Stiffness vs. Carbon



## TehYoyo (Sep 16, 2012)

I know that carbon fiber has a better _stiffness-to-weight ratio_, but is an average all-carbon bike stiffer than aluminum?

Or if, hypothetically, a bike manufacturer didn't care about weight (within reason) and tried to make a stiff bike, would an aluminum bike or a carbon bike be stiffer?


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## laffeaux (Dec 12, 2001)

Either can be made to be made into frames that are super stiff or super flimsy. Neither material is inherently stiffer than the other.


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## stevesbike (Jun 3, 2002)

that's not correct: CF has superior material properties. Stiffness is typically measured as modulus of elasticity, and CF is markedly superior to aluminum alloys in this regard. This is why you can make extremely stiff frames that weigh less than 800 grams with CF. Strength to weight and stiffness to weight are superior to aluminum. 

Obviously design comes into play as well, so comparing 'average' frames isn't very useful, since the goals of the design are maximize a frame along a number of dimensions. 



laffeaux said:


> ... Neither material is inherently stiffer than the other.


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## Zeet (Mar 24, 2013)

stevesbike said:


> that's not correct: CF has superior material properties. Stiffness is typically measured as modulus of elasticity, and CF is markedly superior to aluminum alloys in this regard. This is why you can make extremely stiff frames that weigh less than 800 grams with CF. Strength to weight and stiffness to weight are superior to aluminum.
> 
> Obviously design comes into play as well, so comparing 'average' frames isn't very useful, since the goals of the design are maximize a frame along a number of dimensions.


+1 ^ This! Once again, Steve is right on the money! :thumbsup:


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## Kerry Irons (Feb 25, 2002)

stevesbike said:


> that's not correct: CF has superior material properties.


The second statement laffeaux made was indeed incorrect. But the first statement he made: "Either can be made to be made into frames that are super stiff or super flimsy," was 100% correct. And that was really what the OP was asking. It's what people ask here all the time. And the correct answer to that question is that design is WAY, WAY, WAY more important than material.


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## Zeet (Mar 24, 2013)

Kerry Irons said:


> The second statement laffeaux made was indeed incorrect. But the first statement he made: "Either can be made to be made into frames that are super stiff or super flimsy," was 100% correct. And that was really what the OP was asking. It's what people ask here all the time. And the correct answer to that question is that design is WAY, WAY, WAY more important than material.


Then you should have no problem building a bicycle using snowflakes, liquid detergent, or motor oil.... You simply must start with some type of material that will at least theoretically comply with the physical demands of both its intended use, and the environmental conditions under which it's subjected. Therefore, the proper choice of material is of the utmost importance. Material choice must come first! However, once you've made that choice and the material chosen proves to be compliant with design applications, then the only limitation to those applications will be one's aptitude for ingenuity, creativity, and design capabilities. Bicycle technology is currently at a point where CF material can potentially surpass the physical attributes of aluminum and steel, when it comes to both strength and stiffness. However, the problem with this statement exist within the bicycle's design. Carbon fiber is the only frame material, where once you've selected it, design approaches the level of importance of the material choice, itself.


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## tihsepa (Nov 27, 2008)

Zeet said:


> Then you should have no problem building a bicycle using snowflakes, liquid detergent, or motor oil.... You simply must start with some type of material that will at least theoretically comply with the physical demands of both its intended use, and the environmental conditions under which it's subjected. Therefore, the proper choice of material is of the utmost importance. Material choice must come first! However, once you've made that choice and the material chosen proves to be compliant with design applications, then the only limitation to those applications will be one's aptitude for ingenuity, creativity, and design capabilities.


I guess you cant drink all day unless you start first thing in the morning. 

Do you have a translator for this?


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## Zeet (Mar 24, 2013)

tihsepa said:


> I guess you cant drink all day unless you start first thing in the morning.
> 
> Do you have a translator for this?


Cheers!


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## mikerp (Jul 24, 2011)

tihsepa said:


> I guess you cant drink all day unless you start first thing in the morning.
> 
> Do you have a translator for this?


No kidding


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## mikerp (Jul 24, 2011)

Kerry Irons said:


> The second statement laffeaux made was indeed incorrect. But the first statement he made: "Either can be made to be made into frames that are super stiff or super flimsy," was 100% correct. And that was really what the OP was asking. It's what people ask here all the time. And the correct answer to that question is that design is WAY, WAY, WAY more important than material.


Spot on +++++


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## exracer (Jun 6, 2005)

mikerp said:


> No kidding


Well he rambles for awhile (you know how old folks are). He makes a case for design being important but then makes the inaccurate statement of "carbon fiber is the only material, once selected does design become as important as the material itself".


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## Schneiderguy (Jan 9, 2005)

TehYoyo said:


> I know that carbon fiber has a better _stiffness-to-weight ratio_, but is an average all-carbon bike stiffer than aluminum?
> 
> Or if, hypothetically, a bike manufacturer didn't care about weight (within reason) and tried to make a stiff bike, would an aluminum bike or a carbon bike be stiffer?


I am curious as to what prompted the question. Are you interested in comparing aluminum v. C.F. bikes? I have steel frame, scandium frame and Ti frame bikes and like them all.


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## Dajianshan (Jul 15, 2007)

Stiffness is an oversold metric as a marketing euphemism for "performance" when real world subjectivities fail to sell bikes.


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## Zeet (Mar 24, 2013)

exracer said:


> Well he rambles for awhile (you know how old folks are). He makes a case for design being important but then makes the inaccurate statement of "carbon fiber is the only material, once selected does design become as important as the material itself".


Yes. I am old, but I know about frame materials. Carbon fiber is more design-dependent than all of the other bicycle frame materials, because of its intrinsic anisotropic property. Hopefully, your slow wit, and sharp tongue, will allow your smart arse to get old one day too!


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## Zeet (Mar 24, 2013)

Of course design is very important, but it's useless if a stupid material is selected! The frame material will always take precedence over design.


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## Mr Evil (Aug 12, 2011)

laffeaux said:


> Either can be made to be made into frames that are super stiff or super flimsy. Neither material is inherently stiffer than the other.


Only to a point. There are some hard limits on how much you can increase a frame's stiffness using a given material. Probably the most significant is the limited space that the chainstays must fit through. This means that you will never be able to make a metal frame as stiff as the stiffest possible carbon frame.


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## aclinjury (Sep 12, 2011)

Zeet said:


> Yes. I am old, but I know about frame materials. Carbon fiber is more design-dependent than all of the other bicycle frame materials, because of its intrinsic anisotropic property. *Hopefully, your slow wit, and sharp tongue, will allow your smart arse to get old one day too!*


the older we get, the truer this becomes!


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## Zeet (Mar 24, 2013)

aclinjury said:


> the older we get, the truer this becomes!


Unfortunately, smart arses don't really reach ripe old ages...They tend to get uprooted, early on!


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## aclinjury (Sep 12, 2011)

regarding choice of material vs. design, and which takes precedence. This really depends on the final product.

For example, the highly aerodynamic Lotus bike would not have been possible if not for composite. There was no way for the designer to make such bike using metal. This is not my word. It is the word of the Lotus designers.

But the Lotus is a high end piece of design. On the lower end of the spectrum, i.e., most carbon frame under $5000, I'd design takes precedent over material. 

But the 2 factors (design and material) always tend to feed off of each other though. Usually what happens is that desingers will start working with a material they have available on hands. Then over a period of time they will milk the material, optimizing the material until they can no longer. At that point, the designers start to wonder "wouldn't it be nice if I can have a material that would allow me to do such and such". This is when a breakthru in material will enable the designers to leap forward. Repeat cycle.


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## Zeet (Mar 24, 2013)

aclinjury said:


> regarding choice of material vs. design, and which takes precedence. This really depends on the final product.
> 
> For example, the highly aerodynamic Lotus bike would not have been possible if not for composite. There was no way for the designer to make such bike using metal. This is not my word. It is the word of the Lotus designers.
> 
> ...


No matter how you slice it, you've just got to start out with something that you can at least work with. The material has just got to be applicable to the duties that it's expected to perform, within the environment that it's expected to perform them. You first need some medium that you can at least imagine being a feasible candidate for a building material. You can't build a skyscraper with ice cream!


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## danl1 (Jul 23, 2005)

Zeet said:


> Of course design is very important, but it's useless if a stupid material is selected! The frame material will always take precedence over design.


A statement that is as true as it is worthless. No one asked about chocolate-bar bikes. Within the realistic choices for frame materials and the basic realities of frame design, any of those materials (steel, al, ti, carbon, bamboo, wood) can make a bike more than sufficiently stiff for the motor in question. The differences are come down to weight and a few quantitive characteristics. 



The question of an 'average' frame would likely go to carbon, but it's a meaningless answer, because you can only ride a particular frame, not an average of all frames That is, you can't pedal a number. A tiny-tubed carbon bike would be uselessly whippy (think flyrods) while a very-oversized aluminum bike is very stiff indeed. And vice versa. That's why it is most correct to say that design trumps material.


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## laffeaux (Dec 12, 2001)

stevesbike said:


> that's not correct: CF has superior material properties. Stiffness is typically measured as modulus of elasticity, and CF is markedly superior to aluminum alloys in this regard. This is why you can make extremely stiff frames that weigh less than 800 grams with CF. Strength to weight and stiffness to weight are superior to aluminum.


I wasn't really clear with my statement... My intentions were to say that a frame's material does not define the characteristics of a frame. Aluminum is not a very stiff material compared to steel, but most people equate aluminum with stiff frames and steel with less-stiff frames. The "stiffness" of a frame (or any characteristic of the frame) is based on the design.

Steel, aluminum, carbon fiber, bamboo, cardboard, magnesium, beryllium, and likely other materials have all been used to build bike frames. Each material has it's own properties. Which one builds the stiffest frame? The one that the designer decided to make the most stiff. Knowing the material used does nothing to let you know how stiff the bike will be or how it will ride.

And how "stiff" a bike is in a given plane, may have little to do with how well the bike rides or how fast it is.


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## Zeet (Mar 24, 2013)

danl1 said:


> A statement that is as true as it is worthless. No one asked about chocolate-bar bikes. Within the realistic choices for frame materials and the basic realities of frame design, any of those materials (steel, al, ti, carbon, bamboo, wood) can make a bike more than sufficiently stiff for the motor in question. The differences are come down to weight and a few quantitive characteristics.
> 
> 
> 
> The question of an 'average' frame would likely go to carbon, but it's a meaningless answer, because you can only ride a particular frame, not an average of all frames That is, you can't pedal a number. A tiny-tubed carbon bike would be uselessly whippy (think flyrods) while a very-oversized aluminum bike is very stiff indeed. And vice versa. That's why it is most correct to say that design trumps material.


No. When the topic of potential building materials emerges. The universe is wide open. You must first only consider the candidates which are even remotely feasible. Though all elements, compounds, substrates, matrices, alloys, and composites are on the table, obviously most have to be removed as they are completely implausible for the application. However, some have intrinsic properties that make them probable material candidates. For those implausible candidates, there is absolutely nothing the best of engineers, designers, or scientists, can do for them. They're all eliminated! OTOH, there might be a few potential candidates with applicable intrinsic properties. For these candidates, designers and engineers can begin to assess and plan for possible prototypes. The less viable intrinsic properties innately given to the material, the greater the demand in the ingenuity of design.


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## tihsepa (Nov 27, 2008)

Zeet said:


> No. When the topic of potential building materials emerges. The universe is wide open. You must first only consider the candidates which are even remotely feasible. Though all elements, compounds, substrates, matrices, alloys, and composites are on the table, obviously most have to be removed as they are completely implausible for the application. However, some have intrinsic properties that make them probable material candidates. For those implausible candidates, there is absolutely nothing the best of engineers, designers, or scientists, can do for them. They're all eliminated! OTOH, there might be a few potential candidates with applicable intrinsic properties. For these candidates, designers and engineers can begin to assess and plan for possible prototypes. The less viable intrinsic properties innately given to the material, the greater the demand in the ingenuity of design.


I am confused. You win. I leave you with this.


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## Zeet (Mar 24, 2013)

tihsepa said:


> I am confused. You win. I leave you with this.


:crazy:


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## aclinjury (Sep 12, 2011)

Zeet said:


> No matter how you slice it...
> 
> ....You can't build a skyscraper with ice cream!


oh no?

are these not ice cream scrappers?
View attachment 283517


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## Zeet (Mar 24, 2013)

aclinjury said:


> oh no?
> 
> are these not ice cream scrappers?
> View attachment 283517


Alright! So I lied!


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## exracer (Jun 6, 2005)

Zeet said:


> Yes. I am old, but I know about frame materials.


Yes, I've noticed that.



Zeet said:


> Carbon fiber is more design-dependent than all of the other bicycle frame materials, because of its intrinsic anisotropic property.


Maybe, if you said this in the first place I would have agreed with you.



Zeet said:


> Hopefully, your slow wit, and sharp tongue, will allow your smart arse to get old one day too!


Thank you for your kind words.


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## Zeet (Mar 24, 2013)

exracer said:


> Yes, I've noticed that.
> 
> 
> 
> ...


Caught me on a bad day...I apologize...Have a nice day!


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## serious (May 2, 2006)

Zeet: *The frame material will always take precedence over design.*

For bike builders dumb enough to consider using "snowflakes, liquid detergent, or motor oil" this is true.


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## Zeet (Mar 24, 2013)

serious said:


> Zeet: *The frame material will always take precedence over design.*
> 
> For bike builders dumb enough to consider using "snowflakes, liquid detergent, or motor oil" this is true.


 Nope! It's true for the not so dumb designers too! It's true for everybody!


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## Hooben (Aug 22, 2004)

The frame I had back in the 1980's was a steel Bianchi and when I would climb the bottom bracket was all over the road. The chain would rub on the steel chainstay with every pedal stroke. Yet the bike was comfortable. 

My Cannondale Aluminum from 1987 was fantastic. The aluminum was stiff because of the extreme diameter of the tubing. It stood out like a sore thumb back then. The ride would rattle my fillings. It was rough. 

My Giant TCR Carbon that I own today is stiff. Yet the ride is really nice, not so jarring. I think that the question shouldn't be which is stiffest. The question should be which one do you enjoy more. For me, carbon fits the bill real well.


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## stevesbike (Jun 3, 2002)

look, it's pretty simple. With alloy, there are two basic ways design influences stiffness: wall thickness and tube diameter. Keeping wall thickness constant, stiffness increases with increasing diameter according to a power law, so this is why decreasing wall thickness while increasing diameter can result in increasing stiffness. There's an upper limit to this - I don't recall offhand its specifics but its a ratio of wall thickness to diameter. Beyond this, the tube suffers from buckling (like an empty beer can). Because aluminum is less dense than steel, you can make larger tubes with less weight penalty than steel. Weight and stiffness are competing goals - more material will allow for larger diameter tubes, but at the cost of weight. The ratio of thickness to diameter is a hard limit and there's no way of getting around these design limits that are a function of the material's intrinsic properties. 

This scenario isn't true for carbon because it isn't isotropic. So, there are many more ways to manipulate stiffness, particularly wrt layup schedules. 

Because of its strength and density properties, in principle for any alloy frame of some weight, there exists a stiffer CF frame of the same weight. That's a brute fact. 



serious said:


> Zeet: *The frame material will always take precedence over design.*
> 
> For bike builders dumb enough to consider using "snowflakes, liquid detergent, or motor oil" this is true.


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## danl1 (Jul 23, 2005)

That all has nothing whatever to do with the question asked. You sure can type a lot of words, considering that you can't read.


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## Zeet (Mar 24, 2013)

danl1 said:


> That all has nothing whatever to do with the question asked. You sure can type a lot of words, considering that you can't read.


We've all contributed towards answering the OP's original question. However, just like life itself, this thread has evolved with each participant's comment. Also, typing many words, automatically implies literacy.


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## exracer (Jun 6, 2005)

Zeet said:


> Caught me on a bad day...I apologize...Have a nice day!


No harm, no foul. As you noted I have quite the sharp tongue. I can be one sarcastic SOB and I'm quite the smart azz.

Carbon fiber has given designers far more latitude in the shaping/sizing of tubing, fine tuning of the ride to a greater degree than aluminum, steel or titanium ever did. Computer modeling, stress analysis let you impart whatever traits you want into a frame.


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## Camilo (Jun 23, 2007)

Hooben said:


> ... *I think that the question shouldn't be which is stiffest. The question should be which one do you enjoy more. ..*


This.....


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## terbennett (Apr 1, 2006)

Kerry Irons said:


> The second statement laffeaux made was indeed incorrect. But the first statement he made: "Either can be made to be made into frames that are super stiff or super flimsy," was 100% correct. And that was really what the OP was asking. It's what people ask here all the time. And the correct answer to that question is that design is WAY, WAY, WAY more important than material.


+1... Regardless of whether carbon has a better ratio,the reason the manufacturers give you numbers on a frames ratio and differences in weight is because you can't really feel it. Once someone tells you, then suddenly you can. Kerry Irons is dead on. Design is way more important than anything else. If it weren't, aluminum wouldn't be going through advancements too... Meaning there is still some discovering when it comes to aluminum as well. People thought that aluminum had reached it's point when carbon fiber came out, Not true. A new CAAD10 is way more advanced than the old CAAD8 and that was a great frame. My old 2005 E5 S-Works was an amazing frame too but the new Allez E5 is more advanced with many improvements. The thing is that with the improvements in any material is there really much of a difference in how it actually rides and performs from a real-world perspective? I doubt it, but as long asnumbers are being thrown out there, we're gonna think there is a difference. Again, it's more in the design than anything else. Carbon does have a better ratio, but let's be honest, that ratio is on the higher end bikes. Most peope aren't paying $8,000 plus on their bikes. OP said average carbon; Those aren't average carbon bikes. A $3,000 carbon bike that weighs less than 17 lbs (if one exists) being stiffer than a CAAD 10 or a custom built Tsunami? I think not.


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## Zeet (Mar 24, 2013)

Hey, this is really simple. The same principles apply to so many other things, as well. So ah..If you want to make a kite, the first thing that you'll need long before any plans are developed are the materials. After gathering and assembling the various materials from which the parts will be made, you then have to decide just exactly how you kite will be made and how all of the parts will be assembled together. Without, the initial materials gathered, no plans were ever possible, because you had exactly no idea as to what materials you might have been working with...


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## bigbill (Feb 15, 2005)

Zeet said:


> Hey, this is really simple. The same principles apply to so many other things, as well. So ah..If you want to make a kite, the first thing that you'll need long before any plans are developed are the materials. After gathering and assembling the various materials from which the parts will be made, you then have to decide just exactly how you kite will be made and how all of the parts will be assembled together. Without, the initial materials gathered, no plans were ever possible, because you had exactly no idea as to what materials you might have been working with...


Hey, I guess it is simple. I'm an engineer, tomorrow I'll tell my boss I need to order a bunch of material and then figure out a design to use the stuff I ordered because that's how engineering works, right?


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## Zeet (Mar 24, 2013)

bigbill said:


> Hey, I guess it is simple. I'm an engineer, tomorrow I'll tell my boss I need to order a bunch of material and then figure out a design to use the stuff I ordered because that's how engineering works, right?


No. It's like this....You get marooned on some island. The first thing you do is to assess the island for possible food and shelter resources. If there are bushes, but no trees, you'll use the bushes to thatch shelter. If there is bamboo available, you'll use bamboo. If you find yourself on an island, where wheatgrass grows thick and tall, then you'll already know, the source of both your food and your shelter. You might end up on an island where the only resources for shelter might be mud, sand, pebbles, and rocks. You'll have no idea, until you get marooned on an island, and make an assessment as to all of the possible materials that have shelter, food, or tool potential. Only once you've established just exactly which material you're going to use for shelter, can you begin to plan on how you're going to design your most humble abode. You can't thatch a roof with rocks! You first, must be given a material. You then assess that material for its intrinsic properties. Properties that you will then begin to use to your own benefit when planning a design. You can't use the same design for all possible materials, found. Only once you've selected your material, can you begin to think about a design. Material must come first! Only then can you begin to use your awesome designing powers of ingenuity and creativity, to bring out the very best physical attributes of the material that you've selected.


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## tihsepa (Nov 27, 2008)

Zeet said:


> No. It's like this....You get marooned on some island. The first thing you do is to assess the island for possible food and shelter resources. If there are bushes, but no trees, you'll use the bushes to thatch shelter. If there is bamboo available, you'll use bamboo. If you find yourself on an island, where wheatgrass grows thick and tall, then you'll already know, the source of both your food and your shelter. You might end up on an island where the only resources for shelter might be mud, sand, pebbles, and rocks. You'll have no idea, until you get marooned on an island, and make an assessment as to all of the possible materials that have shelter, food, or tool potential. Only once you've established just exactly which material you're going to use for shelter, can you begin to plan on how you're going to design your most humble abode. You can't thatch a roof with rocks! You first, must be given a material. You then assess that material for its intrinsic properties. Properties that you will then begin to use to your own benefit when planning a design. You can't use the same design for all possible materials, found. Only once you've selected your material, can you begin to think about a design. Material must come first! Only then can you begin to use your awesome designing powers of ingenuity and creativity, to bring out the very best physical attributes of the material that you've selected.


So who is building this bike again?

Gilligan? Or the Professor? 
Oh, your bike will probably get wet on an island. Go with titanium. 

Jeebus.


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## Zeet (Mar 24, 2013)

tihsepa said:


> So who is building this bike again?
> 
> Gilligan? Or the Professor?
> Oh, your bike will probably get wet on an island. Go with titanium.
> ...


Good money goes on the Professor!


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## aclinjury (Sep 12, 2011)

With carbon fiber, one can conceivably build a complete traditional bicycle using off the shelf carbon components for under 10 lbs! 

Let's see a builder design a traditional (2-triangle) bicycle frame and build up a complete bike using all metals components for under 10 lbs. Not possible. 

In the bicycle world, "design" isn't nearly as important as new material. The design of a traditional bicycle frame hasn't changed much. All the parameters such as tubset and joint angles are already well understood (at least for building a traditional 2-triangle bicycle frame). And whenever a change does take place, it occurs because of the introduction of new material.

Exactly what new design has taken place in the bicycle frame within the last 20 years? other than varying the parameters I've just mentioned?


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## terbennett (Apr 1, 2006)

Dajianshan said:


> Stiffness is an oversold metric as a marketing euphemism for "performance" when real world subjectivities fail to sell bikes.


+1......


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## bdavro (Jul 23, 2011)

Hi everyone, long time member and lurker, but this thread has caused me to log my first post as reading the comments here have been driving me crazy.

The design is always the most important for the following reason:

- determining WHAT you want to build (or how you want to build it) will always guide what you build it with.

Certain designs are simply not possible/ practical with certain materials and vice versa. 

As far as which is better, I have no comment.

Anyway, I'm glad to finally be participating instead of just lurking in the ethers.


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## stevesbike (Jun 3, 2002)

The bottom line is this: because of its strength and density properties, in principle for any alloy frame of some weight, there exists a stiffer CF frame of the same weight. That's just a consequence of intrinsic material properties. And that's what the OP asked about in the first place.




bdavro said:


> Hi everyone, long time member and lurker, but this thread has caused me to log my first post as reading the comments here have been driving me crazy.
> 
> The design is always the most important for the following reason:
> 
> ...


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