# Carbon vs aluminum frame with respect to stiffness



## DennisF (Nov 4, 2011)

Is one material generally stiffer than the other?

Thanks


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## laffeaux (Jan 4, 2004)

Either can be as stiff or as not stiff as the designer of the frame intends them to be.


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## ljsmith (Oct 26, 2007)

As mentioned it really depends on the design of the frame. But generally speaking aluminum, due to its large tubes, is the stiffest frame material. Unfortunately it is stiff in good and bad ways and gives a very unforgiving ride. Carbon can be designed to be just as stiff as aluminum where you need it, yet be compliant in other areas so you get a much more comfortable ride. 

Carbon really is the ultimate frame material because it can be designed to give exactly the ride feel that you want. No metal (not steel, not titanium) can achieve this. The only problem is that carbon is much more susceptible to catastrophic damage from impacts than metal frames. In my opinion, if they can ever make carbon fiber that can take impacts as well as metal, then there would be no reason to ride anything but carbon except for price maybe. I believe Cannondale claims their ballisitc carbon can, but I am doubtful.


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## FelixUnger (Dec 28, 2011)

These are all the right answers in regards to bicycle frame building, but to answer the question directly, carbon by unit mass is potentially much stiffer than aluminum.

Trying to make a post with links in it and I need to cross over some magic posting number first.........why I added something that was not really adding anything.


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## FelixUnger (Dec 28, 2011)

Is the magic number 12?


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## eimkeith (Oct 16, 2010)

nope


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## FelixUnger (Dec 28, 2011)

Hmmm what about 15? I tried to have some positive input, but I don't have too much to say. Guess I will circulate on this one for a minute or two and then give up.

There is a thread I want to post in the deals forum, but I can not seem to do it.


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## FelixUnger (Dec 28, 2011)

Not 15 either it seems. 2 more and I am going home for the day.


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## FelixUnger (Dec 28, 2011)

This is number 17 and I promise not to post in this thread again.


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## eimkeith (Oct 16, 2010)

go for broke!


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## DennisF (Nov 4, 2011)

Thanks for the info guys. The backstory: I was having a conversation with someone else who is fairly new to Mountain biking, and he said that he heard that carbon was stiff and had a harsh ride. I rode a carbon bike and had the opposite impression. It was way-cool light, but _felt_ more like my old cro-moly Schwinn than my Trek Superfly AL.

But yeah, what LJSmith says makes sense when I think about it. I guess you can make any material, including steel, as stiff as you want if you use enough of it.

Which brings up another question.

I have another friend who is a big road guy, but has an old low-end REI aluminum mountain bike. It is HEAVY -- I think it weighs more than my Schwinn (which is 29.5# BTW), But to me his heavy aluminim bike is easier to pedal and feels more solid than my Schwinn. Ok, his components are a little better, but actually not much.

So why would you want a flexy frame? Aren't the bumps supposed to be dampened by tires/suspension/legs?

Thanks!


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## heyyall (Nov 10, 2011)

Flex and dampening are different. For example, the seat stays can be configured to minimize the direct transfer of energy of the rear wheel to the rider to make a ride smoother (dampening). Some bikes will have hour glass stays for this purpose Then, a rigid chainstay and bottom bracket minimizes flex from pedaling (flex). Different ride qualities can be accomplished with materials, tube sizes, shapes and wall thickness (butting). 

Now the old al frame you mention probably was built like a tank. No fancy tube work/forming. As such, it is probably very rigid and efficient for pedaling but might beat you to a pulp. 

In general, it is often striking how different bikes made of the same materials can feel while riding. Then when comparing across materials, virtually anything can happen. The most important thing is that you like how your bike fits you and feels. 

Virtually any materials can be configured to give a desired feel.


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## michael1 (Nov 17, 2011)

I am surprised that there are so many posts, but not a single right answer. Stiffness is not a characteristic of the material it is a characteristic of the structure i.e. the frame itself, thus any material can be used to make a structure that is as stiff as you want it. The modulus of a material measures its inherent stiffness. The modulus of carbon fiber composites is way higher than that of any metal alloy used for bicycles. So, generally carbon fiber bikes will have higher stiffness than aluminum bikes of comparable size and shape. However, if you made two bikes the same size and the same strength in every way-one of aluminum and one of carbon fiber-you might find that the aluminum one is stiffer, but I'm not sure. In all applications other than downhill and freeride, carbon fiber is a superior frame material, and it may be the superior for downhill and freeride as well.

As for the 'feel' of the bike, carbon fiber is much better because every part of the bike can be made exactly the way you want it. The tubes can be formed in elegant curves. Plus, the individual carbon fibers can be aligned in different directions to make the bike feel stiffer in some particular directions so you can really fine tune the feel unlike alloys, which are homogenous.


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## mimi1885 (Aug 12, 2006)

michael1 said:


> *I am surprised that there are so many posts, but not a single right answer*. Stiffness is not a characteristic of the material it is a characteristic of the structure i.e. the frame itself, thus any material can be used to make a structure that is as stiff as you want it. The modulus of a material measures its inherent stiffness. The modulus of carbon fiber composites is way higher than that of any metal alloy used for bicycles. So, generally carbon fiber bikes will have higher stiffness than aluminum bikes of comparable size and shape. *However, if you made two bikes the same size and the same strength in every way-one of aluminum and one of carbon fiber-you might find that the aluminum one is stiffer, but I'm not sure.* In all applications other than downhill and freeride, carbon fiber is a superior frame material, and it may be the superior for downhill and freeride as well.
> 
> As for the 'feel' of the bike, carbon fiber is much better because every part of the bike can be made exactly the way you want it. The tubes can be formed in elegant curves. Plus, the individual carbon fibers can be aligned in different directions to make the bike feel stiffer in some particular directions so you can really fine tune the feel unlike alloys, which are homogenous.


laffeaux hit it out of the park on the first try, lj is also right. OK felix had his own agenda.

I dunno what are you talking about. Plus, your example is weak as well, beside the fact that you said you are not sure, yet you are willing to rule every posters before you got it wrong.



laffeaux said:


> Either can be as stiff or as not stiff as the designer of the frame intends them to be.


I agree. Same was said about steel V.S. alu, Ti V.S. Alu, etc. The simple answer is it depends on what the designer wants. There are many other factors than materials alone. Tubes diameter plays key role in weight and stiffness.

The simple rule is if it's the same weight and shape carbon would be stronger, if it's the same size and strength carbon would be lighter.:thumbsup:


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## FelixUnger (Dec 28, 2011)

laffeaux hit it out of the park on the first try, lj is also right. OK felix had his own agenda. 




I thought I gave gave right answer in my first post on this thread-then was just trying to get some links up so I spammed a bit-thought better to do it one after another. Sorry for the posts. Not sure of the purpose, but I assume there were issues here in the past.


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## laffeaux (Jan 4, 2004)

And to expand on my original answer a bit...

Comparing aluminum to steel is a bit easier as both are alloys, which perform more consistently than a material like carbon fiber. If you were to build a frame from tube sets of identical dimensions - same length, same diameter, same wall thickness - and built with identical geometry, the steel frame would be stiffer. The steel frame would be stiffer, and the aluminum frame would be lighter. Steel is inherently a stiffer alloy than aluminum alloys.

However, nobody would build frames out of identical tube sets. It doesn't makes sense. A frame designer would select tube sets that give the desired ride. Thicker walled tubes are stiffer than thin walled tubes, and larger diameter tubes are stiffer than smaller diameter ones - regardless of the alloy. (Tube diameter has more of an impact than wall thickness.) A designer wishing to stiffen a frame selects the appropriate tube set to do so, keeping in mind that the main triangle of a bike is built from three "main tubes" and all of these can vary in diameter and wall-thickness.

So what's stiffer, an aluminum frame or a steel frame? The answer is the the stiffest frame is the one that was designed to be stiffest - it could be made from either material.

Carbon fiber is not an alloy. It's made from strands of carbon joined with a resin/epoxy. It's stiffness is determined not only by wall thickness and tube diameter but also by the strands of carbon per square inch, the number of layers of carbon, the weave of the carbon (do layers run perpendicular, parallel, or at some other angle), etc. There are lots of variables. Two carbon fiber tubes that look identical can be completely differently (unlike alloys). So two carbon frames that visually look identical can ride completely differently.

The person that designs the frame controls how the bike will ride. The material influences it, but the designer (hopefully) knows how to use the material to ride the way they intend it ride. Which goes back to my original answer: any material can be used to make a frame that are stiff or not.

And why would you want a less stiff frame? Part of it is personal preference. Most people are concerned about stiffness at the bottom bracket. I'm not aware of any study that definitely says a stiffer bottom bracket performs better, but many people prefer them. Some don't. Another factor is rider weight, a bike built for a 110 pound rider might feel super flexy and may not last long under a 250 pound rider. And a bike that works for the 250 rider might make the lighter complain that they're feeling "beat up" at the end of a ride.


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