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Discussion Starter · #1 ·
Here's an odd question. I've got a friend who bought a damaged carbon fiber frame. He sells chemicals that are made to repair carbon correctly. I'm sure he did great at that. Here is where my question comes in. He's a newbie to biking and to add additonal strength to the frame, he filled it with a expandable foam that when it hardens it becomes very stiff but light. Is he on to something here or is it likely to make the frame too stiff. I'm assuming they are engineered to flex ata certain rate to absorb shock with enough stiffness to be quick. What do you think?
 

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I'm not aware of any carbon frame that is solid or filled. I've heard of internally ribbing a frame kind of like webbing.

I would think filling would stiffen it up. My understanding of carbon tubing is that the shapes of the tubes make the bike more compliant or stiff. By filling the tubes wouldn't you change the ride characteristics drastically?
 

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I've seen race cars with expandable foam used to fill hollow unibody rails and other areas to help stiffen the chassis up. I never thought it was something you'd do to a bike. In cars, it can make a huge difference in stiffness, I wonder what it would do to a bike. That frame must ride like a brick.
 

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Squirrel Hunter
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Glass Ceiling

vontress said:
...to add additonal strength to the frame, he filled it with a expandable foam that when it hardens it becomes very stiff...
Glass is a very stiff material but that does not make it an ideal material for a bike frame.

vontress said:
...I've got a friend who bought a damaged carbon fiber frame.
He's a newbie to biking...
Always be certain you are no where near him in a paceline. Sounds like an accident waiting to happen.
 

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I actually thought abut doing this. I got a carbon frame thats damaged on the top tube. It looks like it was hit with something. From the outside it looks like its just some pant damage. But when you push on it its kind of soft. Like the fibers are broken on the inside. I'm going to send to Calfee. But the carpenter in me was like I wonder what would happen if I injected foam in the area. Not the whole frame just the damaged area.
 

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Bianchi used to (they may still) fill the seattubes with expandable foam on their RC aluminum frames. I raced on an FG Lite for 3 years with this filling, it did not assplode. I have often wondered why it's not done more.
 

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Some CF builders used to use foam instead of bladders to expand and push the CF into the mold but it certainly wasn't to make the frame stiffer.
 

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Discussion Starter · #8 ·
He didn't really do it to stiffen the frame. He did it when repairing. He really knows little about bikes. This is his first one. When he was telling me the story of the repair, the change in ride popped into my mind. I was just wondering what the more knowledgable thought. Always seemed like a bad idea to me. These product have been available for years and if they were needed, they would be there. Who knows, maybe he's on to something.
 

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pulser955 said:
I actually thought abut doing this. I got a carbon frame thats damaged on the top tube. It looks like it was hit with something. From the outside it looks like its just some pant damage. But when you push on it its kind of soft. Like the fibers are broken on the inside. I'm going to send to Calfee. But the carpenter in me was like I wonder what would happen if I injected foam in the area. Not the whole frame just the damaged area.
the foam wouldn't add any strength to the frame, get it repaired by calfee.
 

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vontress said:
He didn't really do it to stiffen the frame. He did it when repairing.
I don't claim to know a lot about this kind of thing, but I have a really difficult time seeing how filling a tube with foam (of all things) is going to add anything whatsoever to the structural integrity. The foam has absolutely zero capability of offsetting tensile forces and very, very minimal capacity for addressing compressive forces.

If carbon fibers in the tube have been broken, the tube no longer has the ability to take tensile forces. In this case the repair (at a minimum) needs to address the tensile issues. I don't believe foam can do this. This just sounds like a really bad (dangerous) idea for a carbon tube fix.
 

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SRAM is filling their new 10 speed MTB carbon crank arms with foam. I think it's more for the manufacturing process rather than providing structure. It's an interesting idea though. Personally I wouldn't think filling a whole frame with it would achieve a whole lot...





 

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huvia ja hyötyä
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There are applications where foam is used to construct strong and light structures. Model airplanes is one and there are man-carrying aircraft too, where foam is used to keep the skin of the structure in the correct shape for carrying the loads. The skin alone would not be strong enough but, with the foam ensuring the correct shape, it becomes strong enough.

The forces on an aircraft wing are different from those a bike frame meets, though.
 

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It seems that the foam would help prevent the skins from buckling at loads which are at the design limits for compression.
 

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The idea isn't far off - it is used in some carbon rims and in SRAM's crankarms as stated above. However, I would be a little concerned about it in a frame: What happens if water gets into the frame (very common) :confused: :confused:
 

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perttime said:
Model airplanes is one and there are man-carrying aircraft too, where foam is used to keep the skin of the structure in the correct shape for carrying the loads. The skin alone would not be strong enough but, with the foam ensuring the correct shape, it becomes strong enough.
Using an "engineered" foam in this application makes a lot of sense to me, but just as the skin alone wouldn't be strong enough, the foam alone wouldn't be strong enough either. The foam is good at one thing and the skin is good at another. The OP's friend had a damaged "skin" he was attempting repair. If run in the mill aerosol can expandable foam was used (my assumption), nothing of any significance has been done to address the damage to the skin. Until the skin has been repaired, you have "foam alone - without skin". --- I'm not attempting to refute anything perttime is saying; just making a related comment.
 

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I think the thread has "fragmented"...
- OP has a friend who fixed the skin and may want to add foam (if I got it right)
- somebody has broken fibers and is wondering if foam would fix it
- somebody seems to say that foam is useless as a structural element

I was responding to the last: it can be a structural element. But yeah: foam alone won't be much good.


.... there's people who want to build their own carbon frames. I think cutting the shape out of insulation foam sheets and skinning it with carbon is the usual method there.
 

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Discussion Starter · #18 ·
I was saying a friend already repaired the carbon fiber and added foam for additional strength. He sells products for automotive and aircraft uses. The foam I'm assuming he he used was to add strength and support. He would be well educated on the proper use. My concern was that he didn't know enough about bikes to understand how he might change the performance of the original engineering. I believe they use foam in aircrafts to add strength and remain light weight. It might be a good use in bikes.
 

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Discussion Starter · #19 ·
Reading Bicycling mag last night and read about company who puts foam in frame to absorb vibration. This was their big marketing pitch. I forgot what company it was. They were smaller company.
 

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There are 3 types of expanding foam i have used/heard about ,1st is from an aerosol can or ketchup bottle and is a Polyurethane that expands using humidity in the air (very fast or very slow cure), the 2nd is a 2 part polyurethane that foams when mixed due to the chemichal reaction of the products (fairly fast) and the 3rd is a "mastic"( blu tak ?) that expands with the heat needed for a pre-preg cure (fairly hot but i have never used ). I have heard of foam filled alloy tubes before and I think part of the reason o do it was to take the edge off sharp "hits" (damping effect) and stop it "ringing"
 
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