CF tube-to-tube construction, is filler/fillets "less desireable"

[CDollarsign]

As stated previously, the filler reduces the radius for the plies. During the curing process when heat is applied the fibers what to straighten. The filler also allows for better compaction of the ply stack reducing any porosity between the fiber and matrix.

[dddd]

I believe that the stress levels that are trying to pull the bent carbon material away from the surfaces that they are bonded to is inversely proportional to the bending radius of the fibers. So, by building up the "foam" to a larger radius of curvature, there is less de-laminating stress occurring (which might otherwise be the point of earliest failure, i.e. a weak spot in the layup).
The "foam" or "putty" is structural mainly in the sense that it supports the outer fiber layer, transferring loads in other directions besides along the longitudinal axis of the fibers.
With too tight of a radius at the outer layer of fiber, I imagine that there would be more of a tendency for the outer layer to forcefully delaminate away from the underlying tubes and epoxy fillet. This is the "stress riser" area that I believe robt57 alluded to, i.e. the location of the "crimp" that Carl mentioned.

All the photos have me wondering how much carbon dust ends up in the lungs and sinuses of the workers, not to say that brazing or welding fumes are any better for the body(?).

Looking at Parlee's traditional process, I've long wondered how his small-radius carbon joinery is executed, as by appearance it seems there must be some kind of internal reinforcement(???).

[Carl Strong]

Originally Posted by dddd

All the photos have me wondering how much carbon dust ends up in the lungs and sinuses of the workers, not to say that brazing or welding fumes are any better for the body(?).

Hopefully anyone working in either a welding environment or carbon dust environment have proper ventilation and are wearing protection.

Originally Posted by dddd

Looking at Parlee's traditional process, I've long wondered how his small-radius carbon joinery is executed, as by appearance it seems there must be some kind of internal reinforcement(???).

Parlee uses a "trapped rubber" process which you can see in their tour video at the 5 minute mark. They don't need internal reinforcement other than the tubes themselves.

[robt57]

With the foam, my minds eye wonders:

>if the carbon stays bonded to the foam
>how structural the inner foam layer is in terms of resistance to being separated from itself or carbon to foam bond breaking, etc.
>if that even comes into play as far a being needed once the carbon itself has it's cure.

[dddd]

Originally Posted by robt57

With the foam, my minds eye wonders:

>if the carbon stays bonded to the foam
>how structural the inner foam layer is in terms of resistance to being separated from itself or carbon to foam bond breaking, etc.
>if that even comes into play as far a being needed once the carbon itself has it's cure.

Yes, yes and yes.

I would think that the tighter the bend radius of the outer layers, the more that the "foam" needs to have strength/adhesion to support the stress concentration during use.

[Mark McM]

I would think that the foam carries little of the load. When two members act in parallel, loads are distributed between them in inverse proportion to their relative stiffness. As I was expect that the foam is much less stiff than the carbon, I would expect the carbon to take most of the load.

[robt57]

Originally Posted by Mark McM

I would expect the carbon to take most of the load.

As would I, being that is where the epoxy hardens to create the actual structure. But is the foam really merely a form I wonder? Leaning yes, but asking the question too.

If not there would more carbon need to be used. I totally get the physics of the inner and outer hardened layers creating a structure. A truss if you will, thus wider truss supports bigger span and like that. And ga. of members contributing etc. seems reasonable to draw parallels.

[nicrump]

I'm going to quote myself from an instagram post I did a while back. Then I'm going to go make myself a G&T.

p.s. It's not foam.

https://www.instagram.com/p/BZTf0btAjE6/

Every time I apply and sand these fillets I ask myself why I use this stuff. It's expensive, hard to apply neatly and very hard to sand. I use 3M dp420. It is a thixotropic high impact, toughened structural adhesive. It has the highest peel, tensile and compressive properties in it's class. Translation. It sticks like a mofo and flexes just enough to work with the tube and wrap giving what my bench tests reveal is the best possible combination ignoring cost and labor. While I know easier cheaper materials can do the job, I'm stubborn and will continue torturing myself so I can deliver better than good enough. #type5 #customcarbon #crumpton #3M

[robt57]

Originally Posted by nicrump

I use 3M dp420

p.s. It's not foam.

I just Googled how much 3M dp420 costs out of curiosity.

[wallymann]

Originally Posted by nicrump

p.s. It's not foam.

interesting. the guys at guru used some kind of foamy stuff, i'm guessing a less-desireable option compared to DP420.

do you use DP420 as the primary T2T adhesive as well as the fillet material?

[Bob Ross]

Originally Posted by nicrump

I'm going to go make myself a G&T.

Oh, great, as if I needed another reason to lust for a Crumpton!

[dddd]

Originally Posted by robt57

As would I, being that is where the epoxy hardens to create the actual structure. But is the foam really merely a form I wonder? Leaning yes, but asking the question too.

If not there would more carbon need to be used. I totally get the physics of the inner and outer hardened layers creating a structure. A truss if you will, thus wider truss supports bigger span and like that. And ga. of members contributing etc. seems reasonable to draw parallels.

The "filler" if you will is definitely part of what gives the joint structure sitffness and strength, not just because the outer layer needs attachment points to the tubes but also because the curvature of the outer ply tends to nullify it's ability to provide tensile strength.

So by having the bond be continuous "from end to end" instead of at the "endpoints", the curved outer layer can now generate much greater tensile forces which are immediately transmitted to the joint in support of it's rigidity and strength.
Take the filler out of the central area and suddenly things can move relative to each other, reducing the joint's force response to strain (flex).
You would have the gap opening and closing up as the loads were applied, greatly reducing the tensile stress/force in the outer layer.
This is all somewhat analogous to the benefits from laminating layers in a structure, like how even the thin pizza box cardboard that I glue against the inside bottom of cardboard boxes (that I use to store bicycle parts) makes the bottom of the box more like wood.

[robt57]

Originally Posted by dddd

like how even the thin pizza box cardboard that I glue against the inside bottom of cardboard boxes (that I use to store bicycle parts) makes the bottom of the box more like wood.

scavenge some 'Corrugated Plastic Postal Totes'


[BTW Robt57=ttf/Sparky, if you didn't realize]


Having ridden crazy dead stiff carbon BB frames, wonder how dial-able using foam VS the 3M galacticly strong bond plays into the equation...

[dddd]

Yeah, whatever the stuff actually is, definitely you could tune the frame flex to at least some degree using a much lower-modulus filler.
As a weight consideration, and by it's appearance, it looks to be full of air, but making it softer by means of chemistry might actually be useful.

Here's a question, does anyone know how the impact resistance of tubes compares to that of molded frame sections(?), in terms of frames/materials that are used in comparable applications, i.e. price, weight and purpose.