Carbon fiber frame construction
 

Are you interested in learning how to build a carbon fiber frame? I would like to show you how I built my own carbon fiber frame, from raw material to taking it out for a ride. I would like to mention that I learned how to do all this after attending the carbon fiber frame building course ran by Dave Bohm from Bohemian cycles (Tucson, Arizona).

Hello, I build bike frames made out of carbon fiber. I would like to show the process that I follow to design and build such framesets. I'm about to finish a road frame and I will be posting notes on how I'm progressing (including photos). I hope it is informative for you and I hope I can also get help from some of the members of this forum.

To begin the construction of a carbon frame, I'll show you how I make the main tubes (down, top and seat tubes). I use a mandrel which is just a solid aluminum cylinder, the diameter required on each tube and wrapped with strips of unidirectional carbon fiber. I use unidirectional carbon so that I can control the direction of maximum tensile strength and compression (that is the direction in which the filaments of the carbon fiber have been arranged). This results in different layers wound on the mandrel with directions ranging from 0 °, + 45 °, -45 ° to 90 °. In this way I make sure that the tube will respond well to forces in tension, compression and torsion (with the fibers at 45 °). In the picture below, you can see how the strip 45 ° is wound onto the mandrel.

This is the final result after wrapping all layers,

The tube is then placed inside a vacuum bag and subsequently cured in an oven. By the way, I almost forgot to say that I only use carbon fiber that has been pre-impregnated with epoxy resin (pre-preg) as it facilitates the handling of the material lot.

And this is the 3 main tubes look like once baked and removed from the vacuum bag. As you can see, the surfaces of the tubes are a bit rough. This is due to the type of material used for wrapping the tubes for curing. This is dealt with relatively easily by subsequent sanding but I am in favor of sanding as little as possible because, invariably when sanding, carbon fibers are broken and, in a way, the integrity of the tube is compromised. Now I'm trying molds that will leave an outer surface of the tubes completely smooth. The last step is the removal of the aluminum mandrel from inside the tubes. This is done very easily if you have cleaned the mandrel with release agent before the wrapping of the carbon strips. If you forget this step, then I can assure you that you will never be able to take off the mandrel form the inside of the carbon tubes.

In my next "chapter", I'll show you how I make chain stays, dropouts and the rear brake bridge, all out of molds. See you soon.

Great idea for a thread, thanks for taking the time to do this.

What kind of oven is used? Temps? Duration?

Hello. Any oven will do pretty much as long as you have a vacuum line access to the inside of the oven so you can keep your part under vacuum during the baking process. Another important thing is to have a temp controller so you can programe the ramping up and cooling off periods. The duration is clearly specified by the fiber carbon manufacturer but it usually takes a couple of hours to get to max (about 120 C). The whole process takes anywhere between 4 to 8 hours depending on the carbon you're using.

Pre Preg heated in an oven activates the resins and its catalyst.
vacuum Bagging pulls out the bubbles .

the making of a negative mold and Pushing the air-bubbles out with a Bladder inside is the reverse.
the Mold is your Outer surface of the finish product.

Americas Cup Catamaran race Hulls are made in molds , I expect.

Dragon Plate sells carbon fiber tube in all sorts of sizes and shapes and sheet.
https://dragonplate.com/ If you want to Bypass the roll Your Own and go to step 2+.

Can you elaborate on this a bit? All of my years of mold-making experience would lead me to believe that without any "draft" or taper to the mandrel it should be a nightmare to remove from the cured tube, even with the use of a release agent. It obviously works, as a LOT of carbon tubes are manufactured this way. How do you go about pulling out the mandrel? Freeze it to contract the aluminum?

I've only molded tubes using an exterior mold and an inflatable bladder.

Thanks much for this thread!

Rob

Hello Rob,

The trick to realease a mandrel from a cured carbon fiber tube is to have a very polished and smooth mandrel (no need for tapering). You then need to apply 5 to 6 coats of release agent and the last 2-3 coats should be after placing the mandrel in the oven for about 30 minutes. In this way, release agent will have a chance to seep into minute scratches which may be difficult to see.

You might want to contact Enve. They sell head tubes complete with bearing seats.

I'm curious if you've done any FEA of your tubes and layup schedule of the junctures/joints of your complete frames.

One option for cleaning up the main tubes would be Carbon Fiber Sleeves.

Several are available here (not pre-preg).

http://www.sollercomposites.com,carbon fiber,carbon fiber sleeve,Kevlar sleeve, fiberglass sleeves,carbon fiber fabric,epoxy,west system epoxy,nitrile,gloves,nitrile gloves,aramid,fiberglass,kevlar,tape,biaxial tape, biaxial sleeve

They also sell heat shrink tubing. I'm not sure how well it works with oven cured tubes, but it apparently works quite well with 2 part epoxy.

Hello everyone. Now I will show you how I make the dropouts.The first step is to pile up several layers of prepreg carbon fiber sheets (about 30). Once it cures, it will create a solid and very rigid carbon sheet about 3 mm thick, enough to extract very strong and light dropouts. The picture below shows the different layers of prepreg carbon fiber being laid up on top of a aluminum plate which has been thoroughly polished and treated several times with release agent.

Attachment 535418

Subsequently, it's wrapped up in a vacuum bag and on to the oven for curing,
Attachment 535419

And this what it looks like hot from the oven (the brownish color is the result of the resin being absorbed during the vacuum process to the infusion mesh material).
Attachment 535420

The final look of the carbon sheet (apologies for the poor photo quality),
Attachment 535421

The sheet is placed into a CNC cutter so the dropouts can be extracted,
Attachment 535422

And this is what a very strong and light set of carbon fiber dropouts look like,
Attachment 535423

Very cool posts and very helpful to understand all the effort that goes into a custom fiber frame. Thanks!

Have you done any FEA, or ultrasonic tests on your tubes, dropouts, or completed frames?

Hello,

No I haven't but I plan to do FEA in the near future. Of course, the best would be to put the completed frame under a stress machine but I don't own one and I don't plan to buy one anytime soon. I have tested all my frames thoroughly by riding them over all kind of conditions. This is my best reference in terms of frame integrity. I also reinforce all the critical joins with UD carbon fiber overwrapping during the manufacturing process.

Hallo, thank for interesting thread. How much layers do you apply on tubes? And which density (xxx g/m2) of carbon fiber sheet do you use? Thanks.