d87

I have this idea where I take a large piece of balsa wood or dense foam and carve it into the shape of a monocoque frame, like the lotus bike or Graeme Obrees whip.

Then when I have the carved shape I wrap it in carbon in the traditional way using decent epoxy and cloth.

Would this work as a way to build a bike? would the balsa or foam make the ride softer and help take some of the load on the frame. Would the wood change shape inside the frame over time?

Would the added weight of the balsa or foam be a real hindrance?

Any thoughts are more then welcome.

Cheers

gavtatu

try the recumbent forum, or bentrideronline, homebuilders, lots of CF frames going on there !

StephenH

I'm sure it would work. Compared to more traditional ways, I see two issues: 1) A slight bit of extra weight, which wouldn't be an issue except that the main purpose of using carbon fiber is to avoid extra weight. 2) At various places, you need cavities in the frame for seatposts, cranks, fork, and stuff like that, and if the cavity is filled with balsa wood, you have a problem.

It seems to me, though, that carbon fiber would be a poor material to experiment with like that. You'd have an awful lot of time and trouble in building the frame, but would pretty much be guessing on the design. Get a bit too stout, and it's also too heavy. Have a minor defiicency, and you have a cracked frame.

frameteam2003

There are instructions on the web for this type of build.Foam is best as it can be desolived and the frame left hollow.

Doug5150

Using foam or lightweight wood will work for composite molding. Many DIY-ers use balsa wood since (if you are doing long/spindly pieces) it is more rigid than foam would be, it is pretty easy to buy and it still doesn't weigh much.

The wood inside isn't for strength though, it's just for winding the composite layers over. There's no reason to use thick enough wood to separately support the load, since the wood breaks about as easily as the composite will. If you bent the frame far enough to crack the composite, the wood will already be cracked too.

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The "real" way to add catastrophic-safety reinforcement to composites is you alternate in layers of kevlar, such as,,, for a three-layer covering, use two layers of carbon (or graphite) with one layer of kevlar in the middle. The carbon/graphite will snap off clean if bent too far, but the kevlar stays "stringy" and tough, and will hold the pieces together if the epoxy and carbon cracks.

Also--do not use chopped-strand mat, use ONLY woven mat or unidirectional tape.
The chopped-strand mat is cheaper, but is not nearly as strong and doesn't saturate well.
~

zappdog

For those that are thinking of this type of frame build, it will work. I built a balsa and fiberglass mountain bike frame https://unclezapaudiozap.freeforums.o...t875.html#p875
It is the stiffest frame I have ridden and worked out quite nice. The post has full details on the whole build including a couple of mistakes I made. Worth a read.
The balsa wood was more than strong enough to ride before the single layer of fiberglass. Weight is a bit less than the aluminum frame it replaced.