Wooden bicycle build

rickpaulos said:

The very first bicycle was made of wood and there have been countess wood bikes since. You can do what every you want.
Lots of variations on natural material have been used. Bamboo bikes, fibre mixed with epoxies (Schwinn even sold one). There are the engineering student contests where the entire bike must be made from a single sheet of plywood with no other hardware and be rideable. Most wood bikes I've seen are a mix of wood with regular bicycle parts integrated into the design.

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Kevin B said:

I've looked at this on Amazon a few times. Bamboo Bicycle Kit

I have seen books about wood bicycles on Amazon too. This is a very interesting topic.

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axeman88 said:

The challenge with designing structures from wood is that they do very well handling stress along the grain, and very VERY poorly across the grain. Boat builders back in the day would spend considerable time searching for naturally occurring crooks and bends that they could use for those elements of the frame that required a complex shape.

Sawing a curve from a piece of wood that has grain will generally result in some portion of the element being cross grain, and that's where it will break.

Plywood is one way around this, steam bending is another, using wood as a cosmetic cladding over metal is a third.

Designing stressed elements as straight sections of select wood is how the problem is usually solved.
Like how one would build their chukudu:

Even here, time spent going through a big pile of lumber at the lumber yard, to find a few sticks for a project is part of the process. Most of the stuff that they want good money for at your local home center, would have been cut up and tossed in the woodstove by our grandfathers.

I think all of Miyazaki's work is inspiring for those who are mechanically inclined. You may also enjoy "Howl's Moving Castle", "Ponyo", and "The Wind Rises"

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rickpaulos said:

I spotted these on Ragbrai 2021. Actually being ridden! Okay, ridden in an overnight town, not necessarily from town to town. 75 miles on a wood seat? Pass.

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rickpaulos said:

Another Ragbrai. Like the font that matches Trek's for some years. Some wood bikes are works of art. Some are like this:

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Ulu said:

I think we invented welding so we wouldn’t have to do this. But as a curio it has unlimited potential.

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rickpaulos said:

Before picking up any ole scrap wood, check the charts for strengths of each type of wood. Oak, Ash, Mahogany, Pine, etc. I used a bit of Scotch Pine for my first bike related build and it lasted about 10 minutes. I rebuilt using Orange Osage (grows Hedge Apples) & Douglas Fir and that is still holding up after a 1000 miles of abuse by many riders.

Many wood workers go for artistic beauty and mix dark & light. Cherry & White Oak for example.

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axeman88 said:

Your 2nd concept looks the best to me, in terms of strength and efficient use of material. It's similar to a couple designs that you posted for inspiration, and not dis-similar to the 4th concept image you posted. I think that the reverse bow shapes, without the "S bend" of concept 4 would provide the stiffest structure. You might even want to pre-stress the bows either away from, or towards each other. Pre-stressing is often engineered into structures to prevent reversing tensile stresses that lead to fatigue failure. When this method is followed, as a structure is loaded and unloaded, tensile stresses go from moderate to high but never reverse, due to the preload.

You'll want to gusset the joints at the head and the rear axle. Notice how the builder of the bike in pic #3 of your opening post used metal to reinforce the axle joint, and also the stacked laminations at the head. The curve cut into the head structure isn't just decorative, it helps distribute the stress into the frame elements. Also notice how this builder has stacked the laminations of light and dark lumber, which I suspect was steam bent to the desired shape. Laminating, means that flaws and grain structures in individual layers will have less influence on the overall strength of the structure, the flaws are "averaged out". In plywood, grain directions on alternate layers are at right angles. I doubt cross grained layers would be valuable for these designs, but it will depend on the particulars of the structures.

I am seeing structural issues with your CAD developed "stick design". The joints don't look strong enough nor resistant to relative rotation of one element with another. Gussets will probably be needed. They can be thin material if arranged with their widest dimensions arranged in opposition to the loads. Going back to Porco Rosso, those early airplanes appeared to be stick structures, but they weren't only the sticks. The wires were essential tension elements, and the fabric covering stabilized the entire structure. Keep this in mind if you use stick structures.

Since you are using CAD, perhaps you have access to stress analysis tools? Solidworks has a simple FEA package built in that would allow you to visualize the stresses from basic static loading to guide and refine your design before you've committed valuable materials and labor. FEA analysis of 3D models results in images like this, where the colors are referenced to different levels of stress:
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One analyzes their model, makes changes, and re-analyzes until the design seems viable.

There's other methods, ...
As a young engineer, I would sketch individual sections of complex structures and use simple mathematical stress analysis to get in the ballpark, but there is considerable math involved.

There is another method that goes back to the 1930s, that might be valuable. It's primarily for 2D stress analysis, but it seems to me, a bicycle frame is largely a 2D structure. The method uses the principle of Photoelasticity. Simple models are cut from clear material which deforms (strain) in response to imposed stresses from loads. The strain creates visual color fringes which correspond to levels of stress. By first bending some simple structures with easily analyzed mathematical models, one would even have some idea of what level of stress the different colors corresponded to.

In this image a protractor is being squeezed. The local strain, which can be seen via the colored fringing, is proportional to the local stress, which can't be seen, at least while the material is within its elastic range:
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https://en.wikipedia.org/wiki/Photoelasticity
Even if your models aren't cut from cast acrylic, I think you'd be wise to build some simple models, of some cheap fairly homogeneous material, and load them to see where they break, and under what load. When a structure doesn't break consistently in the same spot, it's at least somewhat optimized. I'd guess that your final version would be at least 4 times stronger than your first attempt.

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Ulu said:

50 years ago, we did something called brittle lacquer analysis. I did some carabiners for mountain climbing.

You basically paint your model or your prototype in a very brittle paint.

Then you can do various things to it, like get it very cold and see where it cracks from material shrinkage, or twist it, bend it, & stretch it in different directions to see where the paint would crack: because that’s where your stress concentrations were.

Where you had heavy stress concentration, there would be lots of cracks very close together, and the flow lines of those cracks would show you the pattern of stress distribution throughout the surface of the structure.

Wherever the metal strained, that is where it stretched, the paint cracked. The theory is that where you got all the close cracks in the paint is where the metal was going to crack first.

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SwissGuy said:

I must say, I like the axe carrier concept!

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