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Thoughts on wood.

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Thoughts on wood.

Old 12-03-12, 01:55 PM
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RaleighSport
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Thoughts on wood.

Hi again guys, I know it's been a while since I posted any crazy ideas in this forum but I have been busy!
I really don't have the time right now for the precision of working on my bamboo frame and I probably won't again for quite a while, I've been leaning towards doing some experiments with wood now.. and I've seen some interesting ways to insert things like BB shells, head tubes, etc. but I got some questions of my own now and hopefully you all have some input, so here goes.

1: Threadless bottom brackets dropped directly into a wooden frame, possibly with larger metal washers to keep pressure even and correct for any spacing errors.

2: A laminate composite frame, I don't mean like Renovo either I'm more thinking laminating strips of veneer together in some combination of hardwoods worked out for strength/weight/stiffness ratios ahead of time, I've got the wood clamps and good epoxy.. so I'm thinking this is a maybe unless you all see some terrible flaw I've missed.

3: On a non laminate frame, I've been considering taking a chopped head tube with some length of the top tube and bottom tube still left on it, and using that as sockets for joining a frame with wood tubing, perhaps drilling a couple of holes in the respective tubes for pegging or bolts or whatever to secure it well beyond internal epoxy...

4: Dumping money to have someone CNC me a fork out of a single piece of wood.. I like the idea a lot but it looks costly..

So all input welcome!
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Old 12-03-12, 06:03 PM
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I approached this from a different direction because I'm primarily interested in the acoustic properties of bicycle frames and materials. Since I ended up using a lot of what luthiers call tonewoods I thought I'd add my 2c worth. You are on your own with Qs 1, 3 and 4.

2a: Bonding: no problem as long as you get good impregnation. Use a low viscosity high temperature laminating resin and heat the wood (not the epoxy) before applying the epoxy to the surface. Once the wood cools apply another coat; the first will have impregnated into the wood. If it hasn't you've done it wrong = temperature too low. The smell of the epoxy will tell you if it's too hot. I tried vacuum impregnation and solvent impregnation and heat gave the best results.

2b Choice of woods: The impregnation stiffens and strengthens the wood but adds density. This affects softwoods more than hardwoods as there is more void space, so where European spruce has the best stiffness to weight of any wood (if you can prise it out of the hands of the violin makers), it loses this advantage when epoxy impregnated.

I'm lucky because I live in Australia and we have more high strength hardwoods than the rest of the world (about 600 species of Eucalypt, roughly the same again of Acacia and hundreds in other genera such as casuarina, allocasuarina etc.) Again I primarily chose tonewoods and am currently using:

name, trade name, density*, elastic modulus*, stiffness / density, compressive strength*, toughness*, notes on use.

Eucalyptus grandis, Rose gum, 650, 17, 26.2, 66, 16, Better stiffness to weight than aluminium, titanium or steel, tough, strong, beautiful. Used on laterals for stiffness.

Euc. regnans, Vic mountain ash, 650, 15, 23.1, 63, 20, Sim to above, less stiff but tougher.

Corymbia maculata, Spotted gum, 990, 23, 23.2, 75, 24, Incredibly tough and strong, this wood is harder, stronger, tougher and denser than any Hickory. Used mostly between carbon and boron plies for toughness, can be difficult to bond.

Acacia melanoxylon, Tasmanian blackwood, 650, 13, 20.0, 48, 13, Used primarily for acoustic properties. Exceptionally beautiful wood but difficult to work with as it splinters easily.

Nothofagus cunninghamii, Tas. myrtle, 700, 14, 20.0, 56, 13. Used primarily for tight curves (bends exceptionally well)

* Elastic modulus in GPa, density in kg/dm^3, compressive strength in MPa, toughness by Australian Standard (comes out in Joules).
For comparison, the stiffness per unit weight for steel, titanium and aluminium are all around 25.

Last edited by Mark Kelly; 12-04-12 at 04:58 PM. Reason: Italicising latin binomials
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Old 12-03-12, 06:36 PM
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all the wooden bikes I've seen have been laminated, mostly so the inside of the frame tubes could be hollow. Using more strips is no big deal. Some woods are known to have problems with bond strength, but you probably weren't going to use those woods. Cocobolo comes to mind

I would use an aluminum bb shell and not mess around with making my own. I would also use an aluminum head tube buried in the wood.

I wouldn't build a wood fork, just wouldn't be prudent (I don't usually make puns, but I was sorely tempted in this sentence)
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Old 12-03-12, 07:09 PM
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I've not ridden a wooden frame but the people I know that have say it feels like they're ridding, well, a log. Really dead with zero feel. Unless it will be a wall hanger/art piece I'm curious why you're going this direction.
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Old 12-03-12, 08:13 PM
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I can't see any reason to impregnate wood with epoxy in this app, I wouldn't build a bike out of epoxy, it has a horrible modulus of elasticity. All manner of very high tech wood structures From NASA wind farm blades to flight simulator have been made of wood, and room temperature epoxy is all you need, you can post cure it with a little heat, and that will raise the performance somewhat.
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Old 12-03-12, 08:55 PM
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You are welcome to your opinion, but having done both I strongly prefer the result I get with surface impregnation using a high temperature epoxy.

Your argument, BTW, makes no sense: wood is a fibrous material and the combination of strong fibres with an epoxy matrix is well known as a method of building structures with high stiffness for their weight (specific modulus).

You are in any case wrong about the epoxy reducing the specific modulus, it increases it. I get about 15% by volume impregnation of a 0.6mm hardwood laminating strip, which increases density by roughly 0.2 units (0.15 x the density of the epoxy). The epoxy impregnation appears to increase the modulus by about 30% by increasing the degree of fibre linkage, so if the pre-impregnation density is above 0.67 the specific modulus increases because 1.3*X / (D + 0.2) > X if D > 0.67.

Lastly, the principal cause of failure of epoxy / wood bonds is bondline differential swelling due to moisture absorbtion by the wood. Surface impregnation prevents this happening by excluding moisture. A bicycle is a pretty tough environment for wood - lots of moisture, salt and temperature changes.

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Old 12-03-12, 09:32 PM
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So engineering 101 with wood is that it is a high stiffness (better than anything other than carbon fiber), low strength material/weight. And design wise it is easy to shape, and holds it's shape without needing molds. So you want big aero tubes, like plywood box frame. Take your weight budget, determine how much wood that is, and then design the biggest sections you can, that can still survive in the world.

"2: A laminate composite frame, I don't mean like Renovo either I'm more thinking laminating strips of veneer together in some combination of hardwoods worked out for strength/weight/stiffness ratios ahead of time, I've got the wood clamps and good epoxy.. so I'm thinking this is a maybe unless you all see some terrible flaw I've missed."

There is not enough detail here to really comment on your plan, but try this: a) Think directional, like an I-beam. Not that an I beam would be a good structure, but it allows you to segment your product into tension compression and web or core. So say hickory, walnut, and endgrain balsa wood. Of some arrangement of different thickness plywood in a tube format. Or strips and glass for an aero bullnose, and sidewalls of ply, etc... b) If your big idea is just alternating strips of veneer, in different densities, there isn't enough specialization for most uses. I had an ice axe at one point that had a laminated bamboo shaft, and it basically functioned as hard soft laminations, because of the nature of the materials. That worked in that app, because the material turned out to be stronger and lighter, and more consistent than hickory, which was the main material of the day, or heavy steel pipe. And the solid construction was rugged to resist the kind of damage that can occur in the mountains. But for most bike frame apps, it would be a bit of a blunt design, depending on your use. Now stays, that could be another mater.

"3: On a non laminate frame, I've been considering taking a chopped head tube with some length of the top tube and bottom tube still left on it, and using that as sockets for joining a frame with wood tubing, perhaps drilling a couple of holes in the respective tubes for pegging or bolts or whatever to secure it well beyond internal epoxy..."

What you want to do to transition wood to steel is rely on the glue. I would probably point the stick a little and place it in the socket so that for the most part it is joined to the steel with a significant amount of epoxy in the zone between the two. You want the stick to be max diameter as it exits, and possibly have the edge of the tube tapered with a reamer so it is not a hard point to the degree possible. So say you had 1.125 tubes and sticks, and tapered the stick to 5/8, or 3/4. The rest is epoxy. First the bond will be massive, and second you have a transition where the metal does not bear immediately on the wood. Bolts will make matters worse, if you do the wood right.

"4: Dumping money to have someone CNC me a fork out of a single piece of wood.. I like the idea a lot but it looks costly.."

Wood is relatively easy to shape, but how you cut it does not make it a good idea. I would prefer to fab it out of wood, or better still, use a conventional fork. But others have some good ideas:

https://www.blids.nl/image/Praag_Mepp...6/DSCN8070.jpg

Here is a multi lam, like a bentwood chair. It is a veneer lam like what you were talking about, and wide like what I was talking about.

https://www.rojaksite.com/bonobo-plywood-bicycle/
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Old 12-04-12, 03:15 AM
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I came to this thread expecting somethign very different...
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Old 12-04-12, 03:17 AM
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Also, for reference, those thoughnesse values are on smooth, non-notched specimens, unlike most engineering metals, which suffer either Izod or Charpy impact tests on sharply notched specimens.
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Old 12-04-12, 03:59 AM
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Falanx: thanks for that.

My "rough guide" for toughness was to take one of my wooden tubes and use it to beat a Ti tube* and vice versa. The Ti tube is now terminally dented, the wooden tube is fine.

* Ti tube = 3Al2.5V, CSWR, 34.9 x 0.9 ie typical tube used for bicycle construction.
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Old 12-04-12, 07:22 AM
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Wooden 'tube'?. Do you mean 'bar'?
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Old 12-04-12, 07:25 AM
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Originally Posted by Mark Kelly View Post
Falanx: thanks for that.

My "rough guide" for toughness was to take one of my wooden tubes and use it to beat a Ti tube* and vice versa. The Ti tube is now terminally dented, the wooden tube is fine.

* Ti tube = 3Al2.5V, CSWR, 34.9 x 0.9 ie typical tube used for bicycle construction.
That's not toughness. In fact, it's the opposite of it. Toughness is the ability of a material to undergo plastic work to prevent failure, not the resistance to deformation, and it is quantified by the energy absorbed in propagating a fracture. Seeing as you've not fractured anything yet, you've not actually tested toughness in a measurable way.

That's strength you've just tested, and not in an ubiased way. You've dented a hollow shape with a solid one, which is unsurprising. I wonder how many cracks run down the grain of that wood.
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Old 12-04-12, 07:40 AM
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Mark, Are you sure you're not taking the engineering aspect of things and running with it a bit too much? Getting wood types that are practically pre-imregnated with natural Epoxy and then trying to make them even better by adding real Epoxy seems like a waste of time.
The whole point of wood selection in building structures is to find the right wood in the beginning. I can see trying to improve an inferior wood if that is all that is available, but something stronger than Hickory?
In any event if someone says they don't have time for the precision of building a bamboo frame I don't see how they have time to make such a complex frame as this one is turning out to be.
Here's an un-rideable wooden bike: Typical lugs don't allow sufficient diameter of wood for stiffness.https://www.cycleexif.com/ant-bikes-copper-and-wood

Last edited by Canaboo; 12-04-12 at 07:46 AM.
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Old 12-04-12, 10:41 AM
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Originally Posted by Canaboo View Post
In any event if someone says they don't have time for the precision of building a bamboo frame I don't see how they have time to make such a complex frame as this one is turning out to be.
I was thinking the same thing; actually I was thinking the bamboo project wood() take less time.
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Old 12-04-12, 04:06 PM
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Originally Posted by Falanx View Post
Wooden 'tube'?. Do you mean 'bar'?
No, I mean tube as in hollow cylindrical structure. The hardest part of this has been making the mandrels on which the tubes are formed.

Originally Posted by Falanx View Post
That's not toughness. In fact, it's the opposite of it. Toughness is the ability of a material to undergo plastic work to prevent failure, not the resistance to deformation, and it is quantified by the energy absorbed in propagating a fracture. Seeing as you've not fractured anything yet, you've not actually tested toughness in a measurable way.

That's strength you've just tested, and not in an ubiased way. You've dented a hollow shape with a solid one, which is unsurprising. I wonder how many cracks run down the grain of that wood.
Well, I did say it was a rough guide....

As you point out, this is impact strength rather than toughness. Maybe there's a bit of crossover: there's an awful lot of kinetic energy in the "hammer" tube when it strikes the "anvil" tube and this energy is going somewhere. Some of it goes into my hand, there's an interesting difference between the sting when holding the Ti tube vs the wooden one.

The test is not as biased as you assume: the two tubes were of equivalent diameter and weight per unit length. Of course this means the wood one has thicker walls, but that's part of the advantage of working with wood.

I seem to have hijacked this thread which was not my intention, I was just offering some observations of what I have found worked.

I'd like to repeat what I started with: my primary interest is not in constructing a wooden bicycle, it is in investigating the acoustic properties of frames and frame materials. That has lead me to the use of woods, specifically tonewoods, but they are a means to an end.

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Old 12-04-12, 04:17 PM
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Originally Posted by Canaboo View Post
Mark, Are you sure you're not taking the engineering aspect of things and running with it a bit too much? Getting wood types that are practically pre-imregnated with natural Epoxy and then trying to make them even better by adding real Epoxy seems like a waste of time.
No.

Your question amounts to " Are you sure it is worth taking something which sorta works and improving it?"

I think that answers itself.

Last edited by Mark Kelly; 12-04-12 at 04:29 PM.
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Old 12-04-12, 08:01 PM
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Originally Posted by Mark Kelly View Post
No.

Your question amounts to " Are you sure it is worth taking something which sorta works and improving it?"

I think that answers itself.
I am talking about bike frames here.
I fail to see how a properly made wood frame just "sorta" works. Saying that wood must be saturated with epoxy to be worthwhile practically defeats the purpose of raving about the strength of those unique Aussie woods.
Might as well just dissolve the tree and turn it into carbon.
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Old 12-05-12, 03:15 AM
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Originally Posted by Mark Kelly View Post
No, I mean tube as in hollow cylindrical structure. The hardest part of this has been making the mandrels on which the tubes are formed.
Wow, impressive. What's the wall thickness like, and what wood and epoxy impregnation have you used?


Originally Posted by Mark Kelly View Post
Well, I did say it was a rough guide....

As you point out, this is impact strength rather than toughness. Maybe there's a bit of crossover: there's an awful lot of kinetic energy in the "hammer" tube when it strikes the "anvil" tube and this energy is going somewhere. Some of it goes into my hand, there's an interesting difference between the sting when holding the Ti tube vs the wooden one.

The test is not as biased as you assume: the two tubes were of equivalent diameter and weight per unit length. Of course this means the wood one has thicker walls, but that's part of the advantage of working with wood.
It's not impact strength, either, which is another parameter.

This strength, outright, and as a function of section thickness, you're getting results which are interesting, but not exactly surprising. The wall thickness of the wooden tube is probably at least 6 times that of the titanium one - you said equivalent diameter, you mean equal external diameter? As yielding is a strain controlled parameter and the thickwalled wooden (possibly epoxy impregnated?) tube is incapable of plane stress, then it won't yield, but develop internal sharp cracks you won't see, but the thinwalled titanium tube with its metallic plasticity will be able to deform and absorb that energy. The test is mechanically far more biased than you realise, but as an example of what equal weight structures can do in short term, unconstrained high-loading tests, it's very instructive. Composites don't tend to show their damage til it becomes fatal, remember ;-)

The sting is purely damping as is to be expected. Metals tend not to damp well unless they're full of cracks, or longitudinal interface from precipitates or second phases, like the graphite in cast irons. The wood on the other hand is essentially made of interfaces, fibres in resin.
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Old 12-05-12, 03:55 AM
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OK, so I take away from that that my "testing" is basically no use at all as a guide to the actual real life performance of the tubes. That's a shame, it's pretty spectacular.

Looks like I'll have to bite the bullet and do some actual strength and fatigue testing. That's gonna cost.

There's a bit of info on what I'm doing in my post #2 in this thread.
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Old 12-05-12, 04:24 AM
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I'm not saying it's totally useless, but it's not really applicable to this application - for damage tolerance of freestanding cantilevered structures maybe, it's quite relevant.

Fatigue will be your killer. Wood is actually quite fatigue tolerance, but the impregnation stage will have introduced a sizeabel fraction of astronger, yet more brittle resin, which may cause delamination or rupture of wood fibres. Trees had along time to evolve such structures :-)

If you had two tubes, one of titanium alloy and one of steel alloy, of the same per unit volume strength, and the same wall thickness, despite the steel tube being about 75% heavier, the same impact would dent the steel tube less. It's got twice the Young Modulus of the titanium, so its elastic deflection would be only half, and when the elastic limit is exceeded, the deformation would only be half, too. And most steels work harden faster than titanium.... Just another thought experiment. I'm not sure you could give both a carefully calibrated belting, though ;-)
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Old 12-05-12, 07:20 AM
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Originally Posted by Falanx View Post
I'm not saying it's totally useless, but it's not really applicable to this application - for damage tolerance of freestanding cantilevered structures maybe, it's quite relevant.

Fatigue will be your killer. Wood is actually quite fatigue tolerance, but the impregnation stage will have introduced a sizeabel fraction of astronger, yet more brittle resin, which may cause delamination or rupture of wood fibres. Trees had along time to evolve such structures :-)

If you had two tubes, one of titanium alloy and one of steel alloy, of the same per unit volume strength, and the same wall thickness, despite the steel tube being about 75% heavier, the same impact would dent the steel tube less. It's got twice the Young Modulus of the titanium, so its elastic deflection would be only half, and when the elastic limit is exceeded, the deformation would only be half, too. And most steels work harden faster than titanium.... Just another thought experiment. I'm not sure you could give both a carefully calibrated belting, though ;-)
I disagree that the impregnation phase will necessarily cause more brittleness of the fibers. Plenty of "engineered" wood based products that are designed for flexing repeatedly are made with resin impregnated wood. Bow laminations like "Actionwood" come to mind. The main goal there though is to produce a more uniform, stable product rather than increasing the material properties significantly..
However you have to really flex a material like wood to a significant degree before you start breaking down the material in any way.
A bike frame just doesn't undergo near the degree of flexing that will start to break down wood.
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Old 12-05-12, 08:21 AM
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I don't care whether or not you disagree, it does. This isn't a matter of opinion, it's a matter of materials mechanics fact. You've added an inherently brittle material - epoxy - to a naturally tough material - wood. The toughness of wood comes from having no plastic constraint to the movement of fibres in a low Young modulus material, so that energy can be absorbed. Where do you think those fibres are going to go when the natural 10% void-and-low-density-resin in between them is packed full of epoxy? They won't be able to bow, flex or move. They'll either eventually crack away from the epoxy or collapse or rupture where they meet the edge of a transverse crack.
I didn't say brittleness of the fibres, I said brittleness of the epoxy, which affects the whole, the composite. I'm not pulling this out of my fundament. Materials is what I do for a living, and I'm good at it.


Find me a bow that's undergone a million reversals of deflect in it's life. There are functioning Yumi from 15th century Japan that won't reach that number by 2500. Most bikes will do that in much less than twenty years. That's fatigue. And resistance to microplastic deformation by Bauschinger effect or just plain difficult-to-active slips systems is the only way to withstand it, neither of which wood fibres have. Impregnating to remove that randomly distributed void in the wood component and fill it with a solid is fine for giving you repeatable performance over a few hundred and thousand deflections by constraining those wood fibres. But don't confuse stable over that period with stable over a million cycles.
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Old 12-05-12, 08:36 AM
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Originally Posted by Falanx View Post
I don't care whether or not you disagree, it does. This isn't a matter of opinion, it's a matter of materials mechanics fact. You've added an inherently brittle material - epoxy - to a naturally tough material - wood. The toughness of wood comes from having no plastic constraint to the movement of fibres in a low Young modulus material, so that energy can be absorbed. Where do you think those fibres are going to go when the natural 10% void-and-low-density-resin in between them is packed full of epoxy? They won't be able to bow, flex or move. They'll either eventually crack away from the epoxy or collapse or rupture where they meet the edge of a transverse crack.
I didn't say brittleness of the fibres, I said brittleness of the epoxy, which affects the whole, the composite. I'm not pulling this out of my fundament. Materials is what I do for a living, and I'm good at it.


Find me a bow that's undergone a million reversals of deflect in it's life. There are functioning Yumi from 15th century Japan that won't reach that number by 2500. Most bikes will do that in much less than twenty years. That's fatigue. And resistance to microplastic deformation by Bauschinger effect or just plain difficult-to-active slips systems is the only way to withstand it, neither of which wood fibres have. Impregnating to remove that randomly distributed void in the wood component and fill it with a solid is fine for giving you repeatable performance over a few hundred and thousand deflections by constraining those wood fibres. But don't confuse stable over that period with stable over a million cycles.
You seem to be under the impression that all Epoxy is the same and all '"brittle". You should know better if you're in that line of work.
I can't see how a bike frame is even remotely flexing to the degree that would make it approach what a bow limb or a ski might undergo. It's not remotely approaching its elastic limit unless you are perhaps routinely backing your car into it.
FWIW the natural bow material that stands on its own through the highest rate of cycling with extreme flexion is naturally packed chock full of resin that improves with aging.
It is much more resistant to compression and tensile damage as a result.
Can you show your material facts data to support your stance?
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Old 12-05-12, 12:45 PM
  #24  
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Originally Posted by Canaboo View Post
You seem to be under the impression that all Epoxy is the same and all '"brittle". You should know better if you're in that line of work.
I can't see how a bike frame is even remotely flexing to the degree that would make it approach what a bow limb or a ski might undergo. It's not remotely approaching its elastic limit unless you are perhaps routinely backing your car into it.
FWIW the natural bow material that stands on its own through the highest rate of cycling with extreme flexion is naturally packed chock full of resin that improves with aging.
It is much more resistant to compression and tensile damage as a result.
Can you show your material facts data to support your stance?

Consider me chastened. All marketed structural epoxies are aromatic in nature and fail before 0.2% yielding. I'm not under any impression at all. That is the very definition of brittle. All the same, nice Ad hominem. Would you like me to post a couple of technical data sheets from 3M, DuPont? Maybe a lap-shear from my own company's internal testing? What?

Your next statement concerning the degree of flex demonstrates that you have absolutely no idea what fatigue actually is, so I shall give you my very best laymans: It's also called 'microplastic yielding'. Can you imagine why that term is used? No structure comes within an order of magnitude of the elastic limit if it's well designed, and safety factors used in Civil, Automotive, Naval and Aerospace engineering are set at four for a damned reason. That's your shock load, your never exceed load and intended to be so far in excess of what's expected that normal service loads should never reach *it* let alone the 0.2% offset. The expected fatigue life of a material is an entirely different number and based upon resistance at a set number of cycles, or a minimum value below which it is never witness, but not all materials exhibit a macroscale fatigue life. So no, just because you can't see how the flex in a bike frame doesn't come close to meeting the same percentage of total safe load that you find in a bow, or a ski means not a damned thing,. It just means you don't understand what you're launching off at. Sorry, old bean. But seeing as it's you postulating here, not me, it's you who should be supporting your position. Go read Dieter, P Ashby, RA Smallman, HKDH Bhadeshia.

For what it's worth (how difficult is it to type that whole, really?), that 'natural' bow material that outperforms all others is which particular wood, exactly? No, go cut a sample of it and now look at the section in a SEM. See those huge voids? Not exactly chock full, is it? I'm not going to post images of that either. You can find them with a few moments work on Google for yourself. It's those voids that allow the fibres relief. Can you show some material facts to support your assertion that it's 'far more resistant to compression and tensile damage as a result'? Not 'your'. Some. See, that's the thing about facts. They're not a possession, nor an opinion. There's numbers involved.



Canaboo, I don't know you from Adam, and frankly don't want to. I come here to help people understand, not look for a fight, but if you want one, by all means. I do this on my time, just like Frank, Mark, Massive for example and Warwick used to. This is exactly the kind of bull**** that drives so many people away from here.
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Old 12-05-12, 03:44 PM
  #25  
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I'm very interested in the fatigue question, so if you'll bear with me I have some questions.

The impregnation is not full depth - when I grind an impregnated lamina obliquely, there are two surface layers of impregnated wood and a non-impregnated core, I'd say the proportions are around 20 60 20. The 15% figure above came from a specimen made of a number of layers of Rose gum whose density increased from 0.65 to 0.85 after impregnation and lamination. Thinking about it, some of this has to be the epoxy between the laminations. The laminae are only 0.6mm so even very thin interlaminar epoxy layers may represent a significant part of the 15%.

It it my impression from reading stuff about including deliberate slip mechanisms in fibre composites that as long as the laminae can slip over each other without any part being strained beyond its elastic limit the structure as a whole will absorb energy. Does this sound applicable or am I riding for a fall (literally, I'm expecting to ride this thing up the east coast of Tasmania as soon as it's finished)?
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