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What profile of pole would be torsionally rigid? Answered

Calling all mechanical engineers and physicists. I want to make a pole that can resist twisting forces. The application is a down tube on a bicycle frame. And I want to use bamboo because it's light, very strong, very cheap, and biodegradable. The problem with bamboo is it lacks torsional rigidity because its fibers only run lengthwise. I was thinking of wrapping a bamboo pole with a composite fiber (hemp and glue) at a 45 degree angle like they do with carbon fiber poles but maybe a different shape would be better.

Can anyone think of a specific geometry of pole that is tortionally stiff?

I noticed some bicycles have a triangular down tube. Maybe if I cut bamboo into three strips and glued them together into a tube with a triangular cross section, this might be less twisty. Am I thinking along the right lines here?

My first bamboo bike has a lot of twist and this might be one way to make it stronger. Please let me know what you think.


The issue was many fold. The short answer is Tonkin Bamboo is the species to use for bikes. This is what most people seem to use. Also Boo Bikes use dendrocalamus strictus (aka Iron Bamboo). My frame had some cracks possibly due to harvesting at the wrong season. I've recently gained more cracks and the bike flexes even more. It flexes like a noodle but I can still do an endo no problem. Go figure.

Also hemp sounds like it gets more stretchy after being flexed about 1000 times whereas carbon fiber stays stiff. And squishy wood glue is probably not as stiff as epoxy. All this adds up to a flexy frame. My next one will be tonkin bamboo but I will try hemp and non-toxic wood glue again.


I'm making a three wheeler and my engineering design teacher gave me some advice involving spaceframes and triangles. The plan thus far looks like this upright tadpole trike:

UPDATE #3: I think I've got it! Instructables sent me a box of K'nex and I built up a model of a standard bike's frame. The thing is, when a bike frame twists, the opposing corners of the trapezoidal shape come closer or further apart. So a deep X brace from corner to corner seems to fix most of the problem. I'll try to post a youtube vid of what I mean soon.



Best Answer 8 years ago

Having built several bamboo bikes, might I pass along this most important thought on reducing the torsional weakness of the bike you propose to construct...MATURE bamboo!  Bamboo of different varieties can grow very quickly to have a large circumference.  It is true that the larger the bamboo, the less it will contort to tortional strains.  But for the best rigidity in this regard, definitely want bamboo that is at least 3 years old.  While the wall thickness is slightly more than 1 year old bamboo, the fiber density and ligand concentration is much greater as it matures.  So you have a disproportionately stronger pole for bicycle construction if it is older. 
Hope this advice helps!  Happy riding!
ps- filling the bamboo with a foam forming resin has been known to increase the strength of the resulting frame...but that is beyond my purview.  It might be in a direction to look if all other avenues of research fail (and I like the idea of the wrapped resin soaked cord...but I'd want to keep the beauty of the wood shining through).

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7 years ago

Mr. Mark Kelly over at bikeforums had this to say:


"I would have thought that the longitudinal fibre alignment of the bamboo tubes would compromise their torsional rigidity .

This is supported by data showing that bamboo has a typical ratio of EI / GJ greater than 8 where isotropic materials with moderate Poisson's ratios (eg metals) give values around 1.3. Since EI / GJ describes the ratio of bending stiffness to twisting stiffness, this says that the bamboo has about 16% of the torsional rigidity of an equally laterally stiff metal tube. Since the bamboo tube will have lateral stiffness less than or equal to that of a metal tube of the same diameter and weight, this gives it at best 1/6th of the torsional rigidity.

It is well known that torsional rigidity of frame tubes is extremely important for handling - you don't want the down tube and top tube twisting enough to pull the head tube out of alignment with the rest of the frame.

Am I missing something or is this another case of something which looks like a good idea but really isn't? "


7 years ago

After some more nosing around it could be that the problem with my bike's flex is really the hemp lugs. This article says that hemp lugs get soft and flexy after a while so they only use carbon fiber and fiberglass: http://blog.bamboobikestudio.com/why-dont-we-use-natural-fibers-in-our-lugs/

There are still other natural fiber options to try though: soy protein fiber, bamboo fiber, and maybe something I haven't heard about yet...

Any ideas?


7 years ago

I read that "dendrocalumus strictus, also called “Iron Bamboo”. " is a good material for bamboo bikes.

There is a several month drying process but I don't know what it is.


And the bamboo must be harvested at a the end of the local dry season.

like LEMONIE stated, bamboo of the the right thickness is strong enough,, and, there are several types of bamboo,

we have a 12 ft ladder at home made of bamboo. and with proper wood preserving techniques and care it can last for a very long time (if 10 years is long enough for you that is,,)

Hawaiians treat Bamboo in salt water and fire to preserve it for the long haul.

Use the piece with the thickest wall. This will increase the poles moment of inertia, it's resistance to rotation, and decrease the radius of gyration, decreasing the moment that causes the torsion. Maybe the drawing is wrong but the wrap picture shows one complete cord, counter clockwise, and the other as being cut. This will fix the torsion in one direction, positive Z using the right hand rule, but not the other. You might alternate which cord goes under and over to try and keep it from being stronger i one direction.


8 years ago

Torsion of a tube will shorten its length as well as decrease its radius. Sectioning it into shorter tubes and inserting stiffening discs inside should help.

Second idea is to do something similar to what is done in making ropes: layer upon layer, each twisted into opposite direction.

For your application, it would mean using two bamboo tubes that fit snuggly into/onto each other, pre-twisted in opposite directions and then glued one inside other (or fastened with something prodding them both).

That way fibers of inner tube would be crossed with fibers from outer one, and their elastic forces trying to "unwind" each tube would be put against each other, would compensate each other, while still stiffing complete pole. The pole would have tension that would counter any external torque.

Now, after writing all that, when I think about it, conceptually it is quite similar to your own idea.

A short answer is a good answer so:
Put another bamboo pole inside it and do the cord 45 deegree thing

Bike frames are subject to torsional stress.  For example, when pedaling while standing, one inputs a moment to the handlebars in the opposite direction as the moment created from the pedal force.  This puts the frame in a state of torsional stress. 

The key is that the frame as a system is in a state of torsional stress.  If one were to look at the individual beams of the frame, they would likely see that when the frame has a large deflection under torsion, a single tube only has a small amount of torsional deflection.  The tubes are highly leveraged torsionally.  Increasing the torsional rigidity of a single lateral tube (top or down tube) will help increase the torsional stiffness of the frame but only modestly if other elements are weak.  The torsional stiffness of the total system needs to be increased. 

The mating points of the tubes (bottom bracket to seat tube and chain stays, head tube to down tube) and the stiffness of the tubes in bending are going to likely be the largest contributors to the torsional rigidity of the total bike frame.

I would recommend staying with a circular cross section.  The rope idea is good, but other changes may help more.  First - focus on strengthening the tube junctions.  Second - determine what tubes bend when the bike frame twists and increase the bending stiffness of these tubes - probably top and down tubes.  

Regarding the torsional ridgity of your down tube - a circular cross section will have the highest torsional stiffness when compared to any other geometry of similar size and wall thickness.  Increase the tube diameter, increase the wall thickness, and increase the shear strength of the material.  The shear strength of the bamboo can likely be increased by choosing a more dense (older) bamboo material.  Or, as you suggested add a composite overwrap layer with fibers oriented in the direction of high stress - circumferentially.  For the overwrap to work it must be structural. 

Good luck man.  Your project sounds cool - hope you are successful!

8 years ago

The simple answer is:

Make it a square.

I assume when you say "torsional" stiff you mean as if it were bending in the center, and not twisting about its' own axis of symmetry.  For a simple bike frame (which is what I assume you are building) it doesn't need any torsional rigidity.  That is why you don't see any small road bike frames made from square tubing.  On probably 99% of all bikes made today, they use a circular tube design, thats because the way a bike is designed is that it transfers all it's forces axially through the tubes.

for a pure bending situation a square tube will always outperform a circular one.  But in a bike frame we are looking for axial rigidity.  A circular tube cross section is the best design for transferring pure axial loads about its' length.  It is this very same reason that large suspension cranes are built with circular tubes.  They are not resisting bending when they are lifting something, they however use the combination of the cable at it's top to change the bending load into a compressive load and in compression materials are very strong.

They use the circular tube because it will not buckle under compression, whereas a square tube will.  Just think of a square tube from one side, and you can liken this to a ruler.  if you stand it up on its long end and apply a small force you can see that it will start to buckle.  You can then get an idea for why the square, with only 4 sides, can buckle under compression.  Now for a circular tube you can use the same analogy, but we know there are no flat sides on a circular object.  So in essence there are an infinite amount of flat edges that comprise the circle.  This should make it very clear that if you had two tubes, one with 4 flat sides and the other with a near infinite amount of flat sides, which one will resist the compressive load better?

As an aside, if you are talking about twisting as it relates to the twisting about its' axis of symmetry (like a bottle opening motion), once again the circular cross section will win out.  And that is also a force that you shouldn't be having on a bike,  if you are getting bending, or twisting on a bike frame it is because there is something wrong with the bike.  As a final note, the tubes will not be the weak point on the bike, it is however the junctions of where these pieces meet.  Look at almost any bike frame failure and you should see that almost all of them fail where the tubes meet.

Hope that helps.

I'm currently taking mechanical engineering, and the strongest way to build a down tube out of bamboo would be to keep it exactly the way it is, round.  If you think about your drive shaft of your car, they are hollow tubes, and they resist tremendous twisting forces.

i like your ideas,you could possibly take your first idea one stage further and do a braid like you find on cables or a maypole,making the angle as shallow as possible for maximum torsional strength,but if its stiffness you want then have a look at moulten bikes www.moultonbicycles.co.uk/models.html they have some great space frame designs which could possibly be simply lashed together. good luck.

 The other comments all seem to be on the right track.  If you are serious, it might be worth while to set up a little test of different techniques.  Your idea about cutting up bamboo and reconnecting the pieces in a stronger shape is unworkable.  You would lose it inherent strength of the material.  The amount of fiber and glue you would use to ensure that the strength of the bamboo was at least as strong as before you cut makes it unworkable.  My first thought was bundling three small bamboo pieces together and wrapping them but it would have to be tested to see how strong the smaller diameter bamboo holds up..  

hmm,, another suggestion,, here in the philippines is kawayan tech.... , they produced a bike frame made of bamboo (a mountain bike frame) , i dunno if they will share any info on making the frame , but maybe you can have an idea upon looking on the frames..
 just google it,, kawayan tech

Both Burf and seandogue are correct. Your wraps will work only if they are rigidly fastend to the bamboo and if themselves have near zero elasticity. There are many natural glues out there that can act as a cement and a sort of agrogat (like the resin in fiberglass and carbon fiber). Choosing your glue would depend on the type of fiber you intend to wrap it with. Knowing that you can match the type of glue that will work best with bamboo. I would just get some thick twine, soak it in some wood glue, rough the surface of the bamboo, apply the glue to the bamboo, wrap tight and wait. I would guess with that image you provided you could expect at least a 30% increase in torsional resistance.

Bamboo of the right thickness is stiff enough, I can't see you'll make a worthwhile improvement while keeping it mostly bamboo.
People do use it for scaffolding



8 years ago

In a circular member, torsion results in a sheer stress perpendicular to the radius. Accordingly, to resist that stress, you would want to place your strengthening material in direct opposition to that force. In other words, wrap the member with a material placed at a 90 degree angle to the radius of that member.
Think of the bands around a barrel, same principle here.

no pole is going to be as torsionally rigid as a non-cylindrical design. The wraps have merit, but they're not going to be the end-all be-all.

Make it fatter. A long thin rod with anything up top that can catch the wind is going to twist whether you like it or not.