I had actually thought of trying to build wheels with spokes under tension and compression (a nut on each side of the rim wall) a while back, and an engineer explained to me why it wouldn't make the wheel any stronger.

Bingo. I still think it would be interesting to put the two side by side so to speak to compare the different designs.

If you can't express this problem in numbers then you really dont undersatand it. There is a very simple formula which goes like this:

Ns = PI^2 EI / L^2

Where:
Ns = The theoretical compressive capacity at the servicability limit state
PI = The constant 3.14159
E = Modulus of the material
I = Second moment of area about the neutral axis of the cross section
L = The effective length of the member which is a function of the actual length and the end restraint conditions

If you understand that then you will know that no bike wheel built to the proportions of standard bike wheels will ever be able to gain any significant strength through using the spokes in compression. That includes R Sys and any other wheel. Unless you significantly change the spoke dimensions (this is a geometrical problem) then a spoke simply cannot be of any real use in compression. I suspect the Rsys spokes are at best able to remain stable when the tension is released at the bottom of the wheel and dont actually really go into any significant compression during normal use.

As you can see from the formula, you can increase EI as much as you like (hence Rsys large diameter spokes) but the length is what really matters (hence Rsys restrained end conditions, at least i think they are, maybe), but if you want spokes acting in significant compression, you need them to be VERY short.

Just the 2 cents worth of a structural engineer....

Edit, Having said that I have never actually analysed an Rsys wheel so it is possible they have squeezed some comressive capacity out of them. It wont be much compared with the tensile capacity though.

Oh, this is getting good. :popcorn

I hope you're not suggesting that a spoke can actually work in compression?

By the way I read the other thread about this and read the analysis that was linked to the other thread here: http://www.astounding.org.uk/ian/wheel/. The analysis appears sound right up to the point where the conclusions are drawn. IMO this is an example of getting a computer to a very impressive analysis to come up with some suspect conclusions.

IMO a first order geometrically linear analysis without pre stress is not appropriate to draw some of those conclusions. It may be that a more robust analysis would get the same answer, but on its own I dont think that particular analysis is worth much.

In Layman's terms: A spoke is so thin that it will buckle under any significant compressive load. Thus the load is transferred to the ground via the spokes in the upper part of the wheel. And that load is transferred to the ground through the rim.


** push down on a straw and it will buckle with very little force. pull on a straw and i bet you can not pull it apart. **

Nobody (no-one, not even one) is arguing that spokes support load in compression. What is being said is that it is the reduction in tension in the spokes at the bottom which acts to support the normal load on a bicycle wheel. That truth has been amply explained many times by people who have actually studied the question, performed measurements, and done mathematical analysis.

"The load pushes on the hub which transfers the load to the rim via the spokes on the top of the wheel,which pushes down on the ground. The rest of the spokes keep the rim from deforming/collapsing through tension."

That is 100% (maybe more than 100%) wrong. In truth, the load is carried by reduction in tension of the spokes at the bottom. No, they do not support the load in compression. Nobody ( not even a ******** Labrador retriever) claims that.

"IMO a first order geometrically linear analysis without pre stress is not appropriate to draw some of those conclusions. It may be that a more robust analysis would get the same answer, but on its own I dont think that particular analysis is worth much."

What? You're criticizing an analysis by someone whose name I infer to be Ian and claiming that it is not worth much. Would you explain a bit why you think that this is not a linear problem and how a "more robust analysis" might give different results? Besides, he considers pre-stress.

" push down on a straw and it will buckle with very little force. pull on a straw and i bet you can not pull it apart."

Ok, pull on a straw a whole bunch and then push on the the straw by a half a bunch . Does the straw buckle?


**** I did not use a nasty/dirty word in reference to any Labrador retriever. Labradors, in my experience, are wonderful and very trainable animals and they make great companions and service dogs. It is my favorite breed. However, Labrador retrievers don't spend a lot of their time pondering the greater philosophical questions, and that is not what we expect of them. Even among dogs, they are "lesser" thinkers. The word which I used above, and which has been redacted, is one commonly used to describe those of lesser intellectual capacity. Apparently, Labrador retrievers everywhere have been insulted by my language and for that, I most humbly apologize.

Nobody (no-one, not even one) is arguing that spokes support load in compression

Easy tiger, it seemed relevant to the OP as he seemed confused.

Ok I'll explain why the analysis is misleading. This is a statement that I have copied and pasted from the analysis I linked above:

Related to this is the argument that sometimes gets trotted out - "I bet" they say, "that if you remove the top spoke it has a much worse effect than removing the bottom spoke". They'd lose this bet, and I could do the analysis to prove it.

Actually his analysis shows in great length that the tension in the bottom spoke is "released" when load is applied. It is therefore shown that applying enough load will result in zero tension and zero compression to the bottom spoke. Since the bottom spoke has no force but the top spoke has all of the original pre stress, then cutting the top spoke will have an effect, cutting the bottom will not matter (under that specific load case). If cutting the bottom spoke (carrying no load) does not matter then it cannot be said that the bottom spokes are contributing to supporting the wheel in that particular case.

"In truth, the load is carried by reduction in tension of the spokes at the bottom"

Correct, the tension reduces proportionally to the load applied, and all the forces remain ballanced with the hub nicely in the centre of the wheel. In other words you cant have all the spokes pulling down fully plus the applied load ballanced by the top spokes.

When the bottom spokes have lost all their tension then applying more load will result in additional tension to the top spokes. This has to be the case for the forces to remain ballanced. Since we are talking about "A bike wheel" this is a valid scenario (read Rsys). A model with no pre stress simply can't consider this case (unless added into the conclusions) and results in statements like this one:

"The load pushes on the hub which transfers the load to the rim via the spokes on the top of the wheel,which pushes down on the ground. The rest of the spokes keep the rim from deforming/collapsing through tension."

That is 100% (maybe more than 100%) wrong.

Well actually its not wrong. Just because the bottom spokes release tension for the forces in the wheel plus the applied force to remain ballanced, does not mean that some magic load path appears to the ground. Notice as the tension is lost in the bottom of the wheel the top tension remains the same (near as dam it). It is the riders weight keeping the load ballanced instead of the tension in the bottom spokes. Therefore the riders weight is travelling through the top spokes and the rim to the ground.

An analysis that ignores pre stress is worthless without very careful consideration before drawing conclusions. Otherwise you end up with irate internet warriors who almost get it but not quite (you).

If you carry out a proper detailed non linear analysis with pre stress you will better understand the problem and will also understand how daring the design of the Rsys is because it is designed to remain stable after the tension in the bottom spokes is lost.

Personally I'm sticking with wheels whos tension exceeds my weight by some margin - I consider it structurally more efficient.

Also in his Intro he explains:

"In asking whether the hub hangs or stands on the spokes, I really mean to question where the structurally active spokes are."

He then proceeds to prove that the bottom spokes loose a lot of tension under loading while the top spokes remain with pretty much the original pre stress.

He then concludes that the bottom spokes (now lightly loaded) are more structurally active.

If he had pre stress his model would have showed larger numbers in the top spokes than the bottom and would have been less confusing. Then I would have said his analysis is good. Which it is but his conclusions are all up the screw.

wow, serious reading comprehension fail. engineer you are, reader you aint. who said anything about compresion? If you go back and read my quote you will see I specificly said tension. You have this big education and a sheepskin saying you are an engineer and you think a reduction of tension at the bottom of the wheel is responsible for holding up a load?
Here is a good experiment for you to try on your favorite bicycle wheel. Stand up your bike and place some weight on the seat to simulate a load. measure the tension of the spokes on the wheel of choice with a spoke tension meter. start loosening the spokes at the 3 oclock position moving counter clockwise to about the 9 oclock position without disturbing the wheels position. Check the tension at the bottom of the wheel. has it increadsed or decreased? I am going to say that the tension in the bottom spokes HAS BEEN REDUCED. Is this reduction in TENSION holding up the load any better than before? I will say probably not. Now carry on in your nipple turning until all the spokes at the top are completely free of the spokes. the tension in the bottom spokes at this point will also be 0, by your argument that reduced tension in the bottom spokes is what carrys the load, this wheel should be massively strong right? Would you ride on this wheel? Could you ride on this wheel?
Or better yet, load the bike as before but now take your nipple wrench and tighten the spoke at the bottom. Yes, increase the tension until it is the same as the rest of the spokes. did the wheel collapse? why not the tension is the same as the rest of the spokes?
Did I understand your math problem? (It was math , right?) nope, dont have much more than a high school education. But even with that I didnt have to resort to name calling and insults. And I can see that the reduced tension in the spokes at the bottom of a wheel is the effect of the load being placed on the hub. Your model doesnt take into account deformation of the rim or stretch of the spokes.
You want to argue semantics about engineering in a bicycle forum? Whats the matter wont the engineering forums talk to you?

Yes, but it's even simpler to explain when you consider that a spoke is fastened to the rim only by the outside (on a traditionally built wheel). With this explanation, even idiots (not that BF has any) should be able to understand that the spoke can only have a reduction in tension, and not actually add compression.

"wow, serious reading comprehension fail. engineer you are, reader you aint. who said anything about compresion? If you go back and read my quote you will see I specificly said tension."

Ok, this is what you wrote earlier and is the sort of statement that I was referring to:

"This again? There is no way that a bicycle rim can stand on a spoke or 10 spokes. the nipple will simply push into the rim and puncture the tube.The spoke/rim interface is not made in a way in which a spoke could hold up the hub in this manner."

You were arguing against the proposition that spokes in a normal bicycle wheel support load in compression. I was pointing out that nobody has made that argument. With respect to a normal bicycle wheel, that is.

"Or better yet, load the bike as before but now take your nipple wrench and tighten the spoke at the bottom. Yes, increase the tension until it is the same as the rest of the spokes. did the wheel collapse? why not the tension is the same as the rest of the spokes?"

There is a good chance that the rim would fail and that the wheel would collapse.

If somebody wants to get a decent understanding of how a bicycle wheel works and how the strength elements of a bicycle wheel (spokes and rim) act to support the load, the reference above (http://www.astounding.org.uk/ian/wheel/) should be helpful. Special attention should be paid to the section labeled "Statical Indeterminacy." Brandt's book (of course) also goes into this in some detail. The net result of all this is the inescapable conclusion that the load in a normal bicycle wheel is carried by the few spokes at the bottom of the wheel and the other spokes act to keep the wheel essentially round.

Suspension
http://www.theodora.com/wfb/photos/c...eijing2008.jpg

or

Compression
http://www.nauvoonews.com/collection...rch_bridge.jpg


There is no argument here, nor room for reasonable misinterpretation.

The bridge has elements in compression as well as tension. Those big 'Y' things are mostly in compression.

rim is in compression.

I said "reasonable misinterpretation" for a reason. The spokes in the analogy are quite clear.

I know Scout is offended
http://i72.photobucket.com/albums/i1...ottubsmall.jpg

I was not arguing nor misinterpreting nor trying to start a controversy. It was just an observation -- that's all.

That's a nice looking dog. Here's a picture of Onslow:

http://bikeforums.net/attachment.php...hmentid=150025

(click for bigger)

Onslow looks sad

1) the spokes at the bottom contribute more than the spokes at the top. HOW? They have LESS tension. how can something have less tension and do more work?
2) He also states that the spokes at the bottom pull the hub down. But all the spokes are pulling on the hub ,all in different directions. the load is pushing the hub down which causes the rim to deform ,lowering the tension on the bottom spokes. those spokes are doing less. How do you see an increase in tension in the bottom of the wheel if several of the spokes are relaxed
If you can say that the spokes on the bottom are pulling the hub down , then I can say that the bottom spokes are actually pulling the rim up, and since the rim at the top is attached to the hub with spokes that are in tension, which by the way is higher than the tension of the spokes at the bottom, then the top spokes are pulling the hub up, or the hub is suspended from the upper spokes.

Posted by a person who is actually thinking clearly.

If the bottom spokes relax the hub wont remain central unless there is an external force keeping tension on the top spokes, that force is the weight of the rider.

Hey some people still can't figure if the earth is round or not, spoke theory much to complex!

http://www.theflatearthsociety.org/forum/

This sounds more like a semantics problem than a structural engineering question.