cyccommute 

In both of those pictures, the buckling of the wheel isn’t just because of the cornering. There are other factors that would cause the wheel failure to consider. Ev0lutionz original question was about wheels going out of true after hitting a bad bump and falling over. In his original post he says

Even he thinks that the crash causing the wheels to go out of true is questionable. There are lots of ways for wheels to go out of true and having them do so because of a crash (or extreme cornering) is way down on the list of possible explanations.

Sonofamechanic 

“Way down on the list” as an answer I’m ok with….but not ”impossible.” Of course there are other factors involved…it was a crash…you’ve got things going on all over the place. Is it possible for wheels to go out of true during a crash?….yes, possible.

WizardOfBoz

The cited paper, above, notes how as you overtighten spokes, the wheel becomes metastable. That is it remains round and flat unless there's a bit of a disturbance. This might be a lateral force or even a radial force (bouncing a wheel on the ground). The wheel then finds a new low energy state - the taco. I thought that the paper was pretty interesting.

buckling-and-collapse-of-the-bicycle-wheel.

And, while I agree with those that state that one can have lateral forces (and you can have them in stable straight travel or on curves) I remind everyone that the OP was asking about a crash. The technical engineering term for this is "all hell breaks loose", and the results are dictated by "the bike and your body do whatever the hell physics wants them to do". This is also expressed by some as "Man proposes, God disposes". The point is you could have zero lateral forces, hit a little hillock, get about 2 feet (269.3 potrzebies). You try to correct your direction by tilting the handlebars and when you land the front wheel is 90° away from the direction of travel. This is extreme lateral force and the two options are 1) you go over the handlebars without the wheel failing, or 2) you go over the handlebars as the wheel is collapsing.

Anyway, some may enjoy the cited paper.

Yan 

Hey I appreciate the experiment. However as I said this is buckling, not the effect of lateral force on the spaghetti. My assumption was correct that you are getting these confused with each other.

You can tell the difference the because the spaghetti always buckles in the same direction no matter which way you spin it. It's the result of the amplification of existing imperfections in the spaghetti. If the bending was the result of a lateral force, it would always bend toward the angle of lean no matter how you spin the spaghetti. As you can see this is not the case.

If it is too hard to work with a spaghetti because it is thin and you cannot tell its spin orientation, you can substitute a long metal ruler and see the same result. The ruler will only have two possible directions of bend so it is easier to observe. The difference between buckling and lateral force will be immediately obvious. Whether you lean the ruler left or right, it will always buckle to the same side. If the bending was due to a sideways force, you would expect a different behavior: that it always bends toward the direction of lean.

There is a second way to demonstration the false conclusion of the spaghetti experiment. If you hold the spaghetti completely vertical and press down, you will notice that it also bends. So in this case where is the "sideways force" coming from, under your theory?

____

To everyone else reading this, Cyccommute is not saying there is a lateral force arising from the rider leaning out of plane with the bike. Nobody disputes this.

He is saying that even in an idealized scenario where all mass is in perfect alignment (i.e. a simple free body diagram containing a single straight member), there is still a bending force on that member. His evidence is that the force vector has x and y components, so "therefore the x component must be bending the member".

Maybe someone with more patience can chime in. I've already tried multiples times.

cyccommute 

No, I am not confused about “buckling” and “lateral force”. Buckling is usually initiated by lateral force. There can be rim failures that would result in the wheel buckling due to vertical force but, generally, the wheel is strong enough to withstand this kind of punishment. Wheels are weaker in the lateral direction, although they are still strong enough to withstand a fair amount of lateral force. The article that WizardofBoz linked to details how the lateral force initiates the tacoing (and eventually buckling) of a wheel. The difference between just bending and buckling is a matter of magnitude.

I’m not sure what you are trying to say. The point of my demonstration was that the spaghetti is a nonrigid structure and that it bends due to the lateral component on the rod. Hold one end stead so that it can’t slide and apply a force on it at an angle and the spaghetti bends. The orientation of the spaghetti doesn’t matter.

It doesn’t matter what you replace the spaghetti with. The buckling would not always be on the same side. Going back to the spaghetti, if you exceed the lateral force needed to break the spaghetti, it will break away from the side that you are bent the ends toward. It’s not going to always break to the righ or left.


And how is that any different from anything I’ve said above… other than having the direction of the bend backwards? The rim at the contact patch bends upward away from the plane of the spoke and hub.

It’s a thin structure that is incompressible. It can’t decrease in length so it bends to one side or another. The “sideways” force comes from the inability to push down completely vertically on the strand as well as inconsistencies in the material making up the spaghetti. Even if you could push down on it completely vertically, the direction the spaghetti would be random. In other words, it could bend in any of 360 degrees.


Say what!!!??? That’s exactly what I am saying. There is a lateral force on the rim from the rider leaning out of the vertical plane of the bike. That’s what Heine said and what WizardofBoz said. The latter to which you said

He said essentially the same thing I’ve been saying all along.


First, it’s not an idealized scenario. It’s a common technique to understand the force vectors acting on a body. There is not a “single straight member”. Your argument is using a single straight member…i.e. the angled force vector. I’ll remind you that you “completely agreed” with

You are thinking of a bicycle wheel as being a rigid structure. As a wheel builder, I long ago realized that a bicycle wheel is anything but rigid. It’s a very dynamic structure that is difficult to model. Forces on the rim don’t necessarily translate to the same forces on the spokes. The rim can bend independently of the spokes because the rim isn’t rigidly attached to those spokes. A rim bends upward while we ride down the road which detensions the spoke each time the spoke is perpendicular to the contact patch. (The spoke is also detensioned on either side of the contact patch but the maximum is when the spoke is directly over the contact patch). In a corner, this process doesn’t magically go away so but it does change direction slightly. Instead of the rim bending directly upwards, it bends 90° to the contact patch. How much it bends will depend on the angle of the CG to vertical.

I’ll also remind you that you said

While we don’t agree on magnitude, you are admitting that you can bend the rim sideways with your hand by applying a lateral force! What you can apply by hand is small compared to the force you can apply with a longer lever (your body) and a larger mass (again, your body). The lateral force doesn’t go away just because you happen to be on a spinning wheel instead of a static one.

That the whole system doesn’t just fall to pieces on a corner is something of a miracle. Hell, the fact that the wire spoked wheel doesn’t just aspolde while we ride it, is miraculous.

Yan 

1. Yes. You are confused. You just don't know you are confused. You are conflating two different things here.
2. If you took a marker and colored one side of the spaghetti, you will realize you are wrong. It always buckles on the same side of its strand no matter which direction you lean it.
3. If you stand the spaghetti completely vertically and push down, the direction it buckles in will not be random. It will always buckle on the same side no matter how many times you repeat pressing down. See #2 above.
4. No, you're not saying the same thing as those guys. If you think you are saying the same thing, it indicates you are confused.
5. You are confused here about the terminology, which is causing us to talk straight past each other. See #4 above.
6. You are confused here about how force diagrams work, which is causing us to talk straight past each other. See #4 above. Yes, everything can seem miraculous when one does not understand it.

I have an architecture degree by the way. I'm very hesitant to use this as a cudgel against you because:

a) If I say I'm an architect you'll come back and say you're a chemist, next thing you know someone else says they are a biologist; and then all three of us will list our college transcripts to show what classes we took; and suddenly we're just in a moronic pissing match about d*ck size.
b) and more importantly, this is middle school level physics so profession is irrelevant. Everyone should know this stuff.

But in this case I feel there is some relevance. Believe it or not I did have to take a structural engineering class or two. This force diagram stuff... dead boring. Let's put it this way: we both learned this a long time ago at age 12, but subsequent to that I had a more recent refresher than you. We'll leave it there.

Kapusta

Well, that wheel is on a Lefty which follows its own set of physical laws.

cyccommute 

When in doubt you always fall back on how stupid I am. It’s tedious.

Don’t trust me on lateral force can buckle a wheel? Do you trust Wikipedia?

Don’t trust Wikipedia? How about Michael Ford in yet another paper.

Don’t trust him? Henri Gavin discusses lateral loads extensively.

Let’s get rid of the “buckling” aspect that you’ve dragged the discussion off towards. Let’s get back to your original statement

What causes the bike to corner? A bicycle going in a straight line will go in that straight line forever…unless acted on by an external force. The “force” here is the lateral force produced by leaning the bike. The force to the ground runs from the CG to the ground at an angle but the only way to produce that angle is to impart a lateral force on the wheel. Do you see where the lateral force is coming from now?

No, it wouldn’t. I didn’t rotate the strand. The forces were imparted from left to right of the picture. If I forced the strand over too far to the right, it would break to the left. If I forced it too far over to the left, it would break to the right. The direction of the break (or buckle if you like) would depend on the angle of the lateral force. If bent to the left it isn’t going to break to the left. That’s just not the way the force is going.

So is this “always buckle on the same side” thing material specific? Does spaghetti only buckle to the right? Steel buckles only to the left? Or maybe spaghetti buckles only to the north while steel only buckles to the south.? Do buildings only ever fall down in one direction?

Let’s look at that in terms of a wheel. If the spokes only buckle in one direction all the time due to a lateral force, then all we have to do is orient the spokes opposite that direction. Wheels would never taco! Or are you saying that wheels only ever taco in one direction.

And you say I’m confused.

Try reading what WizardofBoz said again. You said “there is no lateral force on a wheel in a corner”. He said you are wrong. I said you were wrong for exactly the same reason.

And you are overusing the term “confused”. Better yet, why not explain to me how I’m confused. So far you’ve done a right poor job of that.

Forum rules prevent me from saying the words I want to say right now. Let’s just leave it a “Whatchyoutalking about, Willis?”. Describing the force vectors is not all that difficult. The force that runs at an angle between the CG and the ground is a product of the force on the CG and the lateral force. It’s fairly simple. If there were no lateral force, the CG (and the bike) would be traveling in a straight line with the force of gravity act vertically through the plane of the bicycle.

So enlighten me about force diagrams. You say you can apply a force to the wheel to move it when stationary but say that same bending doesn’t occur when the bike is in motion. Doesn’t work that way. If you can bend it by hand, when you put a large force on it, it will bend more. Leaning a bike over with your full weight cantilevered to one side of the bike, as you do in a lean (see WizardofBoz’s picture), puts a rather large force on the rims at the contact patch.

Says the guy unzipping his pants.

Not middle school because that’s far too advanced for someone 11 to 13. And bicycle dynamics aren’t all that simple. There are still professionals publishing papers on aspects of bicycle wheel dynamics that are anything but simple, as demonstrated by the above links. Still, understanding force vectors isn’t all that hard nor is understanding how they can be summed.

Let’s just say that I want to say something that would get me in real trouble but I won’t. I’ll be the bigger man and not hurl insults. Just suffice it to say that, yet again, you are incorrect.

Yan 

I want to apologize for my rude tone earlier. I can tell you are annoyed and you don't deserve that.

​​​​​​I see you didn't marker the spaghetti. I suggest you try it. I know it sounds odious but I promise you, once you color one side of it, you'll be able to tell that the vertical spaghetti buckles to the same side no matter how many times you repeat pressing down.

This is because that spaghetti already has a built in imperfection, an existing slight bend in it, so no matter what you do it's just going to bend in the same way as that existing bend. This is buckling. There is no external lateral force here. Your entire input force is axial. If you take a different spaghetti, that second spaghetti will have a different imperfection so it will buckle in it's own way. But it's always the same. You will never find a spaghetti that buckles in a 360 degree random direction each time you press down, because such a theoretically perfect spaghetti does not exist.

Once you accept this for the vertical spaghetti, you can then move on to the next step which is doing the same pressing with the spaghetti leaning in different angles. You are going to find the exact same result here. That thing is going to buckle in the same predetermined direction no matter which way you lean it.

The only hitch in this is that spaghettis are so weak that once you start leaning them gravity itself might start introducing a sag, which might screw up the result. You can get around this by using something slightly stiffer like a metal ruler.

By the way when I suggest you try something I'm not just pulling stuff out of my behind. Anything I suggest you try, I've tried first. A few days ago when I asked you to try moving your rim side to side 5mm, and you didn't believe me, I actually tried on multiple bikes before I said that. Same when I mentioned the metal ruler two days ago. That's why I get annoyed, because I spent time coming up with tested examples to explain, and then you don't try it, and then you come back with a bunch of contradicting statements that you would have known are incorrect if only you had tried what I asked you to try.

Yes, there is going to be some bending force on the wheel when you are riding. Any time you turn the bars there is going to be some bending force. You are introducing some horizontal force without a lean here. Once you settle into the correct lean, i.e. in a stabilized turn, assuming you ride properly and keep your body and bike mass all centered on the same plane, there is no more bending force on the wheel. At that point the entire thing, you, your bike, the wheel, are one idealized straight member and can be diagramed as such. The only imperfection is that the contact patch of the tire is offset from the centerline very slightly, but it's pretty negligible.

Earlier it was repeatedly brought up by you and others that the failure mode of bicycle wheels is buckling, i.e. if you infinitely tension your spokes eventually the whole wheel will decide to pretzel. This has no relevance to your claim that there is a bending force on the wheel during a turn. It's a completely different phenomenon under a different scenario.

Here's a thought experiment to help you visualize why there is no bending force on the wheel during a turn: imagine you are not riding on flat ground but in a velodrome. The track is sloped, so that although you are turning, the ground is nevertheless horizontal from your perspective. Is there lateral bending force on the wheel? Most people would say no. You are being pressed perpendicularly straight into the ground here. Once you accept that, ask yourself why there would be a bending force on flat ground. There isn't. From a physics perspective the wheel cannot tell whether the ground is sloped. As long as the tire has enough traction to not slide out, the wheel can feel no discernable difference between the two. It has no idea.

cyccommute 

Just to humor you, I did your experiment with two different pieces of spaghetti. In one the bow is to the right and in the other to the left. The one bowed right broke (aka buckled) to the right and the one bowed to the left broke to the right. The one bowed to the right didn’t break towards the left even though the first one broke that way. No one (except you) would ever consider that a structure would fail counter to the direct of the force. There is simply no reason for any thing made of any material to always buckle in the same direction.




That is a very silly statement. Spaghetti is not a uniform material. For that matter nothing is a purely uniform material but spaghetti is less uniform than many others. There is no “existing bend” that is in every piece of spaghetti. There are imperfections through out the shaft of the spaghetti that will have more influence on where the strand fails than the force being applied to the ends of the spaghetti. As you push down on the spaghetti, it bows and breaks. Push down on another piece (or even the left over pieces) and the direction of rupture would be impossible to predict. This is largely because spaghetti is amorphous.

Even in something more uniform (but still with areas that aren’t uniform) like a steel, you can’t predict which way the material will buckle. Ever hammered in a nail and had it bend over? They don’t bend in only one direction. The direction of the bend is determined by the angle of the strike and the material they are being hammered into as well as the crystalline structure of the nail itself. The directions of the bend is going to be completely random. Even under constant, consistent pressure, a bending nail would bend in a random direction.

The bolded passage above is the reason that it is completely random. The imperfections in the material are going to influence on the which way the rupture goes as well as any inconsistencies in the application of the axial force. There are just too many variables to have any predictive capability.

And, yes there is a lateral force. Even if the pressure is entirely axial, the spaghetti (or nail or spoke or crystal) doesn’t compress entirely in an axial direction. The material will bend in one direction or another…as seen in all the pictures I’ve shown…and as soon as that material bends out of plane, there is a lateral force. Same with a bicycle wheel, except in that case you are applying a lateral force to make the wheel turn.


See pictures above in this post and post 50. The spaghetti will break in the direction of the lateral force caused by the bend in the spaghetti. Think about what you are saying: you are saying that if the spaghetti breaks to the left it will always break to the left no matter what kind of force is placed on it. That would require the spaghetti to break counter to the lateral force that is being applied to it. That’s not how force works.

The more rigid the material the more it resists bending but if you apply enough force to make the material bend, it will bend in the direction of the lateral force.

You may come up with examples but they don’t explain what you think they do. Yes, a ruler is more rigid than spaghetti but that only means that you have to apply far more force to it. Yes, a wheel might move as much as 5 mm in either direction but that could be influenced by the bearing adjustment or flex in the frame. Even if I agree on the magnitude…which is the objection I had, not the bending…that means that the wheel bends that much at the contact patch if you use the bicycle as a lever with your CG at the other end of the lever.

My “contradicting statements” are only contradicting your incorrect statements. I realize you mean that I’m contradicting myself but I haven’t done that.

Finally!

And here we go again. Just because you are at steady state, that doesn’t mean that the centripetal force goes away. There is still a lateral force acting on the wheel (and bicycle and rider) throughout the corner. Remove it and inertia takes over and the bicycle returns to a straight line. You keep acting like this (red arrow) is the only force acting on the wheel. Yes, that is the overall force acting on the wheel but…




That is the sum of two vectors acting on the wheel…the normal force and the centripetal force. But this picture isn’t quite right.



The normal force isn’t centered over the wheel. The normal force is centered on the CG…which is why we call it that. The angled red vector in the picture below is a lever arm. Put a lever arm on wheel, constrain the wheel so that it can’t move, introduce a floating rim that detensions spokes under load, and the result is a bending force at the contact patch.




No I didn’t bring up buckling, someone else did. However, buckling of the wheel isn’t just caused by spoke tension. It can result in buckling but that isn’t the only mode for a wheel to buckle. As Ford said above “lateral deflection of the rim may produce lateral, radial, and torsional reactions from the spokes.”. The only way you can produce a “lateral deflection” of the rim is to put a lateral force on it. In other words, go around a corner. Additionally, if the spoke tension is too low, the wheel will also buckle because the rim isn’t strong enough to maintain form if put under lateral stress.

First I really doubt most people would say no. And if they did, they would be wrong. Again, the forces on the wheel are the sum of the normal force and the centripetal force. The rim bends because the wheel isn’t a rigid structure.

Just because the bike is moving doesn’t mean that it is not experiencing gravity pulling the CG towards the ground. Since the only point of contact is the tiny contact patch of the tire, all that force is concentrated on the rim. It can do nothing but bend. It’s even more force on the wheel than you put on it by pulling (or pushing) on the wheel by hand. If the rim bends because of that force, why would it not bend in response to even more force? The traction on the tire that keeps the wheel from sliding out causes the wheel and rim to bend.

The problem here is that you keep using a frame of reference where it can’t be used. Just because the wheel is perpendicular to the surface doesn’t mean that gravity isn’t pulling down on the bicycle and the friction of the tire is counteracting that. The action of counteracting the force of gravity puts stress on the wheel causing it to bend in response. The tire bends as well but that doesn’t mean the rim doesn’t bend at the point of contact as well.

urbanknight

This topic sure inspired a big argument that doesn't help answer your question at all. The important thing is that you have the wheels inspected and trued by a competent wheel builder. It's possible the rim bent enough that a wide imbalance of spoke tension would be required to make it straight again.

Andrew R Stewart 

I think it's time for the warring parties to stand down Andy

Calsun

The spokes on opposite sides of the wheel support the weight of you and the bike. Put too much of a load on one side and spokes can be damaged and then the rim of the wheel. Not a big deal to re-true the wheel but I would remove the tire and check the spoke nipples first.

Yan 

I'm having some trouble following your description. Just to be clear based on your pictures we are talking about vertically standing spaghettis, yes? You're saying the one that bowed to the right broke to the right. That makes sense, as if it is already bowing to the right, and you apply yet more force, then it can only bow even more in the same direction and eventually snap. But then you say that the next spaghetti bowed to the left, yet it somehow snapped to the right? How is this physically possible? If it is already bowing to the left with pressure, and you then apply yet more pressure, it can only snap the same way its already bowing. Maybe you can re-write your description to make it more clear

Did you try repeatedly pressing down on the same vertical spaghetti? Does is always bow the same way or does it bow a different way every time your press? Use the marker color to keep track.

This analogy is completely wrong and probably the reason you're confused. That nail is not standing freely on its bottom tip like a bike wheel. That nail is stuck in wood and is fixed in place through the length of its bottom half. When you strike the top of the nail, that wood is acting as a fulcrum in the middle of the nail and it bends. On the bike, the wheel is just sitting its bottom tip on the surface of the road. It's not fixed in like the nail. You can apply whatever sideways force to the bike you like. The bike is just going to lean freely away in response to your push. It's impossible for the wheel to bend because it just moves away. How do you bend something when you can't even hold it down?

Actually that's exactly what I'm saying. The force being applied is entirely axial. Therefore the spaghetti (or wheel) will always buckle in the same direction. If you had an idealized member, it requires an outside introduced lateral force in the middle of the span to initiate the buckling. In real life objects, there's a small existing curve already built into the spaghetti, a small amount of out of trueness already built into the wheel. No matter how many times you repeat the pressing down experiment, it's only going to buckle more and fail via the existing bend imperfection. That's the weaker direction. It will always fail in the direction that is already weaker. It's never going to reflect and go the other way and fail on the strong side.

Are you serious with this??? Can we go back and try to remember what a lever is? You need three points of fixture to have a lever. When you look at a cyclist from the front, what do you see? You just see a vertical line with its bottom tip balanced on the ground. Apart from the single ground point, all it is touching is air. Can you explain to me how, in any way, this vertical line balanced on the ground contains the three fixture points of a lever? Where is the fulcrum? Where are the lever arms on both sides of the fulcrum?

_____

But forget all of the above. I get you pretty well after all that I think. Let me summarize what I think you believe:

You believe that, if you take some straight object, for example a perfectly straight rod (let's say it is the javelin of Zeus and is an absolutely perfect object), and stand it on the ground, lean it a bit to one side, and then press down axially from the top of it, then it will ALWAYS bend toward the ground, like the bottom end of a hockey stick.

Question: Did I summarize your belief correctly?

Answer this question and then we can continue. I just want to make sure we are talking about the same thing and we are communicating clearly. I don't want to waste time. You can feel free to respond to all the other points higher up, just answer this last question yes or no so we have a clear basis for going forward afterwards.

urbanknight

As long as you check the spoke tension as you go. Having to go too tight or too loose in order to straighten the wheel can be a problem.

cyccommute 

Back before we took the nightmare trip into Physicsland, I did address Ev0lutionz’ question. Given that he could ride the bike for another 150 km, I really doubt that the wheel was tacoed. I also doubt that the crash caused the wheels to go out of true. It’s possible but unlikely. There are just to many other ways for the wheels to go out of true to say with any certainty that the crash caused the problem. If Ev0lutionz had noticed the problem just after the crash, the likelihood of the crash causing the problem would be greater but it would still be questionable unless he had just done a truing job.

At this point, I think it just needs a truing job but it doesn’t need to be assessed for damage.

Bottom line: I’d say “maybe but not likely” to his question…which is what I said so many pages ago.

cyccommute 

Oops. My mistake. I typed it wrong. Both broke in exactly the manner to be expected. If the bow was to the right, the spaghetti broke to the right. If the bow as to the left, the spaghetti broke to the left.

They were marked. I can make them bow in any direction I want but they would bow at random if placed in something that provided a more consistent pressure. They aren’t going to bow always to the right nor to the left nor to any of the other 358 degrees. The angle of the cut on the end would have more influence on the angle of the bow than anything internal consistency (or inconsistency) that the spaghetti has. The same could be said of any material. The direction of collapse is going to be random.

No, the analogy isn’t wrong nor am I confused. The bicycle wheel isn’t just sitting on the surface of the road. It is fixed in place by the friction of the tire just like the end of the nail is fixed in the wood. The wheel doesn’t “just move away” if you push it…not on a surface with friction anyway. Push the bike into the corner at an angle and the wheel response to the force by bending to conform with the road surface. If you exceed the friction, the wheel will slide but that frictional force isn’t small. The tire, which is attached to the rim, will roll a bit because it is, well, a circle. This roll carries the rim with it because the rim is free to move on the spokes. Even the spoke bend slightly due to the lateral force.

And, again, stop saying I’m “confused”. I’m not. Not at all!

Why would an idealized member need a lateral force to buckle and a real object not need the same impetus? Real world objects will buckle if a lateral force is introduced to them. See Ford above. Rear wheels have a built in weakness due to the gearing but even there the wheel will not always taco to the right or left. If you search “tacoed bicycle wheel”, you can find tons of pictures of tacoed bicycle wheels that bend both left and right. Even ones where there is uneven tension on one side of the spokes, don’t necessarily taco to the weak side of the wheel. You can also find examples of tacoed wheels where the tension is equivalent like in a completely symmetrical front wheel. It simply doesn’t fail to the same side all the time.

Think about the implications of what you are saying: If a support member only ever fails in one direction, you could build a building (or wheel) so that you just orient each member so that when it fails it fails only towards another member that would fail in the opposite direction. Additionally, if you used a member where there was a lateral load all you’d have to do is orient the member so that the failure mode is towards the lateral load.

You do realize that levers come in different classes, don’t you. The fulcrum doesn’t have to be located between the load and the effort. In a third class fulcrum, the effort is located between the load and the and the fulcrum.



Using your bicycle picture



The ground is the fulcrum and the CG is the load. The “effort” is the friction of the tires against the ground. The wheel, being the contact with the fulcrum and being a nonrigid structure bends at the rim (and slightly at the spokes).

_____

No, you don’t press down axially. That’s not what is happening in a corner on a bicycle. The inertia of the bike makes it want to go in a straight line but the friction on the wheels puts a lateral force on the tires and wheels while centripetal force pulls the wheels around the corner using that same friction. Since the wheels aren’t rigid, they bend due to the lateral force placed on them as the bike goes around the corner. Although I hate analogies, putting a broom on the ground and pushing down on it to sweep. The bristles bend up and away from the surface.

The force pushing into the ground isn’t “axial”… or, at least, not purely axial…there is an axial component from the normal force but that is not centered over the wheels. The only time the force could be entirely axial would be if the bike is vertical.

Part of the problem here is that you keep want to put the bike in a reference frame where the bike is always perpendicular to the surface. It isn’t. The force vector is at an angle to the surface and you have to treat it as such. Friction keeps the bike from sliding out and that friction is what generates the lateral component of the force.

Asked and answered.

urbanknight

Are you saying the wheel might have been untrue before the crash? That's entirely possible. I'd still check it with a tension meter to be safe though.

Yan 

I'm sorry but can you answer my question more directly. I want to make sure we are on the same page before going forward. Otherwise we will just talk in circles.

If you think my summary of your belief was not correct, then tell me what your true belief is. In that leaning Zeus javelin scenario I proposed, what do you think the rod will always do? Will it always remain straight? Will it always bend? If it will always bend, will it always bend in a certain way? Will it always bow toward the ground or bow away from the ground? Tell me what your belief is. Be precise.

Let me also give you another allowance to make it even more generous: I don't require you to press down on the top tip of the javelin in any specific direction. If you don't like axial, you don't have to do axial. If you think there is some other direction of pressing that will cause the javelin to bend more easily, you can go ahead and choose that way. The only thing you are not allowed to do is to pull up and remove the javelin from the ground. Keep in mind you can only apply force to the top tip of the javelin. You can't touch the javelin anywhere except at the very tip. You can't use both hands and hold it like a spear. You can't grab a part of the shaft with a fist like you're going to throw it. All you can do is to place one finger on the very tip, like in those spaghetti photos you took. Your finger has infinite strength and you can press as hard as you like.

So what is your belief? Does it bend, and how?

djb

I'm defrosting some spaghetti sauce so we can eat pasta tonight, that's my take from all this.

And crash dude, just get a good wheel guy or gal to go over your wheels, and make sure the qr are tightened properly too.

cyccommute 

Could have been. We really don’t know how far out of true the wheels were even after the crash. Given that Ev0lutionz rode for another 150 km without noticing, it can’t have been too bad. I don’t know that I would go so far as checking tension, however. I’d just true the wheel and see what happens.

cyccommute 

I really don’t like to do analogies. Your analogy above is okay for a javelin and it probably wouldn’t bend but it is not a model for a bicycle, bicycle wheel, and cornering. You don’t “press down” axially on a bicycle wheel when cornering. A spear is meant to be used with a thrust so it is pressed down axially into the ground (or opponent). It’s not a pry bar. I don’t want to get into a discussion about spears as we’ve already jumped a thousand sharks using spaghetti.

If you must use an analogy, a regular kitchen broom would be a better model. You angle the broom to the ground to sweep. You apply a downward force at the end of the broom handle to pull the bristles toward you and that action causes the bristles to bend upwards because the bristles aren’t as stiff as the handle. It’s a class three lever in that the load in at the end of the handle, the effort…pulling the broom towards you…is in the middle of the lever, and the fulcrum is the ground. The bristles are experiencing a separate lever because they can bend as well.

This is analogous to how a bicycle wheel reacts to cornering. The rim is the bristles in the above analogy. The spokes really can’t bend all that much because they are constrained by length and limits on elasticity. The rim, on the other hand, is only attached to the spokes down the center line of the rim. And the rim isn’t really “attached” to the spokes. It’s held in place by the tension of the spokes compressing the rim. The rim can easily pivot around those center points that are allowed to float on the spokes. But it will only do so at the contact patch (and a little bit on either side).

Yan 

Can we not use these cop out words like "probably"? Will it or will it not? If you don't know just say you don't know.

But anyway I don't have a problem if you want to change the stick to a different object. The physics are all the same. However when you say you apply force to the broom, it sounds like you're saying you're using both hands. One hand at the end of the handle, a second hand in the middle of the handle. That's not accurately translating the scenario I gave you. I told you you could only apply force to the top point.

If you didn't hold the middle of the broom, but instead only pushed down on a LEANING broom at the very top of the handle with your palm, (1) would the bristles still ALWAYS collapse in the same direction, under your belief system? Or would it collapse randomly in either direction every time your try? (2) If you think it will always collapse in one direction, which direction is that? Concave side or convex side toward the ground?

Please answer the labeled questions.

urbanknight

Understood, but I would be concerned that if the rim got bent from the impact (it happens), it could have caused some spokes to go slack, which will severely reduce their lifespan and thus premature spoke failure. It's also possible (though less likely) the the opposing spokes to those got increased tension, which can cause the rim the fail unexpectedly. I have personally had that happen, making the sheared rim strike the fork (cracking the carbon) and send me into a somersault over the bars. For that reason, I would personally at least check the tension.

Kapusta