KJK

More likely it's hanging from the spokes at the top of the wheel. Spokes aren't very strong in compression.

Jamminatrix

Everything in combination makes a stiff wheel - rim stiffness, spoke gauge, spoke count, hub flanges, lacing pattern, tension, etc.

Spoke count in and of itself is about meaningless. My 16/20 spoke Shimano C50's ride a lot stiffer than most 32 spoke 3x aluminum box rim wheelset I've tried!

Here's a C75:

habilis

@FBinNY: From reading many of your posts, I'm sure you know exactly what you are talking about. Otherwise, I wouldn't ask that you please elaborate on these two statements:

1. "wheel stiffness depends almost entirely on the total amount of steel, namely the cross section X the no. of spokes."

2. "TENSION HAS NO EFFECT ON STIFFNESS."

In Statement 1, you suggest that rim profile counts for little. I was hoping that my deep-V rims at least gave me some strength for their extra weight. (I recently hit a bump in the road and suffered a snake-bite puncture but no deformation of the rim. Could a flexier rim have saved the tube? Just speculating.)

In Statement 2, you suggest that I could loosen the tension on all (or even only some) spokes and it would have no effect on wheel stiffness. If I understand you correctly, this runs counter to everything I've read about wheel building. What am I missing?

bradtx

Frank, I have a fairly flexible wheel set and there is no issue with the brakes rubbing the rim in spite of noticeable deflection between the hub and the ground at the front wheel. I had posted this observation, but I can't find it no matter what.

The wheel set is 28H CXP23 built with 15-16 butted spokes and Shimano 6500 hubs. Your rim maybe stiffer than mine causing deflection further towards the top of the rim (?), but basically I'd just allow more clearance between the brake pad and the rim.

Brad

FBinNY 

Before continuing with this post, read about Young's Modulus, which describes the basic and fundamental relationship between tension and deflection (stretch).

Within the working range the amount a spoke will stretch with added load, ie. your weight, is the same whether the tension starts at 10okgf and goes to 120, or starts at 70 and goes to 90 is exactly the same. So, it's not the initial tension that matters, only the change. So applying the principles of Young/s Modulus, it's apparent that since the elastic properties of the materials can't change, the easiest way improve the stiffness of a given tension structure, ie. a bicycle wheel is to add material (steel) so either fatter spokes or more spokes or both.

There are other things that improve stiffness, for example changing the spoke's bracing angle, but this isn't as easy as it seems at first glance. For example we could increase flange separation (best option), but we're constrained by space available. Or we can increase flange size, but since the spokes are tangent or near tangent, flange size has little effect because spokes are pulling from near the center-line anyway. So it's back to more steel, or as I sometimes write more "spoke". There's no magic here.

Rim stiffness does make a difference, but it's not as great as people would like to believe. Even the stiffest rim is like taffy compared to the stiffness the spokes provide. The rim's main contribution, besides being a tire mounting surface is as a compression ring to hold the spokes under tension. But some added rim stiffness does help because it spreads the loads among more spokes, especially in lateral deflections. Consider truss bridge construction. As spans get longer the depth of the trusses is increased, and likewise deeper or wider rims will be stiffer and spread the load bringing more spokes into play, and thereby improving stiffness somewhat.

The key to understanding stiffness in wheels isn't to think about what it's like statically, but what will happen as it deflects under loads, keeping in mind that it's a system of elements working as a whole, not acting independently of each other.

McBTC

You'll have to argue with the engineers among us about that...

FBinNY 

The entire hanging from top, supported from the bottom debate is nonsense, there are elements of truth and error in both statements.

First, let's end the supported from the bottom error. It's not possible, for the simple reason that the nipple isn't attached to the rim, and the rim can't push it toward center. Try that with a single spoke in a hole of some kind and it'll be obvious.

But the hanging form the top theory is likewise weak. If you press the axle of a built wheel toward the rim, you won't see enough change in the tension of the spokes opposite (the top) to account for the force involved.

Instead of thinking inn terms of either wrong answer, think of a wheel as a system where ALL spokes support the hub at all times. When loaded, the tensions in the various spokes change so there's a net change among the spokes to offset the load. How that change is represented among the individual spokes depends on the radial stiffness of the rim.

Looking at extremes helps. If we assume the rim is absolutely rigid (radially) there will be a reduction in tension among lower spokes, and a symmetrical increase among upper. But if the rim has very little radial rigidity, the area directly above the bottom spoke will move in slackening that one spoke, while then rest will have a very small increase in tension distributed nearly equally all the way around. The reality is that rims are in between those extremes, so you get a blended result, with it looking more or less like one of the extremes according to the rim's ability to spread the load.

KJK

I understand and agree with this totally.

I was just trying to break it down to the simplest form but I may have gone just a little too far. My main point was that spokes don't (for all practical purposes) support a load in compression.

FBinNY 

They don't AT ALL for the simple reason that there's no mechanism for the rim to transfer in inward radial force to the spoke. Spokes can only contribute to the wheel's structure while in tension. Once the tension zeros out, any further deflection would simply push the nipple back into the rim.

Once you realize that the rim can't push the nipple, it helps redirect the mind to more real possibilities.

McBTC

True that it begins with the pretension of the spokes which requires a complete system as all of the spokes together is what allows the spokes at the bottom to be under tension: that's what makes it all work. And, the greater the pretension, the greater the applied compression can be. I'm just passing along what I've read as to the understanding of engineers or most engineers or at least all of the engineers that you can Google on the net (e.g., "bicycle wheel stands spoke compression") that it all works because, a wheel stands on its spokes, not because the hub is suspended by the spokes from the top of the rim.

Looking at the matter by focusing on extremes what you then 'see' is a lengthening of the spokes at the top of the rim, reflecting an increase in tension, and a decrease in length of the spokes at the bottom that are under the greatest amount of stress as evidenced by their loss of tension caused by compression. Among those factors about which there seems to common agreement: to carry more load (withstand more compression) you need a stronger rim to withstand the greater pretension forces; the spokes carrying the load –i.e., those that are under the most compression – are still under tension or they would collapse (a spoke under tension can only support compression loads up to point that it becomes 'slack'); and, the spoke at the bottom of the wheel does not gain compression but rather, it pushes up on the hub as it shortens due to its decrease in tension.

FBinNY 

The phrase "stands on the bottom spokes" is part of the problem. It's a semantic trick whereby someone describes a reduction of tension as a compression. In that way it's analogous to how some in Congress will describe a reduction in revenue, ie. a tax credit for certain items, as an "expense". Yes, it's functionally the same in terms of the net effect, but it's not the same thing.

Moreover, the concept of "standing on the bottom" belies the actual condition, which spreads the changes among ALL the spokes. In the case of a more flexible rim, the tension of spokes near the bottom increases along with all the others.

We can debate semantics forever, but the only realistic models are those that describe the entire system, and discuss net changes in tension bottom to top, including the sides.

If you consider that a deflected rim would ovalize under load, you can see that the hub is also supported from the sides at the same time.

BTW- one of the problems in all wheel discussions is that many people (on all sides) are simply repeating things they've heard and read, and have become very committed to in a way that someone stranded in the ocean clings to whatever floats.

So to those who want to understand this kind of stuff. Don't latch onto partial explanations. Stop, think, draw yourself diagrams and sketches, and try to see the whole picture, not a few pixels.
....

McBTC

Forget about "standing" and "hanging" if that only invokes thoughts of nothing more than a debate over semantics. The real question is which spokes are actually doing the work and that has been calculated by engineers using finite element analysis. The spokes at the bottom are carrying the load so that evokes the use of a word like 'stands' to connote something being supported by something else that is resting on the ground, as opposed to the word 'hangs' that might seem more appropriate if the physics of the matter were more like a hub being supported from something else above the hub. Jobst Brandt ("The Bicycle Wheel") is pointed to as the seminal work on the structural mechanics underlying the functioning of the bicycle wheel.

FBinNY 

I suspect that you're someone without direct expertise in these issues, but apologize if wrong.

You characterize the lower spokes as doing the work, but would be fairer to say that the lower spokes are doing less work.

Imagine a tug of war. Two teams are pulling equally on the rope with the flag centered over the mud hole. On a signal, all the members of one team let go and send the others back onto their asses, taking the flag with them. So how would you describe this. Would you say they pushed the rope? Maybe, and the effect is as if they did, but they didn't push, they simply failed to pull and that was enough.

Likewise with a bicycle wheel. The lower spokes don't support the hub, they simply don't pull it down as hard, and the rest happens. Notice that like with tug of war, the upper spokes don't have to pull any harder, they just have to keep doing what they were all along.

That's why I hate the up from the bottom or down from the top argument. Neither is true.

SquidPuppet

I don't understand how the word "compression" can be used to describe the decrease in tension that the bottom spokes experience. I'm not an engineer, but it seems to me that it would be impossible to compress the bottom spokes. What am I missing?

FBinNY 

YOU are not missing anything at all. Some people use "compression" rather than reduction in tension the same way people would describe spending $6,000 on a $7,000 bike as savings. If that's savings than reduction in tension is compression by the same logic.

McBTC

I am not an engineer either but I think it begins with taking the initial state of the spokes as a given. Looking at the bicycle wheel the engineer begins with the prestessed structure where initially, all of the spokes are under tension. It's like perhaps looking at the compressive strength of a pretensioned concrete bridge.

SquidPuppet

Oh good, I'm not nuts. Sometimes it's very easy for me to get faked out by the counter intuitive bait that mechanical engineering and the laws of physics throw at me.

I've spent more time (much) thinking about wheels, and trying to truly understand them than I have building them. I've reached a point where the building is easy now, but I still try to learn more about what, and how they experience things as a whole unit.

ThermionicScott 

Discussions like these are probably why decades of wheel builders haven't minded not knowing how wheels work, as long as theirs do.

FBinNY 

Actually most skilled and experienced builders do understand the basics. Knowing them doesn't help with the build, but is critical to choosing and matching the components for a better wheel.

As a rule, I avoid the hang vs stand argument the same way I avoid chain lube debates, but sometimes feel the desire to step in and clear the air on public forums, where people are pushed to choose sides between two equally untrue choices.

McBTC

I'm always willing to listen..

ThermionicScott 

The basics, sure. I always get in trouble when I oversimplify what's on my mind.

CliffordK

Here are a couple of diagrams on load stress on spokes.

Ian's Bicycle Wheel Analysis


https://www.******.com/r/askscience/..._the/t3_25mmxp

(oops, lost r e d d i t from my source link, at least that is where Google took me).

What is interesting about the charts is that the highest tension on the spokes is those spokes at about a 45° from the BOTTOM of the wheel.

No simply hanging from the top.

It is also why a stiffer rim should help somewhat with stress distribution. Although, I'm not quite sure how much that is associated with lateral flex as the OP was asking.

SquidPuppet

Do you have a link for that? I'd like to read all of it. Thanks.

McBTC

The science behind spokes | Cyclist

SquidPuppet

Hmmmm. What is causing those bottom spokes to bend? They aren't connected to the rim.