Steel frame, stiff wheels, tuning-fork vibrations

[whatwolf]

I have a Tange Prestige Land Shark road frame from the late 80s. I built it up a couple months ago, and at the time only had one 700c wheelset (Fulcrum 5 CX) so it was doing double duty on my cx bike and the Land Shark (switching out tires once a week or so.) Recently, I decided to go tubeless for cross, so I bought a second wheelset for the road bike: Easton EA90SL. I wanted something lighter.

Well, they're definitely lighter, but they are also very stiff. The front wheel has 20 radially laced spokes with pretty high tension. I noticed after I mounted the tires (Fairweather Panaracer 28s at 80psi) and installed the wheels, that bouncing the fork on the ground causes the fork to vibrate, and that (if you hold the front end in the air) the vibration seems to resonate and lasts a while.

Consequently, the ride is a lot more jarring than it was with the other wheels, and I don't think it will feel good on long rides (this is my rando bike.) Lowering the tire pressure 5-10psi doesn't have much of an impact. I'm bummed because other than that the wheels feel great, especially climbing. I'm curious if anyone has encoutered this sort of issue before and how you dealt with it. Should I just get slightly heavier, higher spoke count wheels to achieve lower spoke tension? I'm pretty sure the Fulcrum 5CX's have even fewer spokes (and the front is also radial) but I never had this problem with those wheels.

[likebikes]

Ok

[bicycletricycle]

Amount of spokes is not directly related to spoke tension, spokes need a certain base level of spoke tension regardless of spoke count.


Perhaps the heavier wheels where just damping your tuning fork like vibration.

Spoke tension is not your problem. The rim dictates vertical compliance of wheels more than any other factor (besides tires of coarse).

[HenryA]

POW

Plain Old Wheels -- will ride better. Build some according to your weight.

[cachagua]

You've probably checked the headset?

Also, as another branch in the troubleshooting, see how that front wheel behaves in another bike. Do the same test and see if the vibration occurs there too.

[Black Dog]

Stiff, deep profile rims make for a harsh ride.

[beeatnik]

^Not always true.

Good Article on Wheel Stiffness:

http://www.slowtwitch.com/Tech/Debun...ness_3449.html

[whatwolf]

You've probably checked the headset?

Also, as another branch in the troubleshooting, see how that front wheel behaves in another bike. Do the same test and see if the vibration occurs there too.

Yes, headset is properly adjusted. Good idea about checking on another frame. I suspect it's specific to this fork/frame though.

I expected these wheels to be stiff, but the resonating vibrations are, I think, out of the ordinary. Maybe I should take a video. I work on bikes for a living and have (test) ridden tons of bikes. I've never had one do this and I'd like to figure it out. Thanks all for the suggestions and info!

[Black Dog]

^Not always true.

Good Article on Wheel Stiffness:

http://www.slowtwitch.com/Tech/Debun...ness_3449.html

I was referring exclusively to radial stiffness.

[Mark McM]

Stiff, deep profile rims make for a harsh ride.

Firstly, the idea that wheels make a meaninful difference to (vertical) ride compliance has already (or should have been) relegated to realm of myth and folklore. The reality is that wire tension spoke wheels are so very stiff vertically, that they provide little vertical compliance. Other components, most especially the tires, but also the saddle, seatpost, handlebars and stem, provide the majority of the compliance, and any compliance difference between wheels is lost in the noise.

Secondly, why should we assume that wheels with deep profile rims should be any stiffer than wheels with shallow rims? Take a look at the directly measured wheel stiffnesses in this web page (http://sheldonbrown.com/rinard/wheel/grignon.htm), and you'll see that often wheels with deep profile rims have less radial (vertical) stiffness than wheels with shallow profile rims. Spokes influence radial stiffness far more than rims do, so wheels with many thick spokes are often stiffer than wheels with fewer and/or thinner spokes, regardless of rim profile.

But what about the argument, "I can feel the differences between wheels when riding, and wheels with deeper rims are less comfortable?" It is easy for people to fool themselves, and make themselves feel sensations just because they expect them. That is why tests involving human subjects should be conducted blindly. When riders have tested wheels blindly, the differences in comfort between shallow and deep profile rims disappear. Here are some reports from Josh Poertner, a former engineer at Zipp Composites, regarding wheel comfort and blind testing:

http://www.slowtwitch.com/Tech/Thoughts_on_science_perception_4571.html

http://nyvelocity.com/articles/interviews/josh-poertner

[Waldo]

Reduce tire pressure.

[sandyrs]

Reduce tire pressure.

Seem the OP has tried that.

Lowering the tire pressure 5-10psi doesn't have much of an impact.

[Waldo]

Reduce it further...

[Mark McM]

Seem the OP has tried that.

If the OP has found that the front wheel audible vibrates when the wheel is off the ground, then he shouldn't ride with the wheel off the ground. Most road shock that actually affects rider comfort is at low frequency, mostly below the audible range. Frequencies in the audible range have less affect on the rider, and will be quickly damped (by the tire) when the wheel is on the ground.

[ColonelJLloyd]

If the OP has found that the front wheel audible vibrates when the wheel is off the ground, then he shouldn't ride with the wheel off the ground. Most road shock that actually affects rider comfort is at low frequency, mostly below the audible range. Frequencies in the audible range have less affect on the rider, and will be quickly damped (by the tire) when the wheel is on the ground.

From the reading the OP I didn't assume the poster:

Meant that the vibration was audible
Rides wheelies
Is a "he"


;)

[whatwolf]

From the reading the OP I didn't assume the poster:

Meant that the vibration was audible
Rides wheelies
Is a "he"


;)

Ha! I wish I could wheelie on my road bike.

[whatwolf]

If the OP has found that the front wheel audible vibrates when the wheel is off the ground, then he shouldn't ride with the wheel off the ground. Most road shock that actually affects rider comfort is at low frequency, mostly below the audible range. Frequencies in the audible range have less affect on the rider, and will be quickly damped (by the tire) when the wheel is on the ground.

There is no audible component to what I'm describing, and the reason I'm not just ignoring it and riding the bike is that it does not go away when the tire connects with the ground again. So if every minor or major imperfection in the pavement that momentarily lifts the wheel creates a vibration pattern that the fork for whatever reason is amplifying, and that therefore persists after the tire touches smooth pavement again, the bike becomes uncomfortable to ride.

Obviously my first action was to experiment with tire pressure, but I have yet to find one that eliminates this effect. I was riding at 75/80 in the front, and have tried as low as 60psi (I'm 140lbs) but below that I'm just going to pinch flat it. I'll probably try a different tire set up tubeless and see what effect that has before trying other wheels.

[sandyrs]

There is no audible component to what I'm describing, and the reason I'm not just ignoring it and riding the bike is that it does not go away when the tire connects with the ground again. So if every minor or major imperfection in the pavement that momentarily lifts the wheel creates a vibration pattern that the fork for whatever reason is amplifying, and that therefore persists after the tire touches smooth pavement again, the bike becomes uncomfortable to ride.

Obviously my first action was to experiment with tire pressure, but I have yet to find one that eliminates this effect. I was riding at 75/80 in the front, and have tried as low as 60psi (I'm 140lbs) but below that I'm just going to pinch flat it. I'll probably try a different tire set up tubeless and see what effect that has before trying other wheels.

For what it's worth I feel your pain- I am generally skeptical of human perception of "ride quality" and it took me a long time to come to grips with the fact that when I used a specific set of wheels (over multiple sets of tires, some of which I'd used on other wheelsets), I always found the bike they were on more jarring to ride. Those wheels now live on my commuter with large tires on them. Ultimately there is no shame in selling the wheels and getting a second pair of the ones you like if you can't reach a workable solution.

[HenryA]

Just borrow any "Plain Old Wheel" for the front and try it for a few minutes. Its too easy to pass by.

[Black Dog]

Firstly, the idea that wheels make a meaninful difference to (vertical) ride compliance has already (or should have been) relegated to realm of myth and folklore. The reality is that wire tension spoke wheels are so very stiff vertically, that they provide little vertical compliance. Other components, most especially the tires, but also the saddle, seatpost, handlebars and stem, provide the majority of the compliance, and any compliance difference between wheels is lost in the noise.

Secondly, why should we assume that wheels with deep profile rims should be any stiffer than wheels with shallow rims? Take a look at the directly measured wheel stiffnesses in this web page (http://sheldonbrown.com/rinard/wheel/grignon.htm), and you'll see that often wheels with deep profile rims have less radial (vertical) stiffness than wheels with shallow profile rims. Spokes influence radial stiffness far more than rims do, so wheels with many thick spokes are often stiffer than wheels with fewer and/or thinner spokes, regardless of rim profile.

But what about the argument, "I can feel the differences between wheels when riding, and wheels with deeper rims are less comfortable?" It is easy for people to fool themselves, and make themselves feel sensations just because they expect them. That is why tests involving human subjects should be conducted blindly. When riders have tested wheels blindly, the differences in comfort between shallow and deep profile rims disappear. Here are some reports from Josh Poertner, a former engineer at Zipp Composites, regarding wheel comfort and blind testing:

http://www.slowtwitch.com/Tech/Thoughts_on_science_perception_4571.html

http://nyvelocity.com/articles/interviews/josh-poertner

I agree whole heartedly with most of what you say here. However, the depth of the rim profile will effect the vertical compliance to some degree. Spoke do not provide and compressive load carrying when they are above the rim section (bottom) that is directly above the road. The load is being supported by spokes in tension at the top of the rim. This makes the rim the sole bearer of the load. the rim will deflect to some degree. The deeper the profile the less defection will occur, all else being equal. When hitting imperfections in the road surface that can be felt differences in deflection amounts effect the amount of force being transmitted to the frame and thus rider and may be perceptible. This effect will be increased as tire pressures increase due to the lessening of tire absorption at higher pressures.

I agree that tires eat up most of the impact, and that most so called differences in "feel" are simply psychosomatic and not worthy of note outside of a proper blind study or better yet actual empirically measured data.

[Peter P.]

I too, am inclined to mostly agree with what Mark McM said.

Wheels without tires are all very similar in vertical stiffness, mostly due to spoke compression upon the rim.*

The tire is the biggest factor but you said you've already reached the limits of tire pressure with regard to trying to eliminate the problem.

I believe the fork's vibration will be damped by the weighted front wheel and the tire's ability to absorb some of that energy.

As has been suggested, borrow a random different wheel if you can and see what happens.

*I know this counters my statement above, but I tend to think it's the radial lacing of the front wheel. Try a wheel with the same number of crosses as your Fulcrum or try 3x wheels. Thinner gauge spokes are not necessary but they may be offer a bonus effect.

[Black Dog]

I too, am inclined to mostly agree with what Mark McM said.

Wheels without tires are all very similar in vertical stiffness, mostly due to spoke compression upon the rim.*

The tire is the biggest factor but you said you've already reached the limits of tire pressure with regard to trying to eliminate the problem.

I believe the fork's vibration will be damped by the weighted front wheel and the tire's ability to absorb some of that energy.

As has been suggested, borrow a random different wheel if you can and see what happens.

*I know this counters my statement above, but I tend to think it's the radial lacing of the front wheel. Try a wheel with the same number of crosses as your Fulcrum or try 3x wheels. Thinner gauge spokes are not necessary but they may be offer a bonus effect.

Spokes do not work under compression. They provide no support under compressive load.

[pbarry]

At the OP's weight, with Tange Prestige tubing in the frame and fork, and a wheel that doesn't flex much, this effect is not surprising. Prestige is pretty unforgiving for lighter weight riders. A wheel with two or three crosses should provide some relief, as would one with a lower profile rim.

The frame was built when all rims were low profile and most wheels were built three cross..

[93legendti]

I have a Tange Prestige Land Shark road frame from the late 80s. I built it up a couple months ago, and at the time only had one 700c wheelset (Fulcrum 5 CX) so it was doing double duty on my cx bike and the Land Shark (switching out tires once a week or so.) Recently, I decided to go tubeless for cross, so I bought a second wheelset for the road bike: Easton EA90SL. I wanted something lighter.

Well, they're definitely lighter, but they are also very stiff. The front wheel has 20 radially laced spokes with pretty high tension. I noticed after I mounted the tires (Fairweather Panaracer 28s at 80psi) and installed the wheels, that bouncing the fork on the ground causes the fork to vibrate, and that (if you hold the front end in the air) the vibration seems to resonate and lasts a while.

Consequently, the ride is a lot more jarring than it was with the other wheels, and I don't think it will feel good on long rides (this is my rando bike.) Lowering the tire pressure 5-10psi doesn't have much of an impact. I'm bummed because other than that the wheels feel great, especially climbing. I'm curious if anyone has encoutered this sort of issue before and how you dealt with it. Should I just get slightly heavier, higher spoke count wheels to achieve lower spoke tension? I'm pretty sure the Fulcrum 5CX's have even fewer spokes (and the front is also radial) but I never had this problem with those wheels.
What is the minimum psi for those tires? 28c at 80psi seems less than forgiving.

[HenryA]

at the op's weight, with tange prestige tubing in the frame and fork, and a wheel that doesn't flex much, this effect is not surprising. Prestige is pretty unforgiving for lighter weight riders. A wheel with two or three crosses should provide some relief, as would one with a lower profile rim.

The frame was built when all rims were low profile and most wheels were built three cross..

^^^^^^^ this ^^^^^^^

[whatwolf]

At the OP's weight, with Tange Prestige tubing in the frame and fork, and a wheel that doesn't flex much, this effect is not surprising. Prestige is pretty unforgiving for lighter weight riders. A wheel with two or three crosses should provide some relief, as would one with a lower profile rim.

The frame was built when all rims were low profile and most wheels were built three cross..

That makes lots of sense. Still, I am surprised at the stark difference in ride quality between two modern front wheels both with low-count radial patterns and similar rim profiles, using the same tires at the same pressure. A more traditional wheelset makes sense. And I will look into the specs of Prestige tubing, I did not know that it was known to be particularly unforgiving. I figured this frame had pretty skinny butted tubes because it is pretty damn light. But whatever the case may be, it's becoming clear it does not like these wheels much. Live and learn.

[jtakeda]

I have 2 prestige landsharks and a prestige Davidson.

While I have never extensively ridden any of them off-road I will agree that 28c @ 80psi is high.

[bicycletricycle]

Firstly, the idea that wheels make a meaninful difference to (vertical) ride compliance has already (or should have been) relegated to realm of myth and folklore. The reality is that wire tension spoke wheels are so very stiff vertically, that they provide little vertical compliance. Other components, most especially the tires, but also the saddle, seatpost, handlebars and stem, provide the majority of the compliance, and any compliance difference between wheels is lost in the noise.

Secondly, why should we assume that wheels with deep profile rims should be any stiffer than wheels with shallow rims? Take a look at the directly measured wheel stiffnesses in this web page (http://sheldonbrown.com/rinard/wheel/grignon.htm), and you'll see that often wheels with deep profile rims have less radial (vertical) stiffness than wheels with shallow profile rims. Spokes influence radial stiffness far more than rims do, so wheels with many thick spokes are often stiffer than wheels with fewer and/or thinner spokes, regardless of rim profile.

But what about the argument, "I can feel the differences between wheels when riding, and wheels with deeper rims are less comfortable?" It is easy for people to fool themselves, and make themselves feel sensations just because they expect them. That is why tests involving human subjects should be conducted blindly. When riders have tested wheels blindly, the differences in comfort between shallow and deep profile rims disappear. Here are some reports from Josh Poertner, a former engineer at Zipp Composites, regarding wheel comfort and blind testing:

http://www.slowtwitch.com/Tech/Thoughts_on_science_perception_4571.html

http://nyvelocity.com/articles/interviews/josh-poertner

Those r interesting articles, apparently, all the differences in bicycles are almost meaningless except tires/tire pressure and how aero it is. Well meaningless from a performance standpoint.

I guess the landshark with Easton wheels is fine and it's all in their head.

Maybe they need a shrink?

[Pierre]

Have you tried tightening the QR? I have something similar happening on my bike and I can quiet it down by simply tightening it up. I've discovered that the hub bearing is ever so slightly loose (MAVIC Kysrium's). With just a bit more force from the QR, it all goes away.

[Mark McM]

I agree whole heartedly with most of what you say here. However, the depth of the rim profile will effect the vertical compliance to some degree. Spoke do not provide and compressive load carrying when they are above the rim section (bottom) that is directly above the road. The load is being supported by spokes in tension at the top of the rim. This makes the rim the sole bearer of the load. the rim will deflect to some degree. The deeper the profile the less defection will occur, all else being equal.

This is an old theory about how wheels bear loads which was disproven decades ago. The rim (which is primarily in bending) is much less stiff to radial loads then the spokes (which are in in axial tension), so loads are not transferred to the top of the wheel. Instead, external wheel loads are supported by the spokes in a small section of the wheel near the ground contact point.

That the wheel deforms at the bottom and that the top spokes carry little of the load has been demonstrated many times, both by finite element analysis, and by direct measurements from wheels (both statically and dynamically). Here's just a few of these demonstrations:

The original work that truly explained how bicycle wheels support loads is "The Bicycle Wheel", by Jobst Brandt, which contains both the theory and a Finite Element Analysis of spoked wheels:
http://poehali.net/attach/Bicycle_Wheel_-_Jobst_Brandt.pdf


Here is a paper that compares a Finite Element Analsys of a spoked wheel with direct measurements of spoke stresses, confirming that the bottom spokes support most of the wheel loads:
https://www.rose-hulman.edu/~fine/FE2002/Projects/Hartz.pdf


Most spoke stress measurements have been under static wheel loading. In this paper, a wheel whose spokes were instrumented with strain gauges was subjected to loads while actually riding, which confirms that the static stress analysis matches actual dynamic stresses:
http://people.duke.edu/~hpgavin/papers/HPGavin-Wheel-Paper.pdf

(Incidently, the above paper also examines the differences in stresses with different lacing patterns, and finds that lacing pattern has little influence on wheel durability)


Here is a paper that compares the change in spoke stresses/strains with different numbers of spokes:
http://www.ewp.rpi.edu/hartford/~ernesto/SPR/Ng-FinalReport.pdf

It can be seen in the data in this paper that spoke/wheel deflections change almost directly in proportion to the number of spokes, showing that the spokes play a larger role in wheel stiffness than the rim (at least for radial deflections)


All of these papers show that the primary response of a wheel under vertical load is a flattening of the rim near the ground contact point accompanied by a signicant compression (de-tensioning) of a few of the bottom spokes, and little reaction around the rest of the wheel. If you look around, you can find plenty of other studies and analyses that agree. You can even test this your self: As we know, the tone (audible frequency) of a plucked spoke varies with its tension - the higher the tension, the higher the tone. Stand a bicycle up on its wheels, and pluck spokes at a variety of orientations around the wheel. Then apply a vertical load on the bicycle, to simulate a rider's weight bearing down on the wheels. Pluck the same spokes again. You should find significant reductions in tone in spokes near the ground contact point, but very little change in tone of other spokes around the wheel.



Spokes do not work under compression. They provide no support under compressive load.

As explained in the "The Bicycle Wheel" and other analyses, spokes can and do work under compression, as long as the compression load is superimposed on a (larger) static tension. While some may argue the semantics of "a wheel stands on its spokes", it can not be argued that the bottom spokes are the primary contributor to the wheel radial stiffness. So at least in regard to wheel stiffness, there is no doubt that the bottom spokes work under compression.

[Mark McM]

At the OP's weight, with Tange Prestige tubing in the frame and fork, and a wheel that doesn't flex much, this effect is not surprising. Prestige is pretty unforgiving for lighter weight riders. A wheel with two or three crosses should provide some relief, as would one with a lower profile rim.

The frame was built when all rims were low profile and most wheels were built three cross..

As the Henri Gavin paper referenced above shoes, wheel stiffness changes only a little bit with spoke lacing pattern. The difference in vertical stiffness between a radial and a 3x pattern is only a few percent - or roughly the same as the difference in spoke lengths between the two lacing patterns. But even still, the radial stiffness of the wheel itself is at best a few percent of the total vertical stiffness, when the tire and other bicycle components are included. So the difference in total stiffness between a radially laced wheel and crossed laced wheel might be a few percent of a few percent - or almost nothing.

[bicycletricycle]

the quoted sources don't seem (to me) to say that spokes work under compression, they do seem to say that the spoke closer to the ground do most if not all the work rather than the spokes on the top. But they are still operating under tension. Am I missing something? (probably did)

Also, that would seem to be in opposition to this study-
http://www.williamscycling.com/assets/images/product%20tech/Bicycle%20Wheel%20Spoke%20Lacing.pdf
which seems to show the spokes on the top absorbing the static rider weight.

if the goal is to make a more vertically compliant wheel than one would want to decrease the height (axial stiffness) of the rim and/or decrease spoke count, if only the spokes in close proximity are bearing all of the load and the load is not being distributed around the wheel than you would want to increase the spoke spacing thus allowing a larger part of the rim to deform.

although the maximum difference you can make here is still much less than what can be done with tires selection/pressure.







This is an old theory about how wheels bear loads which was disproven decades ago. The rim (which is primarily in bending) is much less stiff to radial loads then the spokes (which are in in axial tension), so loads are not transferred to the top of the wheel. Instead, external wheel loads are supported by the spokes in a small section of the wheel near the ground contact point.

That the wheel deforms at the bottom and that the top spokes carry little of the load has been demonstrated many times, both by finite element analysis, and by direct measurements from wheels (both statically and dynamically). Here's just a few of these demonstrations:

The original work that truly explained how bicycle wheels support loads is "The Bicycle Wheel", by Jobst Brandt, which contains both the theory and a Finite Element Analysis of spoked wheels:
http://poehali.net/attach/Bicycle_Wheel_-_Jobst_Brandt.pdf


Here is a paper that compares a Finite Element Analsys of a spoked wheel with direct measurements of spoke stresses, confirming that the bottom spokes support most of the wheel loads:
https://www.rose-hulman.edu/~fine/FE2002/Projects/Hartz.pdf


Most spoke stress measurements have been under static wheel loading. In this paper, a wheel whose spokes were instrumented with strain gauges was subjected to loads while actually riding, which confirms that the static stress analysis matches actual dynamic stresses:
http://people.duke.edu/~hpgavin/papers/HPGavin-Wheel-Paper.pdf

(Incidently, the above paper also examines the differences in stresses with different lacing patterns, and finds that lacing pattern has little influence on wheel durability)


Here is a paper that compares the change in spoke stresses/strains with different numbers of spokes:
http://www.ewp.rpi.edu/hartford/~ernesto/SPR/Ng-FinalReport.pdf

It can be seen in the data in this paper that spoke/wheel deflections change almost directly in proportion to the number of spokes, showing that the spokes play a larger role in wheel stiffness than the rim (at least for radial deflections)


All of these papers show that the primary response of a wheel under vertical load is a flattening of the rim near the ground contact point accompanied by a signicant compression (de-tensioning) of a few of the bottom spokes, and little reaction around the rest of the wheel. If you look around, you can find plenty of other studies and analyses that agree. You can even test this your self: As we know, the tone (audible frequency) of a plucked spoke varies with its tension - the higher the tension, the higher the tone. Stand a bicycle up on its wheels, and pluck spokes at a variety of orientations around the wheel. Then apply a vertical load on the bicycle, to simulate a rider's weight bearing down on the wheels. Pluck the same spokes again. You should find significant reductions in tone in spokes near the ground contact point, but very little change in tone of other spokes around the wheel.





As explained in the "The Bicycle Wheel" and other analyses, spokes can and do work under compression, as long as the compression load is superimposed on a (larger) static tension. While some may argue the semantics of "a wheel stands on its spokes", it can not be argued that the bottom spokes are the primary contributor to the wheel radial stiffness. So at least in regard to wheel stiffness, there is no doubt that the bottom spokes work under compression.

[Mark McM]

the quoted sources don't seem (to me) to say that spokes work under compression, they do seem to say that the spoke closer to the ground do most if not all the work rather than the spokes on the top. But they are still operating under tension. Am I missing something? (probably did)

Yes, the bottom spokes are in net tension. However, they are supporting the vertical load by a compression force which is load superimposed on the static spoke tension. As long as the static spoke tension exceeds the compression forces, the spokes can support loads in compression. This is the entire purpose of a pre-stressed structure: To pre-stress the structure elements in a mode in which they can withstand forces, in order to decreases loads in modes in which they are weaker. This is, for example, how pre-stressed concrete works. Concrete can take very large loads in compression, but only small loads in tension. By pre-stressing the concrete under compressive load, tension loads on the concrete are minimized.

So, As long as the spokes stay in net tension, they can bear compression forces layed on top of the static tension.


Also, that would seem to be in opposition to this study-
http://www.williamscycling.com/assets/images/product%20tech/Bicycle%20Wheel%20Spoke%20Lacing.pdf
which seems to show the spokes on the top absorbing the static rider weight.)

Actually that study is in agreement. Note the first paragraph on page 5, which says:

"The figures are shown for the 3x/3x laced wheel. In the pretension case (Figure 4a), all of the drive side spokes are evenly tensioned to 650 N/mm2, and the non-drive side spokes are tensioned to 390 N/mm2. When rider weight is added to the wheel (Figure 4b), the loads on the spokes near the bottom of the wheel are seen to decrease considerably, whereas the tension in the remainder of the spokes increases very slightly."

In other words, the change in spoke loads which support the weight are the large tension changes near the bottom. Note that the slight tension increases in the remainder of the spokes include horizontal spokes at the front and back of the wheel (which can't support vertical loads at all), plus some of the spokes in the bottom half of the wheel outside of the the ground contact area, which are pulling down on the hub slightly more than before (so these tension increases work against supporting the load). When the vertical components of all the spokes with slightly increased tension are added up, their sum will be close to zero, which leaves only the spokes with tension decreases to support the load.

(Another note about the small tension increases: When the bottom of the rim flattens out, the "flat" section pushes circumferentially outward on the rim, slightly increasing its overall diameter. It is this slight diameter increase which is primarily responsible for the increase in tensions around the rest of the wheel. Also note that largest spoke tension increases are usually in the bottom half of the wheel, at the ends of the "flat" spot.)


if the goal is to make a more vertically compliant wheel than one would want to decrease the height (axial stiffness) of the rim and/or decrease spoke count, if only the spokes in close proximity are bearing all of the load and the load is not being distributed around the wheel than you would want to increase the spoke spacing thus allowing a larger part of the rim to deform.

It depends on what the purpose of making a vertically compliant wheel. Trying to get a "compliant ride" from the wheel is fools errand, since you can't get more than a fraction of millimeter of deflection from the wheel before the spokes slacken, and the wheel becomes unstable and prone to fatigue failures. On the other hand, there is something to be had within that fraction of a millimeter in regard to wheel durability. Specifically, if you use thinner spokes or a stiffer rim, then thae size of the "flattened" out portion of the wheel (in which the spokes supporting the load are compressed) is increased in size, encompassing a larger number of spokes. With more spokes sharing the load, the magnitude of the loading cycles on each individual spoke is decreased, thus increasing the durability of the wheel. This is why wheels with butted spokes are more durable - their extra "stretchiness" allows them to experience lower peak load magnitudes, and to distribute wheel loads across more spokes.

[benb]

How long does this fork continue to vibrate if you strike the wheel and then hold it up in the air?

The fork on my AC Space Horse has given me hell over the last couple years and I want to say it will also vibrate for quite a long time if you run the same experiment.

I'm not at home so can't try it but I've been riding a rental bike with a carbon fork this past week and the difference is so huge in terms of comfort I'm contemplating whether I am ever going to ride my Space Horse again. I'm supposed to be picking up a Domane shortly after I get back and I am tempted to tear down the Space Horse and sell the frame and fork and look for something that is not so overbuilt and fits me better. I've never put a front rack on the Space Horse fork so the fork is massive overkill in terms of stiffness for me.

I've been struggling with on/off wrist tendonitis riding the Space Horse the past 6 months.. I came on vacation and my wrist was pretty bad. I got on the rental bike (Spec. Roubaix) and have put in my biggest week of riding in at least a year and my wrist has been getting better every day, and the roads here aren't great. I'm running more drop & reach on the Roubaix too.

There are other things going on too.. if I hit a rough patch on my Space Horse I seem to grip the bars tighter as the bike wants to wander. It's been pretty noticeable riding the Roubaix that I can just about let go over the bars going through a rough patch on it and the bike goes through the pothole or whatever straight as a laser.. no strain at all on the wrists/arms if you can relax that much, but there is no doubt my Space Horse is too big for me which probably explains why it doesn't track as well as it might.

[bicycletricycle]

there is a compression force on the wheel, balanced by the tension in the spokes, especially the spokes close to the load which are working to pull that flat back to round, but also by all the spokes resisting the increase in diameter caused by the deformation,

no single spoke is supporting load by directly resisting compression, right? it seems that you are saying that as long as the deformation of the rim doesn't completely unload the tension that they do somehow work in compression.

In my non engineer mind I think that as long as the spoke never gets fully relieved of its base tension there is no way for it to start working in compression, plus the inability of the wire to support compression load.

the spokes working to pull that flat back to round do increase in tension, working against supporting the load, but they also effectively push the deformed section of the rim into the ground, supporting the load. No?

I get that the entire spoke system works to resist compression of the rim.

You are saying all the changes in spoke tension balance to zero except the main spoke pointing down and its associated decrease in tension thus a decrease in tension is the only thing supporting the load? I though that the load was supported by that decrease in tension being distributed in various ways to the other (mainly tensioned) components in the wheel.

Ill leave this alone after this








Yes, the bottom spokes are in net tension. However, they are supporting the vertical load by a compression force which is load superimposed on the static spoke tension. As long as the static spoke tension exceeds the compression forces, the spokes can support loads in compression. This is the entire purpose of a pre-stressed structure: To pre-stress the structure elements in a mode in which they can withstand forces, in order to decreases loads in modes in which they are weaker. This is, for example, how pre-stressed concrete works. Concrete can take very large loads in compression, but only small loads in tension. By pre-stressing the concrete under compressive load, tension loads on the concrete are minimized.

So, As long as the spokes stay in net tension, they can bear compression forces layed on top of the static tension.




Actually that study is in agreement. Note the first paragraph on page 5, which says:

"The figures are shown for the 3x/3x laced wheel. In the pretension case (Figure 4a), all of the drive side spokes are evenly tensioned to 650 N/mm2, and the non-drive side spokes are tensioned to 390 N/mm2. When rider weight is added to the wheel (Figure 4b), the loads on the spokes near the bottom of the wheel are seen to decrease considerably, whereas the tension in the remainder of the spokes increases very slightly."

In other words, the change in spoke loads which support the weight are the large tension changes near the bottom. Note that the slight tension increases in the remainder of the spokes include horizontal spokes at the front and back of the wheel (which can't support vertical loads at all), plus some of the spokes in the bottom half of the wheel outside of the the ground contact area, which are pulling down on the hub slightly more than before (so these tension increases work against supporting the load). When the vertical components of all the spokes with slightly increased tension are added up, their sum will be close to zero, which leaves only the spokes with tension decreases to support the load.

(Another note about the small tension increases: When the bottom of the rim flattens out, the "flat" section pushes circumferentially outward on the rim, slightly increasing its overall diameter. It is this slight diameter increase which is primarily responsible for the increase in tensions around the rest of the wheel. Also note that largest spoke tension increases are usually in the bottom half of the wheel, at the ends of the "flat" spot.)




It depends on what the purpose of making a vertically compliant wheel. Trying to get a "compliant ride" from the wheel is fools errand, since you can't get more than a fraction of millimeter of deflection from the wheel before the spokes slacken, and the wheel becomes unstable and prone to fatigue failures. On the other hand, there is something to be had within that fraction of a millimeter in regard to wheel durability. Specifically, if you use thinner spokes or a stiffer rim, then thae size of the "flattened" out portion of the wheel (in which the spokes supporting the load are compressed) is increased in size, encompassing a larger number of spokes. With more spokes sharing the load, the magnitude of the loading cycles on each individual spoke is decreased, thus increasing the durability of the wheel. This is why wheels with butted spokes are more durable - their extra "stretchiness" allows them to experience lower peak load magnitudes, and to distribute wheel loads across more spokes.

[Mark McM]

there is a compression force on the wheel, balanced by the tension in the spokes, especially the spokes close to the load which are working to pull that flat back to round, but also by all the spokes resisting the increase in diameter caused by the deformation,

no single spoke is supporting load by directly resisting compression, right? it seems that you are saying that as long as the deformation of the rim doesn't completely unload the tension that they do somehow work in compression.

In my non engineer mind I think that as long as the spoke never gets fully relieved of its base tension there is no way for it to start working in compression, plus the inability of the wire to support compression load.

the spokes working to pull that flat back to round do increase in tension, working against supporting the load, but they also effectively push the deformed section of the rim into the ground, supporting the load. No?

I get that the entire spoke system works to resist compression of the rim.

You are saying all the changes in spoke tension balance to zero except the main spoke pointing down and its associated decrease in tension thus a decrease in tension is the only thing supporting the load? I though that the load was supported by that decrease in tension being distributed in various ways to the other (mainly tensioned) components in the wheel.

Ill leave this alone after this

Some say whether or the bottom spokes truly work in compression is a semantic argument. However, there are some very real ways that they act as if they are working in compression that bear directly on the matter at hand (stiffness of the wheel). Here's a thought experiment to illustrate how the spokes can work in compression.

Say you had two identical rubber bungee cords. Most people would want to say that bungee cords work only in tension, not compression - you can't push a rope, right?

Conveniently, these bungee cords are both strong linearly elastic. They have a stiffness of 100 lb./in. - in other words, if you pull on the bungee, then for each additional 100 lb. of pull force, the bungee will stretch an additional inch.

Now, hang one end of a bungee cord from an overhead hook, and attach a 100 lb. weight to the other end. The bungee cord will stretch 1 inch. If you increase the weight to 200 lb., the bungee will stretch 2 inches.

Next, hang both bungee cords from the same overhead hook, and attach a 100 lb. to the other ends (the bungees are in parallel). Now, how much doe the bungees stretch? Each bungee will take half the weight, or 50 lb. per bungee, so each bungee will stretch 0.5 inches. The combined stiffness of the bungees is now 200 lb/in.

Now comes the fun part. Remove the weight, and attach the two bungees end to end. Hang free end of one bungee to the overhead hook and stretch the free end of the other bungee until it reaches the floor, and nail it to the floor. Say that the bungees had to stretch a total of 10 inches, or 5 inches per bungee to reach from the overhead hook to the floor. How much tension is in each bungee? Since each bungee has a stiffness of 100 lb/in, there is 500 lb. of (static) tension in each bungee.

Finally, attach the 100 lb. where the two bungees join. The top bungee should get a little longer (stretch), the bottom bungee should get a little shorter (compress). But by how much, and how much tension is in each bungee?
Since we know the combined stiffness of the bungees is 50 lb/in, the deflection should be 0.5 inches. The top bungee will stretch from 5.0 inches to 5.5 inches, and the bottom bungee will compress from 5.0 inches to 4.5 inches. Since we know each bungee individually has a stiffness of 100 lb/in, the top bungee now has 550 lb., and the bottom bungee has 450 lb.

So, we can say that when the two bungees shared the load, and each is supporting half the 100 lb. weight.

Now, some might argue that in fact the top bungee is supporting the entire load, the 100 lb. weight plus the 450 lb. of tension of the bottom bungee. But that doesn't really tell the whole story. Another way to look at is that the top bungee is supporting the original 500 lb. pre-tension of the bottom bungee, plus 50 lb. of additional tension from the weight, while the bottom bungee is supporting the original 500 lb. pre-tension of the top bungee plus an additional 50 lb. of additional compression from the weight.

Is it just semantics? No. The two bungees combined are stiffer than the a single bungee alone, regardless of whether they above or below the weight. The change in load from the weight is shared by both bungees. Effectively, the bottom bungee is acting in compression. If you hang the weight from just the top bungee and it deflects by 1 inch, and then when you connect the bottom bungee and the same 100 lb. weight only causes a 0.5 inch deflection, how can you say that the bottom bungee is not helping to support the weight?


Now, a real wheel gets more complicated. In the above model, we ignored the stiffness of whatever was holding up the overhead hook. And spokes are far stiffer than a bungee cord. The rim the spokes are attached to is relatively flexible in comparison to the spokes (at least radially). When we load a bicycle wheel, the rim flexes inward easily, so that hub lowers (due to compression of the bottom spokes) without the need for the any stretch in the top spokes. Since the top spokes don't stretch much, they have little increase in tension. Most of there tension in the top spokes is just there to balance out the static pre-tension of the bottom spokes.

[HenryA]

I appreciate you sharing this information about the mechanics of bicycle wheels but it generally does not agree with my empirical (and subjective) experience riding different wheels.

I detect significant differences between traditional POWs and the modern (last 20 years or so) fashion statements sold as their replacements.

[bicycletricycle]

the bungee cord model is amazingly informative and is currently hurting my head.

thank you for your time.


I am now on a mission understand how that bottom bungee is supporting half of that load!

if the top bungee was preloaded with 500pound of weight instead of the bottom bungee and then the extra 100 pounds was added it would surely extend an inch.

The bungee is pulling down yet still lifting the weight up!

This world is a big and confusing place.



Landshark dude, do whatever mark mcm says




Some say whether or the bottom spokes truly work in compression is a semantic argument. However, there are some very real ways that they act as if they are working in compression that bear directly on the matter at hand (stiffness of the wheel). Here's a thought experiment to illustrate how the spokes can work in compression.

Say you had two identical rubber bungee cords. Most people would want to say that bungee cords work only in tension, not compression - you can't push a rope, right?

Conveniently, these bungee cords are both strong linearly elastic. They have a stiffness of 100 lb./in. - in other words, if you pull on the bungee, then for each additional 100 lb. of pull force, the bungee will stretch an additional inch.

Now, hang one end of a bungee cord from an overhead hook, and attach a 100 lb. weight to the other end. The bungee cord will stretch 1 inch. If you increase the weight to 200 lb., the bungee will stretch 2 inches.

Next, hang both bungee cords from the same overhead hook, and attach a 100 lb. to the other ends (the bungees are in parallel). Now, how much doe the bungees stretch? Each bungee will take half the weight, or 50 lb. per bungee, so each bungee will stretch 0.5 inches. The combined stiffness of the bungees is now 200 lb/in.

Now comes the fun part. Remove the weight, and attach the two bungees end to end. Hang free end of one bungee to the overhead hook and stretch the free end of the other bungee until it reaches the floor, and nail it to the floor. Say that the bungees had to stretch a total of 10 inches, or 5 inches per bungee to reach from the overhead hook to the floor. How much tension is in each bungee? Since each bungee has a stiffness of 100 lb/in, there is 500 lb. of (static) tension in each bungee.

Finally, attach the 100 lb. where the two bungees join. The top bungee should get a little longer (stretch), the bottom bungee should get a little shorter (compress). But by how much, and how much tension is in each bungee?
Since we know the combined stiffness of the bungees is 50 lb/in, the deflection should be 0.5 inches. The top bungee will stretch from 5.0 inches to 5.5 inches, and the bottom bungee will compress from 5.0 inches to 4.5 inches. Since we know each bungee individually has a stiffness of 100 lb/in, the top bungee now has 550 lb., and the bottom bungee has 450 lb.

So, we can say that when the two bungees shared the load, and each is supporting half the 100 lb. weight.

Now, some might argue that in fact the top bungee is supporting the entire load, the 100 lb. weight plus the 450 lb. of tension of the bottom bungee. But that doesn't really tell the whole story. Another way to look at is that the top bungee is supporting the original 500 lb. pre-tension of the bottom bungee, plus 50 lb. of additional tension from the weight, while the bottom bungee is supporting the original 500 lb. pre-tension of the top bungee plus an additional 50 lb. of additional compression from the weight.

Is it just semantics? No. The two bungees combined are stiffer than the a single bungee alone, regardless of whether they above or below the weight. The change in load from the weight is shared by both bungees. Effectively, the bottom bungee is acting in compression. If you hang the weight from just the top bungee and it deflects by 1 inch, and then when you connect the bottom bungee and the same 100 lb. weight only causes a 0.5 inch deflection, how can you say that the bottom bungee is not helping to support the weight?


Now, a real wheel gets more complicated. In the above model, we ignored the stiffness of whatever was holding up the overhead hook. And spokes are far stiffer than a bungee cord. The rim the spokes are attached to is relatively flexible in comparison to the spokes (at least radially). When we load a bicycle wheel, the rim flexes inward easily, so that hub lowers (due to compression of the bottom spokes) without the need for the any stretch in the top spokes. Since the top spokes don't stretch much, they have little increase in tension. Most of there tension in the top spokes is just there to balance out the static pre-tension of the bottom spokes.

[pbarry]

the bungee cord model is amazingly informative and is currently hurting my head.

thank you for your time.


I am now on a mission understand how that bottom bungee is supporting half of that load!

if the top bungee was preloaded with 500pound of weight instead of the bottom bungee and then the extra 100 pounds was added it would surely extend an inch.

The bungee is pulling down yet still lifting the weight up!

This world is a big and confusing place.



Landshark dude, do whatever mark mcm says

Right?!?!

This is like a Bucky Fuller symposium. :beer:

[bicycletricycle]

I got it,

1. Just think of the load from the hubs perspective, the spokes are trying to pull the hub back to center, much better than imagining the grounds local force on the contact patch.

2. The compression/tension thing is difficult because of the way we commonly use the terms. If you think of compression or tension as the net state of all the component forces than of coarse all the spokes are under tension the whole time.

however

these terms are not made to be used like that, the starting state of the spoke is a net tension. It can take compression load because of this. Compression as a direction of a component of the forces being seen by the spoke, not a net steady state description of the spoke.


also, the spokes act to keep the hub centered and the rim round, this allows the rim to distribute a local compression load tangentially, effectively trying to compress the rim, not into an egg but into a smaller diameter (kind of). Like a rolling infinite archway.


I suspect this is why it would be extremely difficult to increase ride quality through rim cross section, The rim is not being loaded like a plank between to saw horses, it is more like a plank on end being loaded axially. Well, some combination of these two elements.

WHAT!!!!!!!!!!!!!!

I need a nap.

What does this say about prestige tubing and easton wheels. well, maybe that a difference in vertical rim deflection has little to do with their problem.

tuning fork his high pitched, some kind of resonant frequency of the fork, wheel and frame system?

The former zipp guy Josh Poertner's article makes me think you should try a blind test, have someone else change the wheels and don't look down or put some cardboard on your bars. Just make sure it is not your mind playing tricks on you. I know I will know be doing this, to tires especially.

[pbarry]

OK, biketrike and Mark need to start a company or at least have a beer. I sort of get what MMcm is saying, but B/T distills it for me. Go forth and prosper.

[Black Dog]

As explained in the "The Bicycle Wheel" and other analyses, spokes can and do work under compression, as long as the compression load is superimposed on a (larger) static tension. While some may argue the semantics of "a wheel stands on its spokes", it can not be argued that the bottom spokes are the primary contributor to the wheel radial stiffness. So at least in regard to wheel stiffness, there is no doubt that the bottom spokes work under compression.

If the nipple is not bonded or threaded to the rim in some way when you push the rim towards the axel there is no possible way that the spoke can resist the force (i.e. compression). As the rim bends toward the axel the nipple will eventually lift of its bed and the spoke will be slack with no tension or compressive loads. I may be missing something here, and if I am please except my apology for dead horse flogging. I am, of course, speaking only to the few spoke at the bottom of the wheel in the zone of rim deformation.

[Black Dog]

All of these papers show that the primary response of a wheel under vertical load is a flattening of the rim near the ground contact point accompanied by a significant compression (de-tensioning) of a few of the bottom spokes, and little reaction around the rest of the wheel. If you look around, you can find plenty of other studies and analyses that agree. You can even test this your self: As we know, the tone (audible frequency) of a plucked spoke varies with its tension - the higher the tension, the higher the tone. Stand a bicycle up on its wheels, and pluck spokes at a variety of orientations around the wheel. Then apply a vertical load on the bicycle, to simulate a rider's weight bearing down on the wheels. Pluck the same spokes again. You should find significant reductions in tone in spokes near the ground contact point, but very little change in tone of other spokes around the wheel.

Ahh, This is where we are divergent. My thinking on the matter is that compression along a single load path does not start until all tension has been relieved along that same path. In other words you are reversing the load vectors not just altering their magnitude without directional change. Are you saying that if the spokes (rigid) were replaced with a rope you can have compression on the rope along its length?

Thanks for the articles, great reading, and this kind of crazy technical discussion is tons of fun.

[Mark McM]

Ahh, This is where we are divergent. My thinking on the matter is that compression along a single load path does not start until all tension has been relieved along that same path. In other words you are reversing the load vectors not just altering their magnitude without directional change. Are you saying that if the spokes (rigid) were replaced with a rope you can have compression on the rope along its length?

Thanks for the articles, great reading, and this kind of crazy technical discussion is tons of fun.

I think you're on the right path. There are basically two co-existing forces on spokes: The static tension force (which is a pre-existing force that is present when there is no load on the wheel); and a dynamic compression force (which is caused by applying a load to the wheel). All we have to do is make sure the resultant spoke force when the two forces are combined remains a net tension force. In other words, as long as the compression force (from an external load) is less than the static tension force, then the spoke remains in tension, and is able to support external loads. This is why we pre-tension the spokes to such high tensions (100 - 120 kgf, or 220 - 265 lb. per spoke is common).

In regard to your rope question, wire spokes are used in wheels because they are very stiff and strong, can be attached easily to hub and rim, and their tensions are easily adjustable. But tension spoke wheels have been made with cords instead of wire, such as the Tioga Tension Disc (http://vintagemtb.blogspot.com/p/tioga-tension-disk.html):

http://img.photobucket.com/albums/v309/rumpfy/MTB/Tioga%20Tension%20Disk/clear2.jpg

As far as nomenclature: A compression force is a force that tends to shorten a member. A tension force is one that tends to lengthen a member. In the example of the two bungee cords tied end to end, when we applied the 100 lb. weight, the bottom bungee got shorter, so we that's why we call the reaction on the bottom bungee a compression force. And the bungee got shorter in the same proportion as the top bungee got longer, so we know that axial stiffness of the bungee is the same whether it is getter shorter or longer (compression or tension). The bottom bungee contributes to the stiffness of the system, even though it is below the weight.

In the case of the wheel, when a vertical load is applied at the hub, the hub deflects downward a small amount. The bottom spokes must have gotten shorter, so we can say that the external load resulted in a compression force on the bottom spokes. And since the spokes are very stiff (typically 5,000 - 10,000 lb./in.), the spokes will only shorten a very small amount in response to the load. There are several spokes in the LAZ (Load Affected Zone) at the bottom of the wheel, so the combined stiffness of these spokes makes stiffness of the wheel assembly very large - typically 10,000 - 20,000 lb/in, sometimes more.

The engineering term for the inverse of stiffness is called "compliance (https://en.wikipedia.org/wiki/Stiffness#Compliance)", so, the "compliance" of a wheel is beween 1/(10,000 lb/in) and 1/(20,000 lb/in.), or 0.0001"/lb. and 0.0005"/in. So if we apply a 100 lb. vertical load to the wheel, it will only deflect 0.005" - 0.010" - not very much

Regardless of whether we want to call the reaction of the bottom spokes compression or de-tensioning, they compress/de-tension at such small rate with applied force that the have no meaningful affect on ride compliance.

[Peter P.]

Spokes do not work under compression. They provide no support under compressive load.

You're confusing spoke compression of the rim with load compression of the rim, which certainly reduces spoke tension.

[Black Dog]

You're confusing spoke compression of the rim with load compression of the rim, which certainly reduces spoke tension.

My frame of reference was limited to the spoke exclusively. In other words spokes only perform their function while under tension. :o I understand the dynamics of the wheel, but was trying to sort out the syntax and nomenclature of engineering.

[Black Dog]

Thanks MarkMcM, This clears it up. The language barrier has been raised to the ground. I now get the lingua franca from the engineering perspective in relation to compression and tension. Semantics do matter and I am happy to have been set straight on this matter.

I think you're on the right path. There are basically two co-existing forces on spokes: The static tension force (which is a pre-existing force that is present when there is no load on the wheel); and a dynamic compression force (which is caused by applying a load to the wheel). All we have to do is make sure the resultant spoke force when the two forces are combined remains a net tension force. In other words, as long as the compression force (from an external load) is less than the static tension force, then the spoke remains in tension, and is able to support external loads. This is why we pre-tension the spokes to such high tensions (100 - 120 kgf, or 220 - 265 lb. per spoke is common).

In regard to your rope question, wire spokes are used in wheels because they are very stiff and strong, can be attached easily to hub and rim, and their tensions are easily adjustable. But tension spoke wheels have been made with cords instead of wire, such as the Tioga Tension Disc (http://vintagemtb.blogspot.com/p/tioga-tension-disk.html):

http://img.photobucket.com/albums/v309/rumpfy/MTB/Tioga%20Tension%20Disk/clear2.jpg

As far as nomenclature: A compression force is a force that tends to shorten a member. A tension force is one that tends to lengthen a member. In the example of the two bungee cords tied end to end, when we applied the 100 lb. weight, the bottom bungee got shorter, so we that's why we call the reaction on the bottom bungee a compression force. And the bungee got shorter in the same proportion as the top bungee got longer, so we know that axial stiffness of the bungee is the same whether it is getter shorter or longer (compression or tension). The bottom bungee contributes to the stiffness of the system, even though it is below the weight.

In the case of the wheel, when a vertical load is applied at the hub, the hub deflects downward a small amount. The bottom spokes must have gotten shorter, so we can say that the external load resulted in a compression force on the bottom spokes. And since the spokes are very stiff (typically 5,000 - 10,000 lb./in.), the spokes will only shorten a very small amount in response to the load. There are several spokes in the LAZ (Load Affected Zone) at the bottom of the wheel, so the combined stiffness of these spokes makes stiffness of the wheel assembly very large - typically 10,000 - 20,000 lb/in, sometimes more.

The engineering term for the inverse of stiffness is called "compliance (https://en.wikipedia.org/wiki/Stiffness#Compliance)", so, the "compliance" of a wheel is beween 1/(10,000 lb/in) and 1/(20,000 lb/in.), or 0.0001"/lb. and 0.0005"/in. So if we apply a 100 lb. vertical load to the wheel, it will only deflect 0.005" - 0.010" - not very much

Regardless of whether we want to call the reaction of the bottom spokes compression or de-tensioning, they compress/de-tension at such small rate with applied force that the have no meaningful affect on ride compliance.

[bicycletricycle]

I built the bungee cord model.

Thought about it some more.

De tensioning or compressing , the bottom spoke never pushes the rim back to round.

In the bungee model, the bottom bungee is never pushing on the floor , it just pulls on the floor less.

It always remains in tension.

The effect of it supporting the load in the bungee model is just it getting shorter thus putting less preload on the top bungee.

The local deformation of the rim (flat spot at the bottom) cannot be resisted by the bottom spoke, this would be like the bottom bungee pushing the floor down. It does pull on the rim less, but it never has a net push down.

The flat spot is corrected locally by the spokes at the edge of the deformation resisting being stretched (pulling the rim back to round) as well as all the spokes (slightly) resisting the desire for the rim to deform more generally.

The loss in tension in the bottom spokes through local deformation of the rim is not resisted by the bottom spoke directly (they never push the rim out, they simply pull the rim in less, because the are effectively a spring that has gotten shorter)

The rim also does not just hang from the top spokes. Most of the spokes are engaged in supporting the added load of the rider.
http://www.williamscycling.com/assets/images/product%20tech/Bicycle%20Wheel%20Spoke%20Lacing.pdf
Above data shows 24 of the 28 spokes increasing in tension from the static load of the rider.