Spoke Strentgh (My spokes are breaking like mad...)

[Kingfish]

The function of spokes is to provide a modicum of suspension, combined with the rim and tire. It’s a complete lightweight system that works up until the point where a good solid jolt causes the material to fail (the spoke), and it should happen long before the rim will fail and taco.

Radically incorrect. You could think of the rim like a flexy noodle, or like a stone arch with no cement. It's rigidity alone is relatively nothing compared to the loads, in fact, you can just grab a mt.bike rim and try to sit on it and it just collapses under your weight like a pile of goo.

The only way the system works is the stress from the spokes pulling inward. In the same example, you try to sit on the laced wheel, and it could perhaps support 20 of yourself rather than collapsing under some small fraction of your weight alone. This is because the spokes create a system of stress distribution over a wide area of the wheel, enabling the forces to be shared over the whole perimeter.

To have this function work, the spokes MUST all be high in the elastic region of the strain/stress curve, this allows the compliance in the wheel without causing spokes to become unloaded.

A bicycle rim can not put a large diameter spoke into this stress zone, the rim itself collapses (I've tried a few different good eye-lit rims, there is a reason motorcycle rims are about 5x thicker).

Say you set the same ~100lbs of tension on a thick spoke wheel as a thin spoke wheel. Now you load the wheel. The moment event the most tiny bit of deformation occurs, the thick spoke wheel has huge patches of spokes completely unloaded, because they are so low on the material strain curve, that even 0.01mm (or whatever) of compression distance can drop the 100lbs tension to 0lbs, now it can't distribute it's strength through the perimeter of the rim, and it's going to fail from the stress risers that get created in the places that are carrying the stress. You put the same load on the rim with thinner spokes high in the stress curve, and hey! No problem, you have substantial deformation distance you can travel while still staying in tension and functioning to distribute load. The distributed load doesn't create the stress risers that cause failures.

The thicker spoke wheel would be stronger if you could put it in the same part of the stress curve, but for thicker spokes, that can be ~400-900lbs of tension required to achieve it, and bicycle rims simply buckle or pull nipples through at those tensions. Hence why if you have a motorcycle rim you should go for bigger spokes, as they can handle 400-900lbs of load per spoke, putting it in the proper range of the strain/stress curve, and allowing the wheel to distribute loads.

OK, so there you have it: Rims with hub motors are at least 13 gauge, if not 12 gauge. Using a lighter gauge with hub motors is unsafe. I can personally attest to that. :wink:

Wrong. See above.

Luke, radical is a person that expresses or believes in the extreme. Radical is the expression you have used to label me, directly misinform, and debase my contribution. This is an unfortunate choice of words and I have no idea why you would take such a position… :?

I am a Mechanical Engineer, you can find me on the web… it’s in my bio, and I’ve worked with some fantastic clients. So I’ll start simple and work upward in hopes that you’ll understand me better:

Basics:

• Take a primitive Circle, extruded it becomes a Tube: Fold the sides inward or outward and it becomes stronger – it becomes the shape of a Rim.
• A Hub (sans motor) is essentially a smaller version of the above described Rim, though functions as the point of revolution and attachment to the fork via the Axle.
• Spokes are small diameter alloy rods that connect Hub and Rim. Spoke Tension, equidistant constrains the hub concentric to the perfectly round Rim.
• Under normal load, sitting statically, no rotation, there should not be any appreciable deflection with the rim or the spokes.
• Rolling forward, under normal load, there still should not be any appreciable deflection.
• If I hit a small stone, the tire (and inner tube, if applicable) should absorb the majority of the impact, and the rim and spokes should not deflect.
• If however, I impact the curb, a pothole, or some other immovable object at some modest speed, and if the tire is deflected to the maximum, in ideal conditions the point load at the rim-impact will cause the rim to bow inward and the spokes directly in line between the impact point and the hub to deflect outward, whilst the on the opposite side, those spokes go into hypertension, with the spokes at 90/270* into partial tension. High-speed photography clearly displays this action, however slight.

Applying more force:
If the impact is of low enough energy, and the materials we chose for the rim and spokes and hub are strong, though not brittle and having elasticity, the rim and spoke system will spring back to normal, and no harm is done. However if the impact is greater than the system elasticity, we will have deflection to the point of yielding, micro-fracturing, cracking, plastic deformation, and finally failure.

The value of spoked wheels is that they uniquely are suited to provide small levels of shock absorption and deflection in bikes and in cars. This feature was fully expressed during the early years of wheel production when bikes and auto first came into being. Today, it is more difficult to realize the value of spokes because in many cases spokes are often cosmetic (example: car wheels).

Another example of suspension; taking a curve:
Let’s say I’m taking a curve on my deluxe road bicycle and I have my full weight into it leaning wicked hard into this curve. My wheels are deflecting through the axis of rotation perpendicular to the surface in contact; the wheel is warping a measurable amount. The deflection is slight, but visibly apparent. If we drew a line from the point of contact between the road and the tire, then extend it through the hub to the axle, the spokes on top of this imaginary line are bowed, and the spokes at the bottom are in tension. This is active suspension at work.

If the spoke tension wasn’t high enough, the wheel would not be able to withstand the deflection and could fail. If the tension was too high, then likely the attachment at the rim or hub would fail, or the spoke would deformate and yield to the point of failure and break.

What I’ve just explained is elementary structural and materials engineering; stuff we learn in the first year of college. There’s no mystery here.

- - -

Luke, You concluded on your last sentence that I am wrong on a second observation.

I’ll try again to keep this simple for you.

Bicycle rims are not normally designed to handle the mass and momentum of an electric hub motor, and this is made worse by traveling at greater than normal speeds carrying greater than normal loads (e.g. batteries). Adventuresome, possibly risky, though hopefully ~ still safe.

Broken spokes, a personal experience:
The failure I had in 2010 was on a normal bike rim; I had 50 lbs. of batteries draped over the rear wheel in panniers. My bike was tuned before I departed (spokes included). It’s possible the spokes were under duress after I received the bike from Amtrak in Klamath Falls because it was at the bottom of all the unloaded cargo. It’s possible the spokes began to fail when I went on that horrible dirt road to visit my family near Johnsville, or maybe it happened when I was run off the road by reckless youth on their dirt bikes and took a spill, or possibly on the way back out trying to get back to the main road. What I do know is at the summit of the highest pass on the last day heading toward Sacramento that I had at least one broken spoke. I know that when I hit that little dip in the road at Grass Valley, two more snapped right then and there and that ol’ wheel was wobbling all over the place; it’s a wonder I was able to coax it to Auburn and have the entire wheel replaced. They’ve made a lot of changes in MtB wheels in 20 years, namely they the rims are stronger and the spokes larger, possibly better fabricated. This fact is unassailable; it got me home. :wink:

But I am still hugely confused that you think I am wrong with my first-hand experience. How could you possibly know? You weren’t there.

To the best of my knowledge, you’re into racing, not cross-country. For you and racing, your conditions are going to be different than mine. And, it’s not for me to lecture you on how to go about racing and what’s right or wrong with your knowledge. I mean, if I got into racing, perhaps we'd have something in common worth sharing.

But clearly Luke, you seem obsessed with proving me wrong, even though you can’t explain it well.

I’ve seen all sorts in the 30 years of engineering. I appreciate brilliance. I even accept eccentricity and quirky odd behavior, so long as my team mate can produce and there is no malice, no sabotage. I’m here to contribute and share harmlessly along with my little quirks.

I’ve presented the facts, directly quoted right from the supplier the information germane to the topic, and I’ve shared my direct experiences.

So I have to ask: What’s the story?

~KF

 

[Kin]

Summary of story:
_______________
Within conditions bicycle hubs can tolerate, proper tension is essential, and too thick of a spoke will not be sufficiently along the elastic portion of the stress-strain diagram.
KF:
Some conditions require tension suspension [Editors sidenote: I really like the actual word of describing it as a suspension system, they were the words/image I was looking for] that are greater than the wheel can handle. You need a more robust rim to have higher tension system. Hub motors are not ideal because there's a large mass (with inertia) moving around in the center of the wheel, exasperating forces.

KF and LFP, disagreeing while, so far as I can see, actually in principle in agreement [though I for one understand LFP's style and KF's frustration with it].

Edit: Let me reread KF's post to make sure I'm not making a fool of myself. Sorry guys if the story is that I'm not seeing the big picture.

EDIT: ACTUALLY THEY ARE SAYING TWO DIFFERENT THINGS AND MY SUMMARY IS UNEQUIVOCALLY WRONG.
[Summary of difference: KF seems to say the spokes actually do bear some load on compression, a fact which is not in line with LFPs version where spokes always remain to some small extent tensile strain. ]

 

[John in CR]

Ok, so you guys have sold me on the concept that wheels with the thin gauge spokes can be strong, but can they be as maintenance free as a thicker stronger spoked wheel (in an appropriate rim of course)? For me wheels that need truing go in the same category as those with breaking spokes, because I want a wheel as close as possible to the reliability if a car wheel.

 

[Degull]

Having built many wheels myself, I thought I would chime in with some reference info:

http://autobus.cyclingnews.com/tech/fix/?id=tm_1

http://www.dtswiss.com/getdoc/6d884b35-9c2e-4ea8-a4e7-eb8ce3837293/TechnicalDatasheet.aspx

Spoke tension is the key. Finding the right tension for your rim/hub/spoke combo is the tricky part and sometimes you have to experiment to get it right. Following the correct wheel building techniques is also important:

http://sheldonbrown.com/wheelbuild.html

The crap spokes that come on laced wheels from China are a joke.

 

[Kingfish]

Kin: Yes, you are on the correct path of observation with your conclusions. When spokes go into compression, which they must when encountering a load that exceeds the rims’ ability to retain shape (however accomplished), those spokes will deflect and bow outward in reaction, placing spokes at the opposite side in hypertension (and we pray that they do not yield). This is the natural order of things. A great class to take is “Statics”, or in the drafting world the equivalent is “Graphic Kinematics”; one deals with more math-oriented solutions and the other is more graphical. I took the original courses before the inclusion of CAD – though later I wrote code that utilized CAD for stress and strain, but nothing on the order and the sophistication of FEMM.

It is common engineering practice to design components and assemblies, and then test it to failure so that we can confirm that our modeling and comprehension of the physics is on track. The features of design that cause material failure are not always apparent. Frequency and harmonics can undo in the best of efforts: Classic example is the first Tacoma Narrows Bridge where the tension of all the cables was set to be equal, not realizing that in the process they created a perfectly tuned oscillator. When the winds picked up, the bridge resonated to a natural harmonic and self-destructed. I don’t know if this is directly applicable to wheel spokes, unless there is dishing involved which can add another dimension to the challenge. Personally I leave these details to my local Boeing Engineer friend whose expertise in wheelbuilding has no equal. He has a particular process where the wheel is stressed repeatedly so that I won’t have to fuss with re-tensioning in the field.

It’s important to consider the type of activity when selecting a proper rim. I think 9C hubs & wheel assemblies coming from ebikes.ca are fine for the vast majority: Stout, double-walled rims, adaptable to many frame types… it’s a smart move. My type of riding is as an urban-assault/cross-country fiend that also has to double as a cargo hauler (given all the batteries), and therefore I need the best and strongest rims, I need downhill rims that can take the abuse. I want my tires to be phat to absorb the pebble shock and road-chatter. I want my spokes to withstand jarring impacts when I need to take it on nasty dirt roads. Because the spokes I chose are stout, they’re going to be less flexible in normal conditions, hence we’re back to the phat-tire system. The concept worked: I didn’t break a single one last year! Shoot, even the trailer did extremely better than I imagined.

Your activity will likely be different than mine, therefore so will the spoke and rim requirements. But if they are breaking like mad as the OP suggests, I recommend a stronger setup all around.

From the peanut gallery, KF

 

[Kin]

If anyone wants some more resources, I found this:
http://www.youtube.com/watch?v=14UcymYWirc
He's not the best lecturer, but I watched 10 or 15 minutes of it and enjoyed some. Seems decent.

What I was really looking for (but didn't find) was a high speed picture of a wheel hitting a pothole. Another reason why the idea of it ever being in compression interests me is because there's a chance a shorter spoke would have more trouble bending the relieve the compressive stress, if the goal is to relieve some stress so that it's not all born by a single spoke. But it does work that a thinner gauge spoke would find it easier to bow out under compression than a thicker gauge one.

To KF, so my statics course was a terrible course, I didn't absorb too much (my fault), and I feel that I'm learning everything outside of that class. As with the other core classes (taught by young pre-profs who are themselves under the crunch of having to pump out papers) it was a matter of memorizing a few monotonous procedures to apply rapidly during an hour test. As much as they were the most annoying problems, we could have had some more realistic 3D problems because 2D analogs rarely apply (as in this spoke trouble). Intro classes have depressed me a lot in my early engineering schooling. I'm sort of at an awkward place where the next two years I'm promised will be more interesting, but I'm thoroughly distant from the passion.

 

[Drunkskunk]

In a normal wheel system, You can't put a spoke into compression. You can only achieve a point of zero tension. The Spoke nipple is held in place by tension, and the rim can't push on it. The rim can only exert force by pulling outward from the hub. Any movement toward the hub releases tension to a zero point, at which point the rim will let the nipple slide out. Any force applied to the nipple at that point is coming from a rock, a hammer, that big truck that is running you over, ect.

 

[Chalo]

Ok, so you guys have sold me on the concept that wheels with the thin gauge spokes can be strong, but can they be as maintenance free as a thicker stronger spoked wheel (in an appropriate rim of course)? For me wheels that need truing go in the same category as those with breaking spokes, because I want a wheel as close as possible to the reliability if a car wheel.

Thin-spoked wheels are lower maintenance than thick-spoked wheels, because the spokes stay tight without having to have their nipples glued on. The only time you have to true one is when you bend the rim, and need to pull it back into line. Thick-spoked wheels need truing at those times, and also when the spokes work loose because they go slack all the time. If you glue the nipps on to prevent this, then necessary truing becomes much more of a procedure.

If you want a wheel just as low maintenance and damage resistant as a car wheel, you'll have to have one that's heavy and inflexible like a car wheel too. Remember that car wheels are their own kind of pain in the butt when you have to repair a flat, for instance. Bike wheels are cake by comparison.

Chalo

 

[Dauntless]

As my Dad would have said, "There's no way we could know what really is wrong, DAGNABBIT!" Basic chaos theory moves to the practical reality of every day life; we can't say for sure if every spoke is as tight as the one next to it, whether you hit your bumps straight/at an angle/leaning, etc. There's details we can't hope to get our hands on that would offer the REAL explanation. I see people of education stauntly defending certain opinions as facts, only to be reminds of other facts that prove them wrong. That's because they're opinions on bikes were formed as kids, long before they got that education. Somehow education has a way of supporting what we always believed.

I've always thought that too tight was far more likely to break than too loose, I remember some downright rattling wheels people dirtbiked on without breaking anything. Oh, the big opinions some of us expressed about what was GOING to happen. But someone might offer physics to "Prove" that never happened. Bikes missing numerous spokes but not breaking what's left: How could that be possible? PLEASE don't explain to me that it's not possible, I witnessed this impossible thing.

If you have the SLIGHTEST bend on that hub, changing the stress on just a few spokes, think of the effect that could have. What could be going on that you'd never notice?

 

[biohazardman]

Well here is my personal experience and answer to all of these thinner/thicker spokes are better posts. After breaking many of the cheap 12/13 junk Chinese spokes, as many do, on my so far bulletproof but inexpensive Weinmann DM 30 wheels I went with the Sapim 12 ga and have never looked back. The wheels are just a tad bit wider than the Rhinolites and don't have the inserts. These DM30s came out of Florida and were pre-drilled for 12Ga spokes although I am sure that quality 13GA spokes would have done fine for my weight/speed and style of riding but they had 12Ga holes. No broken spokes or bent rims in the last 6K, including a couple hundred trail and dirt road miles, and I seldom even have to true them anymore. Not that I take it easy on them or anything cuz I don't. Total vehicle weight loaded is about 250Lbs. Don't think that your local bike shop can do it right cuz that may not be so. I had my wheels built in a local shop and they came back a mess with a three cross on the motor and spokes seriously bent at the nipple where there should be no bend at all unless you want breakage. I had to tear them down and drill/dremel the holes at an angle to get the spokes to set in the correct angle to the motor so they were straight. Others have used various procedures to do this but they need to be straight or the wheel is not built correctly. Me I don't understand all of this thinner spokes are better stuff at all. Maybe for a road bike or something lighter duty or weight conscience folks that may be so. Heavier loads and faster speeds seem to point towards a stronger wheel to me. Stronger wheels have stronger rims and heavier gauge spokes of decent quality. Look at mopeds for instance they are made for the speeds and weights some travel on their ebikes here on the forum. They are built heavier for the loads and speeds they will travel. You don't see wimpy bicycle 14 ga spokes on them. If you want something to last then overbuild it. Get some heavy rims and use some heavy spokes that actually fit, what they are installed on, or go find some moped stuff that will werq. Always remember your life and well being depend on the parts you choose to build your bike with. This is not to say a well built wheel with 14Ga spokes is not a good thing as it will likely be stronger than a poorly built wheel with larger spokes.

 

[iamsofunny]

This is my wheel I made

ON this bike I made

You could locktite all your spokes tight and true, then fiberglass the entire wheel solid :lol:

 

[liveforphysics]

This is not an easy thing to grasp. I'm trying to think of an example that helps.


Think of a hula-hoop with 36 little elastic threads mounted all tying to spoke holes in the hub at the center. Each thread has been stretched at least 4 inches to reach the hub at the center.

Now apply stress to the hula-hoop that deforms the shape by a few inches in any direction, and watch the spokes and hub position. It all keeps a balance, all strings continue to provide tension on the hula-hoop, none going slack. The hub location will always be found at the location where the sum of all the component tensions in each string sum zero.

Now slit the hula-hoop in the middle between every string tie location, so it's got 36 cuts through it's perimeter, and it's just held together by the tension of the elastic strings (it would be terrible mess of trying to align things to build, I'm not advocating building it, just think about it in your mind). Each little arc segment is held in place by the elastic pulling inward and putting friction forces on the adjacent arc sections. (like how a stone arc doorway key-stone uses normal force from gravity to do rather than an elastic string)

Now flex this system, it can deform, and stay together, the hub stays at the location where the sum of the components equals zero, all members continue to carry stress (inherently if it's together, all members are carrying stress, or it would not be together).


Now imagine the hula-hoop with something that doesn't stretch for spokes, let's say some strong kite string all tied taught to the hub.
Apply any amount of stress to the hula-hoop (causing deformation), what do you see? All the strings on the top and bottom instantly fall slack, the amount of slack being greatest at top and bottom, and become less and less slack as it approaches the middle horizontal axis spoke, where load goes entirely into the two outside most strings, in the axis tangent to the force being applied to the top of the hula-hoop. You're now carrying all of the forces between two strings. (stress riser obviously)

Make the same radial cuts in the taught string hula-hoop and apply any loading to deform it, the stress distribution can only land in tangent axis single points, so the system is of course unstable, and falls apart the moment anything causes deformation.


Of course putting force on top of a hula-hoop isn't how the bicycle rim is loaded (but is a good model to conceptualize the difference in force distribution between spokes with stretch and spokes with less stretch). However, the ability and inability to distribute load effects are the same when loading from the hub to rim, just harder to conceptualize than squashing a hula-hoop.

Relative to the stresses of hitting a curb or pot-hole etc, the rim may as well be a noodle. The strength comes from the load being distributed. The force WILL CAUSE deformation, if you have an imaginary no-stretch spoke, any amount of deformation means you instantly apply all the stresses to two points. If you have an elastic string spoke, obviously you can handle massive deformation and still always be effectively distributing load, but it's load handling would be extremely low of course.

The key to a strong wheel is a spoke with all the required strength, but thin enough to be able to get stretched substantially enough to be capable of still distributing load to a wide area on the rim, sharing between many spokes and eye-lets, while the rim deforms.


This is a wheel with 14awg Sapiam double-butted (butting makes them even thinner in the middle) spokes. The bike lays >50rwhp, is a very stiff hard tail, and handles road imperfections and bumps at >100mph, and gets ridden up and down stairs and all sorts of crap. Rider with gear is 250lbs, bike is 120lbs. This wheel holds up better than any of my early thick-spoke mistake wheels.

 

[biohazardman]

I had no idea that thing had 14ga spokes on it! Pretty impressive wheel build with bicycle spokes and that much power and speed. So, I can see plainly now the choice in spokes, rims, and tires for the build is even more important than I thought. Maybe I will try some good quality thinner spokes next time and save some weight cuz I don't want my next e-bike weighing 125LBS. ;^) Smaller spokes would save allot of drill/dremel time as well.

 

[amberwolf]

I don't know which theory is correct, but I lean toward the way Liveforphysics explains it, simply because of my experiences with many different badly- and well- built wheels (some by me, some from salvaged bikes, in both cases), and my "mechanical intuition" (that isn't always right either). I've read all sorts of theories, some with math I can't understand, on various places on the web over the years.

Almost always, if I have a wheel that has any loose (meaning, not fully tensioned) spokes on it, then riding one of my heavy cargo bikes on that wheel will rapidly destroy it, especially when I hit bumps and potholes with a loaded bike. It quickly becomes unable to retrue it, as the rim bends out of shape beyond the ability of the spokes to retension it evenly into a circle.

Typically, a bad wheel does not have any "tone" to the spokes when struck, or it is a dull one. A good one (taht doesnt' break as easily) rings with "musical" tones on each spoke, and they all tend to be very similar in tone--never exactly the same, once the wheel is trued, but very close. Probably within a couple of semitones at worst. When I have to tension a spoke so much that it's tone is way different from the others, usually because of a slightly bent rim, it almost always winds up failing on that section of the wheel *or* the one diametrically opposite it, once it does fail.

I can *never* get the thicker 12G spokes on the Fusin or 9C wheels tensioned like that--and they go out of true much faster than wheels with thinner spokes like what's on my regular bike wheels.

Given the loads I am placing on my wheels, and the conditions I am riding on, it's not surprising to me that I break the wheels, as the instantaneous forces on them when I hit the far edge of a deep pothole at 20MPH are probably quite high--I expect on the order of tens-of-times the actual weight of the bike+rider+cargo (up to 500-600lbs, almost all of that on the rear wheel) simply pressing down on it, due to acceleration forward and downward. I usually lose a few MPH at the instant of impact, too, so all that energy is ending up absorbed thru the spokes--and eventually breaking them, usually several in a row either at the impact site or on the diametrically opposite edge.

They don't usually fail instantly, either, but rather one may break right then or shortly afterward, then others on either side of it may fail from fatigue as they are stressed further by whatever forces are acting on them, especially if I hit more potholes later before being able to fix the broken spokes.

 

[flathill]

Stop getting all sensitive and butt hurt

A bicycle wheel is a tensegrity structure

``The word 'tensegrity' is an invention: a contraction of 'tensional integrity.'

[From Synergetics [700.011]]

``Tensegrity describes a structural-relationship principle in which structural shape is guaranteed by the finitely closed, comprehensively continuous, tensional behaviors of the system and not by the discontinuous and exclusively local compressional member behaviors. Tensegrity provides the ability to yield increasingly without ultimately breaking or coming asunder.''

Fuller stated as a general principle that ``tension and compression always and only coexist and covary inversely' tension and compression always and only coexist and covary inversely' There is no way to have tension without corresponding compressional forces in the structure. A tensegrity is a continuous tension - discontinuous compression structure. This is as distinguished from traditional structuring which is continuous compression and discontinuous tension.

For Fuller, tensegrities manifested his philosophy: that nature uses tension primarily and compression secondarily (whereas humans often misguidedly do the reverse).

What he means by always coexist
tension and compression always and only coexist and covary inversely'
http://www.rwgrayprojects.com/synergetics/s07/p9000.html

 

[liveforphysics]

Example of how much flex and distortion a wheel can have on a bike rim that is not under anywhere near the stress of hitting a pot-hole at speed (the stress on this wheel is mostly just a ton of torque), and you can see how it gets all crazy by INCHES of rim movement.

How did we know how much rim motion this bike had in operation? In use, the tire left black rubber marks on the surfaces an inch to each side, and and to the cross bar a half-inch forward of the wheel... That's a pretty shocking amount of movement sections of rim had relative to the hub, yet this bike layed over 500ft-lbs of torque to the rear wheel through butted 14awg spokes and never had a failure. It's because when it deformed, it didn't just instantly focus all it's stress on 2 spokes, instantly snapping them or pulling out of the rim like the 10awg spoke builds did. (which perplexed me and I thought about going to THICKER than 10awg spokes, until trying Chalo's crazy-seeming thin-spokes stretched to the yeild point advise, which just solved the rim/wheel failure issue entirely.)

 

[liveforphysics]

If you model a rim like a rigid member, you've all ready failed.
If you view the components assembly as rigid, you're not understanding it.

It's dynamic. Under the various conditions that try a wheels strength (potholes and curbs at speed etc) all these pieces are wildly flexible components with substantial relative motion and deflection, the key to survival is to distribute the stress over as wide of an area as possible rather than concentrating it sharply at some point.

IF you could get big fat spokes up to the top of the elastic strain curve, you would have a super strong wheel, it's exactly what a dirtbike/motorcycle wheel does. However, a bicycle rim simply can't hold the many-hundred pounds of spoke tension required to make it happen with thick spokes.

But clearly Luke, you seem obsessed with proving me wrong, even though you can’t explain it well.

So I have to ask: What’s the story?

It feels like a full-time job just to correct misconceptions here. (I would say all battery/motor/controller info shared here is still >85% misconception/wrong, but that's way better than a few years ago when it was 99% wrong, including myself being among the wrong info spreaders). You present your posts written like it's a page of a wiki written by God to help educate noobs, which would be AMAZINGLY cool if the information in them was correct, but when it's not, it's irksome, because noobs are going to read it and believe it, and that doesn't help enrich the ebike experience at all when more bad info gets spread around.

 

[Harold in CR]

If you model a rim like a rigid member, you've all ready failed.

According to this info, then a CF centered rim, like the road racers use, are not stronger than spoked wheels, and just trying to save weight and air turbulence ?

I have been dreaming up an idea to build a wheel with a disc center, of aluminum. I have rocky roads from my house to the smooth blacktop road, and the blacktop road has potholes, same as roads throughout the world.

I'm trying to think up a way to not have front wheel failure. The back wheels will be disc centered because of the hub motor and the distance from the hub motor to the small diameter rim.

I have a Moped wheel in transit, soon, for the front, but, I keep adding weight to the build, which will be counter productive at some point. Speeds will 50 MPH +/-.

 

[Architectonic]

So what is the optimum size for a bicycle hub with a motorcycle rim, avoiding too much stress on the hub? 13awg?

 

[johnrobholmes]

13ga is what I always use. Anything more and the hub flange will get pulled apart over time. I have seen bicycle hubs stress fractured from 12ga spokes.

 

[John in CR]

Can we resolve the radial spoke question while we're at it in this thread? To me it would seem that radial couldn't flex in the same manner, so it would be more prone to failure. My only experience with radial lacing was on our son's BMX, which was laced with 72 spokes. I was going to show him how to do a wheelie, and when I pushed on the pedals and pulled on the bars and the wheel simply failed at the spoke nipples/rim connection. It was only one day old. Were they tensioned wrong, or is radial actually weaker because there's no way to flex like can be seen in on Luke's bike with a rider with no throttle touch? How do you get dumped on your back in loose rocks and dirt anyway on a relatively flat area?

John

 

[Kingfish]

Under normal conditions with a properly tuned wheel, the spokes remain in tension. Failure begins when the elasticity of the rim is exceeded and the spokes go into compression. Depending on the system, lots of activity occurs.

Compression is a force acting upon an object in the opposite direction of Tension. When a wheel hits an immovable object (call it a pothole) and the force of impact is enough to deflect the rim, those spokes immediately in-line between the point of contact and the axle go into compression, and the spokes on the other side go into hypertension. That’s straight up physics. There’s no lie, no misconception, no other way to explain it – unless it is interruped by miracle, act of god, or disruption of the space-time continuum.

If the impact is severe enough, Compression will continue at least until the tension on the spokes are neutralized, and could continue onward. The salient point that few have brought up are the effects of Acceleration and Momentum and plastic deformation upon materials and assemblies. It’s easy to assume that spokes do not go into compression if you’ve never worked with high-energy physics... which involves very high-speed imaging. In combustion research, we used lasers to strobe and to fluoresce objects and particles in dynamic action. And we studied flames, rocket fuel, explosives, projectiles, and pistons... in some cases to the point of destruction.

Continuing on with spokes in compression: After the compressive forces neutralize the spoke tensions, and if the acting forces still have greater potential, the spoke must go into compression; it is inescapable. There are several new and interesting processes that begin at this point, and all depend upon the mechanical arraignment of the rim and spoke system.

• Rigid spokes, those affixed at both ends, if they were thick and forged into place (mag-wheels for example), well – these are already in compression and outside the model, so we’ll exclude those.
• Rigid spokes fastened at both ends will continue to compress until the fasteners fail, they bend, or they break.
• Semi-rigid spokes are fastened (or hooked) at one end and retained at the other, like a typical bicycle wheel. Most bicycle tires I know have a tube, tube tape, and possibly a rim liner. These will create resistance to movement, and cause the spokes to go into compression. Although even if the wheel was tubeless and without rim tapes, the spoke would still go into compression; it has to before the next action.
  
  Semi-rigid spokes, presuming the hooked end does not fail, compress and resonate in resistance to compression which means that the spokes are bending to a natural frequency, a harmonic derived by the material composition, the physical shape, the environment relative to STP, and the constraining factors such as total energy and fulcrums. Spokes resonate to dispel energy; they get hot, they can deform, they can become momentarily plastic, and they can also become loaded ballistic projectiles. Commonly, the results of spokes in compression are bent, broken, and jammed. But they all were had to go through compression to get there.

Questions?
~KF

 

[liveforphysics]

I don't know if you're insane, badly confused, or just f*cking with people, but there are no meaningful compressive loading in a spoke in a bicycle rim.

Can you tell me by what mechanism the spoke is constrained by to compress? Are you suggesting my 2 passes of electrical tape and my bicycle inner tube at 20psi (pressure I run on the back of deathbike so I get decent traction) are what the nipple pushes against to enable it to experience any compression loading?

Diameter of the head of my spoke nipple is 0.292", surface area is 0.0669in^2 times pressure in the tube of 20psi = 1.33lbs of force the tire pressure in the tube exerts on the back of the nipple. Lets say it's a roadbike wheel at 100psi with a full size mt.bike nipple in it, it's 6.65lbs of force being applied by the tube to the nipple for a best case scenario. Is this force what you believe contrains the back end of the nipple to enable a spoke be meaningfully loaded in compression???


Can you describe the mechanism in which you believe the spoke is constrained in the wheel to have meaningful compression load imparted to it?

 

[Thud]

I get that forces push & pull,

Not to pile on....but given the angles of spokes in relation to the hub & rim....is it even possible to load a 3 or 4 cross spoke pattern in a compressive way?

there is never a straight line to load any spoke from the impact points......so even if the rim is "compressing" no spoke will recive a compresive load....I would call it a "deflecting" load.

The spokes sharing the weight opposite of the impact point are the ones taking a beating with huge preasure increases are the jolt is absorbed.

The geometry of a Mag wheel is a different subject all together...

 

[Drunkskunk]

You can't put a compressive force on a spoke from the rim. If you out inward pressure on the rim, it will simply slide up the nipple, letting the spoke rattle. Even at an angle, its still going to be moving inward along the leingth of the spoke, so the nipple slips free and it can't transfer force.

Its not just the spokes oppisite of the force that take the pressure. Its also the spokes to the sides.