John Hall (Gwin's Mechanic) on Spoke Tension

Currently running DD maxxis 2.4's and friends on the same wheels are running the new Conti's in both Enduro and DH with great success, no punctures over summer riding season. I've also run the Schwalbe Super Gravity before with good success, I just like trying different tyres and based on pricing and availability in Aus 

2.4s in WT variant or a 'standard' version? Have you (or anyone for that matter) tested how a narrower vs. wider rim functions in regards to tyre size? Theoretically WT tyres from Maxxis would benefit from a 30-ish mm rim, on the other hand quite a few DH racers run EX471s...

Just so we don't nerd out on spoke tension and rim stiffness only to come to a point where the tyre is flapping around in the air on a seriously stiff setup, throwing away all that stiffness.

Hey Primoz, I've given this a lot of thought since I first encountered the "hub hangs from the spokes" argument, and I respectfully disagree. If the hub was only hanging from the top spokes, that would mean that the structural integrity of the bottom of the wheel would only be provided by the rim. Simply put, rims aren't that strong. You can go break an aluminum rim with your body weight by applying only radial force and bouncing on it. There's no way a rim would stand up to the radial loads of jumping and g-outs and rock gardens without some extra help.

Someone else shared this video in the thread, but Park Tool has a great video showing how spoke tension changes across a wheel during different use case scenarios by using their tension meter. It's great! You'll see them test the "hanging hub" argument at about 2:30 in the video: https://www.youtube.com/watch?v=7vFlKI_Brts You'll see that when every spoke in a wheel is tensioned, all the spokes are working together to hold up the weight of the rider at the hub. Someone else in this thread shared the term "tensegrity structure," and that's exactly what a wheel is. All the spokes in tension are holding radial and compressive loads, creating an extremely strong structure with a strength to weight ratio close to spider's silk. How does that work?

Here's a simple example- think about a paper coffee cup as a wheel. The open rim of the coffee cup is pretty weak at resisting radial loads. If you push in on each side of the open rim with your thumb and finger, it compresses easily, but watch what happens to the rest of the rim. The sides of the rim expand out as you compress the rim in, into a rough oval shape.

Spokes in tension would prevent a rim like this from expanding out. You can imagine tying strings inside the coffee cup to hold the rim together, and suddenly the rim of the coffee cup is a lot harder to compress. I haven't done that yet, but you can mimic the effect of spokes by putting a lid on the coffee cup. Suddenly, with the lid in place, the rim is really hard to compress with a radial load (thumb and finger). Mind you, the outer lip on the lid only prevents the rim from expanding out- there's no corresponding inner lip on the lid to prevent the coffee cup lid from compressing in. And yet, by preventing the cup's rim from expanding globally (on the sides), we prevent the rim of the cup from compressing locally (under our thumb and finger).

You can only compress the rim of the coffee cup by applying enough force to blow the lid completely off (destroying the "spokes" in this example), allowing the rim to expand out on the sides. I'm not able to embed it right now, but if you want to see my example video, click here: https://www.youtube.com/shorts/rpEDSAkiUPk

Similarly, destroying a bike wheel with radial/compressive forces requires an incredible amount of force. Bernard Kerr destroyed his front wheel at Hardline with pure radial loading by overshooting a 90 foot jump to flat. Ouch. More typically, rims will compress to failure when a highly localized force is concentrated at one small point on the wheel, like hitting a sharp rock and bottoming out your tire into the rim. In this case, the force is so localized that you've generated a huge PSI (pounds per square inch) number at the rim, so the total strength of the whole wheel as a system is less important than the material properties of the rim. Aluminum rims will fail by denting or flatspotting in these cases, carbon rims will crack, and often 1-3 spokes may be loose at the site of the rim failure. In the case of a flatspot or dent, the rest of the wheel is typically still circular and ridable because the rest of the spokes are still holding tension.

WHEELS ARE COOL!!!

Great explanation/demonstration! The cool thing about wheels is the way they distribute the load around a large section of the rim, with the bottom spokes reducing in tension/length and the spokes either side increasing in tension. You can kind of see the evidence of this in a very broken wheel - if you look either side of a very flatspotted or cracked rim you will often see splits or stress marks around the nipples where the spokes have tried to pull through.

Regarding Bernard, I think it was said he impacted the rim beforehand and it might have had a crack. The huck to flat landing could have subjected it to a huge shock, where, like you mention, a carbon rim cracks and breaks into sections. And fails catastrophically.

As for hanging off the top, it's not literally hanging off the top due to the tension in the spokes.
You build up a wheel and each spoke is tensioned by a force that is high enough to hold up the majority of riders on its own (120 kg as the rim limit). With the wheel laying there on the bench, the spokes are tensioned equally. Once you put the rim on the bike and sit on the bike, you're pulling down on the hub. You can't compress the spokes as they don't carry any load in compression. Plus they are pretensioned anyway. What thus happens is the tension of the top spokes increases and the tension of the bottom spokes decreases.
THAT is what I mean by hanging off the top spokes. But as the bottom spokes don't detension completely, they still carry some load. If nothing else, they give stability side-to-side. That's why the rim doesn't fold over. The pretensioned structure is much stronger than the rim by itself.

A demonstration of this would be to lace up a wheel without tensioning the spokes. If you juuuuuust tension them for the rim to not flop around, putting it on a bike and riding around will likely mean a sloppy ride. Because with the bottom spokes not giving said stability, the rim can flop around.

And to make it clear, it's not like the top 3 spokes will have an increase in tension too, the tension will spread around the spokes in the upper half at least, also probably dependant on the angle of the spoke going into the rim - if it's angled against the load direction (downward, at least slightly) or in the same direction (more upwards).

But how does the the load carry around the rim to the top spokes? That would require the rim to be extremely stiff so it can hold its shape to pull on the to spokes. If you take any unlaced rim and pus it on the ground, they are VERY squishy - much like robots paper cup. So if you press at the bottom, that section of the rim must be squished inwards (relaxing the spokes) and that naturally pushes the neighbouring section outwards (increasing tension). The rest of the spokes have very slight changes in tension but the majority is at the top. 

Hey Primoz, if you go to the Park Tool video I linked to, at 3:07 you can see their spoke tension map that shows the top and bottom spokes both losing tension as they sit on the bike. It's the spokes on the sides that gain tension, because the rim is flexing in at the bottom from rider weight and compression and pushing those forces out to the left and the right, like my coffee cup example above.

Hoop stress. The same way a very thin soda can can hold multiple atmospheres of pressure when you shake it but it dents easily when you push on it with your finger. The finger is the unlaced wheel.

Good point on the few local spokes getting detensioned, but the neighbouring ones being tensioned more though. Didn't think about that one. Currently my intuition would be that the neighbouring spokes only cover the deformation itself. You still have the outside force to deal with, which would be spread over the top spokes. But I'm not 100 % sure that is actually the way it happens.

Sounds like an FEA model might give some answers...

EDIT: watched the video, but I was typing at the same time and missed that detail.

It makes very little sense to me for the top spokes to lose tension as it's not the same as the coffee cup (being squeezed from both sides), you load the structure from the middle. The top of the wheel is not loaded at all externally. I'd expect a shift upwards along the bulging out the sides to be honest.

I actually had the same thought! In my coffee cup example, I'm pushing on the top and bottom, but when you sit on a wheel, you're only applying force to the bottom of the rim. So why are spokes at the top of the rim losing tension, too? I don't know, but here's my guess- because they're diametrically opposed to the bottom spokes. I'm guessing that, as the bottom spokes lose tension on the wheel, the hub is able to ease up vertically under the relatively greater tension of the top spokes, which then reduces their absolute tension value.

WT. Yes, I was previously runing 511's and got recommended to switch to 471's, I took some photos and measurements of the tyre shape when I swapped over, i'll try to find them, but essentially the same tyre is a bit more "round" than a 30mm rim, which I prefer, It leans into corners better and I find gives better cornering traction, but has slightly worse braking traction overall (all anecdotal without actual testing).

A lot of DH racers still use 25mm rims, in particular the Specialized team, I've heard they prefer the tyre profile being rounder.

I honestly think there is a good kernal of thought in the stiff wheel floppy tyre idea, It's why cush-core feels so nice on stiff wheels, because it helps stiffen up the tyre from rolling. Tyre squirm is about the worst feeling when pushing hard into corners or compressions. 

https://www.youtube.com/shorts/n7sCD_REkcg

(I can't embed a short for some reason??)

This is a really interesting clip that I constatly think about when discussing wheels, do we want the wheel to flex here and track better, or to be stiff and skip? Is it possible for it to track at all? Obviously probably an extreme example, but interesting never the less.

I was trying to find a video of someone destroying a wheel (pulling multiple spokes) to see where it actually fails. Found one:

Looks like there is something to the coffee cup thing after all. THe spokes that failed are right next to (behind) the point of impact and the rim cracking.

Where's that hat?

I watched the park tool spoke tension video, super super interesting. The whole "hub hangs from the spokes" view of a wheel is totally wrong.

Its clear the wheel is a system, and when statically loaded (i.e just standing on the bike) the rim WILL flex at the bottom, dropping spoke tension on those spokes (since the ERD has reduced) BUT, that tension must be accounted for elsewhere in the wheel, hence we see spoke tension INCREASE at the sides (exactly like the coffee cup explanation, kudos to thinking of that).

The pedalling scenario is exaclty what I thought it would be, leading and trailing spokes increasing and decreasing tension and vice versa for breaking.

Bring this back to our spoke tension debate, my understanding of material engineering (increadibly basic) is that as long as the MAXIMUM tension under loading doesn't not exceed the elastic phase of the spoke (that is the part of the graph before yield strength) and the MINIMUM tension doesn't go close to enough to zero where it looses all structure (see Sheldon Brown deflection test) then the tension on the spoke itself doesn't matter. What matters is the Young's modulus of the spokes in question, so simply put what type of spoke you have. 

When I initially mentioned the 'hanging from the top' stance, it of course doesn't mean 'only the top few, all the rest are there just for show'. The system part is the key here and a spoked wheel really is a marvel of engineering considering how strong and light it is. But yeah, apparently the hanging from the top is not that much of a thing.

As for the stress-strain graph, young's modulus is a part of it, but that only covers the material itself. Steel (210 GPa) has a steeper slope than titanium (120 GPA) that has a steeper slope than aluminium (70 GPA). Carbon fibres themselves are incredible precisely in this metric, in tension the modulus can go to 500 GPa, but off-fibre axis mostly the resin carries the loads, which has a modulus of a few GPa (that's why a layup to handle loads in different axes often has a similar stiffness to metal given the weight of the structure).

That's the modulus, but the modulus is a nominal value not dependant on the shape of the part. Increasing the cross section of the spoke will nevertheless increase the stiffness of it, thus requiring more force to stretch by X mm. So it's not only the modulus, it's also the cross section. Q.E.D., Industry9 aluminium spokes.

Absolutely, I agree with all of this.

I guess my main point is that as long as the spokes don't over tension or undertension during use, the important thing isnt the tension itself, but what type of spoke it is will determine the wheels characteristic (assuming the same rim)

For those of us who lack Buckminster Fuller’s abstract genius, here’s a more simplified explanation of Tensegrity in bike wheels:

https://www.science20.com/the_chatter_box/blog/cycling_for_science_1_te…

I like how these pictures illustrate that the rim flattens and the side spokes try to counteract these forces, it's metioned in the Park Tool video. However, the argument that no spoke can transfer compressive load is somewhat misleading. As shown on the tensegrity structures, what really happens is that the pre-tensioned spokes actually can bear compressive load without being subjected to compressive stress just by decreasing the preload force. Once the compressive force that a spoke carries exceeds the preload force of this same spoke, the spoke actually needs to be subjected to compressive stess to carry load, but it can't obviously. So to conclude, a spoke or a string can carry compressive load if the pre-tension force in this string is higher than the compessive load. or, compressive force does not result in compressive stress inside a solid body if this solid body is already subjected to tensile stress of the same magnitude (or higher) along the same direction. sounds complicated, but it's not. 

So regarding the rear wheel crash video, what happens is that the impact force is so high that the bottom spokes that should carry the compressive load lose all the tension and then you have indeed the "hub hanging off the rim" situation (the side spokes can also contribute to distribution of the load), but the rim suddenly has to carry all the load and fails because it sees way more load than it's designed to do (as TEAMROBOT pointed out) and then folds like the paper cup that he showed on the photo.  

By the way. I just remembered a very interesting book about structural mechanics that is very intuitive and does not require math at all: https://www.amazon.de/face-failure-nature-engineering/dp/3923704437/ref=sr_1_5?qid=1681290431&refinements=p_27%3AClaus+Mattheck%2Cp_n_feature_three_browse-bin%3A4192709031&rnid=4192708031&s=books&sr=1-5&text=Claus+Mattheck I've seen experienced engineers totally drool over it, don't underestimate it just because it has these weird cartoonish pictures and no math. It's written by a German theoretical physist and material failure scientist Claus Matthek, here's his photo, check it out. It's a great present gift too for those interested in engineering or like to build stuff.

Killer boots, man.

Yep, Mattheck is a very knowledgeable person