Carbon rims; harsh, stiff & unreliable…

Which simply proves to me that riders haven’t got a clue what they’re feeling. Though I’m somewhat surprised at Olivier not applying a little bit of his knowledge to the subject and believing the BS.

It is also quite important to get rim stiffness and the stiffness of the spokes and the effect of bracing angle to be complimentary.
What I mean is if you have a very stiff rim that is under supported by the spokes can lead to groups of spokes going slack but if a less stiff shallower rim is used with more thicker spokes then that horrible situation can be avoided as the shallower rim actually deflects less opposite the point of load than the deeper stiffer rim if the lateral stiff for both wheels is the same. Odd huh but true. This is more clearly seen in some road wheels. Deep stiff carbon rims with a very low spoke count can deflect enough at the brake track to cause the nds rear spoke to go completely slack. In that case up the spoke count. This is why more mtb rims should have off set drilling as that problem is entirely avoided.

How many people belive centrifugal force is real rather than an apparent force caused by newtons first law of motion. I think the same applies to that interview.

Which simply proves to me that riders haven’t got a clue what they’re feeling. Though I’m somewhat surprised at Olivier not applying a little bit of his knowledge to the subject and believing the BS.

Or anyone with a more open mind would see that there is a lot going on with a bike being ridden hard and that their oversimplification is causing them to miss the point…

You do realise that it’s lateral, not radial, wheel stiffness that is the bigger issue?

The trouble with having an open mind is that people will insist on coming along and trying to put things in it. Lateral stiffness is just as unaffected by spoke tension as radial stiffness – because the spoke doesn’t care which direction the wheel is being loaded and still obeys Hooke’s Law even when the bike is being ridden hard and there is a lot going on.

http://sheldonbrown.com/rinard/wheel/index.htm#1

bm0p700f

Centrifugal force you say… isn’t that the same as what goes around comes around ??? sorry boys but this has got far too out of hand come on lighten up.

aracer, the biggest professional teams who use every advantage they can get and test back to back are saying spoke tension has an affect on ride quality. Rather than try and shout them down with science, use the scientific approach, i.e. try and work out why people are experiencing a phenomenon which (appears) to contradict the basic facts.

Of course the answer that will come out is that a wheel is a complex system when you look at the aspects of force application from a point on a rim and transfer through the system to the centre of the hub. Yes spokes act in a linear manner (generally, dont forget they properties change as they are stretched due to the change of cross sectional area) but more so you also need to consider the spoke connection to the hub, nipple and the material and design of the rim, all of which will be affected by an increase/decrease in spoke tension.

Which simply proves to me that riders haven’t got a clue what they’re feeling. Though I’m somewhat surprised at Olivier not applying a little bit of his knowledge to the subject and believing the BS.

I know what I’ve felt. The only thing I’ve changed is the rims. The bike is noticeably less skittish on high speed, rough, flat corners, it feels more in control because it’s not what feels like the limits of the tyre grip, due to deflection from stiffness. I can hit, and hold high lines without feeling like the bike is going to wash out, so all in its a positive change.

I’ve also noticed I’ve buzzed the seat stays with the tyre, so all of a sudden ‘something’ is flexing more than an extra 1-2mm as you put it.

It seems to me a carbon rim can be too stiff, to the detriment of the ride.

Using science involves as a first principle applying Occam’s Razor. It is far, far more likely that there is a problem with the measuring device than that physics doesn’t apply. I’m sure the placebo effect is just as great when riding bikes.

To a normal person a wheel might seem like a complex structure, but actually in structural terms it’s quite simple. The change of the x-sectional area of the spoke under tension is miniscule. All the other bits you mention are also normal structural parts which all also obey Hooke’s Law.

p.s. Hob Nob, I’m not disputing what you’re feeling (not here, not now 😉 ) simply commenting on people using low spoke tension to get a more flexible wheel.

well I cracked an LB rim this weekend

punctured on the qualifiying stage at UKGE and thinking I was aaron gwinn !?! tried to ride down, till a loud bang in a rock garden…. crack half way thruogh the rim

anyway , bit of tape over the hookless bead and they resealed, slight wobble to the wheel, but still pretty true

so rode it all day Sunday, took a the first stage easy but in the end I hit everything as normal and it was a rough, rocky, rooty course with a fair few drops and jumps thrown in

I was a bit sad to see the end of the rim but ive had it for 16 months, and its done 2 ews, a year of ukges, dhh races, bikeparks, uplifts and stuff and I can now add it to my list of broken wheels from mavic, shimano, alex etc

and just checked the LB website and they offer an 18 month crash replacement policy for a small fee + postage, which im very happy with!

fwiw, i think they are a bit stiffer, I run my suspension slightly softer, but i also feel they accelerate quicker than my ally rims

It does seem odd to me that for MTB people use their carbon rims for everyday use.

For TT and Tri you have race wheels and training wheels and your race wheels only come out for races and cost about 4 times the price of your training wheels.

but maybe i am missing something?

I find it very easy to ignore the possibility of buying something so amazingly expensive. So the news that they are also a PITA is very welcome.

🙂

If spokes did not deform elastically in a wheel then they would loose tension over time which does not happen so they deform elastically when under tension in a wheel. Your entire post is confusing to me.

I think its a little more complex than that. The spoke as a component is outside of its elastic range, but we have to remember that a spoke is not a uniform material made up of a number of parts from a mechanical perspective (several thicknesses, a bent bit etc) , some of which will still be elastic at certain loads, some not. The point I was trying to get across (albeit badly) was:

At low tensions, the spoke is more ‘stretchy’ (not elastic – the SPOKE is not elastic at this point as that would mean it needs to obey Hooke’s law as someone was kind enough to point out) therefore less likely to suffer a permanent damage from an impact. Pillar spoke publishes the data for each spoke if you want to check it out. You’ll notice that the uniform elasticity ends around 80KGF for pretty much most butted spokes, after this it begins to detoriate.

You’ll also notice that the plain gauge spokes exhibit the kind of more distinctive kink you would expect from a uniform material’s stress-strain curve where the elastic phase ends and they begin to deform, whereas the more butting etc you have on the spoke the more smoothed out the curve becomes as the spoke continues to be elastic beyond the limit of some of its elements.

However, I think its safe to assume that some of the spoke is still in its elastic phase at this point (as you point out bm) but not all. hence why wheels just don’t fall apart. The amount of stretch of the spoke begins to be a falling rate therefore.

I’m not actually saying it works as a useful thing (running loose spokes), you might have noticed that I caveated the possible reasons with maybes and question marks. I kind of thought it might be useful to consider why a downhiller might want looser spokes, although I could have just written ‘You’re wrong’ and got the same level of angst in the responses I think.

What I am saying is that its not as simple as spokes under different tensions yield the same results, thats not going to be true as the spokes will be behaving differently under different tensions. What Im not saying too is that stiffer spokes make stiffer wheels. Oddly all the testing I have seen done on this subject (which normally conclude with the statement ‘tension doesn’t effect stiffness’ actually show a marginal, but consistent gain in stiffness as tension is relieved… I have no opinion on this but it seems odd no-ones looking at it. I guess the change is too small to care about.)

Hob nob I don’t disagree with you for the riding you do a less stiff rim might actually allow your tyres at the pressure you use to maintain a better contact patch. Do you buzz the stays with the stiffer rim or the more flexible one. How deep is your flexible rim and how deep is your carbin rim. I have tried to put an explanation of what you feel in my earlier post.

For me however I have a set of velocity blunt sl rims 28h laced to novatec hubs with sapim grace spokes. These wheels flex, it don’t like them much my rather stiff bike feels like it wollows with them on. Put in a set of stiff wheels and my ability to ride at a higher pace improves probably because I find the bike more control able at speed. My riding is probably very different to yours it just the tracks of East anglia for me.

The trouble with having an open mind is that people will insist on coming along and trying to put things in it. Lateral stiffness is just as unaffected by spoke tension as radial stiffness – because the spoke doesn’t care which direction the wheel is being loaded and still obeys Hooke’s Law even when the bike is being ridden hard and there is a lot going on.

Have you got a reference to show that a bicycle spoke at normal operating tension obeys Hokke’s law? I have no idea whether they are tensioned beyond their elastic limit or not, but the laws of physics can cope even if they are 🙂

A real wheel isn’t simple. The interfaces between spoke and rim at one end and spoke and hub at the other end add a lot of complexity that the oversimplified physics model isn’t considering.

It does seem odd to me that for MTB people use their carbon rims for everyday use.

We use all our good gear every ride :-), no point having it otherwise IMO.

have you got a reference to show that a bicycle spoke at normal operating tension obeys Hokke’s law? I have no idea whether they are tensioned beyond their elastic limit or not, but the laws of physics can cope even if they are

Check out these two spokes’ breaking strength graphs:

the first – A plain gauge spoke is pretty conventional hookes law stuff. Its a fairly uniform spoke and so its behaviour is uniform. The elastic phase is the straight bit at the start, the deformation phase the curved second part. spoke tensions are viable between 50-200 as a rule, but normally in the range of 80-150 so you will see that unless you run a pretty slack wheel (50) you’ve exceeded the elastic limit of the thicker spoke (P13) with pretty much any hit.

the second – a bladed, triple butted spoke. This has a very different curve. You will notice its not got a distinct kink, this is because the individual parts that make up the spoke are behaving differently and the combined effect is much smoother. At higher tensions some of the parts of the spoke are still elastic, some not. This spoke will be far better able to absorb impacts without permanent alteration to the spoke meaning it will stay truer longer, although its not necessarily ‘stronger’.

In relation to this whole discussion on Hookes law, what this means is that if you want a stiffer wheel (discounting for a moment the lateral/radial and torsional stiffnesses and going with lateral as thats what most people think of), as we know you need thicker spokes (no debate there), plain gauge offering the stiffest build as its the most consistently thick. However, in order to compensate for this a lower tension will allow the spoke to remain inside its elastic limits more easily, which clearly is a benefit in terms of the longevity of any wheel.

The problem with any such debate is the basis ‘Pro downhillers use looser spokes to get a better wheel’ is the sort of quote that appears on Pinkbike or similar that doesn’t take into account any of the other variables in play. It may well be true, but they probably didn’t just loosen off the same spokes they might ordinarily use. That technique will be in combination with other factors not mentioned (if indeed at all).

Thanks. That makes sense.

Some of the boffins on here need to have a word with my mate, a shiny new capra with mavic wheels, every ride & I mean every ride it snaps front spokes & they go with a right bang, two in one day recently on the Ard Rock.

Yorky, if his wheels are under warranty send them back rather than dicking with them. They shouldn’t be doing that, they’re mis-built and may fail dangerously.

I was a bit sad to see the end of the rim but ive had it for 16 months, and its done 2 ews, a year of ukges, dhh races, bikeparks, uplifts and stuff and I can now add it to my list of broken wheels from mavic, shimano, alex etc

And therein lies the great carbon dilemma. I have a warehouse littered with dead wheels of all types. Alu ones (fairly chunky at that) that have lasted a single UKGE, carbons that lasted a year or more and still going strong, and everything in between. The question isn’t whether your carbon wheel would have survived better or worse than an alu rimmed one, more whether you’re happy that you paid probably 3 x the price than an alu one for the time you got out of it.

I see a few carbon rims advertised as being indestructible and laugh my ass off. There’s no such thing as an indestructible rim, its a lot of emperors new clothes going on! Carbon rims have their good and bad points. The issue for most people is the cost of getting the good points is higher than they are willing to pay given the bad points still exist (at any price point). It all comes down to how much you want those good things, and how much you’re willing to trade to get them.

theglover – Member

For TT and Tri you have race wheels and training wheels and your race wheels only come out for races and cost about 4 times the price of your training wheels.

but maybe i am missing something?

A big part is, my carbon wheels cost me way less than many people spend on alu ones- they’re not that much more expensive than frinstance a Pro 2 on Enduros hope hoop, despite also having far better hubs, and way cheaper than a Crossmax. That’s a wee bit of an unfair comparison since I build them on used hubs but that’s how it shakes out.

I’ve actually gone the other way round and fitted cheaper, more disposable wheels for racing- there’s been a couple of times when I’ve ridden out a stage on a flat, I don’t mind doing that on my hammered old traversees but I wouldn’t want to do them on my nice wheels 😆

Stan’s new carbon jobbies seem pretty tough going by the footage at 0:55 in this video. Would like to see the same for normal rims and LB jobbies. Makes me think thread lock is a good idea when building.

Check out these two spokes’ breaking strength graphs:

Hmmm, except they show the deflection rate increasing as you apply additional force (as you’d expect from stainless steel or aluminium).

Which is the opposite of what they were posted to illustrate.

I suspect increasing the tension will give a stiffer wheel (beyond a given load anyway) because spokes start to go into compression, therefore don’t contribute to the stiffness of the wheel. This is why up to a point increasing the tension is good.

Unfortunately, beyond that point, you get into the shallower slope part of the stainless steel (or in some cases alu) stress/strain curve, and the strain in each spoke increases more for a given load. This will also cause fatigue as fatigue is controlled by the change in strain.

Other things may come into it, like the resonant frequency of a spoke changing as you change the tension (this may make for a more harsh feel under some spoke tensions) – and I’m sure feel is massively important especially to people at the top level.

Anyway, OP, are you sure your spokes aren’t over-tensioned if they are causing cracking around the nipples?

You’re correct philjunior, those numbers I quoted earlier I pulled off the wrong graph. They would be more like the other way round in fact. In terms of whether or not a spoke is more likely to be elastic at a lower tension or not though that’s not particularly important. I think the first of the two graphs pretty much highlights the point that rocketdog asked which is whether spoke tension alone would be enough to mean a spoke was deforming rather than elastic at a normal tension.

Its interesting to look at how the different spokes behave on the pillar site if you care about that sort of thing… double butted are fairly ‘normal’ but with a much bigger elastic section than pg, triple butted and quad butted you then start to see some weirder results where you get the opposite effect and the spokes reduce in elongation with the increase in force for a while. Anyone who cares to explain that I would be interested to hear your thoughts. [skulks off into a dark corner anyway]

Can I just congratulate those of you who’ve spent your bank holiday monday arguing on the internet about physics.

Well done.

I’m at work 😉

I’m at work

Same here.

🙁

Me too.

OK so heres one for the physics bods to ponder….

Jobst Brandt, in the Bicycle wheel, claims that wheels stand on their spokes, as opposed to hang. Given the stans video earlier that made me think of something I never quite got… how that works. Given a spoke is essentially free floating in its spoke hole, how does the spoke support any weight? Whacking a rim from above is surely the same as applying a force through the rim at the base in a general kind of way? Force is force no matter what direction its applied. Add to this the fact that I once read a (very detailed and techy) study that looked at spoke patterns and stiffness. In that study, it showed that under normal load, the spoke tension is increased relatively (in the grand scheme of things) evenly across all the spokes other than the couple at the bottom, which experienced a massive loss in tension. How would this be the case if the bottom spokes are under compression (hence the loss of tension)? That compression needs to be transmitted to the hub… that force would be applied most directly into the spokes vertically above the hub I guess? If they are also experiencing some compressive forces from the push from below, surely they would also show a loss of tension not an increase? Hmm. Thinking about it though I guess the force goes into the axle not the spokes above… that still doesnt answer the question as to how a free floating spoke would carry any load at all. And more over why is everything else under increased tension if the axle is carrying the load transmitted by the ‘pillar’ spokes?

Surely it makes more sense that whats actually going on here is that the spokes now under increased tension (all the other spokes other than the ones at the bottom) are in essence resisting the desire of the rim to become oval under load, which is what prevents the wheel from collapsing?

Am I missing something really obvious?

EDIT: To highlight my confusion imagine you removed a quarter section of the spokes of a wheel. Now sit on your bike with the 25% at the bottom, then with the 25% at the top… which way round does the wheel collapse?

More nerd reading here

… so I thought that sounds like a fun experiment. So I took an old wheel and cut 7 spokes out (in a row). No issues with it missing spokes down, missing spokes up – notable vertical flex in the wheel! I chopped a couple more out just for the craic. missing spokes down the wheel felt ok, missing spokes up and the wheel was all over the shop (as you might expect) but I could feel the spokes in the base bouncing in their holes. They certainly weren’t supporting any weight.

I’ll come back to the issue about the spoke stiffness under varying tension, as I need to do a bit more research – clearly I was wrong about the linearity, though even with those curves it’s not far off and as pointed out actually working in the opposite way to suggested – stiffness decreases with increasing tension (which might explain the very small variation seen in Rinard’s testing). Something seems wrong though, because given a Yield strength of 520MPa for stainless steel, a PG spoke would yield at 166kgF, rather higher than shown. Some other issues I’ll come back to…

Though I thought I’d try and help with the Brandt standing on spokes thing. He’s not suggesting there that the spokes have an absolute compressive force in them – as you’ve recognised they can’t do that. However they can provide a relative compressive force through the principle of superposition of forces within a pre-stressed structure by a decrease in the tension force.

Essentially the thought process is that with your static pre-stressed structure you consider that the force on each element is zero. Now when the tension on the bottom spokes decreases under loading, within that model that is a compression force. Because the effect is exactly the same as if you had thick spokes capable of transmitting a compression force with zero pre-tension – as you’ve recognised the only significant change in tension is for the bottom spokes and the effect on the hub of them decreasing in tension within that pre-stressed structure is exactly the same as if they were solid spokes with zero pre-tension transmitting a compressive force.

So no, he’s not suggesting they transmit a real compressive force, but superposition of forces is an important concept in structural engineering (lots of pre-stressed structures around – pretty much any bridge built of concrete is). What he’s pointing out here is that the popular idea that when you load the axle the load is taken by an increase in tension in the top spokes is incorrect.

Unfortunately your experiments with cutting spokes don’t prove anything useful, because as soon as you cut spokes you no longer have the same pre-stressed structure which this concept relies on. Though I think you’ve already understood the most important point without realising it.

OK, I get it – so the important factor here is the change in forces not the ‘direction’. Still confuses me more though having tested the two ways up. Without the bottom spokes the wheel is pretty much unchanged, other way up and its useless… which leads me to believe the model is oversimplified as the wheel is capable of surviving without the spokes with Brandt claims are the ones doing all the work, and I don’t believe (for no good reason than I don’t) that there presence in the wheel suddenly makes the other spokes fairly irrelevant, which is what he would lead you to believe was the case when you read the book Which in my head is where I was going.

Thinking more about the concept that extra tension != stiffer wheels, it occurs to me that the testing is fatally flawed surely? if in real life the wheel at the point of lateral deflection (assuming that generally occurs at the contact point) is also experiencing a massive loss of tension as would be implied by the graph I read about, then surely that would effect the stiffness of the wheel? More specifically, if the spokes at the point of deflection are affforded little or no tension due to the compressive load already on them (i.e. they might as well not be there) then at that point does the rim not become the most important factor, followed by the surrounding spokes? And would therefore greater tension within a wheel not lead to a ‘stiffer wheel’ even if the deflection test in lab conditions showed otherwise. Is it possible that all the lab tests are basically bunk because they ignore this important variable?

Just thinking out loud as to why anecdotal evidence might conflict with the lab evidence…. I question everything!

The greater the tension (within reason) the more the supplementary spokes will be involved in supporting the system under dynamic loading.

Spoke stiffness is only dependent on its length, shape, cross sectional area and young modulus. Tension plays no part but in a wheel tension plays a part of sorts. If the deflection at the rim is large enough and typically groups of the nds spokes go slack or for a front wheel the right side then wheel stiffness will drop.

So a wheel needs to be stiff and spoke tension high enough laterally and radially to avoid this under normal conditions. Notice how a separate stiffness and spoke tension they are not the same but both play there part in preventing slack spokes. Slack spokes fatigue quickly and fail and it also how a wheel goes out of true. If the wheel is built properly and tension is high enough and the wheel is stiff enough then thread lock is not needed. Thread lock in my opinion is there to mask an underlying problem of a bad build or inappropriate components.

Creating a stiff wheel is also why I prefer hubs that give the best bracing possible. Every little helps. A lot of this is over thinking what makes a good wheel. Speaking as a wheel builder it is pretty simple. A good wheel is one you enjoy and one that is reliable for the use intended. For it to be reliable the spokes must not go slack or at least it should happen infrequently. This is one reason why 29er rims are wider than smaller rim sizes it not just to change the tyre profile it to increase stiffness of the wheel so it survives use.

A bank holiday taking wheels is time never wasted however the only way to answer all the questions you have is fine to element analysis but I am not sure what benefit the answers will bring.

That’s certainly not what he’s suggesting. They’re only doing the work (by changing in tension) because it is a pre-stressed structure – the other spokes are very important to that pre-stressed structure. Hence as I mentioned above if you cut any of the spokes out it is no longer the same pre-stressed structure so any results are irrelevant.

Though I’ve just checked what Jobst says in his book (you won’t be surprised I have a copy!) and he comes out with the analogy with a wooden wheel which I thought about mentioning, and TBH does a much better job of explaining the concept than I could – I’d suggest just re-reading that section a few times!

Regarding loss of tension at the bottom of the wheel, even if all the load is taken by 2 spokes (checking Jobst’s FEA analysis, he has 3 spokes taking most of the load because the load is at a spoke, but the outer 2 only take half the load of the middle one) , then assuming pre-tension of 125kgF that’s only a loss of ~1/3 of the tension. So the spokes still shouldn’t completely detension under a side-load – if they do you would get a huge decrease in stiffness, but that would also be a non-linear change and something you’d know about. Also a really bad thing in terms of wheel strength if that happens, which is one reason you don’t want low spoke tension.

<sigh> that is also a load of rubbish for all the same reasons that spoke tension doesn’t have a significant effect on stiffness. Unless of course spokes are going completely slack, but I think everybody agrees that shouldn’t be happening.