Carbon rims; harsh, stiff & unreliable…

so are DH teams really experimenting with low spoke tensions, or is that just internet BS?

and wouldnt that make them slower to accelerate

😆

Put a wheel under heavy loads and spokes will repeatedly go slack, it’s inevitable. Look at the forces required to bottom out a long-travel bike (shock curve x linkage progression plus damping losses).

aracer, you have a remarkable talent for putting things across in such a way that it stops others listening. Don’t ever take up teaching!

so no then aracer?

Well if you do make the spokes go completely slack (if you are, then that’s a good argument for having the spokes as tight as possible), then higher spoke tension delays the point at which other spokes contribute. I think that’s the opposite of what you suggested. The tension in any remaining tensioned spokes still makes no difference.

If you’re still listening to somebody who disagrees with you.

Though just in case you are, then let’s do this one I missed earlier:

Well those interfaces are pinned joints. Couldn’t really be much simpler. No problem at all with modelling those.

So I read, and checked and read again (I have a download of Jobst’s book… too cheap to buy it). And I couldn’t figure out why, but something still didn’t sit right. So I kept looking and found this guy’s [/url] analysis. Nothing that out of the ordinary here – results and conclusions match Jobst’s… nothing to see? Well maybe. If you look into it he does a nice job of explaining linear superposition, standing vs. hanging etc. and also explains a few things along the way about the modelling he did.

One thing that jumps out at me is that he’s not used any tension. Instead, he’s told the FEA tool to not allow ‘buckling’ of the spoke –

None of this affects the analysis. If we ignore buckling (or restrain against it), a spoke under tension which is subject to (say) 100N less tension contracts just as much as an unstressed spoke subject to 100N compression (or for that matter a compressive spoke subject to 100N more compression). In the analysis I simply don’t tell the computer to let anything buckle, so I can analyse a spoke with compression without worrying that it’s really a spoke that started out with a tension and now has less tension.

The issue that strikes me is he says that it doesn’t effect the analysis… but it does. His wheel starts from 0 tension and applies compression or tension, which is fine supposing you don’t live in the real world, but in the real world this model doesn’t work, or rather, for it to work there is an un-mentioned constraint that the spoke must be attached (or effectively attached) to the rim. So now I am wondering whether this constraint is there in Brandt’s work too, and if it is how is it effecting the model?

The issue that strikes me is he says that it doesn’t effect the analysis… but it does. His wheel starts from 0 tension and applies compression or tension, which is fine supposing you don’t live in the real world, but in the real world this model doesn’t work, or rather, for it to work there is an un-mentioned constraint that the spoke must be attached (or effectively attached) to the rim. So now I am wondering whether this constraint is there in Brandt’s work too, and if it is how is it effecting the model?

Surely all he has done though is just removed the need for him to model tension? His highest ‘compression’ was -350. If he had modelled in a tension of 400 this would have meant the spoke was at +50, with the rest at 400+(spoke tension).

Differences in the build up of force due to displacement depends on the material properties entered, but id guess for a basic model he has probably just entered a linear value rather than a material properties curve, so suited to his 0 pretension approach.

Note as well, this crossed spoke wheel behaves differently to the radial, with varying distributions of stress around the wheel.

Sort of STATO, but the issue (I think) is as per that Stans video. A carbon rim exhibits that behaviour (Stans or otherwise – they all do it) they are radially soft while laterally stiff. Its what gives them the anecdotally magic ride, its what also makes them a PITA to design for.

If we assume for a second that you can indeed compress a carbon rim much easier than an alu rim, then you start to break the model. The tension on a spoke is only a factor of its elongation. If the compression of the rim in use extends the elongation of the spoke (for its given tension) then the spoke is no longer in tension. This is exactly why carbon wheels _really_ need locking nipples.

So assuming this is correct then the model no longer works, the spoke is no longer in tension… Its then dependent on the ‘other’ means by which wheels hold together – the other spokes are doing the work not the standing spokes. Maybe.

Well yes you are obviously correct if you assume you can deflect the rim more than modelled, but the model is basic and not designed for that application, so you cannot criticise it for that.

Dont know anything about these new ‘soft’ carbon rims. Is deflection possible over a short area, or as a result of ovalisation of the rim? if over a short distance i can see it potentially causing issues with weakness/fatigue. However if more an ovalisation type softness then the spokes across the rest of the wheel will be put under proportional displacement (assuming the hoop of the rim cannot compress) i.e. compression at one location is tension somewhere else?

So……………….should I buy carbon rims or Alu ones ?

I’m not reading all that…

I’ve just built a carbon wheel – should I go for higher, the same or lower spoke tension than I would with alloy?

It doesn’t matter because physics + wheels + hubs + spokes + the real world is very complicated.

You can find an answer to suit whatever viewpoint you have 🙂

Basically the more you spend the harder it will be to realise you have not spent wisely 🙂

You can find an answer to suit whatever viewpoint you have

I seemed to end up a bit under a similar Stans build on tension with the 29er set I did.

cheif – i think to make a spoke go slack by loading a wheel you will be well past rim destruction territory. if built evenly there wont be much vertical compliance at all (basically teh residual in the spoke only).

the olympic velodrome is built basically like a bicycle wheel – which is very cool. I went to a seminar on how they designed it once but all i really remember was that the maths was pretty tricky and the FEM models had lots of pretty moving colours with all teh dynamic loads.

if you strap a stiff carbon rim to a wheel you can run lower spoke tensions to get more lateral compliance as the risk of flat spotting is much less.

you need some lateral compliance to dampen the lateral impacts. otherwise the ride will be a bit ‘jittery’ on the choppy stuff

if you have too high a spoke tension you will have a too stiff wheel and risk rim damage rather than have a compliant wheel that ‘bounces’ off hits as teh only bit of compliance is in the tyre.

for no other reason than its interesting and bloody cool look at gee athertons gap to wall at rampage from 2008? the redbull super slowmo. you can see how far a wheel/frame can deflect. the landing trashed his frame and i doubt the wheel was very round but you can see why some compliance in a wheel is a good thing as he can ride it out. (he was the only one that rolled out of this gap that year)

Is that really the case? Can you compress a carbon rim more than an alu one? I’ve watched that Stans video, and I’m not sure it’s showing what you think it’s showing – they say it’s 3 or 4 times stiffer laterally than an alu rim, but not that it’s less stiff vertically (I’d suspect it isn’t, there’s a limit to what you can do by tuning the direction of carbon fibres). The tyre is completely bottomed on the rim before the rim starts to move (they don’t say what the force involved is, but I bet it’s a lot higher than you’d get in normal use). In the bit just after that where they show an impact on the corner of a rock I can’t see any deflection in the rim, let alone spokes going slack. What they’re actually showing there isn’t that the rim deflects significantly under normal loads (and so makes for a “magic ride”), but that it will be resilient to direct impacts on the occasions when the tyre bottoms.

Yes if the spokes are going completely slack then the model is broken, but that doesn’t happen in normal use – it’s certainly not something you want to be happening, as it will result in the wheel being a lot less stiff and a lot weaker. I’m certainly not convinced it is something which is happening with carbon rims – though engineering in some flexibility in the vertical direction is likely to be a good thing as it will help the rims survive better, there still won’t be enough that spokes are going slack as that would be a bad thing.

You’re trolling me now?

aracer – I’m building a new deflection test rig right now for some new wheels I’m working on, so when I get it finalised I will test some alu rims too and see what the conclusion is! At the moment its only for lateral deflection, but I will figure out how to do a vertical one too at some point. When I do I will publish the data.

What’s on the other side of the bridge?
I hope it’s a cafe.
I don’t know about you but all that building work and walking has made me rather peckish.

Ha, sorry helpful1 the bridge analogy didn’t make the 20min cut… but for what its worth it was a pub.

I honestly don’t think you get much vertical deflection at all. I’ve flatspotted rims before and the spokes were still in tension. Just less where the rim flatted. The load sharing of the wheel working vertically is very uniform when the wheel is well/evenly tensioned. The wheel will only deflect within the elastic limit of the spoke. Which are already under quite high tension and therefore this limit is pretty small. I don’t think the choice of rim matters for vertical loads only other than increasing the risk of plastic deformation of the rim. Look at all the road bike vids. The road bike party vids never broke wheels while imposing some pretty high vertical loads. For point loads, rocks etc, you are considering the strength of the pretensioned rim only for that individual impact therefore rim strength becomes a factor.

I honestly don’t think you get much vertical deflection at all. I’ve flatspotted rims before and the spokes were still in tension. Just less where the rim flatted. The load sharing of the wheel working vertically is very uniform when the wheel is well/evenly tensioned.

If you measure the tension of the spokes of a deformed wheel all the spokes will read different to pre-derformed state. Thats because as tension is lost at the point of impact it changes the balences of forces across the whole wheel.

The wheel will only deflect within the elastic limit of the spoke. Which are already under quite high tension and therefore this limit is pretty small.

Hmm, i dont think this is correct. A wheel will deflect by as much as the force applied makes it. If within the elastic region (spoke and rim) it will return, if beyond elastic it will permanently deform, i.e. buckle. It would be interesting to know (because I dont) how much of a buckle is the rim vs spoke plastic deformation, im guessing the answer would be very different with a carbon rim.

As interesting as the physics aspects are, theory is only as good as the paper it’s written on.

I’d be interested in hearing about people’s actual experience, does the ride quality differ, do they affect the feel/handling of the bike, and does the weight really help acceleration if coming from a heavier wheel set, I know alu wheels can be built just as light at the higher end, but does that lightness sacrifice strength, which is an issue carbon wheels seem to address?

One thing that stands out to me, is that with the onset of affordable carbon wheel imports, we are not seeing many people people with buyers remorse offering to swap thier carbon rims for alloy equivalents, ok the market is still small as carbon equals thousands of pounds in most people’s minds, which simply isn’t the case any more, but good detailed personal accounts from people seem thin on the ground..

One thing that stands out to me, is that with the onset of affordable carbon wheel imports, we are not seeing many people people with buyers remorse offering to swap thier carbon rims for alloy equivalents, ok the market is still small as carbon equals thousands of pounds in most people’s minds, which simply isn’t the case any more, but good detailed personal accounts from people seem thin on the ground.

I think that was the reason this thread was started, most people (general riders) seem to have no problem with them. Take them to the limit and subject to to serious abuse and the failure mode means any damage means the rim is toast.

well heres my crack

i flatted in qualifying at UKGE on the DH run at Ae and it went bang in a rock garden a bit further down

but ran it the rest of the weekend (another 50k) throughout the race with no problems, im not lighweight, lack finesse and the stages had planty of drops, jumps and rocks
https://www.rootsandrain.com/photos/1133057

ive sent the pic to LB, will see what their crash replacement is like after 16mths or so of abuse im pretty happy even if i have to pay full price,

My experience is that you can build a carbon rimmed wheel that is lighter, or for the same weight much stronger than the alu equivalent (you choose whats important to you). I do think there are some drawbacks in carbon rims which while you can try and engineer them out as best you can, they will never be a total substitute for alloy, only an alternative as some people won’t be willing to live with the possible downsides, whereas others have no issues with them at all.

does the ride quality differ, do they affect the feel/handling of the bike, and does the weight really help acceleration if coming from a heavier wheel set

I recently switched from a set of (drilled by me alloy rim) On-One fat wheels to a pair of LB 65mm rims on Fatsno hubs.

biggest change is that the front wheel was 500g lighter and the rear 1kg 😯

I also went from a 4.8″ Bud/4″ Floater tubed to a pair of 4″ FatBNimbles running tubeless – saving another couple of kilos.

Any subtle change in riding characteristics of carbon v alloy are swamped by having a set of wheels and tyres that weigh 5lb less than the ones I was using previously, tbh.

Regardless of weight the tyres are so influential on fat bike riding characteristsics that I’m not sure even if they weighed the same I’d notice anything.

well LB have said rims only 1 year warranty, theyve offered me 10% off a new rim, which ill probably go for!

I had a set of Lb carbon rims on my old bike (ibis mojo HD), I cracked the rear after a few months, and replaced it with another. But when I bought a new bike that needed 27.5 inch rims I bought aluminium rims, for 4 reasons:-

– Cost – I didnt think the LB wheels warranted the extra cost over some superstar DT swiss alu rims. The LB rims werent loads lighter (I think the 26″ LB rim was 50g lighter than the DT swiss 27.5″ rim)

– Hassle of getting tyres on and off -The LB rims were exceptionally tight with hans dampf tyres, so tight that it was a massive struggle to fit tyres. The dt swiss rims are easy.

– Ease of getting replacements – LB rims takes a few weeks to arrive from china, Dt swiss rims can be here in days.

– consequence of damage – carbon rims crack and are a write off if you bash them, alu rims just bend and can usually be bent back.

Cant say I noticed any harshness or negative quality of the carbon rims (apart from those listed above), its just that on a mountain bike where wheels are a consumable, I’d rather go cheap and functional.

Take them to the limit and subject to to serious abuse and the failure mode means any damage means the rim is toast.

A lot seem to be cracking under the intended use TBF.

However the thing that put me off was – as you and julians point out – that they fail in a way that puts them beyond use. I’ve dented, dinged and flat spotted alloy rims but never killed one.

A lot seem to be cracking under the intended use TBF

Sorry, i didnt mean to suggest ‘getting rad’ was outside intended use, just trying to differentiate the failures by riding type, which is mostly under those pushing the limits (where though alu rims would ding but probably still be fine)

chakaping – Member

However the thing that put me off was – as you and julians point out – that they fail in a way that puts them beyond use. I’ve dented, dinged and flat spotted alloy rims but never killed one.

It’s not a big sample but I cracked my LB one exactly halfway through a week in wales and on about run 3 of an uplift at bpw. Stuck a tube in it and carried on regardless. But I’ve broken 2 alu rims in a way that made continued use either impossible (a flatspotted flow ex that wouldn’t hold a tyre on it) or really really brave (a 721 with a load of growing cracks). The bigger difference is just that it took much more to crack the carbon rim in the first place (oh, and that it’s about the same weight as a stans olympic)

TBF, if it hadn’t made such a loud noise when I broke it, I might still be riding on it 😳

More anecdotally, I’ve seen a lot of people with similiar experience, and there’s a few on here- someone who broke a LB one on a waterbar at the trans-savoie IIRC?

So basically; both materials can fail in a way that puts them immediately beyond use, or doesn’t.

I thought that most of the movement towards carbon rims was to get 29″ wheels back to the weight and strength of 26″ ?

Like Big Dummy I’m pretty happy that the OP has given me a very good reason to stop gazing wantonly at kit I can’t afford to replace if it goes wrong! 😆

My luck with rims isn’t great in 15 years I’ve taco’d/ cracked 4 ally; 2x mavic- 717 enduro racing and 321 alps riding , 1x alex- general riding, 1x shimano- commuting, it was a pothole! and now 1 carbon rim, tbf the carbon rim seems totally rideable still
It’s gonna cost me 130 quid for a new rim, but I dont mind paying it as I appreciate the lighter weight for the wide profile (can you even get 38 mm ally rims?)

If anyone contributing to this thread wants to get rid of their stiff, horrible, unreliable (26″) carbon rims, please do let me know.. 😉

I’d be interested in hearing about people’s actual experience, does the ride quality differ, do they affect the feel/handling of the bike, and does the weight really help acceleration if coming from a heavier wheel set, I know alu wheels can be built just as light at the higher end, but does that lightness sacrifice strength, which is an issue carbon wheels seem to address?

One thing that stands out to me, is that with the onset of affordable carbon wheel imports, we are not seeing many people people with buyers remorse offering to swap thier carbon rims for alloy equivalents, ok the market is still small as carbon equals thousands of pounds in most people’s minds, which simply isn’t the case any more, but good detailed personal accounts from people seem thin on the ground..

I touched on this in my initial post. As others have commented too, there are positives and negatives. They do appear to be more zippy, and direct, but I’m not sure the difference translates to actually going any faster in the real world (all other things being equal).

There was someone on here a while ago with some comedy claims – just by bolting some Enve rims onto his bike he went 10% faster. I’ve certainly not slowed down since re-rimming my wheels with aluminium rims, yet I also fee a lot more in control on the ragged edge, as I mentioned before.

I think if you’re not racing or doing silly things, then they would probably be fine, but from my experience now they would be further down the upgrade list over a good comparable aluminium wheelset.

There was also another point highlighted, on the last page, the person who got a flat, and rolled out on the rim, thus cracking it, this is a very real issue when racing too. Say you popped a tyre on a high speed rocky section, it happens, instant flat – your nice carbon rim is toast by the time you stop. I had a real fear at the last EWS, on the last stage in the bike park there is a link up between No Joke & Freight Train down a rock fire road, we were hitting 70kph down there without even pedalling, did often think what would happen if I flatted – not really what you want to be thinking about when racing.

Hob Nob – Member

There was also another point highlighted, on the last page, the person who got a flat, and rolled out on the rim, thus cracking it, this is a very real issue when racing too. Say you popped a tyre on a high speed rocky section, it happens, instant flat – your nice carbon rim is toast by the time you stop. I had a real fear at the last EWS, on the last stage in the bike park there is a link up between No Joke & Freight Train down a rock fire road, we were hitting 70kph down there without even pedalling, did often think what would happen if I flatted – not really what you want to be thinking about when racing.

That’s what Schwalbe were thinking when they released ProCore, I’d imagine. Expensive carbon.

Which doesn’t show that the spokes weren’t totally detensioned when the impact which caused the flatspot happened – I suspect that when you get damage like that they will be. The rim then springs back and the spokes retension.

The wheel will only deflect within the elastic limit of the spoke. Which are already under quite high tension and therefore this limit is pretty small.

Except that when the wheel is subjected to a vertical load the tension of the spokes decreases (for the only spokes with a significant change in tension), so they’ll always be within their elastic limit (*for ben – as much as they ever are within their elastic limit 😉 )

If you’re talking about a vertical impact (I presume that’s mostly what we’re talking about here rather than a sideways buckle – I wouldn’t know, the only wheel I’ve ever damaged the rim on was due to a fish botherer kicking it), then it will all be rim deformation – as above the spokes aren’t getting plastic deformation because the tension on them is reduced.

Well the physics stuff is backed by centuries of experiments and testing which proves that it accurately models the real world – if you’re not convinced by it then I suggest you avoid bridges and tall buildings. Meanwhile human beings are notoriously poor measuring instruments, so if you want to get good results from them you’ll have to be doing double blind testing. I could tell you how much difference it makes to acceleration but you may not believe me. I’m certainly not suggesting there is no difference though.

Haha, yeh I wasn’t poo pooing the science, I’d just like to hear some more real world tales, as for example, you can make a stiff thing 100% stiffer.. On paper that sounds great but if it’s already as stiff as it needs to be, would you notice? Would it become too harsh to ride?

I mean, I could replace the wooden internal frame inside your sofa with carbon or alloys, scientifically it’s a vast improvement, lighter, stiffer, stronger, but you’d never know.

Ok that’s a terrible analogy but hopefully you see what I mean…

A better analogy would be when I was younger, I used to to a lot of suspension upgrades on cars, stiffer is better right, less body roll, more control… To a point.. Go to far and the ride becomes twitchy, you bounce off bumps rather than roll over them, and when you lose grip, on a high speed corner for example, it’s gone in a millisecond, it snaps away so you don’t feel it or have a chance to control it more.

My apologies for misinterpreting you in that case – I agree totally with your suggestion that making something which is already stiff enough stiffer may not be an advantage (and actually your sofa analogy is fine). Your car suspension thing also reminds me of the way we all used to run really high pressures in our road bike tyres, because higher pressure results in less rolling resistance. I used to put 140psi in my TT tyres FFS, happily ignoring that I was wasting lots of energy being bounced around whenever I was on a less than perfect road surface.