Folding Bikes - Anyone broke a front spoke on on a 20" Wheel?

Hi all,

I have just radially spoked my dahon's fron't wheel and so far so good. 80km of bike path/city commuting so far and they have remainde true. Have also dropped ever second spoke for extra weight savings.

The rim is not set up for radial spoking every second spoke because of offset holes but thought I would give it a go anyway.

Fearing the worse, I was wondering if anyone has broken a spoke on the front wheel before? Expecially low spoke count wheels like the Rolf Prima® Accel™ that come with some high end dahons. Should I expect more than a bit of brake rub? Catastrophic failure of the remaining spokes?

Cheers
Derek

Breaking front spokes is relatively rare. But more importantly, is the hub rated for radial lacing?

Mmmm, don't think so but how do you tell?

Mmmm, don't think so but how do you tell?

If the original spoke lacing on the wheel would be radial, then you have a hub ready for radial lacing.

No it was a standard 3x lacing. Sipped every second spoke to save weight, it is a 28 hole rim, now I have 14 spokes, 7 each side. Worked out to around 100g less.

I don't have personal experience, but I have read someone's report about not relacing hubs in a different way than what they were before, and I have asked about radial lacing in the past and it seems that manufacturers do not guarantee their hubs if they were laced radially, unless specifically approved for it. So I didn't build a radially laced wheel until I got hold of an approved hub, and I trust it going 50mph down a hill.

So it seems you did 2 things that are deemed as not the best practice, but what the heck. Just don't go touring inner Mongolia with it, or race too fast downhill. It shouldn't fail suddenly, you would notice a gradual degradation if it did fail. The thing to look out for, is the hub flanges - they are stressed the most, not the spokes. The spokes are absolutely fine.

Actually it might fail suddenly. Since you have a rim that is meant for 28 spokes laced only with 14 spokes, you essentially have a weakened rim (or a rim with failure initiators) at the "empty" spoke holes.

Usually, the lower the spoke count for a wheel is, the higher is the tension on each spoke. If you use radial lacing on a hub that's not made for that, you can get hub failures, like:

http://pardo.net/pardo/bike/pic/fail/FAIL-031.html
http://pardo.net/pardo/bike/pic/fail/FAIL-032.html

A rim failure would probably be worse. Search in the Bicycling Mechanic section for rim failure...

Actually it might fail suddenly. Since you have a rim that is meant for 28 spokes laced only with 14 spokes, you essentially have a weakened rim (or a rim with failure initiators) at the "empty" spoke holes.

Usually, the lower the spoke count for a wheel is, the higher is the tension on each spoke. If you use radial lacing on a hub that's not made for that, you can get hub failures, like:

http://pardo.net/pardo/bike/pic/fail/FAIL-031.html
http://pardo.net/pardo/bike/pic/fail/FAIL-032.html

A rim failure would probably be worse. Search in the Bicycling Mechanic section for rim failure...
I have a few problems with what you say:

How does fewer spokes translate to higher tension? When you measure spoke tension, the only thing that is taken into account is lenth.

How is a rim with empty spoke holes weaker than one without? Since a wheel is a pre-stressed structure, the rim is pulled in on itself; holes do not cause weakening under those conditions, do they?

I would of course agree that fewer spokes = weaker wheel overall since fewer spokes are bearing the load when actually riding it.

Gees those photos are impressive, would have never expected the hub flange to fail like that. Less surprised that the rear wheel failed than the front. Looks like the wheel stayed intact though which is my main concern.

I haven't added any holes so I don't understand how that would weaken the rim? Unless its a questions of the rim now being unsupported at the hole.

I might back off one spoke completely to simulate a break and see what happens. Might even take it for a ride, hopefully nothing more than a buckled wheel and a bit of brake slap.


Mongolia is definetly out of the question even with a stock Dahon, 28 hole hubs and rims are hard enough to find here yet alone a narrow front hub to fit the forks.

I don't know why but I smell a troll in the original post. Nevertheless the followups are yet another examof how the U.S. is falling behind the rest of the world in the well rounded education of its citizens and will one day be an extant superpower. It should not be a point open to debate as to whether the life of a wheel using half of the designed for spokes and laced in a pattern that all agree is hard on components is destined to be short. The only thing keeping the original poster from experiencing any of a number of possible failure scenarios is that everything we buy in America has been conservatively rated in its nomenclature so that when you do things like: overfill a tire or underinflate it for that matter you are not immediately killed or maimed because of your failure to observe instructions. If we accept that at any given instant the weight of the bicycle with rider is 'hanging' from the top of the rim by a number of spokes it shouldn't be hard to conceptualize how removing spokes will leave fewer to share the weight and since the weight of bicycle and rider is unchanged then two spokes are each going to have to support twice as much weight as four would. Each spokes contribution is effectively doubled. I don't know what tires are on the wheel but anyone that concerned about weight should be at least as concerned about the brand of tires and their weight.

H

If we accept that at any given instant the weight of the bicycle with rider is 'hanging' from the top of the rim by a number of spokes it shouldn't be hard to conceptualize how removing spokes will leave fewer to share the weight and since the weight of bicycle and rider is unchanged then two spokes are each going to have to support twice as much weight as four would. Each spokes contribution is effectively doubled. I don't know what tires are on the wheel but anyone that concerned about weight should be at least as concerned about the brand of tires and their weight.

HIf along the general lines of your post you educated yourself and read up on wheel theory, you would know that the first sentence quoted above is incorrect, and therefore the subsequent conclusions are invalid.

Well, 200 km later and no dramas. Here are a couple of photos for those are interested.

My wifes flat bar road bike has a radially spoked front with 20 spokes, I measured the the distance between spokes holes on the rim and it is actually larger than the resulting distance on my half spoked wheel, thats what gave me the idea in the first place.

Cheers
Derek

That doesn't look like many spokes to me! I've considered halving them myself before - maybe on a 36 spoke rim but 16 somehow doesn't seem enough. I'm sure it's fine but I wouldn't be doing any bunny-hops off any kerbs anytime soon.

your followup in particular is a good example of the ignorance that seems to concern you so much


I don't know why but I smell a troll in the original post. Nevertheless the followups are yet another examof how the U.S. is falling behind the rest of the world in the well rounded education of its citizens and will one day be an extant superpower. It should not be a point open to debate as to whether the life of a wheel using half of the designed for spokes and laced in a pattern that all agree is hard on components is destined to be short. The only thing keeping the original poster from experiencing any of a number of possible failure scenarios is that everything we buy in America has been conservatively rated in its nomenclature so that when you do things like: overfill a tire or underinflate it for that matter you are not immediately killed or maimed because of your failure to observe instructions. If we accept that at any given instant the weight of the bicycle with rider is 'hanging' from the top of the rim by a number of spokes it shouldn't be hard to conceptualize how removing spokes will leave fewer to share the weight and since the weight of bicycle and rider is unchanged then two spokes are each going to have to support twice as much weight as four would. Each spokes contribution is effectively doubled. I don't know what tires are on the wheel but anyone that concerned about weight should be at least as concerned about the brand of tires and their weight.

H

Semantics. I am 49 years old. In the 70's when I was learning about bicycles I was taught that the weight hangs. Fine, that that is now discounted, that was not the point of my post but thanks for bringing it to my attention I shall be Googling this. But if removing half the spokes of a wheel does not affect its reliability, why stop there? Why not four spokes 90 degrees apart? Are you also going to tell me that rotating mass at the circumfrence of the wheel (i.e. the tire) which has always been considered of more importance than weight of hub or spokes is now discounted as well?

H

Sorry, I really don't want to pick a fight, that is futile, but there are quite a few errors, myths and exaggerations in your post... I pointed out only one, only because I don't like to see myth propagated. If you read my previous post, I didn't say the wheel strength is unaffected by removing spokes, quite the opposite.

Sorry for the late reply. I was away for a few days...

How does fewer spokes translate to higher tension? When you measure spoke tension, the only thing that is taken into account is lenth.
Spokes are essentially very stiff pretensioned springs with the Young's modulus (E) as the spring rate, until they deform plastically. Using only half of the (same) spokes means the (hopefully) elastic deformation/elongation of each individual spoke during each revolution of the wheel is significantly larger than if you use the "regular" number of spokes. Since you don't want any spoke to be completely without tension, you need a higher tension on each spoke.
The pros confirm the higher spoke tension:
"Due to the low spoke count, spokes are adjusted to more than double the tension of spokes in traditional wheels."
http://www.dahon.com/news/dahonnews/11212002.htm
"And as with virtually any low spoke count/high tension design,..."
http://www.cyclingnews.com/tech.php?id=tech/2006/reviews/shimanoduraace
"In any case, I haven't rounded a nipple even when building low spoke count wheels to the tension limit."
http://yarchive.net/bike/spoke_tightening.html




How is a rim with empty spoke holes weaker than one without? Since a wheel is a pre-stressed structure, the rim is pulled in on itself; holes do not cause weakening under those conditions, do they?
Any structure with stress concentration points is weaker than the same structure without those stress concentrations. A stress concentration point/area can be a hole, clamping point/area, sharp angle, crack tip ...

http://en.wikipedia.org/wiki/Stress_concentration

If every spoke hole of the rim is used, it is still a stress concentration point, but the total stress is spread out over a larger number of spokes and therefore a larger area.

Thanks for those links Rincewind, facinating reading. Will get out my spoke wrench and give all my wheels a bit of a tighten in light of some of the discussions.

Cheers
Derek
(http://www.bikeforums.net/member.php?u=10191)

Yeah thanks, that was very clear... I haven't seen anything on spoke tension with lower count spokes so far... or perhaps more likely, I have but just forgotten about it since I havent built any wheels with low count.

now the question is how much faster have you gone saving all that weight? has it really made any difference but weakening the wheel?

I can't be sure as I also swapped over to a set of 120 psi stelvios when I respoked the wheel. How ever a while back I swapped over from brass to aluminium nipples (keeping the same tubes and tyres) and I could tell the difference when accelerating although it was not huge. I guess the 100 grams saved would also make a difference although it probably only translated to 50-60g at the rim as it counts less as it gets to the hub from what I have read.

I just like the idea I am not carrying around the extra spokes and nipples and the look of a radially spoked wheel. If I had the money and the inclination I would probably have just got a Dahon with a set of the rholfs, but I like to tinker :-)