My 36 needs spoke tensioning/replacing every 4 months

Unicycles and Equipment
1

Does anyone else have to get their 36er spokes tensioned or replaced every 2-4 months? I have had two KH 36s for about 18 months and have had to tighten or replace spokes every 2-4 months. I use the 36s about 8+ hours per week. Just 10 weeks ago I had one of them rebuilt with a Schlumpf hub (it’s fantastic) but I am already having to retension the spokes.

As far as I know the best quality stainless steel spokes have been used and the wheel builds appear to have been very high quality, typically with steel ties where the spokes cross.

Is there anything I can do to stiffen up the wheel, or keep it stiffer for longer?

Perhaps some strong carbon fibre spokes would help?

2

Did you build the wheels or did a shop do it?

4

A shop had done it every time. udc Australia built 4 of the wheels and I have a local bike shop build one and make the repairs/retensioning.

As I said earlier, I believe the wheels have been well built. Perhaps I am stressing the wheels more with my riding style (I like to muni on the 36) and I might be heavier than the average rider, but don’t tell me to go on a diet.

I would like to try new wheel designs, perhaps carbon spokes.

5

A some point I picked up a Park’s Tensiometer. I had a Schlumpf hub (36") built up by Bronson: it is a wonderful wheel build that is both true and stable. I just checked the tension on the spokes (2 mm round steel) and they appear to be 21-23 on the tension gauge. These numbers correspond to 760 to 950 N if I am reading it right (10X chart value.)

By comparison, I have another wheel that was originally built by UDC and has been trued by myself. I have had several spokes break on this second 36" wheel. When I check the tension (after not messing with it for a long time) the readings range from 18 to 25 on the tensiometer gauge. I have now readjusted them to balance it better to give readings from 20-24, which I think is a reasonable range. It is reasonably true and is probably a better build now.

You could get a Parks tensiometer and see what your wheel looks like. You could ask your LBS to give you readings. I suspect a good wheel build will have relatively uniform tension with nothing too high or too low. It would be very interesting for you to list the readings of all your spokes in order. Large discrepancies in spoke tension would probably lead to spoke failure. You could keep track of the spokes to see which ones break. Both too high and too low tension may lead to failure. I have read that too low tension is often a problem because it leads to more flexing and breakage, but I have not direct evidence of this effect.

Scott

6

Where are the spokes breaking? Spoke tension isn’t the only part of this equation, but what it sounds like to me is that the wheels are NOT being built properly – in particular, the spokes aren’t being “prebent” to conform to the spoke line, and/or not being strain relieved during the build (intentionally and briefly overloaded). If both of these steps are done correctly, the spokes should outlive you. I have a set of spokes on a mountain bike that have outlived seven rims (I just swap the rim as they wear out from rim brakes and retension).

Here is the short version of what happens with metal fatigue. Consider a straight length of stainless wire held between your hands. You can bend it a certain amount and it will return to its original shape (elastic deformation). If you bend it more than that, it will yield and take a new shape (plastic deformation). The region where it deforms elastically if further divided into two regions, in the first one the metallurgy does NOT change as you flex it. If you flex it further, the material work-hardens and will eventually fatigue and break.

Here is the counter-intuitive part – if you want to flex it in the shape where it is undergoing fatigue you can instead bend the spoke (plastic deformation) once into that shape permanently, and then afterwards when you flex it the further deformation is elastic and in the region where no hardening occurs. This is usually referred to as “strain relieving” the material.

During the wheel build, the builder needs to perform a number of steps to deform the spokes in this manner and strain relieve them so that during use they do not undergo further fatigue.

See http://sheldonbrown.com/wheelbuild.html and scroll down to “Seating and Stress-Relieving the Spokes”. I prefer to use a lever to bend the spokes to conform to the hub and the spoke line early in the build process, and then after reaching full tension grabbing the spokes in groups of four and squeezing the heck out of them to relieve them. I then retension and do a final truing.

I’ve seen Bronson’s work and he builds a good wheel. I build all my own wheels (and a good number for friends); any mass-produced wheel that I’m faced with I would first loosen the spokes up, check/correct the spoke lines as necessary, and then retension and strain-relieve. Btw, I typically run 1200N tension on all of my unicycle and tandem wheel builds. Quality stainless spokes have no problem with this high a tension, it is more of a function of whether the hub flanges and rims can handle it.

You should never have to tighten spokes once true. Be sure the wheel builder is treating the spokes with something like the Wheelsmith spoke-prep (or boiled linseed oil). When building a large wheel (eg, 29er, 36er) care must also be taken that the
spokes don’t twist (wind-up) as the wheel is tensioned, or that they are untwisted as a final step. I typically mark the spokes with a marker near the nipple so I can see if they twist. The last step is to turn the nipples in the loosen direction to untwist them.

7

I can vouch for the wheelbuilder municycle.com.au use in Melbourne. I’ve had numerous wheels (7 at last count, from 24" to 36" geared) built up by the same shop over the last 3.5 years, and never had an issue with any of them. The only thing different that I can think of is that I’ve never had any of my spokes tied where they cross.

The wheelbuilder is very experienced, and has built a heap of unicycle wheels, along with countless bike wheels over the years (he builds Cadel Evans’ wheels when he’s in the country).

Unless they’ve changed the shop they use, the wheelbuilds should be A OK.

8

Very informative and interesting, thank you.

9

Yes, Ken, I would agree whole heartedly. The Melbourne wheel builder that udc au use has always delivered good stiff wheels.

I just find that after a few months (well just 10 weeks in the last case) the wheel becomes sloppy. It would appear to me that a 36" unicycle wheel sustains much more stress than a bicycle wheel. Forward, backward and twisting torque. So, it doesn’t seem too strange that a 90kg unicycle rider would stretch the 36 spokes, after a few months muni and idling.

I guess I was looking for some ideas on how we might make the 36" wheel stiffer. I don’t believe there has been any issue with way the wheels have been built.

How about 36 carbon fibre spokes, or a solid carbon wheel?

10

Haha, alas no, i’m not Ken (Looi)…my name’s Dan.

As for making the wheel stiffer, maybe a 36er carbon aerospoke? Haha.

11

you can try tied and soldiered spokes, but it’s pretty controversial as to whether it actually does anything or not. Read up on it first. It’s a lot of effort, but if it fixes your problem. . . . Also expect the shop to give you a cock-eyed “what the hell are you talking about, are you an idiot?” look if you decide to ask them to do it. It used to be more popular, and my the bike shop my brother used to work for did it upon request. “Young folks” (like me) usually haven’t ever seen it.

12

In case you don’t know. Carbon is very nasty stuff if it breaks. And it breaks very easily.

13

Regarding fixing-and-tying spokes: It does nothing to help make the wheel stiffer. The proof is to look at your existing spokes where they cross and see if there are signs of them rubbing lengthwise. If they are not “moving”, then fastening them isn’t going to change anything!

Regarding carbon spokes: The only thing that will make a wheel stiffer is a different tension. The stainless steel spokes are plenty strong for this; they are capable of much higher tensions than the rim or hub are, so as long as you are reaching that tension, the stainless steel spokes are just fine. Again, this has nothing to do with your breaking-spoke problem. Potato-chiping a wheel is a sign of too low a tension. Carbon spokes will also be more prone to impact damage as a result of UPDs on rough terrain.

You’re not “stretching” spokes (that is, not to the yield point). 90kg rider? Heck, each of those spokes has over 100kg of “force” on them. The total forces from all the spokes are measured in tons on a properly-tensioned wheel. Nothing a 90kg rider could do would change this, and if you DID twist that wheel enough to overcome these forces, you’d get a potato-chip shaped wheel, not a broken spoke. Take your spoke tension gauge and have a friend measure the change in tensions as you sit on the wheel holding onto something; I’ll bet you can barely measure it. Tension-spoked wheels are one of the strongest structures there are for their weight.

Ken

14

Tied and soldered spokes were never done to make a wheel stiffer. Well, I should say that it wasn’t the original purpose, but some people got it in their heads that it would make the wheel stiffer. Anyway, it was started with large wheels on “penny farthing” bikes because when a spoke would break it could become very dangerous to have a sharp rod of metal sticking out. It was continued with smaller track wheels to facilitate quick rim changes. It is a pretty way to add a little weight to a wheel, and it would still work for the original purposes, but not to add any stiffness to the wheel. Jobst Brandt did some tests a while back that pretty much put the argument to rest.

To be honest, I’m not sure I always agree with Jobst’s conclusions, but he is far smarter than I am so I’m willing to concede.

15

I made a thread a while ago about my spokes breaking too much.

Ken seems to know more about the fine details of spokes than I explored but the main thing I learned was that spoke washers are often overlooked and can help prevent this annoyance, depending on where the spokes are breaking. I’ve only broken one spoke in the nearly three years since I posted the stainless steel M3 washers fix.

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ahem like I said, it’s controversial. The weight is minimal. I looked them up again before I posted, and also found the test you were referring to. . . note that that test was NOT on a 36er, and a 36er may in fact be closer to a penny farthing than not. Just thought I’d throw the idea out there, didn’t think it’d provoke a rebuke.

17

Hey Richard my 36er has never needed touching since new and still runs perfectly true and it is mostly used for muni.Something fishy going on with your wheel.

john

18

Spokes are “designed” for a certain thickness hub flange (the distance between the mushroom head and the 90 degree bend near the head). It seems that spoke manufacturers make this distance too long for the typical hub; I’d guess because if they made it too short, the spokes wouldn’t fit at all. The purpose of the washers is to “take up” that extra distance and hold the 90 degree bend tight against the hub flange. If it isn’t held there, then it flexes during riding and will eventually fatigue and fail at the bend. DT makes some very nice brass washers for this purpose – Bike Tools Etc. - 1000's of bicycle tools and parts for the home mechanic! . After tensioning the wheel, but before final truing, they should be set in place with a gentle tap from a punch with a concave head, eg, Bike Tools Etc. - 1000's of bicycle tools and parts for the home mechanic! .

Ken