Good questions, Twente Muni. I didn’t design the rim, I just bought it. Maybe someone else will know more about how they came to be that way. But a few thoughts: I’m approaching it as an art project more than as a structures problem. I liked the look. And I figured it might not be all that bad if, as you note, we’re seeing it more often. I was curious myself about how it would work with that much material missing.
I wouldn’t assume a priori that stiffness is the main goal. For a piece that gets tested most by large sudden impacts–like a trials wheel–compliance can help limit the maximum forces experienced. (The guy from Webcyclery mentioned that some trials riders prefer single-wall rims over box-section because they seem to have more give on hard landings.)
Also consider that the load from the tire is applied through the beads. Having the spokes in a line in the middle of the rim puts them as far away from there as they could be, with the rim loaded like a beam supported at its ends. I haven’t had many rims fail, but the last couple have been box-section road bike rims that developed fatigue cracks near the spoke holes.
Anyway all I can tell you in real-world terms is that it seems fine so far and hasn’t collapsed under it’s own weight yet.
Pretty much standard on Fat bikes (take a look at Sidd’s Darryl), and there’s loads of those out there. I would think that on a wider rim, spreading the spoke holes out like that may make for a more stable build, but really have no idea :). I’m not an engineer, and hope to never be one.
Spokes pulling in the middle of a wide drilled rim will pull the “bridges” in deforming the rim. Spokes near the sidewall are pulling against a much stronger structure and the lateral stiffness of the wheel is taken care of by the rims width.
That is a nice start. 32/48 builds aren’t as tough as many people think. What tire are you planning on using?
Yeah, that’s probably the key consideration right there. In any case, the rim doesn’t seem to need help from the spokes to keep from bending to the side. It’s plenty stiff in that direction by itself, as wide a section as it is.
Thanks! That’s something I was really curious about, specifically how to know what spoke length to use. Reading through old threads, some reported using a single spoke length but others said that there were big differences between long and short lengths. I stared at it for a long time and made charts and graphs, and by my reckoning there are eight (I think) distinct ways to arrange the spoke directions and the locations of the skipped holes. Two of them are good, spoke length difference less than two millimeters, and one of those uses longer spokes than the other, depending on whether they cross the long way or the short way. The other six are not good at all, close to 5 mm difference in spoke lengths for my rim and hub. I think there’s a chance that some got it right and others did not, and one could pretty easily dive into it and not know why it wasn’t working out.
Several that I might use but a Hans Dampf SG is going on first. (Won’t fit in that 24" KH frame though. Actually surprisingly close but no go.)
It does; the bracing angle is what gives the wheel side-to-side stiffness. The wide rim might be very rigid in that direction, but that just means that more of the rim will deflect under a load, instead of a smaller section.
The obvious answer is to run the spokes to the far side of the rim instead of the near side; I don’t know why more rim manufacturers don’t do it. Rims like that do exist, for example alienation’s insurgent rim.
That profile comparison is a nice resource, and thanks for the recommendation. Are you saying that a 62-559 Big Betty would be shorter than my 60-559 Hans Dampf? I spent some time reading the muni tire review thread and everyone seemed to love the Hans Dampf SG–as of late last year anyway. But most of those reviews come from riders here in the US and Big Betty tires might be less widely available here.
Twente Muni commented not on the spoke angle but on the hub, spokes, and rim forming a quadrilateral rather than a triangle as they would be without the offset, if the spokes joined the rim in a line. There’s still enough spoke angle with the offset that when the whole rim or a substantial part of deflects, it has to warp in a cone shape with one bead stretching and the other shrinking. Which has to happen to some small degree, but it won’t be happening much. Spokes are a lot springier than that. That’s what we like about spokes.
Having the wheel right here beside me, I can tell you that it isn’t noticeably less stiff than other wheels I have. And since the tire on this wheel will be inflated below 20 psi, compliance of the tread w/respect to the hub won’t have much to do with rim deflection. This seems like a problem that isn’t really a problem.
It’s still a quadrilateral unless you could tie or solder the spokes somehow where they crossed, but it doesn’t seem like they would actually pass that near to each other. Maybe the extra spoke angle would help with a flatland bmx tire at 100 psi, or it might just look cool. (Not that there’s anything wrong with that!) I doubt I’d notice it with a fat low-pressure muni tire.
They’re two different points actually, but as above I don’t think either of them is a real problem. I like my new wheel. If you guys don’t, you don’t have to ride on it.
Looking again at what I posted yesterday, it comes across as more preachy and defensive than I’d have liked. Sorry about that Twente Muni and MrImpossible. I blame the heat.
MrImpossible, thinking about that Alienation bmx rim again, I’ll be very surprised if we don’t see some fatbike wheels like that before long. Once the idea is out there…
It doesn’t have anything to do with the shape being a quadrilateral vs. triangle; spokes are in tension and work like springs. It doesn’t work like a rigid structure.
If the spoke tension pulls in a sideways direction, there will be a restoring force that fights any deflection. Imagine the wheel gets pushed to the right -
the spokes pulling to the left will be stretched, and will pull harder to the left, fighting the deflection
and the spokes pulling to the right will lose tension, and so pull less hard to the right, which is the same as pulling harder to the left (which can be confusing).
The sharper the spoke angle, the more the spoke tension has a sideways component and helps keep the wheel rigid in a sideways direction. The more vertical the spoke, the less sideways component to the tension, and the more the spoke only works to keep the wheel rigid in the vertical direction.
Yup, no argument with that, MrImpossible. Divide by the sine of the angle to find the change in spoke tension needed to offset the lateral force.
With the Qu-Ax hub and the Rockman rim’s 9 mm offset, the spokes narrow 24.5 mm from hub to rim, around 6 degrees from vertical. The small angle approx seems good enough for arm-waving here, so 18 mm more by offsetting the spoke holes the other way wouldn’t double the apparent stiffness but ought to help with no obvious cost. (Change in spoke length is minimal, a mm or so.)
I haven’t tested Hans Dampf so I can’t answer.
An other that I own,tested and liked is the continental rubber queen a little less large than Bigg Betty but very nice to ride.
Thanks for the recommendation, Philippe. I think the problem with 26" muni is that there are too many good tires for it! It almost makes me wish that they wore out faster.
I asked about the size of that Big Betty because unless there’s a huge reason not to–like another 26" tire that’s just as good but fits inside the KH24 frame–then I should use the Hans Dampf I’ve already got. I spent more on it than I paid for an entire unicycle when I bought my first one last year, so I really ought to at least wear the shine off it first.
Maybe someone else will know more about how they came to be that way. But a few thoughts: I’m approaching it as an art project more than as a structures problem. I liked the look. And I figured it might not be all that bad if, as you note, we’re seeing it more often. I was curious myself about how it would work with that much material missing.
