mauin1x

I got a set of Origin8 TA42 rims. The joint on them was kind of lopsided and I straightened them out as much as I can. Is there a certain way to lace the wheels where the spoke tension brings the joint together better?

Duragrouch

The first set of wheels I ever built, newbie, no instructions, I accidentally had the spokes crossing above the valve hole (rather than "correct" parallel), no problem, plenty of space for the pump head, but that also meant 180 degrees across the rim at the joint, same thing. These welded steel rims, not an issue, but the same thing might work, but I don't know how much force it will pull them together with, you could trig it out assuming a given spoke tension.

All the spokes crossing, you would think would have a tendency to pull everything together, but I think it all cancels out. Except perhaps local to the joint like I noted above, and even that I am questioning in my mind, I'd need to draw it out.

For the joint to come apart, the rim needs to grow in circumference, and all the spokes are fighting that.

FBinNY 

It doesn't matter.

The rim is compressed by the cumulative tension of all the spokes and not by the combined action of any two.

Imagine the compressive force of the water trying to crush a submarine's hull. Now, mentally consider if, instead of pushing from the outside, you were to generate the same effect by pulling fron the inside.

FBinNY 

A nice example of the difference between slide rule engineers, and those trained in the digital era.

No deep analysis needed, think about, or look up, hoop stress and apply comparable logic.

Andrew R Stewart 

Most wheelbuilders (both hand done and machine done) place the valve hole between a pair of spokes that radiate away from each other (to have the greatest access to the valve when inflating). To end up with the valve between two spokes that cross over each other along the way to the hub is very well known as a sign of a newbie or poor lacing awareness. Agreed?

On a 36 spoke wheel this open over the valve spoke pair results in the seam (typically being diametrically opposite the valve hole) placed between two spoke that do cross over each other. This is as the OP seems to want. We all know that 36 spoked wheels are suppose to be strong (whatever strength means in a wheel).

Back in the 1980s 32 spoke wheels began to be the choice of performance riders (and those who wanted to look the part too) due to slightly less weight and aero drag. This spoke count places the rim seam between two spokes that radiate away from each other, exactly like that of the valve hole two spokes. In the last 30ish years we don't see any number of 32 spoked wheels suffering from rim seam opening ups. Sure there's some that had a pinned seam twist and distort the brake track but the forces to do that already potato chipped the rim and the seam's condition is a moot point then.

Francis has it correct. The rim, regardless of spoke pattern, count, rotational placement WRT of rim seams is under significant compression. This is why a pinned seam is a perfectly good way to join the rim extrusion. And if the rim has after pinning brake track machining the seam will nearly disappear as a braking disruption/grab. Andy

FBinNY 

To add---
Even if it isn't machined, any misalignment at the joint will rapidly be worn smooth by the brake shoes, and undetectable soon enough.

The move to welded and machined rims was always more about marketing one-upmanship than function.

Andrew R Stewart 

Here I have a bit of disagreement.I find the machined brake tracks to be far nicer than the ones from the 1970s/early 80s. But I'm rather picky about my brake feel. I stayed away from rims like Weineman A124/concaves because their ground down welded seams were so often undercut, the grabiness at the seam was annoying. Plenty of pinned seams had one rim end a tad narrower than the other or slightly offset to one side. Sure these will smoothen out over the miles, I guess I am not as patient as some are

I do agree that welded seams have less to do with "strength" and more with manufacturing process and costs but happens to have a marketing hook (welding is considered a sign of strength, right or wrong). Andy

Duragrouch

Yeah, what I was talking about was any "lateral" (circumferential) pull by the spokes, figuring that all cancels out. I wasn't sure if crossing spokes at the seam would help at all, probably not, unless an extreme spoke angle at the rim. I did conclude exactly what you said, if the spokes are pulling in radius/diameter, there is also strong force trying to reduce the circumference. Hoop stress definitely applies, but when I think of that term I think of hoops around wooden barrels, sort of working in reverse. And automotive stuff I designed to use press fits for assembly (then welded to hold in place, but the "hoop" takes the loads). I'm definitely a graphical thinker first, then use math to prove; For a critical application, no matter how good I am, I never just trust my instincts, I science it out.

FBinNY 

OK.

Let's do the math quickly. Off the cuff calculation follows.

Hoop stress (or compression) equals the pressure X the diameter.

Since the "pressure" is from spokes rather than a fluid, we start by converting spoke tension to a comparable unit. Assuming 100kgf tension and spokes about 2" apart at the rim, we have a "pressure" of 50kgf/linear inch. With a diameter of 26", the hoop compression equals roughly 1,300kgf.

The spokes pull the rim sideways along it's circumference by a vector of tension X the sine of the angle from radial. Conveniently, we don't need to know the angle because simple geometry tells us the sine equals the hubs radius (about 2cm)
/spoke length or roughly 0.15 depending on the hub. That makes the circumferential force equal to roughly 15kgf.

So, the net effect is that the rim is compressed at 1300kgf plus/minus 60kgf (2 spokes pulling each side at the cross) between alternating spoke holes. I think we can all agree that it's therefore safe to ignore where the crossed pairs are.

Just a bit more math proving something you didnt want or need to know.

Duragrouch

No, I appreciate the above, thanks for that. I had to walk through it slowly, didn't see how total radial force and pressure, equals rim circumferential force, without using Pi somehow, but walking through the stress equations for a tube (simplified, assuming thin wall, which we are), some units cancel, the rest line up, yep, I got it. I've long since been away from industry for decades, plus I've blown a few circuits since then too, takes me quite a while to go through things these days.