Need help with odd wheelbuilding problem
 

Hi. I'm building a custom front wheel from a 48 hole hub with a large flange and a 48 hole 20-inch BMX rim. I'm doing a 2 cross pattern. Have it all laced up now but not tight, about one turn of the nipple onto the spoke. The problem is that the spokes protrude through the rim in 4 different lengths (I'll do my best to explain..) They're grouped in 4's as you go around the rim. The first spoke is just the right length, the next goes through the rim a little bit more , the third maybe an eighth more, and the fourth spoke threads are all the way through the rim hole, way too long. Then the pattern repeats, all the way around. The two longest spokes are the ones that go through the hub from the inside out, the two shortest ones go through the outside in. I went through the spoke calculators several times and came up with a spoke length of 156mm which is what I'm using. Can anyone shed light on what may be wrong?

sounds like you laced it wrong. probably started the second side in the wrong hole.

I'm somewhat limited by the rim holes which are staggered left and right, and also angled forward and back to match the spoke angle. But I'll take it apart and redo it. There's nothing odd about the rim and hub, so it has to work somehow.

Most BMX 20" ERD's are in the 392 - 396 range.
Most 48 Hole High Flange Hubs are in the 58-62mm range.
Most 48 Hole High Flange Hub Offsets are in the 31.00 to 34.5mm range.

For two cross your spoke lengths would be in the 172.00mm to 176.00mm range.

Dude...you are probably doing EVERYTHING wrong!!!

=8-)

https://www.mrrabbit.net/wheelsbyfle...plications.php

Grab the spreadsheet a little ways down...

=8-)

Don't unlace it yet. You'll have to later, but first take a moment to analyze the situation as it is.

When spokes come up different heights in the nipples like yours it's because the hub, rim and spokes are out of phase. When it's in pairs Hi-Lo alternating it's because the hub is rotated with respect to the rim and it can be corrected by slackening and re-tightening evenly.

But when they come up in 4 lengths it's usually because the spokes on one flange are out of sequence with respect to the other. Sight or lay a rod across the flanges and you'll see the the two flanges are half a spoke position apart so sequencing around the hub the spokes are right, left, right, left (radial) or right flange, counter clockwise, left flange cc, right flange clockwise, left flange cw. (it could also be the mirror image - R-cw, l-cw, r-cc, l-cc).

When you look across the spoke pattern just outside the flange you'll see that your aren't right which is your problem.

Usually this can be fixed by unlacing and relacing one flange only, but not always.

In your case though there's a second issue. Your spokes are too long. With the right length spokes you'd see half high and half low, since yours are all high and higher you need to figure out how much too long and re-lace with new spokes.

If there's a bit of tension you can average the various excess lengths and shorten by that. Or go back and check your spoke length calculations. My guess is that you used a 3x length and laced 2x, or something similar.

I think I chickened out too soon. I tighened up all the "short" ones first, and the long ones pulled down. Now they're all even, all around. My hub flange is about 100mm, hub center is 30, my ERD was 396.

That's what it sounds like to me too. When I've done that on a typical 32 spoke 3 cross build it comes out with two tight spokes followed by two loose spokes. I've never had 4 distinct lengths show up.

That makes more sense...with a larger hub diameter. Hopefully you are not too short - 156mm versus 159 at 100mm diameter and 30 offset.

=8-)

My measurements are from memory, I've got the exact ones written down somewhere to the tenth. Actually the spokes could be 1 or 2 mm shorter, but they're still ok. I've got the wheel trued now to about 30 thousanths.

The wheel's looking badass now. Thanks for all the replies.

I found a similar thing to be true when I laced a larger hub to a 20" wheel. The outer spokes actually protruded into the nipples less than the inner spokes. When I thought about it, the reason was fairly obvious- a spoke with the head on the inside of the flange has a steeper angle to the rim, and therefore a longer distance. I didn't lace them on the last cross on this wheel, since that would have involved bending the spoke more than I was comfortable doing, and the width of the flange was great enough that the spokes didn't touch.

20" wheels are fun.

After studying it for a while I grabbed the hub and turned it hard towards the leading spokes. The leading (short) ones came up and the trailing ones (long) pulled down. That's when the light when on. The way I started lacing the hub rotated it a bit. Once i tightened the short ones it rotated the hub and pulled on the long ones until they were all even.

In my case, I screwed up a bit. The heads-in spokes went in different directions on the left and right flanges. If I had laced them in the same direction, then the length difference would compensate by a tiny bit of hub twist like yours. Oh well, live and learn. I'll do it right the next time.