Fat Bike Wheel Build- Help me avoid mistakes

[mtb_frk]

Since the other recent wheel build thread had so many great tips, I figured I would post my most recent planned build. I built the front wheel already, but was a little surprised about ending up with the same length spokes. First attempt I ended up with spokes that were too short. I failed to take into account that the spoke holes were not on the center line of the rim, but 19mm apart. Seems like the wheel came out ok, but now I want to move on to the rear.

Rim specs are Sun Single Mulefut SL 80mm 26". I measured the ERD at 552mm as labeled using two spokes, nipples and washers. As mentioned there is a 19mm offset between the spoke holes.
The hub is bitex 197mmm, which i measured the following:
Left and Right flange to locknut= 52mm
Left/Right PCD= 58mm
Putting this into spokecalc, it works out 270.5 for both sides. Does this make sense?

Hub info here, which matches my measurements.
https://www.bikehubstore.com/product-p/fb190.htm

[Mark McM]

You didn't mention spoke lacing pattern, but based on your calculated value, I'm assuming 3x on each flange, and spokes on each flange crossing to the opposite side of the rim.

I put your numbers into Spocalc, and also did my own calculation from the standard spoke length formulas (from "The Bicycle Wheel" by Jobst Brandt), but when entering the numbers, I found what might be descrepancy in the definition of the spoke hole offset/width measurement. Spocalc asks for "width between spoke holes", but the the Sun rims spec. of 19mm appears to be the spoke hole offset from center, in which case the width between spoke holes would be 38mm.

Comparing the Spocalc numbers with the standard Brandt formulas, I get:

Spocalc lengths:
0 width between spoke holes: 269.1mm
19mm width between spoke holes: 270.9mm
38mm width between spoke holes: 273.0mm

Brandt spoke length formulas
Spoke holes offset 0 from center: 269.1mm
Spoke holes offset 9.5mm from center: 270.9mm
Spoke holes offset 19mm from center: 273.0mm

The numbers above are based on an ERD of 552mm, spoke flange diameters of 58mm, flange offsets (from center) of 46.5mm, and flange spoke hole diameter of 2.4mm, with a 32 spokes in a 3x pattern, and spokes crossing from left to right between flange and rim.

So I would double check whether the spokes holes are actually at a width of 19mm apart at the rim, or whether they are at an offset 19mm from the center of the rim.

[mtb_frk]

Originally Posted by Mark McM

You didn't mention spoke lacing pattern, but based on your calculated value, I'm assuming 3x on each flange, and spokes on each flange crossing to the opposite side of the rim.

I put your numbers into Spocalc, and also did my own calculation from the standard spoke length formulas (from "The Bicycle Wheel" by Jobst Brandt), but when entering the numbers, I found what might be descrepancy in the definition of the spoke hole offset/width measurement. Spocalc asks for "width between spoke holes", but the the Sun rims spec. of 19mm appears to be the spoke hole offset from center, in which case the width between spoke holes would be 38mm.

Comparing the Spocalc numbers with the standard Brandt formulas, I get:

Spocalc lengths:
0 width between spoke holes: 269.1mm
19mm width between spoke holes: 270.9mm
38mm width between spoke holes: 273.0mm

Brandt spoke length formulas
Spoke holes offset 0 from center: 269.1mm
Spoke holes offset 9.5mm from center: 270.9mm
Spoke holes offset 19mm from center: 273.0mm

The numbers above are based on an ERD of 552mm, spoke flange diameters of 58mm, flange offsets (from center) of 46.5mm, and flange spoke hole diameter of 2.4mm, with a 32 spokes in a 3x pattern, and spokes crossing from left to right between flange and rim.

So I would double check whether the spokes holes are actually at a width of 19mm apart at the rim, or whether they are at an offset 19mm from the center of the rim.

Yes you are correct the spoke holes at 38mm apart. I forgot that bit of detail, as it’s been a few months since I ordered the spokes for the front. Also, the spokes will remain on the same side of the hub/rim, so they will not cross the center line.

Thanks for the help! I’ll re-enter the numbers into spoke calc with the 38mm vs 19mm. Doing this it works out to be 266mm.

Is it normal for a disc hub to have the same length spokes on both sides?

[tellyho]

Roger Musson's book has great guidance on offset rims and weirdness that goes with fatbikes. I built a crazy rear wheel for my Pugsley last winter correctly, using his methods (offset hub with offset rim - had to 3D print a spacer for the initial lacing and tensioning).

[mtb_frk]

Ran the front wheel through the park tension app.

https://www.parktool.com/en-us/wta/ws65d664a16314f

Looks pretty even, but was wondering if the relative tension from left to right side looked ok, being that is a disc wheel. Spokes were the same length.

Same mulefut rim, with the 150mm pub (bitex) hub.

[Mark McM]

How round a true was the rim before building? This will affect the tension variations. Or rather, if the rim isn't perfectly straight a true before building, there will be a compromise between getting the wheel straight and getting the tensions even.

Assuming the rim was straight and true to start with, the tension variations you have will probably be okay for a reliable wheel, but with a bit more work you could probably get them more even. If the rim wasn't straight and true, then the tension variations could merely be a result of irregularities in the rim.

As far as the right/left tension balance - as I recall, the hub had equal left & right flange offsets, so the wheel should have no dish at all, and the right & left spokes should have the same average tensions. In your numbers the right & left tensions are only about 0.5% different, so they are basically the same, as expected in a non-dished wheel.

[mtb_frk]

Brand new rim so hopefully it’s round. It has a bit of a stubborn hop at the rim seam, but it worked out eventually.

Thanks again for the help! I’ll be moving on to the rear wheel soon.

[JAGI410]

Most of those Mulefuts have a little seam hop. You'll never notice it with a 4-5" tire and 5-10psi though, so don't go nuts fighting it!

[oldpotatoe]

Originally Posted by mtb_frk

Brand new rim so hopefully it’s round. It has a bit of a stubborn hop at the rim seam, but it worked out eventually.

Thanks again for the help! I’ll be moving on to the rear wheel soon.

Even tension far more important than perfect roundness on a truing stand, particularly with a disc brake rim.