DRAFT 36 Hole EBike Motor in a 28 Hole Rim

This is a draft but I'm probably not going to finish it so... here's a half baked ible. Hope it helps

https://endless-sphere.com/forums/viewtopic.php?f=3&t=39652

The Problem: this electric hub motor has 36 spoke holes but this rim has 28 holes.

More specifically 16" rims with an ISO diameter of 305mm and 36 holes seem quite rare.

The solution: 28 60mm spokes in a zero-crossover, radial-starbust pattern. The spoke/hole arrangement is 3 spokes, skip one hole, 4 spokes, 3 spokes, skip one hole, 4 spokes, 3 spokes, skip one hole, 4 spokes, 3 spokes, skip one hole, 4 spokes.

In retrospect 60mm might have been a little bit short and next time I'd try 64mm.

I used these calculators:
https://leonard.io/edd/
http://www.ebikes.ca/tools/spoke-calc.html#lacing-options

And this example: https://endless-sphere.com/forums/viewtopic.php?f=3&t=39652

Step 1: Decisions

Spokes are normally laced in one of several overlapping patterns such that they are able to transmit torque (this is why some bikes have radial, non crossing spokes on the front wheel and non-drive side). So naturally one would be inclined to use a crossover pattern. However, somewhere along the line, someone discovered that when combined with the wide spoke flange diameter of a hub motor, wheels in the 20" range and smaller, crossovers are both difficult and unnecessary: Motor wheels don't need crossing spokes in 20" or smaller rims.

So I decided to go with straight radial spokes. This simplified the build significantly.

Yes I could have done the 3D trigonometry to calculate the spoke lengths. No I didn't want to spend a lot of time on the math to discover that I'd made a little mistake and have to order new custom spokes at over a $ apiece. Fortunately there are spoke calculators. Unfortunately, most of them don't handle hole mismatches. That being said, since I decided to go with a radial spoke pattern, the spoke length difference is within the margin of error of a slight angle difference due to skipped holes. So I chose to use dead reckoning: Radial spokes are all almost the same length even when accounting for slight dishing and skipped spokes so I decided to just ordered one length.

Step 2: Dimensions

With a relatively simple plan laid out I got to work getting the details.

First I looked at what the online spoke calculators wanted, the critical numbers are:

Flange diameter, flange spacing, effective rim diameter, dishing, cross pattern.

Most of these were readily available from my hub vendor.

The rim required manual measurement. Ypedal did the best job of explaining how to measure effective rim diameter so I'll refer you to his guide here: http://www.ypedal.com/wheelbuilding/wheelbuilding.htm

My measurement came within one millimetre of a template 305 rim so I went with the default value of 291mm.

Now, since we have a hole mismatch, the hole offset in the rim will not merge properly with the standard left-right spoke pattern. 16 of my spokes pointed the "wrong" way. This could be an issue but for the moment it seems to still fall within the margin of error.

Step 3: Calculators

I used two calculators and compared the results.

Some things to watch out for are options like Elbows In and Dishing.

Elbows out turned out to be untenable with such short spokes. When actually building the wheel, all elbows in was the only practical option due to physical constraints.

I used the default dishing of 5mm for my particular hub motor.

Both calculators spat out spoke lengths between 59 and 62 so I rounded this to 60mm. Standard practice is to round up but I was afraid of having to grind down long spokes so I went with a low average. This seems to have worked out but slightly longer spokes might have been better.