This tool design has been around for a while but I thought I'd pass along what I did for those folks who may not be familiar with it.

The outer "wings" represent the intended tire radius at the top. I can swap them out with others for different radius sizes. I impression stamp the size on them.

The 3/8-16 threaded rods are cut to represent the intended section-width of wheel/tire combos. Additionally, I cut lengths that can be adjusted in / out until interference occurs to verify the "real-world" cavity I have to work with.

On the work-bench, I pre-adjust various back-spacing's. After it's set up, I install and spin while observing the clearance results.

I'll be running Tru-Spoke Steelies. They have a standard back-spacing but for an extra 15 bones, they'll back-space to suit. I found I can run their standard spacing in front but will have to specify the dimension on the rear.

I hope this is useful to someone

Dawg-A-Saurus.