I don't have any experience with pop-ups, but when I ordered a spring-type 3500 lb axle from Dexter, I gave them my frame width (center of frame rail to center of frame rail), and what hub face to hub face width I desired. They responded later that my hub to hub needed to be wider, to accomodate the brakes and ezy-lube axle fittings I wanted. Luckily, the measurements they gave ended up being the max width my tires would sill fit under my fenders - this was a larger axle than my original ( I didn't want to have to re-mount the welded-to-frame fenders).

- red square shows hubface and frame width measurements
- dexter axle.jpg (27.28 KiB) Viewed 624 times