Hi Ben
I put a straight edge (broom handle) across the bottom of the chassis rails.
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The Kirkham has the bottom of the 4' side pipe only 10mm higher than the bottom of the round chassis rails.
With my new ride height 4.5" at the front and 5.5" at the rear, the bottom of the side pipe is 140mm off the ground.
Hope that helps a little
Cheers
Kent