Backspace Guage
Here is a view showing the device I constructed to aid in measuring
backspace clearance for my wheels and tires. I drilled a piece of wood (15.5"x7")
to fit the lug pattern making one side 7.5" to center and the other end 8"
to center to provide the inner edge of 15 and 16" rims, respectively. In this
case the bolt pattern is 5 on 4.75" to work on a GM pattern. The large center
hole is to clear the protruding hub. Size is not critical only that it is large enough
to allow the wood to rest flush on the hub as the wheel would.
I then drilled a piece of 1/4" board (or whatever) 4"x12"
marked in 1/2" increments to screw to the first vertical board. This horizontal
piece is then adjusted inward in increments, spun full circle, and tested lock to
lock to see if it hits anywhere. You can add another vertical piece to this to add
the 5" or so additional tire radius to the test if you wish or just use a tape
measure.