[tim80z]

Backspace is the distance from the wheel mounting surface to the inside of the inside bead. So for a 7" rim 0 offset would be 3.5" backspace. 3.75" backspace means on a 7" rim is equivalent to -0.25" (-6.35mm) offset. More backspace is equivalent to a more negative offset.

- Tim.

[jono165]

tim80z wrote:

Backspace is the distance from the wheel mounting surface to the inside of the inside bead. So for a 7" rim 0 offset would be 3.5" backspace. 3.75" backspace means on a 7" rim is equivalent to -0.25" (-6.35mm) offset. More backspace is equivalent to a more negative offset.

- Tim.

Tim, that's working the wrong way, the smaller the backspace number the greater the negative offset (ie the the mounting face of the rim is a smaller distance to the back side of the rim.



3.75" backspace on a 7" rim is -6. But 0 offset is 4" backspace.

Its the measurement from the mounting face to the furthest point from the mounting face on the back side of the rim. The bead lip is usually around 1/2" so weld on beadlocked 7" rim is effectively a 7.5" wide rim. Its a bit of a crap way to measure a rim because there's variation between wheels, an alloy usually has a wider lip. But offset is always the distance from the centreline so the thickness of the bead lip is irrelevant.