Correct, sort of...the words are right, the interpretation is off.
Lets say the offset is 40 mm...so, is that 5.0" of BS?
Offset is in mm, and BS is in inches...plus, they do not REALLY measure the same thing, so I need additional info to make sure the damn thing will fit, etc.
They are both ways of measuring the relationship of the rim to the hub....but from different sides.
If you lay a flat bar, a yardstick, whatever, across the rim (The rim's laying on its side..., the side that faces the brakes is facing up...), and then measure from the inside of the rim, up to that flat thing across the rim, THAT'S the BS.
Like in this pic:
This is the BS changing, with the rim width staying the same....notice the hub getting closer to the centerline as the BS increases?
This shows the offset relationship to the centerline of the wheel, positive or negative offset.
This chart gives an approximation (Not exact...) of the relationship between BS and offset.
Notice that as the rim gets wider, the offset and BS do NOT stay in lockstep...as the extra rim width changes the relationship...the wider rim can keep the same BS, and just have less offset for example.
Measuring Wheel Offset
To calculate offset you'll need the following measurements:
Wheel backspace
Wheel Width
Wheel Center line (outboard flange to inboard flange measurement / 2)
Subtract:
Wheel center line from Wheel backspace to get offset. If backspace is less than the wheel centerline the offset is negative
If backspace is greater than the wheel centerline the offset is positive
Tip:
To convert from inches to mm multiply by 25.4
To convert from mm to inches divide by 25.4