Okay, I may as well throw my hat into the ring on this one. Buckle up; it may
be a long ride.
Back when I was a kid in '67, Ansen Sprint mags and "Firebird" (Verduro) Green
had just hit the market place. I NEEDED those Sprint mags to compliment that
new color I'd sprayed my '57 T-Bird. So I went to Caldwell Racing Treads in
Pasadena, CA. (Still there, BTW), to get a set of the Ansens. When they
started talking about offsets for my car my face glazed over so they explained
what it was to me.
Visualize a wheel. Now turn it sideways so the outside is on one side and the
inside is on the other. Draw an imaginary vertical line down the middle on
the wheel. This plane is the wheel's "centerline", half way between the
outside and inside of the wheel. This is the base of reference for offset.
Now, with your sideways wheel and your centerline in place, picture the
"mounting surface" of the wheel, a parallel plane to the centerline, where the
wheel mounts to the vehicle.
The distance between these two planes is referred to as offset. If the
distance >from< the wheel's centerline >to< its mounting surface goes toward
the wheel's outside it is measured as "positive" offset. This outboard
mounting surface moves the wheel toward the middle of the car. Conversely, if
the distance >from< the centerline >to< the mounting surface goes toward the
wheel's inside then it is measured as "negative" offset, This inboard
mounting surface moves the wheel toward the outside of the car.
The problem with "offset" is that it is difficult to measure in the real
world. When I was having PS Engineering make the kidney port wheels for my
Tiger I discovered the miracle of ""backspace". It's very tangible and easy
to work with. Take your wheel and set it face down. Place a straight edge
across the wheel from lip to lip. Measure perpendicularly down from the
straight edge to the mounting surface. That's your backspace. That's it!
Simple and it gives you all the dimensions, along with wheel diameter, to
orient and measure for wheel fitment.
We could get into tire specs, which would be the missing link to actually
determining if a wheel/tire combination would fit the space we have available
in our wheel arches, but that's another story...
Darrell
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