You give me way too much credit here man. Why is 4.5 back space on a centered 8" rim? Why not 4"? And, why would you add that to the 1.5 inch front space. All I ever did to calculate back space was lay a ruler across the back of the wheel, and and measure down to the mounting surface.

I need a little more basic theory here. I could find it on the internet, but then you wouldn't get to be my teacher. Thanks.
Sam

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First backspacing (the easy one)........

An 8" rim is 8" inside bead to inside bead.

Backspacing is measured from the wheel mount point to the "outside" of the rim........

So you need to convert from inside measurements to outside measurements.......

But luckily the difference is always around 1" (+- a fraction here or there)

So now your 8" rim measures 9" outside to outside. So with a perfectly centered wheel mount point you're going to be half of the outside to outside measurement or 4.5"

Offset is the difference between the center line of the wheel and the wheel mount point. Positive offset means the wheel mount point has moved to the outside of the wheel from the center (more backspacing compared to a 0 offset wheel) and negative offset means the wheel mount point has moved to the inside of the wheel (less backspacing compared to a 0 offset wheel)

So a 38mm positive offset (or 1.5" ) moves the wheel mount point out (adds to backspace) by 1.5". If the center line wheel would have 4.5" backspacing, this wheel has 1.5 + 4.5 or 6".

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I get it. Thanks.
Now I did go on line and read about the effect of back space on steering and handling, and it seems the correct offset is most important on the front, as this effects the scrub radius, or where the center line of the tire is in relationship to the intersection of the ground by a line drawn through the upper and lower ball joints. This, of course changes as the overall diameter of the wheel tire combo changes. It seems that the steering works most naturally, that is toe, caster and camber have their designed offset when the center line of the tire is on the intersection of this line through the two ball joints. And, even how much air is in the tires will effect it.

It is hard to believe that this Dart was originally designed to have 13" wheels. This car is so far off design specs by now, it seems as if I would have to start over by measuring everything and determining a custom offset for this particular combo of parts. I know when I took the cop car wheels off, which were 15x7 with 4.25 offset, and went to 15x7 Dakota wheels with a 3.75 backspace, the handling degraded. This changed from a positive offset,(almost neutral) to a negative offset, if I understand it right. Correct me if I am wrong.

Offset is not so important on the rear. But, it does effect how the car tracks through corners. It seems racers will have a different offset on each side for particular tracks, depending on the configuration of the curves and straightaways. I am going to worry about theh front first. Any idea on how to figure the centerline of the steering knuckle on the ground? it seems a pair of jack stands and a couple of straight edges might me a start.
Sam

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