Wheel offset

[Tomsecks]

Ok, question about wheel offset. I want to finally figure out what size rims I need for my car. BUT have an issue with calculating the backspacing. Backspacing is measured by laying a straight edge acrost the lip of the rim, and measuring to the hub mating surface. BUT the rim width is measured from inside the lip. To center my tire properly, won't I need to know the width of the rim lip? Would this not be needed if a wheel was listed with offset and I wanted to figure out the backspace too?

[PRRC]

Back spacing and off set are two differant measurments but achive the same thing. Offset is a measurment from the center of the rim.So if you had a 8" rim and the offset was listed as 0mm it would be the same as 4.00 back spacing.Back spacing is a measurment from the mounting pad to the outer rim lip.What kind of car are you trying to figure rim sizes?

[TonyL]

always check here

It should answer any questions you have.

[Tomsecks]

Wheels will be for a 70' Dart Swinger. Car will be mini-tubbed in the rear. Going to go 285/40/17 in the rear on a 9.5" rim. Up front probably 245/45/17 on a 7" rim. Most likely going with a set from Centerline, possible Daggers.

My only grip is that backspace includes the lip thickness. I measured an aluminum wheel and it was like 9/16". So when figuring this out on paper I need to know that thickness to have the proper backspace. Do I not? I've yet to find manufactures that give that info. Thus far I've yet to reach the proper person at Centerline who can give me an answer.

[astroracer]

The backspacing is always measured over the inboard flange. The backspace measurement does NOT take into consideration the width of the flange...
What the flange width DOES affect is "offset". An 8" wide rim is measured on the "inside" of the flanges, where the tire bead seats. The overall width is measured across the outside of the flanges. Lets use your 9/16ths as an example. An advertised 8" wide rim will measure 9 1/8" overall. Specifying a 4" backspacing will produce a "negative offset" in the wheel of .56" (9/16ths), which moves the wheel and tire out, toward the fender. If you want a "zero" offset wheel you would have to specify a 4 9/16ths backspacing. This will put the wheel mounting face directly on center of the rim width.
Keep in mind that the flange design is an industry standard. The wheel manufacturers can play around with the flange widths a little bit but most of them are going to be very similar. They have to be to fit any manufacturers tires AND the tire machines. Any major deviations will cause problems with one or the other.
Mark