One more time/ wheel offsets...

[Guest Frank280z]

I went to a local parts shop today to inquire about a fwd rim. I found an ADR 16x7 with a 40mm pos. offset. Theoretically this would not fit. But it only has a 4" back spacing. I'm pretty sure these cars can clear that....Can they?? I'm going to take the car down tomorrow and trial fit the rim. But my confusion is with the pos. offsets. Another rim from the same manu. states a 40mm offset. 16x7 w/ measruring a 5 1/4" backspacing. Go figure. Is the info brochure inacurate? Frank

[Guest scca]

most newer rims are 37 or 40mm offset.

this is a metric way of measuring. all US and racing wheels are usually measured in backspace from the rear. most all 3 pce wheels are measured this way as well.

 

they will fit-- you will possibly need coilovers for clearance and a huge 1.5" adapter to bring them out close to the fender lip. Evan did the honda conversion adapter before but this is a one time thing and IF you ever change rims the offset better be the same or your custom spacer/adapters will be useless.

 

------------------

Mike

mike@fonebooth.com

http://www.fonebooth.com/brakes.html

raceparts and brake upgrades.

[Danno74Z]

Here are some formulas for you to figure backspacing or offset if you have one or the other.

 

Offset = Backspace - (ovr.width / 2)

 

Or:

 

Backspace = Offset + (ovr. width / 2)

 

E.g. for the wheels above (40mm offset=1.57") (mm x .03937)

 

Backspace = 1.57" + (8" / 2)

= 5.57"

Approx. 5 9/16"

 

Danno74Z

 

[This message has been edited by Danno74Z (edited November 09, 2000).]

[pparaska]

Actually, that's not quite right.

 

Check this out, I "borrowed" it from the Fiske web site:

 

Backspacing is measured from the inboard lip of the rim to the mounting surface. Note that this the part of the "OVERALL RIM WIDTH" inboard of the mounting surface, including the lip width, not just the "RIM WIDTH" part.

 

So the calculations are:

 

Backspace = Offset + (Rim width/2) + (lip width)

 

or

 

Backspace = Offset + (Overall Rim Width/2)

 

and

 

Offset = Backspacing - (Rim Width/2) - lip width

 

or

 

Offset = Backspace - (Overall Rim Width/2)

 

I thank Jim Biondo and Fiske for keeping me straight on this.

 

BTW, the lip width is usually 1/2" (~13mm) on most modern AL wheels, so it makes quite an impact if you're trying to fit that last little bit of wheel against a spring perch, etc.

 

 

------------------

Pete Paraska - 73 540Z - Marathon Z Project - pparaska@home.com">pparaska@home.com -

[Danno74Z]

Pete,

 

Thanks for fixing that for me. I knew I'd blow it trying to explain it. In Frank's example, I needed to add 1" to the width of the rim (the lip factor). If my math is correct, Frank's rim has a backspacing of approx. 5.57". And you are right about the inboard space it's very tight but I think I have found a good wheel to use with my coilovers.

[pparaska]

No biggie. A year or so ago, I thought of backspacing and offset the same way as you stated. Sometimes that 1/2" wheel lip is important though.

[Danno74Z]

Frank,

 

I did find this very interesting site. You can enter different values for tire size and offset. A before and after snapshot.

http://www.tolan-hoechst.com/cars/tirecalc.htm

 

Danno74Z