Anyone want to back up my offset math?

[Oscar]

ok so hypothetically (mostly) from some not very precise measurements of my car the other day it looks like the wheel mounting area (flange) to the outer portion of the rear wing is about 4.25 inches.

 

I have run across a very attractive set of wheels that are 15x8.25 0 offset. (again lets call this a hypothetical exercise in determining needed offset).

 

From what I understand offset it is the measurement from wheel centerline (positive numbers show offset towards the outside edge, negative numbers show an offset towards the inside of the car).

 

so in this scenario if my clearance is 4.25 inch to the wing edge and with a zero offset wheel I end up 4.125 inches added from the flange to the outside edge of the car that means I have .125 inches to spare.

 

To complicate matters I threw in a one inch spacer. which now make the flange to edge of the wheel distance 5.125 subtract my allowable 4.25 and that leaves me with and extra .875 (23mm).

 

so if I can find the same wheel but in 15x8.25 +23et I would be on easy street...right?

 

Heres a quick drawing. Any input is appreciated. fyi I'm no longer a track day guy and although I like spirited driving I hardly ever go super legal on the roads. but if you have some tid-bits on scrub radius or similar on this type of wheel please share.

 

http://img.photobucket.com/albums/v74/Ahhsk/ScreenShot2013-08-25at43948PM_zpse7507616.png

Edited August 25, 2013 by Oscar

[xcarguy]

Okay, I'll ask the first question, what is the purpose of the one inch spacer? Would it not be simpler to just remove the one inch spacer and use the zero offset or am I missing (probably am) something.

[xcarguy]

Here are the specs for my wheels I use for track day events. The back spacing is 4.5" (4.5" measured from the outermost inside-rim edge of the wheel to the wheel mounting flange). With a 10" wheel, this moves the mounting flange of the wheel .5" off of the wheel's center (.5" toward the inside/back of the wheel). However, the advertised offset is a -25.00mm. One would think the offset-with a 4.5" backspace-would be a -12.7mm, not a-25.00mm. Hmmm.

 

Just to clarify, wheel offset is the measured distance between the wheel mounting flange and the wheel centerline (positive to the outside and negative to the inside).

 

Here is the link to my wheels with advertised measurments:

 

http://m.summitracing.com/parts/sum-571-5161ps

Edited August 26, 2013 by xcarguy

[Oscar]

I used spacer when I really should have used adapter.

 

Ok here is what I think your wheels would fit like on my car:

 

I agree with your 12.7mm. Its not the first time I have noticed a discrepancy between offset and back spacing and thats one of the reasons I posted my math to make sure I wasn't way off.

 

http://img.photobucket.com/albums/v74/Ahhsk/ScreenShot2013-08-25at74226PM_zps084692c8.png

[Oscar]

how wide do your rear fenders go out from the tub?

[xcarguy]

My rear fenders are 12" wide.

 

As for the discrepancy, wheel widths are actually measured (I think) from wheel bead-to-wheel bead (the inner side of each wheel where the tire actually mates to the wheel. Therefore, a 10" wheel (with lip or bead roll, for lack of a better term) may be closer to 11" wide overall. My 10" wheel, for instance, with its 4.5" BS and -25mm offset would then make sense (lip or bead roll equal to approximately .5" So, .5" BS + .5" lip = 1" or 25.4mm, hense the -25mm offset.

Edited August 26, 2013 by xcarguy

[Oscar]

That makes sense. I'll add a small lip to my diagram next time I measure out a potential wheel.

 

I'll measure of my rear fenders this week they look pretty wide but I don't think they are 12". Maybe bigger fenders will get me that extra .875".

[xcarguy]

Just went out to the shop and confirmed my suspicion; my 10" wheel is 11" wide overall. On the opposite side of my 4.5" BS, I have 6.5" of wheel; 11" total.

[Oscar]

Well that settles the backspace math problems I ran into. thanks!

 

Oscar

[Oscar]

Decided to go with a 15x8 +20 wheel all around (although I will likely order the 8.25" rears once I get an intimate understanding of the clearances).

 

I just have to make sure I have lug studs vs lug bolts and then I can order some adapters.

 

This set up looks like I will be just a few tenths of an inch inside the fender which should work out nicely.

 

http://img.photobucket.com/albums/v74/Ahhsk/ScreenShot2013-08-26at81211PM_zps39029419.png

[xcarguy]

With the wheel setting outside of the fender, have you checked to make sure you won't have fender/tire interference (rubbing)?

[Oscar]

It should be just inside the fender edge if my calculations and measurements are accurate. and there should be plenty of room on the backside. If its close I could always machine a few millimeters off the adapter.

 

have you ever seen a 7 with lug bolts instead of lug nuts and studs? I can't order the adapters till I find out and I wont have time to get to my storage unit until the weekend. I'd say it's likely that its a nut/ stud set up but I guess I can wait.

Edited August 27, 2013 by Oscar

[xcarguy]

Had a Birkin; lug bolts in the rear. Changed them to studs. You'll need screw-in studs

[Oscar]

I'm going to make an attempt at getting the car home this weekend. Then pull off a wheel and get some good measurements... somewhat after the fact considering wheels will be here next week ha.

 

Hey Xcarguy, do you still have those Proxes R1Rs? Does WCM sell fenders?

Edited August 30, 2013 by Oscar

[xcarguy]

  Oscar said:

I'm going to make an attempt at getting the car home this weekend. Then pull off a wheel and get some good measurements... somewhat after the fact considering wheels will be here next week ha.

 

Hey Xcarguy, do you still have those Proxes R1Rs? Does WCM sell fenders?

 

I still have the R1R's but they are 15" and won't fit the 'look' of the WCM fenders. Sending you a pm.

[Oscar]

ahh, I like the cycle style rear fender look. I guess larger rear wheels go along with that. Thanks for the pm.

[Oscar]

http://img.photobucket.com/albums/v74/Ahhsk/Super%207/IMG_2236_zpsfa786c4b.jpg

 

http://img.photobucket.com/albums/v74/Ahhsk/Super%207/IMG_2237_zpsde779e1a.jpg