Need some wheel math tutoring (:

[70vert]

I am trying to assess my options to resolve an issue with my front tires rubbing when making a hard right/left turn. I've temporarily resolved it by raising the torsion bars a bit but prefer it lowered. I have Cragar 5-spokes that are 17x7. My assumption right now is that if I can get a wheel that will "pull" the outer edge of the rim in, so it doesn't stick out as much that will solve my problem (I know I'll need to measure clearance from lock to lock). But this is the only 7" they sell in 17" wheel and I suspected I'd have issues with an 8" so didn't go that route. But maybe not. I thought I understood the Backspacing aspect but then throw in various Offset, which is in metric for some reason, and I'm stumped. Here is a recap of my Cragar options with 17" rims:
  * 7" width - 3.75" Backspace & -6mm Offset > what I have currently
  * 8" width - 4.50" Backspace & 0 Offset
  * 8" width - 3.50" Backspace & -25mm Offset
  * 9" width - 4.75" Backspace & -13mm Offset
  * 9" width - 5.50" Backspace & +13mm Offset
They make a 10" width but clearly that won't work on the front.
I'd prefer to stay with Cragars as I like the look. And they do offer custom offsets, for a ton of $$ but an option. And actually I would really like to go back to a wider tire (see below), more rubber on the ground!

I'd read somewhere that a higher offset will move the wheel inward so since most of these are negative offset I assume that does the opposite and not what I need.

If anyone has a link or can explain how this wheel geometry works I would appreciate it. I read several posts on the site but didn't find any that really go thru the details of the measurements.


As a side note, I may have other issues related to a 4-wheel disc set up I had installed a while back. Prior to the discs I had 14" ralley wheels with 235/60 Goodyears which were 25.1" diameter/tall. I initially got a 235/50 which were 26.3" diameter (so about an inch taller) for the 17" rims. But they rubbed all the time so switched to a 225/55 which is 25.9" diameter, still taller but better. I don't know if there are specs I could check for left-to-right wheel mounting surface. Or from wheel mount to fender edge, etc. To confirm mine is still correct or not. I know there is not a lot of room on the E-body front for wide tires but I do see some claiming they are running 255/50 even!!

For now I'm just wanting an education on wheel measurements so I can proceed to analyze options. Thanks for any feedback!

[autoxcuda]

17x7, 3.75 backspace, 225/55/17 = 26.7" diameter

15x7, 4.25 backspace 225/60/15 = 25.6"  diameter (tire that typically fits e-body's)

The 3.75 backspace pushes the rim out 1/2" more than typical

And the tire is 1" more diameter. Which pushes the tire edge 1/2" more toward fender when turning.

You need at most 4.25" backspacing with a 17x7. It would be helpful to reduce the diameter also.

A 17x8 with 4.5 backspacing with 225/55/17 would move the outer edge only 1/4" inward than what you have now. And 225/55/17 is sorta narrow tire for a 8" wide rim.

Basically, those rim choices suck for your car.

[70vert]

@autoxcuda yeah I knew they weren't perfect but Cragar doesn't have a lot of choices and going custom was over 4 times the cost so not an option.

I get Backspacing, but still wanting to understand how Offsets impact the position of the wheel.

[Bullitt-]

The way I understand it offset is just the distance from the center of the wheel mounting pad is  + or -
Zero offset would be the middle

   See if this does you any good

https://www.wheel-size.com/calc/

[autoxcuda]

@autoxcuda yeah I knew they weren't perfect but Cragar doesn't have a lot of choices and going custom was over 4 times the cost so not an option.

I get Backspacing, but still wanting to understand how Offsets impact the position of the wheel.

Offset is the measurement away from the center line of the wheel created between both inside tire bead edges.  (Edit)

Backspace is the measurement from outside bead of rim to wheel hub mounting surface. Taken from the rim edge toward the center of vehicle.

Here's a really good graphic tool that you can plug and change numbers into to see what happens:

https://www.rimsntires.com/specspro.jsp

[Bullitt-]

Offset is the measurement from the center line of the inside tire bead edge.

  not the bead.... center line of wheel to pad .... look at your example 

[Dakota]

If you haven't gone through this thread, I'd suggest you do so to see what other folks have used.

https://forum.e-bodies.org/wheels-tires-brakes-suspension-and-steering/12/e-body-tire-sizes-what-fits-whats-too-big-what-looks-good/4016/15

I've always liked the look of Cragars but as you've found out they aren't available in a size that worked well on my car (70 Challenger). 

[autoxcuda]

Offset is the measurement from the center line of the inside tire bead edge.

not the bead.... center line of wheel to pad .... look at your example

Yes. Confusing the way I wrote it. The centerline of the rim is half the distance between outer and inner rim beads measured from the inside of the rim lips where the tire bead sits against. 

Offset is the distance from that determined centerline.