Track Width between 9.5" and 10" running 255 tires for Time Attack

Ok, so this is a lot of data dump and if you respond please read it all because I'm aware this topic is covered in a lot of threads, however I can't figure out an answer to this nuanced specific question. I was hoping some of the experienced/professional track rats here might have some advice. BLUF: I'm guessing the answer is the difference is so minor I shouldn't worry about it, however I want to do my homework.

Background and Assumptions:

• I'm working my way into time trials and time attack with a 2011 WRX STi
• Purchasing wheels soon and based on class rules, I can't run a tire wider than 255
• Most 255 tires have a tread width of 9.5"
• GRM testing concluded the best wheel width is the same width as your tire's tread width (not section width)
• The fast STi's in GTA and Gridlife appear to run 10" wheels vice 9.5" with 255 tires

I hit the books and my best guess is that perhaps the wider 10" wheel is close enough to the 9.5" tread width that there isn't a significant difference in tire performance while potentially offering a track width advantage for the car. Obviously the offset of the wheel will have a large impact on track width of the car.

Work I've done and the wheels I'm looking at:

• Current OEM Wheel is 18x8.5" +55 Offset
• Considering:
  • 18x9.5" +40 Offset
  • 18x9.5" +35 Offset
  • 18x10" +18 Offset
• By taking half the wheel width and subtracting the offset I get the amount of wheel extending beyond the hub
• Then I subtracted the OEM Wheel comparable value to see how much additional wheel I get beyond the OEM wheel. 
• Then I multiplied this by two to account for each side.
• I got the following value for how much each wheel adds to the total track width of the car:
  • 18x9.5" +40 Offset   Adds: 1.96"
  • 18x9.5" +35 Offset.  Adds: 2.36"
  • 18x10" +18 Offset     Adds: 4.25"

Insert comment that "this is insignificant and I should just focus on driving better." No fear, I understand improving driving skills is the most important mod, but I don't want to leave anything on the table as I gradually modify the car.

Based on the weight transfer formula's in Fred Puhn's book "How To Make Your Car Handle", the most significant factor to reducing the amount of weight transfer is track width because it's the denominator by which other values are divided [weight transfer due to roll center height = (front CF x front RC height)/track width]. Therefore if I can increase my track width it does not seem trivial. 

Questions I can't answer:

• Since I'll be running the same tire size regardless of wheel, does any of this actually matter?
• i.e. if you increase the wheel width, but not the tire size, does your car's track width actually change?

Assuming the wheel width matters and offset is the most significant factor in increasing track width:

Concerns I have with significant Offset changes:

• Fred Puhn's book talks about the extra stress on bearings due to large changes in offset, defined as more than 2 inches
• The +18 offset offers the widest track width, but is close to 2 inches of change from OEM wheel offset
• The book is older, so are modern cars any better at handling increase stress on wheel bearings and negates most of this concern?
• Does the impending change in steering and toe geometry outweigh the track width increase

Okay, that's alot, but any help you experienced/professional guys out there can offer would be greatly appreciated. I also would love to know if I've found myself in some rabbit whole that is irrelevant due to some other factor or real world condition I have completely missed. 

Summary of two main questions:

• Does running a 255 tire on different width wheels (assuming same offset) change track width (9.5" vs 10")?
• If Offset is the primary contributor to track width then how much is too much (ignoring clearance issues)?
• Is my math totally wrong and I have lost my mind?

Thank you for your time if you made it this far!

In reply to Subi Fighter :

So.....To give a simple answer.....

"most" will run a wheel that is slightly wide (the 10" in your case) because the little bit of stretch it gives the tire can/seems to offer a little better response from the tire due to the pre stretched side wall.

As for track width...the simple answer is go as wide as you can. Yes you'll be increasing the loads on the bearings. You'll also be increasing the scrub radius which will increase tire wear. Neither of which you are likely to notice IMO.

In reply to Asphalt_Gundam :

Copy overthinking it haha. Thanks for taking the time to read through my insane post. The more I've read and rethink my way through it, I keep running into run a wheel that's the same as your tread width and ultimately the total width of whatever is touching the ground is the effective track width. Specifically, a 9.5 vs 10 inch rim on the same tire likely have no to very marginal difference on track width. 

Thanks for the thoughts and time. I think I finically decided to just goes with the 9.5" wheel to ensure I don't run into clearance issues and I can always go bigger later. 

10" wheels will almost certainly be marginally better for the tires. If not in outright speed, in wear and feel. Some tires might even be happiest on even wider wheels. My personal opinion is that minor track width changes are pretty insignificant. That said I'd probably be trying to keep it as narrow as possible (that might just be my autocross brain though). Can you not get higher offset 10s? The ones you listed will have ~10mm more inner clearance than the 9.5s. I also think the wheel bearing thing is largely over hyped, though Subaru does have something of a history with weak wheel bearings I think. In any case if you're doing a lot of track time, wheel bearings become something of a wear item anyway, it's just a matter of how frequent the replacement interval is.

In reply to dps214 :

Thanks for the reply! I'm sure there are a variety of other offset 10" wheels. I mainly was looking at that specific +18 offset because there is a Time Trials car of the same model running those wheels. The driver told me he was able to fit with rolling fenders and running 3 degrees or more of camber. I ran the numbers and measured out the clearances on my car. The clearance seems extremely tight. I just started to feel I would getting a bit ahead of myself. The testing I can find (GRM and not a lot else I can find) points at the wheel width equaling the tread width for best performance. Most the Sanctioning organizations and classes I'm looking at will confine me to 255 tires with tread width's of 9.5" anyway. You raise a good point though, I'll take one more look at some other offset 10" wheels. The 9.5s are just the safe "good enough" choice at this point. 

In line with the pack as a general statement on going a touch wider with the 10".  I'd probably do a bunch of measurements since you have the wheels.  TO minimize scrub radius issues with wider setups, I'd been told to do your best to keep that inner tire edge as close to where it was OEM as possible.  This presumes you have the space to work with and the offsets aren't too far out of what is available.

Thank for all the input. I had not thought through the scrub radius aspect yet. I ended up going with the Titan 7s that are 18x9.5 +40 ET. The more I looked into everything it seems like a great option. I might be giving up a little without a 10" wheel, but also it keeps me from going to extreme with my offset. I run top hats that do tilt the struts in a little which alters the steering angle inclination so I think my scrub radius will remain pretty similar to oem. Most of my camber still comes from the lower strut attachment point, so I don't think I have the SAI angles to compensate for the +18 ET wheel. Additionally it seems like SAI change can come with some dramatic effects that are a greater problem than scrub radius. All in all, I'll take a stepping stone approach without making too big of a jump at once. Also the Titan 7s look great!

Point of nitpick, wheel width has nothing to do with track width, only wheel offset.  Track width is measured from the center of the contact patches, whether the wheels are 10" wide or 2" wide.

In reply to Pete. (l33t FS) :

I love to nitpick! So this was a question I had on my deep dive of this whole issue. I felt like a crazy person and had to draw it out so many times haha. Eventually I figured it out haha....maybe...

I don't know if this helps answer the question, but it seems most of the STR guys use 245s rather than 255s on a 9.5" rim, but I could easily be wrong.