What offset for 15x9's and 225's on an FC?
 

I need some advice. I'm days away from ordering new custom track wheels (Kodiak) and I'm not 100% sure on what offset to get (I can specify ANY offset). I'll be getting 15x9's to go with 225/50/15 or 225/45/15 tires. I'd like to get them as close to the suspension as possible without rubbing and I'd like to avoid using spacers if possible, but I'd rather have them be a bit too close than too far away. I've got Tein Flex coilovers.

Any advice on offsets (can be based on 16" and 17" sizes too)?

Who recommends such a wide rim for that size tire? Hoosier is at 6-8" or 7-8.5" depending on tire size both for the A6 and R6.

As far as offset, not sure. On a 15 x 7 with the 225/45/15, I'm at +22mm and clear the spring seat by just over 1/4" in front. Do some reverse math???

I used 16x10 with 0 offset (5.5" backspacing) and it required a 3mm spacer to clear the front struts. The rear fit fine.

When we did my 15x8 steelies we pushed the center as far out as we could and welded them up. The results fit perfect with a 1/4" spacer to clear the stock calipers.
If I was to do it again I'd go 16" wheels as there are a lot more options in slicks when you go that direction (Which I think all track cars are doomed to go).
my$0.02 (As I ponder the jump to 18"x 12.5" wheels to use porsche cup slicks)

-Trent

A little background: back to back testing on a Miata with 15x7's, 15x8's and 15x9's, all the same wheel type and the same 225/45/15 R comps has shown that each additional inch of wheel width was worth an extra 1-2 seconds a lap. I've seen a bunch of pics of this and it fits quite well with little to no real visual stretch to it. Most 225/45/15's that I've seen, or at least those that I'd consider running, are reccomending up to an 8.5" wheel (ok, so I'd be 1/2" out, big deal) and my current 225/50/15's up to a 9" wheel, so I'b be maxing that out while leaving the door open for the potential to run wider tires as they might become available (or maybe 275/35/15's with a widebidy).

Right now I'm thinking 20mm, as I'm pretty sure that'll fit and maybe let me run Hoosiers later on if I so choose (their 225/45/15 is about 1" wider than the NT01's I plan to run next).

I figure if I'm getting custom wheels, I might as well go as wide as is reasonably possible.

That's substantial. Maybe I need some new rims as NASA doesn't regulate wheel width in PT. What kind of R comps were used? What was the total lap time or track length approx? Obviously 1-2 seconds is a bigger deal on a short track. With the wider wheel you are adding unsprung weight, but a bigger contact patch, I am surprised to hear such a giant improvement. Was testing all done on the same day? Same driver? New tires each session? If they started with the 15x9 on sticker tires, I could see that being much faster by the time they got to the 15x7 and the tires had been heat cycled.

I got the info from posts on miata.net, but I'm not sure if there was ever a thread created with the results or anything like that. IIRC it was all done with Nitto NT01's in 225/45/15 on 949Racing 6UL wheels (the owner of 949 was the one doing the testing). It was probably Buttonwillow, but I don't really know for sure, SoCal for sure though. I do believe it was true back to back testing, but others have confirmed that the 9" wide 15's are faster with 225/45/15's than with 8" wide wheels. As for weight vs. grip, grip is almost always the faster choice (there's a limit obviously). I'd gladly gain a pound or two per corner for an extra inch or two of width. Besides, the Kodiaks won't exacly be "heavy".

I'd get a wider tire if I could get one in that sort of diameter, but as I'm sure you know, there's not really anything untill the 275 Hoosier, and that'll require a widebody. I'd love a 245/40/15 NT01 or R888. There's a 235/50/15, but I'm don't want to go that tall.

The problem is that for 8" wheels or bigger, your only real options are custom wheels, which gets expensive. There's also the SSR SP1 Professor and Work Meister S2 available in that size with appropriate offsets, but due to Kodiak being "local", it makes more sense for me to go with them (exchange rate, duty, shipping, adds a lot to the cost).

My idea has always been to try and match the rim width to the tread width of the tire. Given an inch is 25.4 mm in width, a 225 should like a 9" rim. I've run a lot of tires on smaller rims and you do feel the difference once you step up to the same width. I don't think going any wider will get you any advantage.
I'm currently running a 235/40/17 on a 17x8.5" rim and a 255/40/17 on a 17x9'5" rim to good effect.

-Trent

So shiny, not that I'd normally go for that sort of thing. Judging from the price I was quoted (Canadian dollars) and the exchange rate, the price per wheel in US dollars could be a bunch less than the listed price online. Would be worth an email if you're thinking about it to know the real price.

http://www.kodiakracingwheels.com/images/rt-15-d.jpg

I was always under the impression that the conversion factor was rim width + 1 in inches should be matched to tire width in mm's.

I don't think I should be running 205 and 225 on a 17 x 8, 17 x 9 setup, that just screams way to thin tires to me.

Wheels are nice and shiny.

How's your outer clearance? How much wheel peeks out of the fender, or does that not really matter to the SCCA / NASA folks in your experience?

None peeks out. SCCA rule for IT says something about contact patch as measured with chalk dust can't protrude beyond fender as viewed from above. Production rules say tires have to be covered and I don't recall what the CCR says and don't recall any mention of it in the PT rules. Then again, I have never been concerned about any of this so don't have the rule memorized.

This shows how well the tires are covered, completely stock fenders - not rolled or flared.

I'm actually on a +35mm 15 x 7 with a .5" spacer, which equates to +22.3mm. Team Dynamics did not offer what I wanted, so I had to use a spacer.

http://img443.imageshack.us/img443/8...4337wo8.th.jpg

Sweet, I've been wondering what was the maximum legal offset was on a 15 x 7 and a 225/45/15 tire. I had previously thought it was +24, now I know +22.3 works.

that is inline with mazda's fitments over the years. fc is 205 on a 7", fd is 225 on an 8" , same with rx8, the 3 etc etc

Wouldn't Mazda's method be more in line with this:

than direct conversion from in to mm?

There is no simple conversion for rim to tire, since it depends a lot on the sidewall height and construction. Normal, taller tires have much "bulgier" sidewalls, which will behave differently than the relatively straight sidewalls on lower profile performance tires.

Tread width and sidewall width are usually about .75"-1.5" different(15mm to 35mm). I like the rim width to be the same as the tread width. In many cases a street compound 235 is less than 235 mm at the tread. With race compounds and slicks you often find a 235 is right around 235 mm in tread width. Most good tires sites will list the tire sidewall width and treadwidth for race compounds.

End of the day don't sweat it too much. Buy the rims that will fit on the car. Tires will still work on a wider or narrower rim, just not optimally.

-Trent

GRM did an article on upsizing wheel widths, and came up with the same conclusion. I believe it was in the February 2008 issue.

Well I finally got the wheels today and before leaving, I took one out of the box and weighed it on the postal scale and it came out to be 13.2 lbs. Not too shabby.