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Thread: Value performance analysis of 15x8 vs 15x9

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    RaceTape Ninja Force McCocken's Avatar
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    Default Value performance analysis of 15x8 vs 15x9

    I'm looking to move into a different set of wheels for my dual-use '92. I understand 205s square up on a 15x8, but I am wondering if a 15x9 would offer a better value in terms of optional sizing and performance? 205s on a 9" wide wheel would be a little stretch of the sidewalls, but is it a detriment to performance? If I wanted to try 225 or "plus" sized 205 tires, the 8" wheels introduce slip. Seeking some feedback or advice, thanks!
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    Senior Member fatbillybob's Avatar
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    look at the Hoosier tire chart for the A7/R7. You can run up to a 275x15 on 8" wheel. On the Hoosier chart there does not seem to be an advantage to a 9 x15 and a 205 on a 9 sounds wrong. You want the tire to be well supported on the wheel. If you run a spec tire for example see what the manufacturer recommends as a starting point. And a 8" should be lighter unsprung weight than a 9. Sometimes you cheat a little like in the old days of SCCA T1 we ran 315 square on 11" rears and 10" fronts because we did not see a fall off in laptimes despite the wrong sized front and it was easier to have 4 tires the same logistically. RyanC who used to be around here (held T1 track records in the day) was the lone T1 racer who ran smaller correct front tires at ACS because he found a slight advantage to doing so but no one else did.

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    http://www.trackhq.com/Banners/yellowsitesponsor.gif emilio700's Avatar
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    One additional factor you may not have considered is NVH and utility. The wider tire/wheel option on the NA chassis will tramline, be harder to steer at low speeds as well as add NVH properties. This is the trade off for the significantly higher grip they provide.
    Our dailies at 949 tend to run 205's on 15x8 for that reason. Still plenty of grip but a better street solution. Also, the 225/45's are all 200tw so focused on grip at the expense of civility. In 205/50/15 you can fine some medium grip tires with much quieter treads, lower rolling resistance, longer life, etc.

    If grip is job one however, the choice is simple: 225/45/15 on 15x9.
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    RaceTape Ninja Force McCocken's Avatar
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    Excellent. Appreciate the feedback gentlemen!
    Yer pal,
    Force

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    If the setup is just for races, I wonder how much time can people get from 225 on 9 vs 205 on 8 on a 2 mins track? Lets say NT01 or RC-1? Thank you!

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    http://www.trackhq.com/Banners/yellowsitesponsor.gif emilio700's Avatar
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    Quote Originally Posted by wxywb021254 View Post
    If the setup is just for races, I wonder how much time can people get from 225 on 9 vs 205 on 8 on a 2 mins track? Lets say NT01 or RC-1? Thank you!
    My friend, that question was answered about a decade ago.

    Optimum wheel width is equal to or next size wider than actual tread width.

    The question of more vs less tire depends on power, car setup, weather conditions and track. More power needs more tire but even that is affected by course layout.
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    Quote Originally Posted by emilio700 View Post
    My friend, that question was answered about a decade ago.

    Optimum wheel width is equal to or next size wider than actual tread width.

    The question of more vs less tire depends on power, car setup, weather conditions and track. More power needs more tire but even that is affected by course layout.
    Understood. The reason I am asking is that I have 205 + 8" set up right now and I am wondering if worth it to upgrade to 225 + 9”. My car is a ~170 whp MSM. I tried to find a more numerical answer since if for example the difference is 2+ sec on a 2 min track, I would want to switch. If its only less then 1s, I will hesitate for now. But I understand that you were saying it depends on cars and tracks. Thank you!

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    http://www.trackhq.com/Banners/yellowsitesponsor.gif emilio700's Avatar
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    Quote Originally Posted by wxywb021254 View Post
    Understood. The reason I am asking is that I have 205 + 8" set up right now and I am wondering if worth it to upgrade to 225 + 9”. My car is a ~170 whp MSM. I tried to find a more numerical answer since if for example the difference is 2+ sec on a 2 min track, I would want to switch. If its only less then 1s, I will hesitate for now. But I understand that you were saying it depends on cars and tracks. Thank you!
    Do it.

    You will not find the definitive numerical answer you are looking for, because it varies. Besides adding stickier tires, it's the single biggest gain in mechanical grip you can do.
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    The Real Captain Slow Red_5's Avatar
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    Don't NAs need some massaging to get the 225s on x9" wheels to not rub? Might just be a fender roll, not sure if a pull is required.

    I was planning to start on 8" 6ULs for my '91 Miata project and maybe get some 9" wheels with track tires down the road when the car is making more power.
    99 Mazda Miata SuperMiata #515 - AKA Sparky SOLD
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    Quote Originally Posted by emilio700 View Post
    Do it.

    You will not find the definitive numerical answer you are looking for, because it varies. Besides adding stickier tires, it's the single biggest gain in mechanical grip you can do.
    Thank you for the suggestion!

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