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      10-21-2012, 02:10 PM   #1
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Playing with PSI to Address Tire Flex...

I recently switched over to 17" setup for track days. Figured I'd save in the long run with cheaper 17" rubbers but one thing I noticed is that the taller profile of the 255/40-17 (vs 255/35-18) has considerably more flex during high speed turns. At certain points it just felt like the tire was was going to roll off the rim! I tried playing with pressures but couldn't find the sweet spot. Anyone have suggestions on how to address this?

Setup is 17x9.5" et35 square, Hankook RS-3 255/40-17. I run 30 cold, and hot gets up to 38psi.

Is it just that the RS-3 sidewall is too soft to begin with?
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      10-21-2012, 03:12 PM   #2
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There's a little arrow on the side of the tire (I'll get a pic shortly) that shows how far the tire should roll over. I don't remember exactly, but I would run between 38 and 41 hot on my RS-3s.

EDIT:

RS3 by M detaR, on Flickr
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      10-21-2012, 03:21 PM   #3
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Quote:
Originally Posted by Rated M Roadster View Post
There's a little arrow on the side of the tire (I'll get a pic shortly) that shows how far the tire should roll over. I don't remember exactly, but I would run between 38 and 41 hot on my RS-3s.
Interesting, and you haven't had any flex issues?
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      10-21-2012, 03:46 PM   #4
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Quote:
Originally Posted by DSC_OFF View Post
Interesting, and you haven't had any flex issues?
Not with my 18s. Are yours rolling past the arrows?
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      10-21-2012, 04:00 PM   #5
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Quote:
Originally Posted by Rated M Roadster View Post
Not with my 18s. Are yours rolling past the arrows?
Have to check again but was having this issue with my 17's
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      10-21-2012, 04:18 PM   #6
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Do you have a pyrometer? I just got one, and will be using it going forward after my less than stellar experience running tall 275/35 and 245/40 tire on too narrow a rim.

I tried all kinds of pressures and couldn't avoid the flex issues. I couldn't find a pressure that gave me reasonable temps across the tire. The rim issue exacerbated the problem, but the taller tire and increased sidewall height (and softer sidewall of the PSS vs. the Star Spec) is probably an issue on it's own.

Tires sound like they're well sized to the rim in your case. But there's a half inch more sidewall height as compared to your 18s in the same /40 size. Diameter is more or less the same of course.

I'd play with more pressure, up to 41 hot, as Rated M suggested, but I'd check the pyrometer readings as well (and of course feel). There's only so much pressure can do though. A half inch more in sidewall is going to have some impact. If pressure/pyrometer-based adjustments can't solve this, then the options are probably a stiffer side walled tire (NT05 or Star Spec) or a different profile. I know it's an expense, but can you run 265/35s without rubbing?

Guys I run with at the track who run the RS-3 (I was considering this tire) love the tire. But they did also say had a softer sidewall compared to Star Specs or NT05s, and some had gone with a lower profile to dial things back in (always compromises).

I've decided to run 235/40s and 265/35s with NT05s on my 9" and 10" rims for the track vs. 275/35 and 245/40s as the increase in section width is more or less negated, IMO, by the increase in flex and associated issues (and lack of confidence and traction, and 5 seconds of lap time) the come with it. I moved the PSSs to street duty only (and appropriately sized rims).
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      10-21-2012, 04:45 PM   #7
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I have not noticed a sidewall flex difference between the RS-3's and the Star Specs? Grip yes. As suggested; use the sidewall wear indicators as your judge and play with pressures.
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      10-21-2012, 07:30 PM   #8
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Quote:
Originally Posted by Caddyshk View Post
I have not noticed a sidewall flex difference between the RS-3's and the Star Specs? Grip yes. As suggested; use the sidewall wear indicators as your judge and play with pressures.
Good info, I'll take your opinion any day over a few guys at the track.
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      10-21-2012, 07:48 PM   #9
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I have 3 days on the RS-3's and I can't feel any real difference accept the RS-3's appear to have more grip when heated up? The Dunlops are great too! I've found they both work with the same pressures and I check the roll over arrows after every session.
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      10-21-2012, 07:54 PM   #10
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I'm not comparing Dunlop vs RS3, rather the increase in profile as a result of going down 1" in rim size while maintaining the overall diameter. I only mentioned the possibility of the Hankook's softer sidewall as a potential reason behind why the tire flex was much worse than I thought it'd be...
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      10-21-2012, 07:57 PM   #11
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The installer has commented to me that they have never seen a sidewall as stiff as the Star Spec. I just don't notice the difference in my sizes.
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