Rear end "swimming" at high speed with new 40 series tires

Rubrignitz · 10-18-2009, 06:21 PM

I recently switched from 305/35 (drag radials no less) to Nitto NT555 street tires in 285/40 size. I was just on my way back home from a weekend trip on one of the really deserted toll roads. A modified S2000 came up on me at 80mph REALLY fast and wanted to run.

We squared off at 80 and ran up to ~120mph where I noticed my rearend swimming around. I never noticed this with the 35 series drag radials or the 35 series on 20'' wheels that I used to run.

Is this a tire issue??? My car handles beautifully up to this point and I've never noticed it before at this speed.

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Sleeper_08 · 10-18-2009, 09:07 PM

I'm running 285/40/18 all round and get up to close to 140 MPH at Mosport and it feels stable. Can you describe what you mean by "swimming".

Rubrignitz · 10-18-2009, 09:15 PM

What tires are you running sleeper? Nothing, and I mean nothing has changed other than the tires and I get this rear swimming i.e. "not as responsive" feel. When I move the wheel at that speed it now feels as though the rear takes time to catch up...swims...

Rubrignitz · 10-18-2009, 09:17 PM

Ah, I see your running kdw2's. I never had trouble with those tires at speed. I'm not very happy with these nit 555's.

Argonaut · 10-19-2009, 01:18 PM

I've only used the 555RII (the track tires, 305/35/18) and they are stable at high speeds. I have heard a few negative comments about the 555 street tires related to their handling before... interesting.

Rubrignitz · 10-19-2009, 01:41 PM

I found it interesting as well. Nothing has changed with the cars setup other than new rear tires so it's directly attributable to the tires. They're fairly new (<200 miles)...so maybe they'll get better. I was leisurely cruising at ~70mph then up to ~100/20 made a lane change and it felt like the rear wasn't following the front...just doing it's own thing back there. Very disconcerting... I never noticed this with either the 35 series dr's or the 35 series KDW2's I used to run. The car tracked very well at speed with those tires.

Sam Strano · 10-19-2009, 02:01 PM

Tire construction is different... I'd recommend more air pressure and/or more shock in the rear of the car. The shock will simply help make up for the presumably softer sidewall. The air will actually help stiffen the tire itself.

FWIW, this is a classic example of why adjustable shocks are something that should be considered. You can make a change to a car, that seems like it wouldn't matter, but does. Having adjustment allows you to dial the car in around any changes you make.

Rubrignitz · 10-19-2009, 02:34 PM

Thanks for the input Sam (btw, I purchased the d-specs and steeda comp springs from you about a year ago). I'm running the d-specs about midway firm/soft. You suggest I firm the rear more?

BTW - I planned to purchase brake pad/rotors from you this week but one of the rear pads contacted rotor Friday night and I was forced to make an emergency repair Saturday morning, replacing the rear pads/rotors with duralast golds. I didn't have much choice. I'll still get with you later on the front pads/rotors...

REDFIRE_GT_BEAST · 10-19-2009, 02:46 PM

I have those tires and i know what you mean it has good grip i guess and yea i got the same feeling but nothings wrong trust me already took it into the dealer and a place called whell tec they all tell me that this is better rather then bad i dont know how but yea i get the same feeling but now im used to it. As far as tire pressure its not that cause these are nitrogen tires and ive tried that already on a trip back from chicago all the way down here to tampa and about midway tennesse i put more air then when i got back i noticed the air pressure was 40 and the guy told me they were nitrogen so scratch that idea.

Derf00 · 10-19-2009, 02:46 PM

What PSI are you running at? KDW's get a different feel depending on what PSI you're running.

I run mine at 37psi. If you run them at 32 you will DEFINITELY feel loosey goosey in the seat at high speeds. I don't think the sidwall is as stiff as other brands of tires.

BTW I've used KDW's for over 8 years and been through 3 sets so I know how sensitive they are to PSI.