Rear end "float" on straight at lime rock

[jakell2010]

OK,,this is difficult to explain, but I did a track day the other day,at lime rock and coming off the downhill into the right that leads to the high speed straight away, downhill is 85mph to the straight which topped out at 110mph, when the car would level out . The reared of the car would float ( or rock) left and right like a boat, then settle in and perform as directed.

This car is a 99, with 80k on it, it is box stock, with z rated 18" tires. Except for an aftermarket nose that resembles a GT3 front end.

It only does it on this one part of this specific track. I am assuming the spoiler is up at that speed.

Any ideas, the instructors said he's never seen it before and has been in cars with older vintage and more miles,,,other than struts

[ekam]

Likely that one of your shocks are shot, you can quickly check them for leaks.

[Lobo1186]

how do you check?

[ekam]

Look underneath the car and see if there's oil residue on the shock body?

[Pat]

I've felt the back end of my car get light at high speeds (130+.) I'm a little surprised you'd feel it at 85 mph. It's probably related more to the suspension than the aero.
Any idea if the different front bumper cover provides more downforce? I suppose if it does provide much more, the rear could feel light in comparison. But I think that's unlikely.

[Topless]

If your GT3 nose has a chin spoiler it might be enough to unbalance the aero on the car. Most likely though your struts are toast at 80K. Time to look at M030 or PSS9 suspension.

My car was unusually squirrelly in braking zones with no other symptoms a while back. Bad front motor mount. Once I changed it the car was rock solid again.

[insite]

Are your tires sumitomo by chance?

[jakell2010]

The shocks,,, LOOK new,,,it's an odd thing because it's a hard down hill right hand bend, when the car decompress' it feels like the rear floats,,,other than that it's perfect,,,tires are NITTO EXREME ZR-nt55 set at 28 f 36 r

The shocks are original, the issue is,,,I didn't want to any major money into the 99 986,, waiting for 09 Cayman S to come off lease and be in my $ range with LSD

A set of Bilestiens with alignment and corner balance are going to come in around $2500-$3000

[blinkwatt]

Originally Posted by jakell2010

The shocks,,, LOOK new,,,it's an odd thing because it's a hard down hill right hand bend, when the car decompress' it feels like the rear floats,,,other than that it's perfect,,,tires are NITTO EXREME ZR-nt55 set at 28 f 36 r

The shocks are original, the issue is,,,I didn't want to any major money into the 99 986,, waiting for 09 Cayman S to come off lease and be in my $ range with LSD

A set of Bilestiens with alignment and corner balance are going to come in around $2500-$3000

Dude a full RoW 030 suspension(shocks,springs,front and rear sway bars and bushings) shouldn't cost you more then $1.2k from Sunset Imports. And installation for all those should be $400-550 max. $100-200 for an alignment.

[blue2000s]

Originally Posted by blinkwatt

Dude a full RoW 030 suspension(shocks,springs,front and rear sway bars and bushings) shouldn't cost you more then $1.2k from Sunset Imports. And installation for all those should be $400-550 max. $100-200 for an alignment.

Depends on labor rates, but at $110-120/hr the install is typically $800+

[jakell2010]

The nose does have a chin spoiler,,,I didn't think of that,,,but I would think that if that were the source, then the faster it went the more unstable it would become?,',.

Topless,,,,,how did you diagnose the front motor mount?

[Topless]

Originally Posted by jakell2010

The nose does have a chin spoiler,,,I didn't think of that,,,but I would think that if that were the source, then the faster it went the more unstable it would become?,',.

Topless,,,,,how did you diagnose the front motor mount?

I discovered a small coolant leak from my water pump which was still original w/95k miles. Better safe than sorry I had it changed along with a low temp T-stat. While in there you can see the motor mount was torn w/30k miles on it. Out it came. The car does shift better now and is solid again under heavy braking.

Aero is a little tricky because it is difficult to test unless you know someone at JPL. The general rule is to match front and rear to maintain balance but I don't know how much difference a GT3 nose with chin spoiler would make. You are right that aero imbalance should get worse as speeds increase.

I'm not familiar with that model of Nitto tire. If you mean these: http://www.nittotire.com/Tire/nt555 They are pretty good. Run them 36psi Hot f&R for good results. I have a set of NT-05s on my street wheels now and like em.

With 80K on your original struts they are almost certainly gone. My brother had 65K when his car got a case of the wallows. When we pulled the struts and removed the springs there was only dampening left on one rear. The rest were toast. A fresh set of ROW M030 struts & sways would be around $1800 installed and aligned and give you 95% of the benefits of running PSS9s.

[Eric523]

some good ideas mentioned. First thing I thought of was having too much "toe out" in the rear suspension alignment. When was the last time the car was aligned?