For 2+ years I was trying to eliminate understeer on 2004.5 G35 MT coupe.
So recently I end up with Enkei RPF1 and Toyo's T1R 265/275-35 18" which also increased Final Drive to 3.685.

With wider front track understeer is gone now. I was expecting heavier feeling on stirring wheel, but if anything it feels either the same or lighter. During testing, as curve speed increased my steering wheel has become lighter (lift?), however, rear wheels felt like rear is dancing, bouncing and moving left and right so I was afraid to lift the throttle and risk spin-out. Yes, I was scared ...

I wonder if this new problem in rear is actually low profile tires attribute - stiffer sidewalls on harsh bumpy road surface or did I introduced power oversteer?

Changing Tokico D-Specs from 4/4 to 3/6 helped but didn’t eliminate oversteer.


So how to correct oversteer, should I try/install:

Lower front tires pressure (from 35 to 34 psi cold)
or increase rear tires pressure (from 35 to 36 psi cold)

Narrower tires in front (T1R from 265/35-18 to 255/35-18)
or wider tires in rear (T1R from 275/35-18 to 285/35-18)

Stiffer roll bar on front (revised Z from 1333 to Stillen 1706 [med])
or softer roll bar on rear (OEM G from 385 to 378 350evo)

Stiffer springs on front (H-Tech Z from 358 lbs to 504 Basics? )
or softer springs on rear (revised Z from 427 lbs to 375 H-Tech Z)

Raise the front hight (from 26.06 inch to 26.5? [OEM G hight is 27.32 inch]) but how
or lower the rear hight (from 27.5 inch to 27.1? [OEM G hight 27.95 inch]), by trimming ruber bump-stops

Install OEM rear spoiler
or rear wing

Change my last alignment:
Camber -1.4/-1.9 F -2.0/-2.0 R
Toe +0.03/+0.02 F +0.03/+0.02 R
Caster 8.5/8.6

Something else
or change nothing, drive slow and take all that aggressive driving to track - but then what?

Also, would lighter rear, removing all this junk from trunk (amp, sub, carpc, ...), make oversteer worse?

Thanks

 

fix the toe

edit
The tire pressure changes you suggested are backwards, but you shouldn't really change them anyways on street as too low or too high will be dangerous in rain and bad for wear. I would say put the toe back to factory specs for starters and then maybe look at camber. Those are the cheapest and can be done without changing parts.

 

^ +1, our FSAE car has the same problem with grip in the rear. We dialed in a very slight toe in for the rear... really helped with the power-on oversteer. From what I understand, toe-out in the rear is mainly used on awd,fwd cars to get them to rotate.

A bit more front camber would probably help as well. And to answer your last question.. yes taking the crap out of the trunk will make it worse. Less load on the rear tires = less grip.

ChiShi

 

Per Hunter rep., negative toe denote "toe out", positive toe denote "toe in". Confusing



I think Hunter 611 is spiting out toe in inches. I have toe IN, total toe in is +0.05” or just a little less than 1/16". Nominal OEM for G35 is 0.04” total toe in on front and 0.11” total toe in on rear wheels. I think Z have same nominal OEM in front and less than G total toe in on rear wheels (0.075” ?). So I am within spec, although per betamotorsports posts I thought to change my front toe in, to toe out.


Tire pressures, see http://www.wtrscca.org/tech.htm page. I have infrared Pyrometer but haven’t confirmed tires temps yet, hopefully this weekend.


Camber change would need ~$800 arms, so not yet, I need to wear out set or two of tires first
During winter, for snow, I am hauling around bags of salt/soil in the trunk, but there are some posts on the net suggesting that lighter rear of the car decreases oversteer!?

Thanks

 

Sounds that a car that you need to put the power on to keep it balanced is the way to go.

What did your instructor think of the rear end's behavior? It might just be that the car is right now and you need to get used to it.

I would try a softer rear swaybar if the new alignement doesn't help. But that will put more load on the outside front, potentially sending you back into understeer...

 

Kolia, instructor said nothing yet. Will try again on Pocono next month.

You might be absolutely right because few days later it feels better … or maybe just predictable, because something in the rear is still floating. Its hard to describe this oversteer of mine. Its like bump-steer on the rear and funny thing is that front becomes light. I think that if I let go throttle that I would loose control. It’s happening only at high-speed (100+) curves and if it is oversteer its “Steady state oversteer”. Low speed curves I can now carry 10+ miles faster than before.

Only softer rear sway bar that I know of is 350evo and I cant find one for a while now. I was thinking about Stillen sways and set them to medium/low, which will give me +28/+7 % compared to what I have now.

I will do alignment again this weekend, but what should I aim for?

 

What you are describiing is a neutral handling car. A well balanced car WILL oversteer if you lift out of the throttle - that's exactly the behavior a neutral handling car will show. An oversteering car will be obviously oversteering in the situation you describe, not "threatening" to oversteer.

My guess is that you've become so used to an understeering car that a more neutral handling car appears to be loose when its really working as it should.

 

Does the car have an under-car diffuser? No rear spoiler I presume?

At 100+ it IS possible that there's enough lift to cause the car to lose grip in the rear. The 350Z isn't terribly proficient at covering up the bottom to make it aerodynamically stable.

Also is the car corner-balanced with you in the driver's seat and "track" weight?

Just another thought.

 

No rear spoiler – no diffusers
No coilovers – no corner-balance
No camber and no toe kit
Only alignment shop manager in the driver's seat (similar weight), but it made no difference because I am at the max camber with G35 coupe OEM adjustments.

I thought about aerodynamics being changed if underbody air passage have changed and decreased ground force with wider front track. But I have zero knowledge on the subject.

I am going to order and install under-car diffusers. Before swapping the sway bars, I wonder if the front strut bar would have any effect on decreasing oversteer? Thanks

 

Wait, so the high speed oversteer starts to happen right after you changed tires?

 

where we can buy the under-car diffuser? Any pictures?

 

The HACK, yes, right after four RPF1 9.5 18” +15 and T1R’s 265/35 275/35 were mounted.


kennyl5123, you can order diffusers at Infiniti dealer ~$180
For pics of diffusers (on someone else car), please see this g35driver.com thread.

 

Get it fixed soon, Pocono's grass is not flat at all. I went on there, very bad news.

 

Then I will have to echo what one of the other poster said, that you're just not used to NOT having the inherent understeer built into the staggered set-up...So now a more "neutral" handling car feels like it's about to oversteer?

No way to really tell over the internets.

 

Saturday I did alignment again. In last 2 years I did 6-7 alignments and each of these machines was spitting out different numbers (and without potholes). And I tried all kinds of both Hunter and John Bean machines. It’s very hard to find a shop that you can have confidence in their work and equipment. I think I found one yesterday because now, for my daily driving, car handling is almost perfect.

. . . | . . . |2007-Shop A -|--- Shop B---|-- Freedman--|
Nominal . . . |BEFORE| AFTER|BEFORE| AFTER|BEFORE| AFTER|
. . . | . . . | 03/21| 03/21| 03/27| 03/27|05/12 | 05/12|
Camber
−0.50°|Front L| -1.6 | -1.6 | -1.4 | -1.4 | -1.6 | -1.6 |
. . . |Front R| -1.6 | -1.6 | -1.9 | -1.9 | -1.7 | -1.7 |
−1.25°|Rear L | -0.5 | -1.3 | -2.0 | -2.0 | -2.2 | -1.5 |
. . . |Rear R | -1.4 | -1.2 | -2.4 | -2.0 | -1.6 | -1.4 |
Toe
+0.04”|Front L| +0.12| +0.07| +0.04| +0.03| +0.35| +0.05|
. . . |Front R| -0.03| +0.00| -0.02| +0.02| +0.25| +0.05|
+0.11”|Rear L | +0.27| +0.11| +0.03| +0.03| +0.15| +0.05|
. . . |Rear R | +0.23| +0.03| -0.03| +0.02| +0.05| +0.05|
Caster
+8.00°|Front L| 7.8 .| 7.8 .| 8.5 .| 8.5 .| 8.2 .| 8.1 .|
. . . |Front R| 7.8 .| 7.8 .| 8.6 .| 8.6 .| 8.3 .| 8.2 .|

LOL, I feel silly now, all I needed was good alignment. Thanks for those that replied.

 

Case closed !

 

Is there anyway to adjust the caster? mine is off by 1 degree side to side

 

No, per G35 SM FSU-6:

 

SPL adjustable upper A arms will allow +/- 1 degree.

But in your case, you might need lower links in the front ?