Topic: Is this tire sidewall flex or the rear suspension?

[Sly Bob]

Going into a corner hard, be on an on-ramp or in the twisties, I feel a sway in the rear end. This feels as though maybe the sidewalls of the stock GXP tires are flexing and frankly it is a little unnerving. Has anyone else experienced this and what has been the fix. I believe the car has done this since the car was new and I would like to get this fixed before I get a tune.

Tires or suspension? Have those with a Probeam noticed this before the installation and if so was it lessened after install? I have a backbone and although I love it and what it did to the car, it didn't do anything to lessen this issue.

[Uranium-238]

Does it feel like the rear end is moving in a vertical circle? Used to get that all the time with my Redline, the ZOK bar seems to have done away with it.

[snaponbob]

Tires or suspension?

Yes.

Tires. The Goodyears are not much better than rim protectors. Make sure you have 29PSI in them - MORE and the car gets "edgy, LESS and the handling gets mushy. These tires are 45 section tires and are not really very firm. Tires really are the OTHER set of springs. When you hear NASCAR or IRL teams talking about changing pressures, they are fine tuning the suspension's overall spring rate. Really. It is a fine balance between altering tire pressures for the TIRES and adjusting for spring rates. Street cars are not so finely tuned that the tires are part of the total spring package. That said, when the car is set up for competition (i.e. Solo) and driven accordingly, suddenly the tires DO matter. Example; I attended an Evolution driving school for Solo. Rather than using the Hoosiers excessively, I decided to run the Goodyears. I had just reinstalled the Konis. After the first pass I could not understand the bouncing the car was doing. I checked the tire pressures, raised them, and was REALLY alarmed how bad the car was. Assuming the Konis had "died", I checked them for leakage. None, so I installed the Hoosiers. POOF - all was right with the world. The tires were bouncing the car SO hard the even the instructor (a multi time Solo national champion) thought there was something wrong with the car. The stock tires are junk. Just be aware of it, as the only REAL fix is some better tires.

Suspension. The Bilstiens are decent up until about 7/10th driving and then they start to show that GM almost always under damps their cars. Also, there is plenty of rubber in the suspension to allow the four corners to absorb a lot of the road noise and harshness. Add to that the characteristic flex in the rear of the chassis and you get that "cycling" of the rear end in those big sweepers. Also, Kappas understeer, so as the rear tries to step out the front is dealing with slight understeer, and the two ends start to compete and neither is helping the other. Install a ProBeam. Trust ALL of us on THAT. The difference will be stunning. AFTER you do the ProBeam, get used to it. If you want the car to be a bit more responsive and front/rear neutral, order up a GXP/Z0K rear sway bar, brackets, and bushings.

[Sly Bob]

Thanks for the insightful post Bob. My next mod was gonna be the GMPP tune but now it's the Probeam.

I need to get this tightened up before I go any further.

Talking tires down the road. Would a little lower profile tire on a 19" rim help or would a decent set of Direzza Sport Z1 Star Spec tires do the trick?

[nomead]

I just glad to hear that it's not just me!
I ordered the probeam + backbone a couple days ago figuring that would help with this. Of course with all the positive reviews/comments I've been hearing here I knew I wanted them anyway. I am still running on the stock rubber, so I guess that is next on the list.

[Ben L]

   re the stock tires.  My GXP handled like a pig and very unpredictably in early autox attempts on the factory tires.  The only thing they are good for is burnouts.  

[sol_man]

   Ditto....

[deepwater805]

About 18 months ago I went into a 45 mph curve on PCH at Sycamore Canyon near Malibu at 90 mph. The rear end felt like it wanted to slide, so I shut it down muy pronto. The next day I ordered the Z0k suspension. After installation I took the same curve at 100 mph, and it never wagged a bit. It did however, take a week for my adenalin to come back down. 

[Kelu]

Same in my Opel, I hear waterfall making some noises when I push it in the corners and I do it pretty often, I drift my back just a little bit and then I hear it.

I have only a thick custom backbone made of steel. I will get ProBeam with the help of G8tr.

Should I go for Z0K also?

[snaponbob]

Thanks for the insightful post Bob. My next mod was gonna be the GMPP tune but now it's the Probeam.

I need to get this tightened up before I go any further.

Talking tires down the road. Would a little lower profile tire on a 19" rim help or would a decent set of Direzza Sport Z1 Star Spec tires do the trick?

19's will not help the handling of the car at all. More rotating mass to control, more unsprung weight, etc. Pure bling. ALL sorts of good tires available besides the stock rubber.

[Ben L]

 

If handling is the goal, larger diameter is not better, for the reasons Bob cites.

My engineer/racer pals told me that the effect on the moment of inertia associated with increasing diameter by one inch is quadratic, meaning to the fourth power! 

I am a functional retard when it comes to math, and have no idea what all that slide rule jockey gobbledygook means.   

My takeaway was that the car has to work harder to accelerate a larger diameter tire.  And that's bad. 

For example, gear ratios are keyed to the intertial moment associated with the stock tire diameter.  You might find fourth and fifth kinda weak by plus sizing diameter.

For these reasons, when selecting dimensions for custom CCW 3 piece race wheels for the FFR Daytona Coupe, we could have had 18" or even 19" wheels.  The make 'em in those sizes, and we had plenty of height in the fenderwell, and it would have looked "cooler."

But because the whole idea of that car is to go fast very quickly, we went with 17s, for all the reasons Bob states (plus a larger selection of various R compounds).
 
Also when you get up to 19s, you might find choices in tires become more expensive.  Just my two cents.

[lil goat]

Believe or not the Drift guys want as much traction as they can get, the Sears Sky runs Eineki 18X9 wheels and the Gardella Solstice runs 18X9 OZ Allegra's. They get free tires so they could easily use a bigger wheel if it served any function, they don't. These cars have 700 and 500 horse power respectively.

[Sly Bob]

That clears up a lot of misconceptions that I had, 18s it is. Thanks for the info guys.

[LatinVenom]

Kelu, the pro beam is equivalent to the Z0K brace. Why you may want to add is the rear sway bar from the Z0K if Pontiac ever start making more again.