Problem with 18" PS2's (not N-Spec)

[mjw930]

I've been fighting a problem with a set of PS2's (non N-Spec) on my Boxster S. The biggest problem manifests itself as a high speed shimmy (vibration) at 70mph+. It doesn't get a lot worse as the speed increases but it will travel through the entire car. I have had the 2 fronts replaced under warranty and it helped but didn't clear the problem.

As strange as it may sound I think the very soft sidewall of the non-Porsche spec PS2's combined with the relative stiffness of the Boxster and the Boxster's inherent drop top flex sets up a sympathetic vibration in this tire/size combo. I've had the wheels checked for true, the road force comes in at single digits and it's literally balanced to .05 oz on the top of the line Hunter 9700 machine (calibrated just before we put my wheels on it).

For reference the combination is Kinesis K18R's in the proper Boxster 18x8.5 and 18x9.5 size. The tires are 225/40x18 and 265/35x18 PS2's (not N-Spec)

Another thing I dislike about this combo is the effect it's had on the handling. Even though the car generated extremely good corner force the feel and turn-in are quite sloppy. The slip angle on these tires makes the rear twitchy and the front sloppy. On top of that the tramlining borders on dangerous. Not what I expected after my previous experience with the PS2's on other cars. Having felt the N-Spec PS2's on a 911 I'm becoming a believer that there is an internal difference between the N-Spec and non N-Spec tires of the same brand.

I may be going to either the N spec Pilot Sport Ribs or some other brand real soon. I want the best performance I can get and I spent top dollar on the best stuff I could find, Kinesis K18R's and Michelin PS2's. I'm quite upset that $5000 later the car rides and handles worse than it did with worn AVS sports on Mille Miglia replica 17's

Thoughts?

[TriGem2k]

Originally Posted by mjw930

I've been fighting a problem with a set of PS2's (non N-Spec) on my Boxster S. The biggest problem manifests itself as a high speed shimmy (vibration) at 70mph+. It doesn't get a lot worse as the speed increases but it will travel through the entire car. I have had the 2 fronts replaced under warranty and it helped but didn't clear the problem.

As strange as it may sound I think the very soft sidewall of the non-Porsche spec PS2's combined with the relative stiffness of the Boxster and the Boxster's inherent drop top flex sets up a sympathetic vibration in this tire/size combo. I've had the wheels checked for true, the road force comes in at single digits and it's literally balanced to .05 oz on the top of the line Hunter 9700 machine (calibrated just before we put my wheels on it).

For reference the combination is Kinesis K18R's in the proper Boxster 18x8.5 and 18x9.5 size. The tires are 225/40x18 and 265/35x18 PS2's (not N-Spec)

Another thing I dislike about this combo is the effect it's had on the handling. Even though the car generated extremely good corner force the feel and turn-in are quite sloppy. The slip angle on these tires makes the rear twitchy and the front sloppy. On top of that the tramlining borders on dangerous. Not what I expected after my previous experience with the PS2's on other cars. Having felt the N-Spec PS2's on a 911 I'm becoming a believer that there is an internal difference between the N-Spec and non N-Spec tires of the same brand.

I may be going to either the N spec Pilot Sport Ribs or some other brand real soon. I want the best performance I can get and I spent top dollar on the best stuff I could find, Kinesis K18R's and Michelin PS2's. I'm quite upset that $5000 later the car rides and handles worse than it did with worn AVS sports on Mille Miglia replica 17's

Thoughts?

THe PS2 Tire in General is a soft tire regardless of the N Spec or not. It is more of a comfort tire

[mjw930]

Originally Posted by TriGem2k

THe PS2 Tire in General is a soft tire regardless of the N Spec or not. It is more of a comfort tire

While they may be a softer tire than many of their competition, branding them as a comfort tire doesn't do them justice. In the proper application there are very few "street" tires that can match them in most performance metrics. I have them on my Volvo V70R and they transformed that car. On GT3's and Carerra GT's they are without match. It just seems that the construction of the PS2 225/40x18 and 265/35x18 versions aren't a proper match to the Boxster's configuration.

[larez2]

mjw930, you sound very knowledgeable about tires and wheels. I think you might be onto something. I have 18's - turbo twists. I had the same vibration/shimmy that you describe, most noticeable felt in the steering wheel. My tires at the time were Pirelli P-Zero Asymetrico N-rated. I got the tires balanced on a Hunter 9700 (and rebalanced). I got an alignment. I paid the local dealer to check the suspension and steering components and take it for a road test - the tards charged me 50 bones and said they "observed normal steering wheel and suspension feedback" and that nothing was wrong. I searched the message boards and saw that other people were having similar issues. I just lived with it.

Then the tires wore out I bought a set of new tires, Pirelli Nero's non-N-rated. The shimmy/vibration? Gone.

I don't know why there was the difference, it's feels like it's a different car. Maybe it has something to do with one of the tire models' sidewalls being softer as you describe. In my case, with the non-N-spec tires, there was no observed shimmy, and with the N-spec tires that I had before did have a shimmy.

HTH and good luck,
Larez2

[TriGem2k]

Originally Posted by larez2

mjw930, you sound very knowledgeable about tires and wheels. I think you might be onto something. I have 18's - turbo twists. I had the same vibration/shimmy that you describe, most noticeable felt in the steering wheel. My tires at the time were Pirelli P-Zero Asymetrico N-rated. I got the tires balanced on a Hunter 9700 (and rebalanced). I got an alignment. I paid the local dealer to check the suspension and steering components and take it for a road test - the tards charged me 50 bones and said they "observed normal steering wheel and suspension feedback" and that nothing was wrong. I searched the message boards and saw that other people were having similar issues. I just lived with it.

Then the tires wore out I bought a set of new tires, Pirelli Nero's non-N-rated. The shimmy/vibration? Gone.

I don't know why there was the difference, it's feels like it's a different car. Maybe it has something to do with one of the tire models' sidewalls being softer as you describe. In my case, with the non-N-spec tires, there was no observed shimmy, and with the N-spec tires that I had before did have a shimmy.

HTH and good luck,
Larez2

I have the same shimmy on my car and dealer says it is normal and may be caused by wheels/tires. I have had 3sets of different wheels 17s & 18s and so many different tires on my car that i cant even count. Shimmy has never gone away. Can't find anyone that can explain it. Ive used both N Rated and Non N Rated tires. Same thing from both. In fact the Bridgestone S-03 were the only tires that minimized it to the point where i would have to pay attention to feel it.

Anyone with advise or an idea as to what it might be?

[MNBoxster]

@TriGem2k & mjw930,

What is the mileage on your cars? Do you Track them? What are the Roads like in your area(s)? Have you ever had non-standard sized Tires or wheels on your Car(s)? Did you buy your Cars New or Used?

Happy Motoring!... Jim'99

[mjw930]

Originally Posted by MNBoxster

@TriGem2k & mjw930,

What is the mileage on your cars? Do you Track them? What are the Roads like in your area(s)? Have you ever had non-standard sized Tires or wheels on your Car(s)? Did you buy your Cars New or Used?

Happy Motoring!... Jim'99

2003 Boxster S w/ 10000 miles purchsed used with 8600 miles on it. A 4 wheel alignment and complete suspension inspection was done by my dealer @ 9500 miles after the Kinesis rims and PS2's were installed, they gave it a clean bill of health.

I live in Florida but the previous owner lived in rural PA (I suspect my roads are a lot better then his were). The car came to me with 17" turbo twists in 17x7 and 17x9 sizes (aftermarket wheels installed by Brandywine Porsche when the car was delivered new) and the original AVS Sports. On the 1000 mile drive home from the purchase the car exhibited only moderate vibration in the rear at speed above 100 mph. To my knowledge the car has never been damaged.

To this point the car has had 17x7 and 17x9 wheels with the stock 205 and 255 tires and 18x8 and 18x9.5 wheels with the stock 225 and 265 tires.

I think I know where you are going with this, the car's suspension is perfect as are the rims. Runout on the Kinesis rims is less than .010 in all directions. To my knowledge it was never tracked though I have done one Autocross.

[TriGem2k]

Originally Posted by MNBoxster

@TriGem2k & mjw930,

What is the mileage on your cars? Do you Track them? What are the Roads like in your area(s)? Have you ever had non-standard sized Tires or wheels on your Car(s)? Did you buy your Cars New or Used?

Happy Motoring!... Jim'99

2001 Boxster, 43,430miles, Car has never been tracked. The roads in California aren’t bad at all. I’ve actually noticed that the shimmy occurs more severe on different types of road surfaces, the car only shimmies around the 3,000-3,400RPM range. I’ve had 3 types of different wheels on the car. The first being 18" turbo Twist that came from the factory. Sold them only because I had an incredible offer on them. I was given a set of 2001 17" Boxster wheels as part of the deal. Now just recently (2-3months ago) bought a set of 18" O.Z. Racing wheels. I’ve gone through over 10sets of different tires/brands/models. I’ve used standard sizes for the most part. The only time that a non standard size was used was on the 17" Boxster wheels. I ran a 225/45/17 in the front rather than a 205/50/17. Right now I am thinking about pushing a 285/30/18 in the rears on my 18X10s for my net set coming up soon. The car was bought new with an extended warranty that just ended 9/10/05 .

Oh almost forgot the car has had 3 different suspension set ups as well.... 1st being the stock. 2nd was H&R Springs with Bilstein Shocks. Now running Bilstein PSS9... Next comes the H&R Coilovers....

[TriGem2k]

Originally Posted by TriGem2k

2001 Boxster, 43,430miles, Car has never been tracked. The roads in California aren’t bad at all. I’ve actually noticed that the shimmy occurs more severe on different types of road surfaces, the car only shimmies around the 3,000-3,400RPM range. I’ve had 3 types of different wheels on the car. The first being 18" turbo Twist that came from the factory. Sold them only because I had an incredible offer on them. I was given a set of 2001 17" Boxster wheels as part of the deal. Now just recently (2-3months ago) bought a set of 18" O.Z. Racing wheels. I’ve gone through over 10sets of different tires/brands/models. I’ve used standard sizes for the most part. The only time that a non standard size was used was on the 17" Boxster wheels. I ran a 225/45/17 in the front rather than a 205/50/17. Right now I am thinking about pushing a 285/30/18 in the rears on my 18X10s for my net set coming up soon. The car was bought new with an extended warranty that just ended 9/10/05 .

Oh almost forgot the car has had 3 different suspension set ups as well.... 1st being the stock. 2nd was H&R Springs with Bilstein Shocks. Now running Bilstein PSS9... Next comes the H&R Coilovers....

Jim, any thoughts?

[MNBoxster]

Originally Posted by TriGem2k

Jim, any thoughts?

Hi,

AHA...! Different suspensions... ! My Guess is that there's a problem with your Ackerman Angle or your Scrub Radius. I know I sound like a Broken Record on this, but if your car's alignment is set to Factory Specs they very probably no longer apply.

Unfortunately, I do not have a clue as to what the correct specs may be. But, once you go away from the Factory set-up, you also go away from Factory Specs. The AfterMarket Companies don't offer substitute specs for their set-ups, which is unfortunate ( I mean Geometry is an exact Science and you can't just ignore it). I don't want to Harp on you or Single you out, but this is the issue I have been speaking about - Major Kudos to you for being astute enough to realize the problem - many wouldn't have.

My strongest suspicion is that your Scrub Radius has been messed up and your Tires are essentially fighting each other, causing the Shimmy. I mean you have already eliminated the Wheel/Tire combination.

All is not Lost however. Depending upon how Anal you want to be in solving this, I would go to an Alignment Shop specializing in Race Set-ups and let them have at it on an Alignment Rack. I suspect the proper Tie Rod and Toe Adjustment will at least better, if not eliminate, the issue. It could even be a Steering Rack, or Tire Pressure adjustment.

But, I also have a haunting feeling that the bushings may be involved. Since there isn't any alternative there (that I know of), a total solution may not be in the Cards.

But, again, depending on how Anal you want to be, there are a plethora of Bushings out there and one may work, even if it's for another application ( Ironically, probably a Yugo... ) - hope you have a Micrometer and Caliper, if not, you can borrow mine. Best of Luck...

Happy Motoring!... Jim'99

[TriGem2k]

Jim, I never thought of it being an alignment problem, i think u have something there. Once u mentioned it i realized it never started doing this until i had the car realigned at the dealer. I think you just solved my problem.

Now to find an Alignment shop i trust.....