Feedback on how the car should feel

[rteichman]

I have had my car now for a few weeks (which translates into a few days of driving since I only use it on nice weekend days). This is my first Porsche, and I had only test driven a couple of boxsters before I bought this one. Even though this car was owned by a forum member who gave it a clean bill of health (I bought it from the dealer he traded it into), and I had a PPI done I still have "worries". I guess its my nature.

The car handles fantastically around curves and the engine just purrrs. Its beautiful. HOWEVER I the performance on straight roads and rough (i.e. not perfect pavement) roads concerns me. I am hoping that what I am feeling is "normal", but I would like your input.

I guess the best way to describe the issue, is that on straight roads there is a very minor vibration in the wheel/car and the steering feels "skittish". You feel it, but you can't see the steering wheel vibrate, its that minor. Also its not the same feeling as tires needing balancing. The steering is totally responsive. the tinyist input in either direction and the car goes that way (which is what i would expect). Many years ago I had a similar feeling on another car, and the dealer realigned (it was in spec already) it with at a different setting to have the front wheels "fight each other" and thus get rid of that straight line jitter. I'm not sure I'm describing it right. Also any uneveness in the road tends to reverberate through the car a fraction of a second too long, and worse you feel it through the steering a second longer than I would expect. Interestingly enough, none of these things occur in a curve. The car handles perfectly then: perfect handling, no vibration, no problems with rough roads.

So is this normal or something that needs to be adjusted? What are your thoughts?

Rob

[MNBoxster]

Hi,

It's somewhat difficult to diagnose something which can be so subjective.

That said, it's a '99 (mileage?). This means that there can be some normal wear to any number of the suspension components such as Bushings, Shocks, Springs, etc. None of these things will last forever, and any one of these could be your culprit.

When was the last alignment done? Tires Balanced? Was it a Full 4-wheel Dynamic Balance? (on a Hunter 9XXX Machine?) What size Wheels? It is not at all beyond the realm of possibility that it could be a Rear Wheel alignment issue. How old are the Tires?

From what you describe, it doesn't sound too serious or expensive to correct. Have a Quality Alignment Shop, (not the Tires Plus or Firestone), preferably a Race Shop have a look. They'll probably correct it in no time.

The resonance of the Car you mention can simply be due to the fact that it is a convertible, not as rigid as a coupe. Strut Tower Braces go a long way to improving this. Good Luck!...

Happy Motoring!... Jim'99

rteichman, I am sure you will get some other good advice from forum members, but my only imput would be to get a really good digital tire gauge, and frequently check your tire pressures. This time of the year you can get huge temp swings and this can greatly effect your tire pressures...and possibly handling.

[rteichman]

It has 43,000 miles on it with 18" wheels and brand new tires. I believe the car had an alignment recently (don't have the paperwork handy) but I have no idea of the quality. I should also mention that the wheels/tires were put on after the alignment. Thanks for the advise I will try to find a good shop and schedule an alignment.

Any suggestions on finding a good alignment shop in my area (Lehigh Valley, PA)?
Rob

[Perfectlap]

new wheels and tires won't affect the handling after an alignment.
At what speed do you feel the vibration?
I get the same thing around 65mph but I put it down to some flat spotting on my tires.
You do have to keep in mind tha a more responsive car is also a more sensitive car and you'll feel imperfections in the road in a much loud and clear way then you would driving along in a 4 ton sedan.
I suspect there is some suspension maintenance needed in your car if new tires have been mounted and your alignment was recently done. I doubt its the springs and shocks, when those go the symptoms don't really seem like what you've described. could be a bad wheel bearing some where.

[MNBoxster]

Hi,

I hate to disagree with Perfectlap, but handling can be adversely affected by the Tires. The alignment won't be affected, but the handling, especially vibration issues at Speed can well come from a set of Tires, New or otherwise

No Tire is perfectly round, nor is any wheel, they each have a High and Low side to them, an eccentricity. The inherent eccentricity is called Runout. A proper Tire Shop will mount the Tire so that it's Low Spot matches the Wheels High Spot, this reduces, but may not entirely eliminate the Runout problem, leaving some vibration in the Tire at some speeds.

In addition, a Tire is a Dynamic thing. It changes due to Forces exerted on it. A Tire may have a minimal Runout when not under load, but, due to variations in construction, may actually become more out-of-round once it is loaded.

This is why I recommend a Dynamic Balance of the Wheels/Tires. This differs significantly from a standard Spin Balancing. A Dynamic Balance puts the Wheel/Tire under Force by spinning it against a Loaded Road Wheel. This allows the Tire to be observed as it would actually run on the Street. Typically, Wheels/Tires are observed and corrected for 50MPH, but any speed can be selected and checked.

A Company named Hunter Engineering manufactures this type of equipment, the latest generation being the GSP9700. I would not have my Wheels/Tires mounted and Balanced by a shop which does not use this equipment. You can see a detailed explanation of all this here: http://www.gsp9700.com/pub/technical/5098T/5098T.pdf

One problem is that not every Wheel/Tire combination can be made to work. There are limits to the eccentricities which can be adjusted for. I once bought a set of Dunlop SP Sport 8000 Tires for my Esprit from Tire Rack and two of them were out of Spec when checked on the GSP9700 (which gives you a Graphic Printout of the Tire, it's Specs and it's Footprint). The Shop refused to mount the Tires as they have high Quality Control on their work.

I had a heck of a time getting Tire Rack to exchange them. They don't check the Tires for this and so did not consider it to be faulty. It took two more pairs from them to get it right.

Interestingly, the Manager said that they see much more of these problems with Tires bought from Discount Houses such as Discount Tire, Tire Rack, Costco and the like, over those sold by the Tire Manufacturers' Outlets and Franchises, by a factor of about 5:1, a significant amount. He speculated that the Tire Manufacturers keep the best Tires for their own Outlets and Franchises...

Happy Motoring!... Jim'99

[Perfectlap]

yes you are right I mispoke, I meant to say that the alignment numbers should not change because you are changing tires.
But yes one set of tires will affect the handling vs. another set entirely. Going from Conti Touring tires to streetable R-compounds was a great example.

and yes the manufacturers aren't stupid. They know every batch of tires of the same exact tire aren't always the same. Even motorsports teams fail to catch a bad batch until the driver is out on the track so I wouldn't doubt that a discount broker like Tire Rack would even bother checking. But if you have the time they will get it right eventually. To some their time is less valuable then the money they may save which can be a whole lot with $250 tires (EACH!) now the norm on high end sports cars.
BTW, my preferred bicycle tires are now $50 each. Our dependence on oill...

[insatiable]

I'm not sure if my experience matches yours but I described it in the above reference.
Not much input though, I think most Americans must drive on good quality roads!!
I noted with interest the comments received to date and my first reaction was that any balance or alignment would surely show up on a smoothe road as well.
Cheers, I'll follow this one with interest.

[denverpete]

I guess I'm struggling a bit rectifying the description of the problem with the fact you've only driven it a few times. Don't get me wrong, if you feel something then by all means get it looked at. This is your baby and you need to be happy with it.

I just know that after I've been out of my Boxster for any length of time (which is rare), I tend to be a little more sensitive to every little perceived sound, vibration, or performance issue. I keep thinking - "was that there before", "did it always make that sound", "why isn't an apple called a RED". You know, standard stuff.