Thanks for steering me to the RL 996GT3 forum. I spent a bunch of hours over there last night reading any relevant threads with the word "vibration". That forum is great -- way better and more technical than the 997GT3 forum where they spend more time debating collectibility and the latest rage seems to be putting all the cars back to stock, you know, for originality/speculative value....

I did find a couple threads that mentioned vibration, but the threads that were most interesting (and similar to my situation) seemed to suggest that wheel/tire roundness and balance were at the root cause. Go figure. It was a decade ago, but one guy had multiple "bad" Michelin tires -- couldn't get rid of the vibration until he'd finally replaced multiple tires. Another guy (Cayman owner, I think) had a nearly identical cyclic/oscillating vibration and ultimately tracked it down to a wheel/tire combination.

So now I'm thinking I might search for the best tire place in town and let them have a go at measuring runouts and rebalancing all the wheels (for a 5th time).

That bent tuning fork picture is pretty incredible -- looks to be the (narrower) rear one -- I used reverse image search and saw it was on Tom's legendary GT3 build thread. My guess is the PO or shop lifted the car from that arm. I've got to say, however, I don't see how that would cause a vibration as long as the alignment was in spec. Maybe because the caster would be different side to side? But again, that should be picked up in the alignment. Regardless, it's a good prompt for me to review my tuning fork arms as they were among the few suspension pieces I did not change out....

Take a look at this thread:

https://rennlist.com/forums/boxster-and-boxster-s-986-forum/924441-oscillating-vibration-at-high-speeds.html

It's probably the only thread I've found that specifically calls out the cyclic/oscillating nature of the vibration I'm having.

The oscillating nature of the vibration comes from two individual, separate vibrations coming into and out of phase with each other.


I still think you have two wheel tire assemblies that are not properly balanced, not perfectly round or have road force variations.

I would pay particular attention to the point made by wheel dynamics in that rennlist thread.

I keep going back to the oscillating vibration, and how I suspected you have two separate energizing sources of vibration that have frequencies that are very close to each other.

The tire sizes listed for that vehicle are
front 235/35 R19
Rear 305/30 R19

Theoretically the rolling diameter should be very close to the same.
But if the rear tire diameter differed slightly from the front, they would not be spinning the same speed and the two different vibrations could oscillate by going into and out of phase.
Probably a long shot.

Agree with you, Javadog. I'm going to do another deep dive on the wheels/tires.

Darn you JV, quick copying me before I say it!

He pisses me off. Always making sense coupled with that annoying clarity of thought.

It's not a long shot, at all. It's almost a dead certainty.

I've seen this more times than I can count. Not only are there usually circumference differences from front to rear (even if they start the exact same size when they are new, the front and rear tires wear at different rates, so always end up with slightly different diameters) you can get the same cycles from the difference in rotation speed from the left and right sides, when the car is driven in a shallow turn, like on an interstate.

The point I was making about the wheel vendor's comments earlier was that it's not uncommon for a tire balancing job to get screwed up because the operator doesn't know what he's doing, or is sloppy, or the machine isn't a good choice for the specific wheels in question. Wheels and balancing machines have evolved, they aren't always compatible with each other.

Decided to get new wheels.

I think these are 28s or 30s.
https://uploads.tapatalk-cdn.com/202...d773171df4.jpg

American Tire and Wheel says my wheels were imbalanced and I'm good to go now. No bends or run out.

Test driving shortly.

I know, really gets old after a while! SmileWavy

Got the wheels road force balanced again this afternoon (attempt #5). Very interesting as the rear wheels are now almost weight free. I'm hoping that means the installer rotated the tires on the rims for best balance, but I didn't see the typical installation "soap" residue, so not sure. The fronts mostly have the weights in similar, but not identical positions and amounts as before.

It definitely made an improvement. The steering feels amazingly smooth/communicative. The felt vibration through the chassis is greatly reduced at 85mph and the passenger seat no longer visibly shakes. It also seems to cycle a little slower. It's not perfect, but it's much improved.

Debating whether to have the shop try again, but not sure I want to mess with it and risk having it get worse...

I'm amazed at 1) how sensitive the car is to wheel balance, and 2) how difficult the wheels are to balance. There's got to be some secret "Porsche thing" or calibration that's not being done on the balancer.

JavaDog: Sounds like you've got experience in this department -- any tips on how to get wheel balance done right?

I used to do them myself, using a friend’s machine.

Failing that, you just have to look for someone with the latest equipment, plus the willingness to spend whatever time it takes to do it right. You have to make sure they don’t have any problems with using the correct adapters and always make sure they spin them again after putting the weights on and get an absolute zero balance. Close is not good enough.

The other thing I would do sometimes is calculate where the weights were going to be and measure a little more carefully to tell the machine where the centerline of the weights was. The way some machines calculated that in the past wasn’t all that accurate. I also put weights on both sides of my wheels. I was more interested in a perfect balance than hiding the weights.

Lastly, when you change tires, consider spinning the wheels up without any tires on them to see where the heaviest points on the wheels are, and marking them. Then talk to the tire engineers for whatever company you choose to buy from and get them to tell you which of the colored dots is which. Do you have to decide whether to go for putting the heaviest part of the tire where the light is part of the wheel is or I’m using the road force dot if you have run out in the wheel.

I also tried to pick lighter tires, there’s often a variation of several pounds between brands. My go to brand was usually Bridgestone but I had good service from Pirelli’s as well.

Very helpful. Thank you.

And by the way, I went back and checked: full credit to you for being right (and consistent) about the wheel balance issue from the outset.

Very frustrating/confusing when wheels are rebalanced multiple times and yet the problem persists. I can't help but wonder if there's also something weird about the tires...

Some tires have a tendency to flat spot when they are parked for a length of time, especially parked when hot. It may take more than a couple miles to get them perfectly round again. Sometimes, they never are truly perfect again.

The more usual explanation though, is the balance job is just not done very well.

Do those wheel balancers do single plane (static) balance or can they adjust for couple unbalance?
Since the weights are only placed in one plane I think I know the answer but hesitate to ass-u-me .....


In my old shop I had a shenck CAB 820 to balance rotating equipment.
http://en.pasio.eu/fileadmin/pasio/downloads/RC1057eCAB820.pdf
slick as all get out but wouldn't work for wheels ;)

Point is, if all they're doing is single plane balancing and the wheel has heavy spots 180 from each other but one on the inside and one on the outside, I doubt they would detect or correct.

On most cars that would be like trying to pick the fly crap out of the pepper, but when taking it to this level ....

http://forums.pelicanparts.com/uploa...1594183525.jpg

http://forums.pelicanparts.com/uploa...1594183525.jpg


http://forums.pelicanparts.com/uploa...1594183465.jpg

http://forums.pelicanparts.com/uploa...1594183465.jpg

I have never seen a balance job on a car wheel that didn’t put the weights in two planes. Motorcycle wheels are often balanced in one plane, though.

The reason why you want a road force balance is because the simulated weight on the sidewalls will uncover any imbalance due to a faulty tire belt seam(s). I had at least one bad tire brand new that I bought from Tire Rack. I just replaced the pair. I think they were Michellins. I think it can happen with any brand though.

so they place the weights on the inside edge of the rim as one plane and the middle of the rim as another? I can see how that could work but it could require more mass to be added for couple unbalance.
Is that placement calculated by the machine? I've never operated one of those, just this type:

http://forums.pelicanparts.com/uploa...1594214335.jpg

fer giggles I checked my balancing certifications: it's been 20 years for shop balancing and 12 years for vector field balancing.
Haven't done either in a long time, and I've slept since.


http://forums.pelicanparts.com/uploa...1594215032.jpg

As you suggest, you want the weights as far apart as possible, to minimize the weights needed. In the old days this was easy. Wheels had little offset and they clipped the weights to the edges of the wheel rim.

Now, wheel offsets can be huge, the outer rims may not be compatible with clip-on weights nor have room for stick-on weights (and some people are too vain to tolerate the look of a weight on the outside of a wheel), so they put the inner weights on the inside edge of the rim and the "outer" weights on whatever flat space is available on the inside of the spokes.

There are lots of things wrong with modern wheels, that's one of them. Styling over function... Diameters are too large, widths are too wide, tire profiles are too low, everything is too damn heavy.... I don't know which group is dumber... the car designers or the buying public.