Good stuff -- that all makes sense. I think I've gone to Stage 4 very quickly. And I suspect my next stage is catastrophic and may take out a bunch of expensive bits...

It's a good thing I'm out to pasture, what took me 30 years to learn can now be done with a phone.
Only better.


I used to have power
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In some ways, I think those with the experience are only more valuable, particularly as they become more scarce.

As the newer equipment (I'm talking plant equipment) increasingly comes with condition monitoring sensors, it's likely the old timers that will best know how to troubleshoot a situation that falls outside of what the equipment expects to be wrong.

Things seem headed toward the part-swapper model in general.

But I do like having access to so much information (and data) in my phone.

Agreed, but try telling that to the young engineer/MBA who just got promoted to be your boss ;)
The world is changing


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I think I found my problem! https://uploads.tapatalk-cdn.com/202...f99d940fbc.jpg

Do tell.

Is that part of a valve?

I have absolutely no clue where that belongs on a car. Toilet valve = yes. Car = no

So, upon further reflection, I don’t think the picture of the item above was actually my problem. But it may have been part of the solution.

That disk is a brake pad vibration dampener. There are six of them on each front caliper (one for each piston) — the rear calipers appear to use a slightly different, but similar setup. The brass “spider” fits into the bore of the brake pistons and the other side is glued onto the back of brake pad thus fixing the brake pads to the pistons.

You may remember at the beginning of this thread that I’d mentioned that I’d replaced my rotors thinking that they might be causing the vibration issue. When I replaced the rotors, I also did the pads, but I did not replace the vibration dampeners on the new pads. I’d done some reading on the forums and plenty of guys left them off — especially the track guys — as those dampeners require you to remove the entire caliper from the rotor to change the pads, versus just undoing the single large retaining bolt on the caliper and sliding the pads out the back.

I’d noticed that I was getting some clunking from the front end over certain harsh bumps but I wasn’t sure what it was. I’d suspected that it might be the pads, but it might also be a suspension clunk from the front — hard to tell.

Having replaced all those little vibration dampeners this weekend, I can assure you that they are needed if you don’t like clunking. In combination with the pad retaining spring, they very positively locate the brake pads to eliminate all pad-related rattles. Very important for NVH control.

So it must have been the giant pads clunking around, making weird vibrations? After all, those pads weigh around 10lbs and were possibly being vibrated at certain speeds. Enough to shake the car?

Maybe. Although, I don’t think so any more. Why?

Because the vibration existed before the pads were changed — I went back and checked my notes. But I am happy the clunking is gone!

So what about the vibration? Well, I’m happy to report it’s very much reduced (dare I say gone?). More updates in a bit...

Need help with a strange cyclic car vibration
 

So, as mentioned, I think I’ve got the vibration greatly reduced (gone?).

A couple weeks back, I left off with a video of what I was pretty sure was a failing water pump making a big racket. Here’s that video again:

https://youtu.be/w2kK6qhib5k

And here’s the after video:

https://www.youtube.com/watch?v=QptdomcIo2E

BIG improvement and thankfully no damage from the failing pump. Side note: Replacing the water pump on a Mezger engine is kind of a PITA — mostly because it requires removing the rear bumper, exhaust, rear engine support bracket and a bunch of other stuff before you get to the pump. Whatever, it’s done. Sounds good.

In addition to less noise, replacing the water pump reduced felt vibration at idle and a little bit at speed, but I still had the weird cyclic vibration.

I’d also mentioned a few weeks back that I’d thought my front strut bushings were shot. I could hear noises with the chassis ears and the rubber looked funny. So I set about swapping those out this weekend.

On Saturday I started with the left front wheel because it still had the front CV axle attached — remember, I’d removed the right front CV axle and replaced it with a stub axle from a 2wd Carrera. I figured that removing both front axles from the equation would at least rule them out as the source of the vibration.

The old strut mount was definitely cracked, but it was not falling apart — the center section still appeared to be fused to some extent — I certainly couldn’t push it out by hand.

While I had the strut removed to swap the top bushing, and since I’d bought the necessary tools and replacement parts previously, I also decided to replace the left front wheel bearing. Many hours of work later, I had reassembled the front left suspension with 1) a new strut mount, 2) a new wheel bearing, and 3) no CV axle. I meticulously tightened all bolts with a torque wrench as per factory specs. Time for a test drive.

Result: Very little, if any change. Maybe slightly less vibration. But not much. Still cyclic. I was pretty bummed.

I got started fairly early on Sunday doing the same job on the right front side, except this time I was much, much faster. Like the other side, the right strut top bushing was cracked, but didn’t seem completely failed — it looked about the same. Same with the wheel bearing — it felt tight, no obvious signs of pitting when removed/disassembled.

And it was at this point that since I had the calipers off, I figured I would reattach the vibration dampeners from my old brake pads (you keep your old brake pads just in case, right?) and eliminate the aforementioned clunking on bumps. In retrospect, I wish I’d only done this pads on the right side, but since I was working much quicker I added the dampeners on the left side pads too. Final work done: 1) new right strut mount, 2) new wheel bearing, and 3) twelve brake pad anti vibration buttons installed on all four front pads.

Since the car was already in the air and since it was now a 2WD car, I decided to run it in the air and look for vibrations. Here’s a video of the results at a wheel speed of 85mph:

https://youtu.be/eMTID77fxAE

It looks and sounds pretty good. There were very small vibrations that don’t film well, but nothing shocking. Even my aforementioned “jump roping” rear axles looked fine this time.

But it was raining yesterday, so the test drive had to wait until today: And the preliminary results are very good!

The car feels great actually — Very tight and smooth and controlled over bumps and no clunks. And perhaps most important, the cyclic vibration appears to be gone, even on grooved pavement.

I did some vibration logs and while there were still peaks, the intensity levels were down around .25Gs or less — which is about where I’d said was the threshold of what I felt when driving my wife’s car. I still think there’s a bit of vibration in the back, but it’s nothing compared to the cyclical vibration — totally livable. There’s a small wiggle in the back from hard throttle applications — I suspect I’ve got something a bit worn in the rear suspension that’s causing a toe change under acceleration. But it’s fine.

So what was it? Well, I think it had to be either the right front strut mount or the right front wheel bearing. Hard to say which. I’m inclined to say strut mount, only because the wheel bearing seemed tight, and again, no obvious damage on the races. Why was it cyclical? I have no idea. Maybe I’ve got a similar vibration in the rear except and the very small difference in tire sizes caused the vibrations to cancel and “beat”. Dunno.

Given the age of the suspension, I may also tackle the rear strut mounts too — apparently they’re known to go bad too. Maybe that tightens up the rear and further eliminates vibrations.

Could the brake pads have been a contributing factor — it would seem like yes, except that the vibration preceded the removal of the anti-vibration buttons from the backs of the pads.

I need to do some more driving, but I’m pretty excited — it seems too good to be true after all the trial and error of getting this sorted.

Fingers crossed. Thanks for all the good suggestions and feedback.


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Suspension feels much better with the new front strut mounts (and sounds better with new water pump), but the annoying cyclic vibration is still there on the grooved pavement at 85 mph.

Current hypothesis is that one or more ball joints/bushings is allowing the suspension to oscillate on grooved pavement. Admittedly, I don't know why it would be cyclic. Maybe both the front and rear are both loose and the cyclic nature reflects different resonant frequencies? Don't know.

Anyway, I've elected to replace the most common wear items in the suspension - - basically everything except the "tuning forks", aka thrust arms, which apparently are less prone to wear.


List of new parts includes:
Front lower control arms (upgraded to GT3 for more camber/track)
Front tie rod ends
Rear shock top bushing
Rear lower control arms (upgraded to solid thrust arm bushing for less toe change under accel/decel)
Rear toe arms

Will look to set alignment at GT3 settings. My current alignment seems to be heavy on toe in front and rear. Can't help but wonder if that makes a difference.

Place your bets if any of these parts/changes make a difference.

After spending a lot of time and money swapping parts, I'm left with a very nice handling car that still vibrates oddly on the grooved stretch of road I frequent.

In addition to a substantially new set of suspension arms and bushings, almost every rotating item from the transmission outwards to the tires was checked or replaced. About the only thing I didn't get around to replacing was the rear wheel bearings and the shocks/dampers (which all seem to be doing their job just fine).

New front strut mounts made the biggest difference -- they seemed to half the feel of the vibration. Most everything else -- meh, not much change.

As I have no further ideas of what could need attention on my car, I'm going to give up and accept that it does not appear to be in my control. I think there's just something unusual about the combination of grooved pavement and my car/tires (at the right speed) that sets up a weird harmonic. With all new suspension parts, the car feels great on "normal" roads, BTW.

I was telling my saga to a friend with a well-setup 964, and he says the same grooved stretch of road makes his car go nuts -- not with vibrations, but darty squirminess. Another friend (who's wheels I borrowed) is going to drive his car on the same stretch and report back. I'm hoping he has a similar issue as me.

I did learn a lot about the 997 suspension including:
- Most of the 997 suspension parts interchange with the 996 and vice versa
- 996 GT3 lower control arms are materially cheaper than 997, so many people elect to save money by buying the "older part" -- there is a slight difference in the bushing thickness, but the aftermarket sells spacers to make all the parts fit the 997
- All the 997s and 987 Caymans and Boxsters use the same steering rack (and also share many other parts)
- The front suspension on the AWD models (4S and Turbos) has a different front knuckle that positions the strut at a weird angle so as to provide clearance for the front axle -- this weird angle may contribute to relatively short strut mount life on such models as the strut flexes the bushing more when turned.

Wish I could have reported back with a surprise smoking gun. Who knows, maybe I'll stumble on something in due time.

Thanks to everyone that offered suggestions.

Wow, didn't see that coming, as the newest 911 I've worked on was an SC and it didn't have those new-fangled gizmos.
But I know engineers don't put dampeners on things unless they have vibration problem they need to dampen.

Wierd. As you know the 997.1 is virtually identical to the 996 underneath. I have quite a bit of experience with 996 setup for the track and have never had vibration issues. There was recently a thread on Rennlist where a 996 GT3 owner was getting similar vibrations to what you describe. I'm not sure if he ever sorted it out... Go have a look it's on the 996 GT3 forum.

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Yep, those little disks keep the brake pads from clunking over bumps and when changing forward/reverse direction. They probably also help with squeal, but I didn't have that issue without them.

Thanks, I'll check that out. I've spent some lurking time in the 997 GT3 forum and the 996 Turbo forum, but not the 996 GT3 room.

Good news is, except for that one stretch of grooved road, the car feels pretty darn good. I'd be thrilled to find a solution for the few miles it vibrates on, but I've made my peace if I don't.

I originally replied in this thread that you may be having worn suspension bushing-related vibrations. I got a good look at 9-year old suspension bushings last year. I would say they had probably less than 5 years of life left in them. The rears, probably less.

I changed my front control arms and "dog bone" looking toe link arms on my 987.2 Cayman, also for more negative camber. The car is transformed. I also changed the uprights/ steering knuckle to accept the 6-piston Brembo calipers. Front suspension is really easy to change.

I suspect the rear suspension was a huge PITA. And I have no idea how to get all of the plastic interior pieces off of the car in a 987.2.

Pic from another Rennlist thread. Bent tuning fork arm was the culprit. Too subtle to notice. https://uploads.tapatalk-cdn.com/202...ef56c21095.jpg

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Glad this thread popped up. I have the same exact issue on my 987 Cayman. Will have to investigate a bit further when I get some time.

My suspension definitely feels better than when I started this whole process:

Key changes / upgrades:
- New DSC suspension profile -- it firmed up the shock profile
- New front strut mounts -- this was my most worn part
- GT3 Front Lower Control arms w/ 7mm spacers on each -- stiffer bushings, wider track, more camber, new ball joints all sharpened things up
- New rear lower control arms with Elephant solid thrust arm bushings -- adjusted to lengthen wheelbase slightly, should be no change under accel/decel (except from rubber toe arm bushing flex)
- Upgraded to thicker GT2 rear sway bar
- Factory GT3 alignment (vs lazy Turbo spec)

Net result on handling: As one would expect, grip was moved forward (largely due to the bigger rear bar and more front camber). Much better turn-in and balance (but not to the point of oversteer).

Also, completely agree that rear suspension is harder to work on than the front -- that goes double for a 2wd car where you don't have the front axles to deal with -- 2wd front suspension would be a breeze.

Just doing a rudimentary "good enough to drive to the shop" alignment on the back is a serious PITA without a lift. The back adjuster bolts are near impossible -- turning the eccentric bolts and tightening them is really finnicky; the front is much easier as you can just reach the tie-rod adjuster with the car on the ground.

But my weird vibration is still a head scratcher...

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.