Need help with a strange cyclic car vibration

[brainz01]

Originally Posted by sammyg2

Since it's belt-driven (side load) it'll go downhill fast, good plan to change it now. If you can hear it, then it's way past bad.


On straight coupled pumps, there's a strange phenomenon:

• First stage bearing failure, usually not detectable without high frequency analysis.
• Second stage failure, starts to appear in regular spectral analysis but very low amplitude, likely not detectable by ear without stethoscope (yes I got one, still)
• Third stage failure detectable by ear, temporarily gets quieter with addition of grease, barely detectable by feel. Shows non-synchronous peaks between 4x and 8x coinciding with inner or outer race frequency or fundamental train frequency (cage, ball spin etc). Still low amplitude.
• 4th stage failure easily detected by ear and feel, larger non-synchronous peaks with side-bands and harmonics (haystacks).
• 5th stage may get quiet. May not be detectable under normal diagnostics

Note that is my list and others may see it differently or have different definitions of the stages, there is no O'fficial written in stone version that I am aware of.


On straight (in-line) coupled pumps, assuming the alignment is good, the shaft can be supported by the coupling and magnetic field so when the bearing balls disintegrate into little black pebbles and fall out, they no longer make noise. The shaft is essentially riding on air.
But shut it down and then try to re-start it and all heck will break loose.

Been bit by that one once. Uno.

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...

[sammyg2]

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

[brainz01]

Originally Posted by sammyg2

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.

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.

[sammyg2]

Originally Posted by brainz01

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

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

[brainz01]

I think I found my problem!

[javadog]

Do tell.

[sc_rufctr]

Is that part of a valve?

[asphaltgambler]

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

[brainz01]

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...

[brainz01]

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.


Sent from my iPad using Tapatalk

[brainz01]

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.

[brainz01]

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.

[sammyg2]

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.

[Nickshu]

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.

Sent from my Galaxy S20+ using Tapatalk

[brainz01]

Originally Posted by sammyg2

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.

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.

[brainz01]

Originally Posted by Nickshu

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.

Sent from my Galaxy S20+ using Tapatalk

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.

[rusnak]

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.

[Nickshu]

Originally Posted by brainz01

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.

Pic from another Rennlist thread. Bent tuning fork arm was the culprit. Too subtle to notice.

Sent from my Galaxy S20+ using Tapatalk

[pdxfj]

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.

[brainz01]

Originally Posted by rusnak

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.

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...