a comment....

I'm on my way to BRBS, and I've noticed an unpleasant vibration occurring (at its worst) around 80 mph. It is independent of engine r.p.m and road surface.

It feels somewhat like a shudder. It lasts for about 1 second, goes away for a second, and then returns for a second, and so on. It can be felt in the seat, the steering wheel, and my 'black box' on the dash quivers.

There isn't any vibration below 75 mph. It continues as the speed rises about 80 mph and beyond. The pitch increases with speed.

I got new PS2's a few days ago, on the front.

I would think that if it were a front tire balance issue, it would not occur in waves, a second apart - rather it would be constant at the speed it occurs. Unless, of course, somehow the back and front wheels have a imbalance which synchronizes at 1 second intervals.

The only other observation - it seems to lessen when the car is turning, even if it's only a slight turn - the type you'd find on a divided highway.

2005 987, 55k miles, all suspension and steering components are original.

Any thoughts?

At that hypothetical 80 mph, I'd say it is wheel balance. Did the shop you used balance the wheels with a Hunter Road Force machine?

Nope, they didn't have a Hunter. It can be hard to find them, I use them when I can. Last time on a Hunter, my front wheels were noted to be true and not out-of-round.

I'd have thought a wheel balance issue would appear before 80 mph.

Based on your response, I should try to get them re-balanced with a HRF machine. Just googled it, and there's an online HRF locator, Cool.

BTW, the speed limit was 70, traffic was travelling at 75, and of course, passing takes it up a notch or two.

if the new ones were tightly bound for shipping it might take a while for them to "round out." By putting on the miles, when you do have them rechecked you ought to have a more valid set-up.

or 3 depending

1. balance
1 1/2 tire OOR
2 Drivetrain vibrations. I get a lot of those.

Now, you said it is not dependent of engine RPM. How did you validate that? Did you shift while maintaining 80 mph? That's what i do. Its one way i diagnosed my failing motor mount.

Typically, the sensation you describe is not front whels. It is either drivetrain or rear wheels. Front is a steering wheel shake.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

I found a place that will rebalance my front tires with a Hunter Road Force tomorrow. That will either fix it or not.

I'm inclined to agree with Grant that the vibration in the seat would suggest something more/other than the front wheels. But it's a good and easy place to start.

I'll likely get new rears towards the end of the trip - needless to say, I'll have them balanced at that time.

BTW, looks as though I'll get about 18k miles out of the rears. Sound typical of PS2's? (SS aren't available in my size, darn)

Incidentally, i feel the Hunter simply covers up for operator incompetence. My preferred tire place never needs it, and many others still cant get it right even with the RFB thingy. You need to use your eyes to see how its seated, to make sure the 'high point" is balanced with the same for the wheel, and to see treat squirm.

I'm still suspicious of drivetrain vibrations.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Along with some of what the others had said, I'd also suggest checking the tie rod assemblies. TRW makes decent outer ends, but their inners, leave something to be desired. I'm about to replace the assemblies on our 986 which has about 60k (round town miles, no aggressive driving) and the inner ends are completely shot. Just a thought.
regards

Roger:
Was this present before the new fronts? If not, I'd think this strictly a new tire issue.
Another thought--how long since your last alignment?

I don't recall it being there when I last drove the car at speed (last fall). Given it was AFTER the new tires that I noticed it for the first time, I'm suspicious the balance might be off. I'm surprised, though, that it can be felt in the seat.

I've not had an alignment done in a number of years, but I'm reluctant to get one, given it tracks straight and true, the tire wear has been even and there's a possibility a new alignment may not be as good as what I have now. My experience with alignment shops has been hit and miss.

I'm interested in the normal replacement interval for tie rods/control arms. In my BMW's when those started to go (which was often) the first clue was steering wheel shake. Having said that, this isn't a steering wheel shake so much as what feels like a chassis vibration.

I've noted Grant's comment about drivetrain vibrations - though that's a possibility, I'm going to try to rule out the simpler things first. Weird that the vibes come and go in one second intervals. You'd think a wheel balance issue would be constant once the 'bad' speed is reached.

You don't have access to another set of tires and rims, do you?
Years ago when I bought a set of Kumho XS tires for track service I had a bit of vibration with them on. To confirm that set of tires were the issue, I put the street tires on back to back and no vibration; back to the installer it went. Just a little bit of adjustment made a noticeable improvement especially at speed..

I've had a couple of times the new tires felt just fine at (practically) any speed only to vibrate after racking up some miles. The cause is as the tires wear the balance changes. The techs tell me the Cayenne is the worst offender: Big tires. But the other models can experience this too.

Give the tire treads a feel. If you feel any evidence of scuffing, feathering, etc. I would add "alignment" to the list of suspects.

A pattern across each block that is diagonal to the tire's rotation. I associate scalloping more with excessive, progressive tread noise than imbalance vibration.

everyone who suggested tire imbalance. Top honours go to Marc W.

The fronts were off a bit .25 - .50 oz, but the rears were off 2.5 oz.

When the rears were installed about 16K miles ago, I had them RF balanced. When the technician asked today if I wanted him to check the rears, I almost said no, given I'm thrifty, I'll be replacing them soon, and I figured the original balance was still good. Clearly I was wrong (about the last bit).

I think the front and rear imbalance synchronized in one second intervals, thus amplifying the overall imbalance. That's just a guess, but it would explain why the vibration occurred at that interval.

I have a few more hours of driving to get to Charlotte airport and Little Switzerland, which will give me a chance to ascertain whether it's truly all better. My quick run back from the tire shop suggests it is.