Help diagnosing vibration at freeway speeds
 

As mentioned in another thread, my car sat for about 18 months while I worked on an electrical issue. That's done and the car is running very well. However, I'd like your help in chasing down the cause of vibration that becomes quite noticeable at freeway speeds. When I first took the car out (on the way to get it emissions tested) I drove it on the freeway for about 5 miles to warm it up. The car started producing a severe shake at about 55 mph and above. The shake comes through the steering wheel, so I assume that the vibration is coming from the wheels or suspension. Before driving, I made sure the tires were properly inflated.

After, passing the emissions inspection and getting the car all legal again, the vibration has lessened somewhat. So, I assume that part of what I was feeling was just the tires and other parts of the car 'limbering up' after sitting in one place for so long. I've had the tires balanced. The front tire on driver's side needed an additional 1.5 oz of weight. All the others were spot on. The 'Road Force Balancing' told a similar story, except this time it was the front passenger side tire that needed adjustment. They rotated the tire 180 degrees on the wheel to get it into balance. Any ideas how sitting 18 months would cause the tires to come out of balance like this?

Rebalancing the tires has helped -- a lot -- but I'm still noticing a vibration I swear wasn't there 18 months ago. It could be my mind is playing tricks on me. What's next? Going by the book, I'd look at alignment, but I've not done anything to the car that should cause the alignment to change.

Sitting on the weight of the car without jacking the thing up [and taking the load off the tyres] is a recipe for disaster tyre wise. Just sitting on the tyre for about 2 weeks without movement will cause a temporary flat spot that will recover after a few miles but months.....? At some point in time the rubber will not recover its former shape but how long that is I really do not know.- if I was to hazard a guess I would say no longer than two months. Once that point of no return has been crossed the tyres are junk unless someone has a way to re-profile them. Inflating the tyres to their maximum rated pressure and leaving them like that a while may help them recover a bit. I doubt that force balancing will do anything remedial longer term if they have developed a permanent set.

Once the tyre is flat spotted it should be obvious why the wheel is now out of balance but putting different weights will not cure the problem of a wheel that is no longer truly round to within acceptable tolerances.

Fred -- Thank you for the reply. Looks like I made an expensive mistake (again).

David,

If the vibration is not too annoying suggest you try to see if you can "run it out". Part of the problem is that air naturally diffuses through the rubber walls so unless you over inflate and check/top up the pressure every week or two the benefits gained are lost.

Last year I left my cars unattended for 6 weeks. but as I departed I only anticipated being away 2 weeks. The tyres on my 928 and my Cayenne recovered from that no problem but it had me worried.

Also remember if you had them "rebalanced" then as they recover [as they will to some extent] they may also go "further out of balance" to some small extent. The problem with wheels out of balance is that the faster you go the worse it gets and then of course there is the "personal sensitivity factor" in play.

Although there are no hard and fast rules, I reckon that if one cannot get the vibration to run out after 30 minutes or so of motorway type running chances are they will never fully recover.

For long periods of non running there is no better way to protect the tyres than to get the car on axle stands and the tyres away from direct sunlight if the car is not stored under shade..

+1
I always just inflate to the max psi on the sidewall before storage. I've never had a tire flat spot from just storage doing this.

. .

David --

See if you can borrow a set of tires/wheels from another local owner to try on your car.

That would be my first step after verifying that the originals are still round (using dial indicator or crayon. and also verifying that the wheels are true and round. I've found that the quality of balancing varies by the machine and the technician's knowledge. You've done this already with the road force balance, but still good to keep in mind.

This past winter is the first when I've put the car un stands with tires completely off the floor. The previous couple hibernations had the tires pumped up to 50 PSI, and it took more than a couple miles of driving to make them round again. The car sits against a garage wall during the winter, so it took a little thinking about how to get stands under the against-the-wall jack points. But I did it, and the results are much better.

The method -- raise the outer rear away from the wall with a couple extra blocks on the jack for height. Place a stand under the front away from the wall. Lower the jack, and rock the car to get the front against the wall up to receive a stand. Lift again from the same point, lower the away from the wall front stand to the same height as the other front stand. Raise the rear by the rear crossmember, and place the two rear stands.