mpuzach

First, a little history. When I bought my new '03 4.2 back in June, my salesperson very appropriately cautioned me that I may notice a slight shake or vibration in the steering wheel at highway speeds. He had test-driven the car after its PDI and noticed the symptoms which he said were due to the fact that the car had been sitting in inventory, never driven, for several months before I finally bought it. He went on to say that since it had not been driven (and probably not even moved much), one or more of the tires probably developed slight flat spots that would disappear after a few hundred miles of driving.

Upon driving my new car home from the dealership (Rector Motors, Burlingame, CA), the symptoms became immediately obvious. At around 50 m.p.h., the wheel began shaking and it didn't let up until around 65 or 70 m.p.h. Funny thing, though, while it would shake <i>most of the time </i>I was driving within the vulnerable speed range, every now and then it would not shake. It came and went with no apparent pattern. The critical thing is that the symptoms I was experiencing seemed <i>exactly </i>like those expressed by many others here, including those that led to the eventual buyback of you-know-who's 4.2.

What I want to share here is that my vibration problem was <b>solved</b> today, thanks very much to the diligence of service advisor Hector Contreras, service director Carter Tyson, and the service team at Rector. Here's what happened:

When, after putting on the first thousand miles or so the shaking didn't disappear, I took the car in. They rebalanced the wheels and the shaking disappeared for a couple of hundred miles at best. I took the car back at around 2000 miles; their suspension specialist did a 4-wheel alignment and checked out whatever suspension adjustment parameters they can affect and pronounced the car healthy. When I picked up the car it was behaving no differently than when I brought it in. I immediately turned around and asked Hector (service advisor) to come for a ride with me. He observed the continuing symptoms first-hand and gave me his commitment that he would do whatever it took to get the problem resolved.

We set an appointment for yesterday. Hector got the shop foreman involved; they started by re-checking the wheel balance (on their recently-acquired Hunter). All 4 wheels were within 1/4 oz. of being perfect <i>in the standard balance test.</i> It wasn't until they ran a "load force" balance check (one of the unique capabilities of the Hunter) that they found <i>all four wheels out of balance by as much as 3/4 oz. </i>There was absolutely no question that this was enough to cause the vibration. But there was more. As they were examining the wheels and tires, they found that one tire had an ever-so-slight flat spot in the tread area, one that would likely change as the tire heated up/cooled down. They decided (without prompting from me) that the only right thing to do was to replace the tire, since it likely developed the flat spot while the car sat in inventory. They put on a new Conti, re-balanced ("road force"), and installed the new set-up; they then swapped the other front wheel/tire assembly with the virgin spare (after, of course, doing a road force balance on it, too) in order to have equal wear (actually, no wear) on both front tires. That brought us to the point of no known tire flat spots, all 5 wheels road force balanced, and a still-fresh 4-wheel alignment.

I'm pleased to say that my car has never felt better. In fact, on the 80 or so mile drive home it was absolutely perfect. Of course, I now have to hope that it holds, but I'm very encouraged. The steering and tracking are fantastic, the best ever.

Bottom line: If you're experiencing vibration or steering wheel shake, insist on a road force balance on all 4 corners with a Hunter GPS. One thing I learned through all of this is that not all techs know how to use the Hunter to its full capability.

In addition, check tires for flat spots. I don't know the procedure, but the dealer does.

Finally (and this may be the most important info of all), Hector was very specific in saying that the vibration problem <i>is not </i>widespread and is definitely not "inherent" to A6s as some here have suggested. As such, he had no doubt whatsoever of being able to correct the problem. It now looks like he was absolutely correct.

Hope this helps.

PCH_4.2

nice write-up; i may need to get mine checked too (on my own expense).

PaulW

I've been around Audiworld forums for about 3 years now, and around Audifans.com for about 12 years.

I don't think I've ever heard of anybody discuss tire flat spots except on the A6 forum on a vibrating 4.2. It's odd that it's the only Audi car that flat spots tires.

So what did they do to fix your car? They removed the forcing function that is exciting the vibration. The forcing function is so small now that it can't excite the natural frequency that causes the vibration to the point where you notice it.

Remember, this is a brand new car from the factory, it shouldn't have these problems with wheels, tires and residual imbalance of the two.

I predict that within time (I will say six months or less, probably less) that it will come back. Not that I want to come back, but it will because they did not do anything to fix the problem except balance your wheels to a very, very low residual imbalance criteria.

Flat spotting on tires? How many times has that been used and to tell you that there are no problems with A6's or S6's? It's just not true.

We need to change the stiffness of the suspension to change the resonance frequency of the front end so the car operates further away from it. At normal wheel residual imbalances (like from tirerack or any other place that might balance your tires), this will not provide a high enough input to feel the vibration.

One company that I work for is in CT (too bad I'm in Chicago) has vibration analysis equipment, the first thing that needs to be done is to take it into the car that vibrates and take vibration spectrums to determine the frequency of vibration.

Is it 1X, 2X, 4X etc of wheel rotational speed? Once that's figured out, changes could be made and further vibration analysis could be performed and the frequencies could be eliminated.

Here's a vibration analizer:

<img src="http://www.mtillc.com/images/vibe.h2.jpg">

See the probe on the right side? It's a acceleramoter, the machine measures vibration in displacement (expressed in inches), velocity (expressed in inches/second) and acceleration (expressed in inches/second squared). Here's a screen shot:

<img src="http://www.precisionspindle.com/images/Csi.jpg">

It is displaying frequency in CPM (counts per minute) on the X scale, which would be frequency of the vibration, and velocity on the Y scale, expressed in inches/second.

Note the distinct frequencies you can see? We could put it on almost anything in the car to pick up the vibration and determine the frequency.

Amplitude would be used to compare after changes were made, using the same monitoring point at the same speed, on the same road. It's an accelerometer that plugs into the machine that you put on the equipment that you want to measure.

This is normally used for rotating equipment such as pumps, motors, fans, turbines, etc, but will work on anything.

Maybe Audi should hire me?

If somebody is in Chicago with a vibrating 4.2 and wants to check this out, I might be able to get some hand held equipment to start mapping this out. We could determine the root cause of this problem and possibly provide a simple fix.

Paul

TheBrit

What are your thoughts on suspension stiffness changes? Do you mean something like bushing changes?

I never had a problem in my A4, but my A6 vibrated from the very first tyre swap, as does the RS 6 (sometimes). The S8 didn't vibrate until after two sets of tyres, and even then it was very, very faint most of the time and under very rare circumstances a little more noticable.

I also noticed that the vibration is very dependant upon weather, colder air and rain make it worse. Hard cornering and rapid acceleration (especially through tight corners) makes it much worse briefly - are these likely indicators of it being a suspension problem?

I wish I could offer my services in diagnosing, but Chicago's just a little bit too far away for me. If you do find anything out that may help pinpoint the cause, please post on the RS 6 forum as well as in here. Thanks!

PaulW

I was thinking more about this, can't sleep tonight...the first step would be to hook up the vibration monitoring equipment and basically do a resonance search.

Drive from a stand still to over 80 MPH and see what frequencies change the most. If I had the equipment in the car, it would be easy for almost anybody to see as the car passed through a resonance frequency, that frequency on the monitor would continue to change both up and down (down as the resonance is passed), while the 1X (one times) frequency probably wouldn't do anything but get larger and larger and at very small rate. This would be the 1X from tire/wheel residual imbalance.

Once we figure out what frequency of concern we have (subsynchronous, synchronous (which is 1X), or what multiple of synchronous), we can then change things.

I would put a stock set of tires and wheels on that had the stock width, which is a 205? width for the A6....then see how amplitudes changed, also measuring weight of the wheel and tire combo.

I was also thinking of taking essentially a small sand bag and tie wrapping it (something that weighs 5lbs or so) to the lower straight control arm and see if that has any effect on amplitude.

After gathering data, changes could be made, the first I would most likely try is the bushing change posted by ??? with the A8 bushings.

Or another thing you could do is preload the control arm bushings by loosening the control arm to subframe bolts with the car jacked up, then tighten them again and put the weight of the car back on them.

You're not suppose to do this, because the bushing can't move once it's locked down, but this would provide signficant preloading of the bushing and another test run could be performed. Changes in amplitudes and frequencies could be recorded to see what changes. If it's the same frequencies that are the resonance frequencies, then start looking at those bushings.

You stating that it changes with the weather and corner makes sense to me, the stiffness of the components change, which will change the natural frequency of the system.

I would just develop a whole troubleshooting plan before starting. After the data is collected, at least you could see what frequncies have the largest change in amplitude during it's pass through resonance.

I have a great troubleshooting books for resonance problems on rotating equipment (pumps), and have been involved with it for a long time. I bet there is a bunch of ASE papers on the internet already that tell you what you could do to fix it.

Maybe everybody will need to get H&amp;R's!!! That's a nice fix!

pw

RobertV

After a day or two, the vibrations come back. A local service manager told me that he has seen more "bad" Conti and Perreli tires in the last 2 years than he has seen in 20 years. I even have probelms with a new set of BBS wheels and Pilot Sport AS.
I do wish you the best, but I have been there three times and it has always come back.
Keep us posted!!!

ICONCLS

Brit, can I infer you still have that post tire-rotation vibration then?

ESN

Now, if only AoA monitors the Board, as is generally believed.

no.radar

I'm in Chicago! Almost makes me wish my 4.2 vibrated so I could come and play w/your stuff!

ZCD2.7T

You sure you wanna use the words "play with your stuff" and "vibrated" in the same sentence, no.dot?