High speed vibration - need help tracking down

[Turbowned]

Hey guys,

I've been living with an incredibly irritating vibration at speeds between 40-80+mph. It seems to be ever-present, but worse, or more noticeable at some times than others (no particular rhyme or reason to when or why it's worse or better). Doesn't appear to be any better or worse on accel, decel or coasting. I've pretty much ruled out it being a wheel/tire issue as I've tried two different sets of wheels, two different sets of tires, road-force balanced the first set of wheels to the first set of tires, installed four brand new tires and road force balanced them, checked the wheels for bends/runout etc. No wheel/hub corrosion.

I'm thinking it must be either brakes or driveline vibration. All of my brakes were done this year; the rear rotors are new, new pads, and freshly lubed rear slide pins; rear calipers rebuilt two years ago. Front rotors resurfaced this year with new pads and new caliper pins and hardware. All the pistons seemed to compress fine. Doesn't mean they're perfect, of course, but the vibration was there before and after the brake work.

Front suspension is all new, for what it's worth. New struts all around.

Engine/trans/diff mounts are original. Driveshaft bearings/cv joints original. RF axle was replaced with an inexpensive new one about two years ago (currently my #1 suspect, followed by the driveshaft). I do hear a low faint humming from what I'm thinking is the driveshaft or rear diff but it's always been there since I bought the car almost four years and 35k miles ago. Car has 103k on it now.

How would I check for a braking/driveline related vibration? I tried driving it in the air with the traction control turned off but the car didn't much seem to like that. I'm pretty much trying to avoid having to take it to the dealer and paying them to find out for me... Any ideas would be greatly appreciated; thanks!

[bethridg]

If it's a front axle, I would think you should feel it in the steering wheel. Or if it's wheel balance related you could feel it coming from the corner(s) of the car, similar to having snow packed wheels. If it's a resonation from the center of the car, like under the center console, it's most likely propshaft related. My vibration, like you described, was caused by a bad front CV joint on the propshaft. Could also be a misaligned propshaft if you've had related work done or a failed center support bearing.

Another user had these symptoms recently but I haven't seen any follow-up. There are few suggestions in that thread.

http://www.rs6.com/showthread.php/33222-New-RS6-6-speed-owner?p=288192&viewfull=1#post288192

Engine mounts would throw a code and vibration would fluctuate with rpm. Diff mounts would be an on/off clunk when the drivetrain is loaded/unloaded. Braking related could be felt in the brake pedal (warped rotor).

[PCMacDr]

Here was my vibration problem
Thank God I made it home in one piece

18231

[bethridg]

Here was my vibration problem
Thank God I made it home in one piece

18231

Alignment issue? Some uneven wear on that inner edge.

[mrdave]

Looks like a really bad alignment, possibly combined with improper tire pressure. That "DWS" indicator on the outer edge is fully intact while the inner edge is worn to a nub.

[G2]

Here was my vibration problem
Thank God I made it home in one piece

18231

Sorta scary there.

Check wheel bearings in addition to control arms. This is very hard to do and takes a deft touch for the bearings. Best way is to remove the wheels, brakes and axles. PITA but I know it works.

Another way of looking at it: if you're past 100K miles, then the bearings are due. They can become both tight and/or loose. In my case one of each. Have replaced bearings on 3 or 4 RS6's so far.

It will steer and handle so much better also.

[G2]

The humming is likely (also) the rear diff. Change the oil to 85-140. I prefer Amsoil as most now. You can also try their 190 race oil. That WILL dampen noise

Vibration is most likely the prop shaft. Put a straight edge on it. Remove the shaft and check CV's and center bearing. Be sure it's aligned during installation. They are usually crooked and/or the spacers are missing (corrects for up/down).

Unlikely the cheap axle. The bearings do tend to get a bit loose moreso than OE. Haven't seen any issues on a few cars so far including my own.

[lswing]

The humming is likely (also) the rear diff. Change the oil to 85-140.

Preferred reason for not using the stock (I believe) 75w90?

[Turbowned]

Haven't been on here since Google Chrome started blocking the page. I haven't been able to take any action to try to track it down, yet. I know my front rotors are warped, dunno if that'd cause a vibration?

I changed the diff fluids a couple years ago; kept 75w90 in there. No noticeable difference. Thought it was a wheel bearing for the longest time but the bearings are quiet. Made me think it might be a driveshaft u-joint or center bearing. Gonna try to swap out front axles soon and see what happens. Failing that, I'll drop the driveshaft and replace the u-joints and center bearing.

[G2]

Preferred reason for not using the stock (I believe) 75w90?

Thicker oil dampens noise. Its more appropriate for the high torque. Will only use 75-90
on a stock 4cyl or 6cyl car.

Have ealao used 190. Haven't tried the 250wt yet...

[lswing]

Thicker oil dampens noise. Its more appropriate for the high torque. Will only use 75-90
on a stock 4cyl or 6cyl car.

Have ealao used 190. Haven't tried the 250wt yet...

Aha, thanks.

Had vibration caused by out of balance tires, and then worn bushings on front control arms, couple possible causes.

[fukinavit]

Thicker oil dampens noise. Its more appropriate for the high torque. Will only use 75-90
on a stock 4cyl or 6cyl car.

Have ealao used 190. Haven't tried the 250wt yet...


Be careful on thicker grades, i would stick with stock grades and change more regularly. sure a thicker grade might quieten the diff a tad but unless your towing lots, drag racing or living in a very hot climate year round best to stay away.
pretty common here in the winter to drop between minus 25-30 deg c, a thicker oil will offer zero protection until warmed up properly and on short drives will probably never get near its operating temperature.

[G2]

Haven't been on here since Google Chrome started blocking the page. I haven't been able to take any action to try to track it down, yet. I know my front rotors are warped, dunno if that'd cause a vibration?

I changed the diff fluids a couple years ago; kept 75w90 in there. No noticeable difference. Thought it was a wheel bearing for the longest time but the bearings are quiet. Made me think it might be a driveshaft u-joint or center bearing. Gonna try to swap out front axles soon and see what happens. Failing that, I'll drop the driveshaft and replace the u-joints and center bearing.

IE allows unfettered login, unlike Google.

In order to help pin point the issue, we need some descriptive verbiage. What is the frequency? When does it exactly occur? Is it subtle or extreme? Does weather conditions affect it? Video clip? (you can upload it to 'Audi RS6' on Facebook if that helps)

Unless the axles are shot, it won't help reduce vibration. At 60mph wheels and axles rotate at only 800rpm or so. A drive shaft rotates at thousands. Reason they are balanced. VERY different frequencies. Also ensure motor and trans mounts are OK.

Wheel bearings can be extremely noisy, but rarely vibrate.

I'd be looking at prop shaft alignment, in addition to the center bearing. Especially if the noise or vibration Get the shaft straight as possible. Sometimes the spacer shims are missing- effects the vertical alignment. In my experience nearly all are not correctly positioned. Use a straight edge if your eyes are not laser calibrated like mine ;)

[G2]

Be careful on thicker grades, i would stick with stock grades and change more regularly. sure a thicker grade might quieten the diff a tad but unless your towing lots, drag racing or living in a very hot climate year round best to stay away.
pretty common here in the winter to drop between minus 25-30 deg c, a thicker oil will offer zero protection until warmed up properly and on short drives will probably never get near its operating temperature.

Sure its common sense to think about what viscosity to use, especially when extreme cold is a factor. The issue of protection is always in hot conditions, when oil thins out. The rear diff gets surprisingly hot, not just from all the power. The exhaust pipes are right next to it, radiating tremendous heat. Similar situation for the center diff, with the cats right beneath. Main reason the seals leak oil. They get cooked. I've racked my car countless times after a drive and am always surprised at how hot everything is.

I don't want to rebuild a differential if it can be avoided. The oil I use is a cheap hold over fix. Five plus years on one car. Same situation on the transmissions; three should have been junked by now, including my own. All 3 to 5+yrs. Not simply thicker, but simply better oil.

Next time someone drains the gear oil and it looks questionable, will the status quo OEM spec suffice? Is being put in a box by OEM builders who's sole purpose is to get said vehicle past warranty, and to meet/beat CAFE (fuel MPG) standards, really worth listening too? Remember, there's Bean Counters involved. Thicker oil cost more.

Guess my question is what is the basis for recommending against, if anything other than a climate concern?

[G2]

Aha, thanks.

Had vibration caused by out of balance tires, and then worn bushings on front control arms, couple possible causes.



Don't think these are related to the OP comments, but never know. If/when your tires are balanced again, be sure they are 'hot balanced'. Numbnuts Big Box Retail tires shops used to make my balance worse 1/2 the time. I have high tech tire machines. I can usually get a nice non bent wheel assembly down to 1 gram on both wheel planes. Don't know of a single shop that does a High Resolution balance. Industry standard 7gram. Not adequate or acceptable for an Audi. Use RoadForce, assuming they can get it down to 3 grams. Cheap tires don't help. Get tires balanced every 5K miles or 1X per year min.

Anything over 3-4 grams and I'm feeling it. Most people don't/can't notice it. In the long run your suspension, wheel bearings and shocks sure will. Here's where I say "ask me how I know".

Worn suspension won't per se cause vibrations. Surely will exacerbate it.

[Turbowned]

It's not wheel/tire, I know. Wheels were checked for straightness, all were very good. Tried three sets of tires, Continental, Michelin, Bridgestone. Road force balanced wheels, tried two sets of wheels even. I am starting to think it's something with the front brakes, because sometimes stepping on the pedal the pads chatter and sometimes they don't. Also, sometimes the car vibrates more than others. Haven't had time to have the wheels off again and check the brakes; last time I looked the pads were all an even 7mm but that's as far as I've gotten. When you apply the pedal initially it feels like warped rotors, but then when you give it more pedal it smooths out.

[Jimmy]

Motor mounts leaking/on their way out?

[lswing]

Motor mounts leaking/on their way out?

That's a darn good point. Still sounds a lot like when my control arm bushings were worn, slight wobble, bad under braking, thought it was rotors at first.

[Jimmy]

That's a darn good point. Still sounds a lot like when my control arm bushings were worn, slight wobble, bad under braking, thought it was rotors at first.

I only chime in because I 'JUST' did both lower control arms on the allroad, and my mounts were spotted leaking last month and the subsequent driving brings me to today with the vibration under load.

I have a slight wobble too due to the HP2 calipers that tend to leave pad material on the rotors causing the shudders...but that is only on braking from highway speeds.

Its something you might want to have looked at, or look at yourself if you can get under there to see the mounts.

[P1054]

That's a darn good point. Still sounds a lot like when my control arm bushings were worn, slight wobble, bad under braking, thought it was rotors at first.

Just curious, which control arms had failing bushings that caused your problem? Uppers? Or did you replace all of them at once?

[G2]

Warped rotors will greatly exacerbate wear and tear on suspension. Loose suspension won't cause a vibration, but will greatly accentuate the condition if the rotors are even slighly warped or there is material/pad transfer.

Can't get to the first page due to the Decepticon page warning that relentlessly pops ups. Admin??

Just realized almost missed an opportunity to plug myself. Might consider my rear BBK. Takes a load off the front end. Car really clamps down. You almost have to over drive going into corners to work them much. Hopefully will make the front binders last a bit longer also. My tires are holding up much better despite driving like a nutter all summer.

[boca rat]

I was going through this until car was totalled a few weeks ago. Pretty well the entire drivetrain and suspension had been replaced in the last 5 years, except for main driveshaft. That was the last thing, before strating over with the older already replaced components. Ugh, it was getting expensive. At least mine was from 70-80mph only, which is OK cause I usually cruise at 85-90...