periodic vibration in car

[goodboat]

What might be the cause of a periodic low-frequency vibration in a car. The vibrations sound a bit like muffled wind-buffeting, on the edge of being unnoticed, and pausing every second or so. The frequency does not change; the vibration just comes and goes. Can be felt, just barely, in the steering wheel, but the sound seems to come from the whole body of the car. Seems more noticeable when car and tires are very cold. Poor tires increase it, new Pirellis lessened it. Have had this '99 Mercedes 4-Matic wagon from 82,000 miles to 126,000 miles. Alignment has been checked repeatedly, 4-wheel alignment (supposedly). This car has full-time AWD but different percentages of power to front and rear, 40 vs 60 percent, I believe. If I take the car to a dealer and ask this question, I'm always told the mechanic can't hear or feel the vibration.

 

[patprimmer]

The tyres are forming flat spots when parked for an extended period. As the tyre warms up it reverts to round

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[Zapster]

Pat most likely described the source of the problem in his post.

Every car vibrates; the question is: is it excessive for the make and model?
Could you be hyper sensitive to this sound/vibration? I can understand a mechanic not being able to hear the vibration; however, I wouldn’t think the workplace environment has damaged the normal sense of touch/feel for most mechanics.

If you are not over sensitive to the noise, start looking for something that the tire vibration may be exciting. If a body panel has a natural frequency that matches the tires forcing frequency, you may be getting some noise/vibration from a difficult to locate source. There might even be a factory bulletin that describes this problem, has your mechanic checked?

 

[GregLocock]

Try inflating the tires to 50 psi before parking overnight. let them down to your normal pressure before driving off. If that is better then Pat's theory is the right one. There are dozens of other possibilities.

Alignment won't help.

Is it speed sensitive?




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[goodboat]

Thanks, Pat and Greg. I have of course considered the flat spots. But I wonder if we aren't overemphasizing my observation that it's worse after a cold sitting-period, because, for example, last night my son (also a driver of the car, who has noticed the vibration) and I were driving for 25 miles, over half an hour, after seeing a movie (2-hour sit-time, about 20 degrees F), sharing observations of the vibration (sound and feel) all the way, and still observing it as we arrived home.
Hyper-sensitivity, yes; but as you see I'm not alone in noticing the problem. It doesn't keep me from appreciating this as the best of many good Mercedes cars I have had; a sweetheart all-round. On the other hand I think I forgot about the periodic vibrations pretty much entirely on a recent Thanksgiving trip of 200 miles.
What, by the way, is tire-pop? Saw the phrase in another eng-tips post, and wondered if it refers to an event that follows a certain build-up of strain from misalignment, which relieves the strain.
I searched the forum before posting my thread and found a post referring to two different frequencies of vibration, from wheels turning at slightly different speeds, coming into sync every so often, as explaining the periodicity. But how can two or four new tires be rotating at different speeds? So that led me to wondering if, in the Mercedes's presumably complex front and rear differentials, which apply different proportions of engine power to their two sets of drive wheels, there aren't parts revolving at different rpms. (How do they do that, anyway, if not through clutches?)
All to add to the interest of the problem. Here's one more item: My last used Mercedes 4-matic wagon, 1991, a very different and more complex set-up, wore rear tires unmercifully. There's some indication that this newer one, which relies on the braking system instead of clutches to apportion power, also wears rear tires.
I hope you've been patient enough to read all this. --Goodboat

 

[goodboat]

I didn't reply to Zapster by name in my last. Meant to. Zapster, would your notion of a sympathetic vibration explain the periodicity?
I think in a noisier car one might not notice this vibration at all. But most drivers, not including my wife, do notice this one.
And if it weren't periodic, one might not particularly notice it. Since it is periodic, vibrating for about a second, then resolving for a moment, then vibrating again, it penetrates the smoothness and quiet of the car, and makes me think somethingn's a little amiss, perhaps related to that tire wear.
I have more than once very suddenly discovered bare steel on the inner portions of my rear tires on these two 4wd wagons, and had a blowout on the Mass Pike last year while towing a boat, with "performance" tires with less than 15K on them.
Well, so it goes. Any thoughts will be appreciated. --Goodboat

 

[patprimmer]

Many years ago I had a set of roof racks n my VW type 3 with an air cooled 4 cylinder boxer engine. The roof racks were whisper quiet.

I the obtained a Toyota Cressida. The very same roof racks kicked up a racquet.

Same deal with the tyres. I used Toyo R600s on the VW. Whisper quiet. In the Cressida the tyre roar from R600s was quite intrusive. On the Toyota needed a tyre with a much less aggressive tread paten.

Different tyre pressure or different weight on each wheel will give different speeds at each wheel. That is why we should not run locked centre diffs on dry hardtop roads.


Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[JohnLear]

The description reminds me of a vibration I had in some car or other (can't recall, maybe Alfetta). From memory this was caused by a loose / worn drive shaft central bearing, joint or support (two piece drive shaft).

 

[Thecardoc]

Well, this should be fun. I've seen the engineering people often fret over designing something that a technician would not assemble the way they want it to be done and risk a problem here on this site as well as on the job. Through-out my career as a technician I have had to deal with situations where everyone else simply throws guesses, and treats "us" as if we have no real talents.

The "flat spotted tires" is essentially a guess Pat. While it may be somewhat likely in some cases, simple details about how the reported problem was written actually rule it out. The problem from a techs standpoint is that a tech could replace the tires, and then when the problem is still there (as it is) now the tech and the repair shop are often expected to find and fix the noise, for free! The shop could also be expected in many cases to give the money back that the customer spent on the tires. I can go into greater detail later, but both of these scenarios work to drive potential great automotive technicians, who could solve goodboats problem into other fields.

From Goodboats original post.

"What might be the cause of a periodic low-frequency vibration in a car. The vibrations sound a bit like muffled wind-buffeting, on the edge of being unnoticed, and pausing every second or so."

At what speed does this occur, and what speed ranges will this definitely NOT occur?

Does turning either by going around a bend, or simply swerving inside your lane have any impact on the sound as it is occurring? (lessen, or increase as compared to turning one direction over the other)

Does surface wind direction/speed play a role in the noise?


"The frequency does not change;'

This line right here rules out tires.....

"the vibration just comes and goes. Can be felt, just barely, in the steering wheel, but the sound seems to come from the whole body of the car."

Seriously, if "the entire body of the car" seems to be involved in the noise, then it would make sense that it can be felt with your hands through the wheel. So, lets concentrate on the entire body, and NOT the steering.


"Seems more noticeable when car and tires are very cold. Poor tires increase it, new Pirellis lessened it."

I call this "tainted intuition". In the shop this kind of information would not help me diagnose the problem. In fact, it easily could lead to to looking at tires, when they should already be ruled out!


" Have had this '99 Mercedes 4-Matic wagon from 82,000 miles to 126,000 miles."

The noise, or sensation your experiencing, when did it start?


"Alignment has been checked repeatedly, 4-wheel alignment (supposedly)."

More tainted intuition. Concentrate on the problem right now. What's been done in the past has absolutely no bearing on the situation.


"This car has full-time AWD but different percentages of power to front and rear, 40 vs 60 percent, I believe."


Not relevant to diagnosing the problem at this time.

"If I take the car to a dealer and ask this question, I'm always told the mechanic can't hear or feel the vibration."


No surprise there. How have you set the stage for them to experience it? Right now I could ask you if you could tell me if we drove from YOUR HOUSE, to a specific location would we definitely feel the problem? Your answer could easily be yes, and one of the possible reasons for that is the fault creating the vibration could actually be the surface of road your regularly driving on. Don't laugh, or attempt to deny it because that has been a real diagnosis many times! What we cannot do is simply guess (like the flat spotted tires). We have to spend time and investigate the issue. Now, one might say if its the road why doesn't the frequency change? The answer there is simple, speed limits, and the fact that no one has investigated this possibility yet. Until someone has, then its in play.

Now a few last questions. Do different people inside the car experience the symptom the same or differently? In other words, if your son drives, and you ride can you move around to different seats inside the car in an attempt to get a source direction? What about adding passengers, and/or changing tire pressures? How does that impact the issue? If you cannot force a change in the issue, by any of these, steering, passenger weight, speed, etc. Then the problem is NOT tire, road related. Now you have to actively be looking at things like plastic air dams that control airflow through the radiator (if the problem is in the front of the car). The plastic shield that is connected to the body under the engine is one such panel.

So before you go back to the shop, again.. Lets see how well the vibration can be observed and then explained here. That way you practice painting a clear picture of exactly what I would expect to find when I drive your car to experience the problem. Be prepared once we reach that point, that you may have to ride with the technician to show him/her the problem. One thing, use hand signals to show when you feel it, and don't feel it. Trying to say "there, did you hear that" will only be an interruption to the techs observation.

 

[goodboat]

cardocs give me homework to do, diagnosing this vibration problem. but for now, in answer to your questions, cardoc:
Observed this problem from the first of my ownership of the car until now.
Speed range to be more carefully observed, but noticed at all normal driving speeds, say from 45 to 75 mph. I will check more carefully whether I feel/hear it under 40.
Pretty sure wind and road surfaces unrelated. Certainly present with no wind other than relative wind, and on very smooth new road surfaces.
My "frequency does not change" statement perhaps embarrasses me. I certainly wouldn't swear that it is the same at different car speeds. It hardly could be, no matter what the source, could it? What made me make that statement? Perhaps that it does not rise and fall as the vibration comes and goes, and it is always quite low, so that you aren't tempted to hum the pitch.
We hear it from both front seats and I think from the rear seats but will have to check.
Anybody else think driveshaft center bearing a possibility?

Mason Smith

http://www.adirondackgoodboat.com

 

[Thecardoc]

"Anybody else think driveshaft center bearing a possibility?"

Since that turns at 3-4 times the wheel speed, NO. You said its a low frequency.

 

[goodboat]

cardoc, would it be possible for a driveshaft, turning at one rpm, to whip eccentrically at another rpm?

Mason Smith

http://www.adirondackgoodboat.com

 

[ivymike]

Seriously, if "the entire body of the car" seems to be involved in the noise, then it would make sense that it can be felt with your hands through the wheel. So, lets concentrate on the entire body, and NOT the steering.

Okay, but the description seems consistent with how I would have described a problem that I once had with a Honda Civic - which went away abruptly when I got new front tires.

More recently my Toyota Scion had started making a sound which could be described as "yum yum yum yum" but only between 45 and 50 mph. Anything above or below and there was no noise. There was very minor steering wheel vibration that accompanied the noise. For a week or two my wife and I debated whether the road had gotten rougher on the hill leading away from our house. Based on my experience with the Civic, I had the (worn anyway) tires replaced - and *poof* the problem was gone.

 

[Thecardoc]

Too funny.....

One would expect that performing accurate diagnostics should generate more support than the suggestions that simply tossing parts guessed correctly seems to.

Bad tires make a specific kind of noise depending on exactly what is wrong with them. Mike you described (loosely) cupped wear, or a condition otherwise known as combination wear. Replacing them may well have gotten rid of the noise for now, but what did you do to prevent the next set of tires from wearing the same way?

goodboat. Harmonics from vibrations can occur, the first question is, if the drive shaft is whipping eccentrically, wouldn't that be the primary vibration? Now which way would you expect a harmonic from that vibration to occur, faster or slower?

 

[pmrobert]

Dumb suggestion: crawl under the vehicle and see if there's any excessive play in the driveshaft(s)/halfshaft(s)? Cheap, easy, probably rules it out. On a related note, I have a Mazda RX-8 that now, on it's second set of Yokohama ES-100s, has developed noticeably more tire noise at about the same age/wear on both sets - ~10K miles. Guess what I won't be putting on next rubber change? I also have a Ford Ranger with mild very cupping on the front Michelins that has an odd vibration in the steering wheel/chassis existing only at 15-20 mph. Data points only, very anecdotal, etc....

 

[ivymike]

ha. nothing at all... but I especially did not pay some guy $100 to tell me that everything looks just fine on his alignment machine... The only time in my entire driving history that an "alignment job" ever turned up anything, they promptly told me it wasn't an adjustable parameter, by the way I needed an air filter, and that I shouldn't go more than 3000mi between oil changes.

I think 55,000 miles is enough to ask from the factory set of tires. $100 is about half the cost of a new set, so unless an "alignment" (of the stars, perhaps?) can get me 60%-75% more tire life, thanks but no thanks.

 

[patprimmer]

I don't recall recommending a change of tyres.

When presented with a collection of anecdotal contradicting evidence I often guess the most likely easy to check cause.

I think observing things like a rough approximation of tyre temp when parked, how long parked, a rough approximation of tyre temp on restart and time to smooth out is a really simple NO COST test that anyone who belongs here could figure out for himself, or Greg test or to take it to extreme jack the car up overnight and see if the problem exists next morning.

It could simply be the OP expects unobtainable level of smoothness from the car.

It could be the normal response to a road surface.

It could be the resonance of any component of the car.

It could be a resonance set up in the exhaust and only transmitted at certain exhaust pipe temperatures due to change of fit due to expansion.

Many of the stated symptoms may not exist or may be irrelevant or not associated with each other.

If believed every statement describing a problem, I would have spent a lot of time looking for complex answers and not solving simple problems.


Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[goodboat]

Well, thanks all. Let's close the thread. Fact is, as one of you hints, I do not belong here. Am a boatbuilder/novelist who likes to understand physical things, not an engineer. Have learned a little about disgnostics from you all. I am not really any closer to understanding my vibration but may be equipped to observe it better now and do some tests. Again, thanks.

Mason Smith

http://www.adirondackgoodboat.com

 

[Thecardoc]

"I am not really any closer to understanding my vibration but may be equipped to observe it better now and do some tests."

Your welcome. Being able to take these steps right here will allow you to make progress if it's possible. Take notice of the possibilities that Pat outlined, they are all in play until they are either confirmed or ruled out by proper diagnostic steps. BTW, you live in a beautiful area. I was up there in Mid October (Malone) to teach a class. Spent one day sight-seeing in Lake Placid.

IvyMike.. I think what you said right here sums up the real problem as well as anything.

"but I especially did not pay some guy $100 to tell me that everything looks just fine on his alignment machine..."

During my entire career as a tech I've had to deal with partial instructions from other technicians as I was learning to be a technician. Varying shop management expectations. Instructors who may or may not have been techs. Disdain, and little or no respect from manufacturers and especially their engineers. All the while we had to juggle with the expectations, as well as the needs of each vehicle owner, which amounted to anything but a simple set. While one person would expect any problem to be solved, and services performed to maintain their car because their transportation was the most important thing to them, the next could easily be someone that would speak out exactly as the rest of the thoughts as you presented here.

" The only time in my entire driving history that an "alignment job" ever turned up anything, they promptly told me it wasn't an adjustable parameter, by the way I needed an air filter, and that I shouldn't go more than 3000mi between oil changes."

That pretty much amounts to a situation where as a tech, the more schools that WE (I) attend, and the more WE (I) invest in equipment (which increases the CODB, and ultimately our price) the better we serve one group of customers, while a second set (yours) is in fact less happy with us. Your not alone because we struggle with that kind of pressure all of the time inside this trade, but its only been in the last couple of years that we have been afforded a way to address this problem. Basically, the cars we can fix, your own your own with the rest of this. As the cars are getting ever more complicated, the number of techs that are out there is shrinking. Part of the cause of this is that the cars don't break as often which is because the quality of the entire fleet has increased dramatically. Today its normal for us to have to solve problems that we will never see a repeat of. That takes even more personal training and investing than ever before. If we come across someone that we cannot make happy by doing that, then we have learned to simply not try. We take care of the ones that need and appreciate us, and are left to hope the others don't darken our doorstep.

 

[NormPeterson]

Every time I read the original post I can't help but think that this vibration might be the difference between two vibration modes of nearly the same frequency. That would probably require some sort of frequency analyzer and would perhaps be something to try if nothing mentioned thus far turns up the culprit.


Cardoc - only a few years ago I had the opportunity to work attached to the service department of a local new-car dealership (between engineering jobs). I got to meet technicians covering a wide range of talent and attitude, and a variety of personalities sitting on the shop side of the service counter. Separately, I personally know a man who in all probability would have never risen above being a mid-level "B" tech had he remained in the business (and who had also worked counters in independent shops from time to time). Point being that diplomacy can be a hit-or-miss proposition, so I'm inclined to give customer Ivymike the benefit of the doubt here, based on inputs other than my being an engineer. Don't take it personally.

Perhaps the members of an engineering forum are not generally representative of the driving public at large, and those with mechanical or automotive experience less so. A minimal explanation that may be satisfactory to nontechnical people may not be sufficient. This is a "soft topic" more along the lines of what somebody like Mitch Schneider might write about, and I don't have any quick answers.


Norm