Periodic vibration in Mercedes 4

[hhy2k]

Hi everybody! First post, so please bear with me. I have a problem with my car for years. Out of experience I can say that almost all problems with card can be solved by sitting on the sofa and keep thinking hard enough, but this one got the best of me. I need ideas... so here it goes:

Periodic vibration at higher speeds. Starting at about 110 km/h (70 miles/h) and gets stronger and faster up to 140 km/h (90 miles/h). It starts to smooth out over that speed and barely noticeable from 170-180 upwards (yes, in Europe). The vibration is pretty strong , shakes the change in the glove box, shakes the entire body of the car, seats, dashboard, everything.

It is periodic with a wavelength of about 3-4 seconds at 140 km/h. The length depends on the speed of the car AND NOTHING ELSE! So it shakes, gets smooth and shakes again.

What effects it? Barely anything except the quality of the pavement. If that is smooth (concrete) the vibration is less. The car has a stick shift, 5 speed, an inline 6 cylinder engine built in with two monts. There is a mont under the transmission as well. The differential is mounted to the body of the car. The drive shaft is made from two pieces, coupeling to the transmission and the differential with flexible disks. The two pieces joins in the middle of the car where it is supported by a small bearing. There is also a u-joint next to it.

The shaking won't change for a gear change so the engine speed is indifferent. It won't change for an open or closed clutch either! It won't change for bends in the road or braking and accelerating. The road surface has some effects.

I am lost. Tires are changed multiple times, rims (steel) were rotated, suspention elements, tie rods, shocks, shock mounts, steering elements all had been changed. Flex disks and drive shaft bearing had been changed. These all had minor effects if any, problem is still there.

Can this be a motor mount problem? What could cause a periodic vibration or what method is right to go after this. It might be even a suspension problem, nothing to do with the drive train! What would you do next? Can this be a combination of vibrations it being periodic but very predictable?

Thanks for all your inputs.

 

[romke]

based on your description it looks like that when the wheels and driveshaft rotate within a certain speed range something in the car is brought into a vibration - a part that has a natural frequency that is more or less equal to the frequency of the wheels turning. normally that is not going to happen, since all the frequency generating parts (driveshaft, differential, engine etc) are mounted through rubber dampening fixtures that should prevent it to happen. the car body itself also has stiffening beams in its structure that should help prevent it. maybe a thorough check of all the mounting components and the bodywork itself (welds that have come loose) may reveal where something is wrong.

i do not think the drivetrain itself is at fault - if that was the case the change of various elements in it would have solved the problem. one thing i would particularly pay attention to is the mount of the differential/angle drive. i would check all the rubber mount components and also check whether the tightening torque is in accordance with what is should be.

 

[hhy2k]

Thank you very much for this input. This car has a multiple link independent rear suspension. That means the differential is fixed with bolts to a subframe in the rear. That subframe is mounted to the car with huge rubber beds but also connected with multiple (10) links to the wheel assembly. So in theory, the back end of the system is pretty sturdy if you add that the multi link suspention hold the wheels steady (no loose parts and aligned) and the differential should be kept in check by the axles, subframe, coils and (new) shock absorbers. Also, the shaking is more pronounced in the front of the car.

On the other hand, the transmission is damped only with a fist sized rubber element connected to the frame, much closer to the middle of this system. While that element is old, seems to be in a good shape. Motor mounts... Well, this is an old car...

May be I should add, that the shaking doesn't feel like something is loose. It is more like when something violently shaking the car.

If I understand correctly, you would go for the transmission mount, the motor mounts and the rear subframe rubber... Probably in this order...

The biggest question in my head, before I start throwing money at an (in other aspects great) car, if this is a drivetrain mount problem, or a suspension problem. As the suspention is checked so many times and found sound + the vibration is periodic and it is present in a wider speed range than a tire vibration (between 110-180 km/h) + the opinion of "romke", I would say, this will be a drivetrain mount problem.

Any second opinions? thank you!

 

[BrianE22]

The coming and going sounds like 2 frequencies modulating each other. If it's a tire or wheel problem, the vibration gets worse when the frequency of each tire/wheel is in phase. Have you swapped out the wheels or just rotated them? I'm thinking a rim or two might be bent.

 

[hhy2k]

I have not swapped out wheels (rims). I have mounted multiple sets of tires and the wheels were rotated from rear to front or spare randomly. It also means that the wheels had been checked visually by multiple mechanics. The rims are made of steel. I understand your point, but how would you explain periodicity. I assume wheels turn pretty much at the same speed in a straight line.... Also, there is no side to side shimmy in the steering wheel, but it shakes with the car.

The car is also buttery smooth up to 110 km/h, 70 miles/h (tire problems should come out slower speeds than that I guess)

Oh, one more thing: this car has ASD, which is sort of a limited slip differential system. ASD is supposed to regulate slip in the differential assembly with oil pressure. This system is not functional in this particular vehicle (Fault code). There is no noise coming from the differencial, what so ever. I don't see how could this add to periodical vibrations, but who knows? All wheels are turning at the same speed, as the ABS works properly.

 

[nebdirtturner]

Just throwing this out but we had a State Patrol unit that had a bent wheel and it did exactly the same thing, you went fast eough and it shook, slow down and it went away. Even a small amout of off center weight on an item spinning at that many RPM will start to shake a lot. I think BrianE22 hit the nail on the head.

 

[FeX32]

For sure that is wheel related vibration. 60-70mph is usually when 1st order vibration from wheels or tires starts to excite the base.

 

[MikeHalloran]

Agreed, wheels are suspect.
Visual inspection is not sufficient.
Put a dial indicator on each axle and measure runout of each wheel rim, inside and outside, radial and lateral.

While you are preparing for that, you need to remove the wheels and scrub the accumulated mud/tar off of the rims, something which I think is still part of an 'E' service, and probably not often done.



Mike Halloran
Pembroke Pines, FL, USA

 

[hhy2k]

Oh boy. Forst of all, thank you all very much for your inputs. Consensus, it seams, is about a wheel related problem. So here comes my doubts:

Wheel runout problems come out about 40 mph. The car is very smooth up to 70.

Balancing have been done at least 10 times in the past 4 years in 10 different shops while either rotating or replacing tires. The 5th wheel was always involved. The problem never changed.

I never heard a periodic vibration issue caused by wheel balance, runout! How and why it smooths out if it vibrates in the next second?

I never heard of a balancing problem present in such a wide speed range (from 75 to 110 mph) untol the vibration will get so fast that it can't shake the car anymore.

I will check the radial and lateral runout with the wheels mounted. That has never been done before.

 

[BrianE22]

A bent rim can be perfectly balanced, statically and dynamically. Force variation comes from the associated runout.

Wheel hop natural frequency is usually between 9-11 hertz which for most cars would be between 60-70 mph. I believe the next natural frequency is the oscillation about the steer axis but I don't remember what frequency that typically is. It's possible your 3rd vibration speed range is caused by the next mode of vibration for the wheel/tire/suspension.

The coming and going of the oscillation is caused by there being 2 frequencies close together. You're getting a modulation producing a "beat" frequency. Suppose, for instance, that one wheel hop natural frequency is at 10.5 hertz and another is at 10.7 hertz. The modulation would produce a frequency of 10.7 - 10.5 hertz or .2 hertz. The period for that would be 5 seconds. You'd feel the vibration coming and going every 5 seconds.

It does seem strange, though, that no one would have thought to check for mounted wheel runout. Maybe your shock absorbers are in real bad shape?

 

[hhy2k]

Really good clue! Right now I am measuring vibrations on all the 4 corners of the car. I will post results soon. Shocks are new though and so are shock mounts.

 

[Tmoose]

Hi hhy2k,

by "tires were changed" to you mean new tires, or existing tires get swapped around?
How long has this been going on? It did it suddenly develop when you went on a trip and your teenaged kids were left home with the car?

"wavelength of about 3-4 seconds" . I take that to mean 3.5 seconds between the peaks of highest vibration you feel.
3.5 seconds/cycle is about 17 cycles per minute. If my 20 year old Volvo has tires that are about 25 inches tall and spin at about 840 rpm.
17 cpm is about 2% of 840 rpm. A 2% change in tire diameter would be about 0.5 inches. Not inconcievable for worn mismatched tires.


Any "couple", two plane or dynamic unbalance, or even significant single plane unbalance would be inphase on both tires every 3.5 seconds.
One result could be to try to wag the tires back thru the steering linkage. Whether the force is so large it significanlty deflects suspension parts, or the suspension has looseness that allows small amounts of unbalance to achieve large vibration amplituies is hard to say.
yes, less than perfectly round whee;l/tire assemblies will cause 1X rotation (or other frequencies, depending on the shape of the deformationforce no matter how well they are balanced.
Hunter claims theire force balancing compensates for stiffness variation in the tire, which would be leigitimately invisible to a conventional balancer.

When measuring vibration will you be getting frequency info? Frequency information is key to accurate troubleshooting.

 

[hhy2k]

I had a long conversation with a tire shop. They said again and again, periodic vibrations can't be the result of wheel balancing. The vibrations are either present or not (at a constant speed). More than that, they also said that a badly balanced wheels has symptomes at a constant speed only. A slightly higher or lower speed would make the vibration disappear! By the way, that is my experience with tire imbalance as well. In this case though, the vibration is always there over 75 mph, only the frequency will change and the periodicity will be faster.

I get questions about the car and part's age. This is an old car, but it os a good car with amazing engeneering. The problem we discuss is there since I bought the car 4 years and 30 000 miles ago. In order to track down this problem I have changed tires at least 5 times (different tires!), changed shock absorbers, shock mounts, tie rod ends. The car had been checked by professional shops for every suspention element and even minor problems were addressed. I have been changed two flex disks in the drive line and the one suspention bearing in the middle.

I did not change wheels, but as I have 5 of them they are going to various places on the car, or in the trunk. I never had a change in the vibration behavior by doing anything with the tires. The wheels are made of steel, not aluminum.
I haven't change motor mounts.

Now to the measurements. You can see 5 mesurments below, each line with 4 pictures represents one measurement, the lenght of every measurement is 30 seconds, the car travels 140 km/h (!). The app I used runs on an iPhone. The phone lies flat (like on a table) over the four corner of the car, over the wheels. The fifth measurement is in the middle of the car! The order of the measurements is as follows:

1st line: middle of the car.
2nd line: right front
3rd line: right rear
4th line: left rear
5th line: left front

RED represents x axis, side to side motion
GREEN represents y axis, front to back motion
BLUE represents z axis, up and down motion

The last drawing is the motion pattern I read out of the diagrams. Your conclusion might be different of course.

 

[hhy2k]

First pic... Sorry, i can add only one at the time...

 

[hhy2k]

One more thing! Someone asked about the real time of the periods. As the pictures I already posted are smaller scale summaries, here I post a full size one where the periods are very clear. This is the side to side motion of the left front wheel for an exact 30 second period at 140 km/h (87 mph).

Thanks for every inch of though put into this problem. I really appreciate it... But it's fun, right?

 

[hhy2k]

To bad there is no post editing. The last pic is for the RIGHT FRONT wheel.

 

[GregLocock]

I'll wait til you publish your vibration data.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[hhy2k]

Hello again. I already published the diagrams. They are in the attached pictures. My conclusions after some math, measurements, counting and tire size consideration are as follows.

At 140 km/h (39 m/s) the vibration base band is 6 seconds! This is the periodicity. The vibration frequency is about 90/6s (15/s). Tire rotation is 120/6s (20/s). Drive shaft rotation is about 360/6s (60/s). //engine speed is 3600 rpm (60x60)// >>> note: this is a manual transmission! Symptoms are the same in any gear (3,4,5,N), depressed clutch has no effect either.

The tire rotation speed (120/6s) does not match the vibration frequency (90/6s)

The gear ratio in the differential is about 3:1

 

[GregLocock]

no, pictures of waveforms aren't data.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[hhy2k]

Using TMOOSE's analogy (thanks for that post):

10cpm (6 sec wavelenght) and by a tire rotation speed of 1200 rpm the wheel diameter mismatch is 0.833 percent. At this tire size this makes about 5mm, 3/16th of an inch difference in diameter. This is very possible, although I do not have mismatched tires on one axle. They are different in the front and the rear, but they are matching pairs (the car is RWD)

Also, how would this explain the 90/6sec periodicity if the tires turn 120/6 seconds?