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.

 

[hhy2k]

@GregLocock

I worked from these pictures and found all the data I posted. I am happy to provide more data if you need and if I know (but I really dont know how old is the captain, so don't ask that question please but seriously, what is the data you are missing?

 

[GregLocock]

numbers, in the form of columns of time and the instantaneous vibration (ie the numbers plotted on that graph). I think you have done a great job at analysing it thus far, I'd just like to look at the raw data.

I've worked on 2 heterodyne or beating problems in my 20 years of automotive NVH, they are tricky.

Cheers

Greg Locock


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[hhy2k]

Well, columns are good, so here it comes:

Vehicle speed: 140 km/h, 39 m/s, 87 mph
Wheel speed: 20 rps, 1200 rpm (120/6s)
Vibration base wave lenght: T=6 seconds
Vibration frequency: 15 Hz (90/6 seconds)
Drive shaft speed: 3600 rpm (60/s and 360/6s)
Engine speed (no effect)
Transmossion speed (no effect)
Vibration is present over 110 km/h

I can't match the wheel's 20 rpm to the 15 Hz vibration frequency. May be nobody can!

 

[GregLocock]

No, the actual datapoints.

s

v
0

0.0000
.00152​

0.0012
etc etc

typically a csz or tst file

Cheers

Greg Locock


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

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[hhy2k]

Oh OK, that column! I'm sorry for being such a bonehead. I have this longer run recorded.

 

[Tmoose]

The tire shop " 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).
Would you say, daring to risk an age discrimination lawsuit, based on superficial appearances Is it likely that the tire person could have accumulated significant tire service experience back in the 70s, before the manufacturing tolerances of some mundane components gained a decimal point or two? When even Honda was still using lug centered wheel assemblies? (I'm guessing your Benz's wheels are hub centric.)

The tire shop " 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).

Did they say that 10 times?

How about this...
An Internet clown says "All 4 tires on a car are not rotating at the same rpm when traveling in a perfectly straight line when traveling at constant speed because even if brand new they (he tires) are not the exact same diameter."
An Internet clown says "All 4 tires on a car are not rotating at the same rpm."
An Internet clown says "All 4 tires on a car are not rotating at the same rpm."
An Internet clown says "All 4 tires on a car are not rotating at the same rpm."
An Internet clown says "All 4 tires on a car are not rotating at the same rpm."
An Internet clown says "All 4 tires on a car are not rotating at the same rpm."
An Internet clown says "All 4 tires on a car are not rotating at the same rpm."
An Internet clown says "All 4 tires on a car are not rotating at the same rpm."
An Internet clown says "All 4 tires on a car are not rotating at the same rpm."
An Internet clown says "All 4 tires on a car are not rotating at the same rpm."
An Internet clown says "All 4 tires on a car are not rotating at the same rpm."

An Internet clown says "All 4 tires on a car are NOT rotating at a constant rpm."

I'd position myself on the highway enjoying the "periodic vibration" at speed in either the left or right lane.
Then, when traffic permits, I'd turn the wheel about 1 inch at the rim to cross all 3 lanes in one gentle swoop carefully noting if during the swoop the period of the vibration changes. When traffic permits I'd return to the originsl lane, again noting the vibration. If a section of the highway has a gentle curve a similar test is conducted automaticallly.
Sometimes I can "catch" the vibration at constant point, because by luck I have set the problem tires at very nearly the same rpm, so the unbalance and/or runout are also maintaining the same phase, whether the relative phase is maximizing (high vibration) minimizing (low vibration) or some intermediate effect.

Then I'd measure (not eyeball, even a tire specialist's eyeball) and record the radial and axial runout of all 4 tires and rims while mounted on the car.

https://groups.google.com/forum/?hl...t.autos.rod-n-custom/AOibJtReV_A/NWuP8k46cxwJ

 

[hhy2k]

I think I got the message. Although I don't think he said they all turn at the same speed (if that would be the case, your ABS and all those stability control gadgets were pretty much wasted money). He only said, at a constant speed (a 10-15 mph range) a vibration is either there or not and as this is true for each and every wheel, he doesn't see how a periodic vibration could happen in such a wide range of speed (and I add that the characteristics of the vibration never change). The turning test I have done many times. I never noticed any change in the vibration amplitude or frequency because of a slight turn. The only thing had any effect ever are the very smooth surfaces under the car.

Honestly, I should really do the runout test on the car as you suggest, it is just so hard to grasp the bent wheel concept after all that tire and wheel swapping for years... While I always had the same symptoms. On the other hand if the geometry discrepancy is on the car itself, that would have never came up by switching around the rims, no matter how well the tires were balanced or checked for runout on the balancing machine.

This is clearly the next step I must do. Thank you very much.

 

[BrianE22]

Have a friend drive your car while you ride next to it in another car. Look at each of the 4 wheels to see if one is shaking more than the others.

 

[DanEE]

Or get a GoPro or other high def/hi frame rate camera and make video recordings of the suspect areas to try to narrow down the source. Would be easy to use a GoPro underneath the vehicle to observe drive train and suspension areas.

Jay Leno's Garage and other automotive videos makes extensive use of Go Pros. Have noticed quite a bit of detail captured on vibrations or shake that builds up in various areas of a vehicle, for example frame/bumper to body differential movement at road speeds, especially on older vehicles.

 

[hhy2k]

All run out had been checked out to be less than a millimeter with the wheels mounted on the car.

I move on to the drive train.

 

[BrianPetersen]

All well and good that the runout was measured.

On a smooth road, have an assistant in another car drive beside you while you drive at the problem speed, and have the assistant tell you which wheel(s) is plopping up and down.

 

[GregLocock]

OK, I got that data, I'll have a play around with it. It looks as though you used a 100 Hz sampling frequency. What did you set the anti aliasing at?

Cheers

Greg Locock


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

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[GregLocock]

Here's a waterfall plot of the data, and a synthesised signal of similar characteristics

here's the function to generate that synthesized signal

for i=2:length(t)

if t(i)<20
f=20.5;
else
f=20.5-(t(i)-20)/60;
end
G(i)=sin(t(i)*2*pi*f)+.8*sin(t(i)*2*pi*(f+1/4.5));
end

On the pretty plot frequency is vertical, 22 hz or so at the top, and time runs from right to left (sorry)

Now, the interesting thing is that the waterfall of the synthesised signal does not display as I'd have expected, as the speed ramps down the beating reappears and disappears, quite strangely. The resemblance to your data is reasonable. Sadly i don't think this really gets us much further.

Cheers

Greg Locock


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

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[Sloss]

Very interesting thread gents.

The one thing that jumps out to me is that the wheels are the same ones.

I suggest borrowing a set of good wheels/tyres that don't give these vibrations and go for a run.....