Vibration diagnosis via sensor data

[JHZR2]

As noted in my new car thread, my 1991 350SD has a vibration that starts to come on around 50 mph and gets worse and worse until a peek at around 70 to 75. I have been hesitant, however I have found that it does start to smooth out around 85.

The two items that seem to be in play are a potential issue with the drivetrain and a potential issue with tires being off-balance. The car had a little bit of control softness at speed so I replaced all four shocks with new Bilsteins.

To date I have done the following:
- Replace shocks
- Replace tires to a matched set
- Rotate tires including the spare

Shocks did nothing nor did replacing the two old tires with two brand new tires of the same model as was on the other axle. I have a new full-size spare, and I noticed that one wheel took a very large amount of weight to balance. So it was my hope that by swapping out the wheel with the higher weight requirement, I would be able to obtain a smooth ride. It seems like there might have been some slight difference in terms of what speed the vibration started or where they got worse but it was not substantial enough for me to be able to make a determination of it. I did move various tires to various positions, with no substantially indicative result.

I have an accelerometer app on my phone, So I decided to take some data, at multiple speeds verified by GPS. At the bottom you'll see data at 55, 60, 65 and 70mph respectively. Below that is the best info I could find on these tires in my size.

The trend appears inline with the speed increases, and given the revs/mile, seems to jive with the frequency Id expect at 60mph. Around 13.6Hz.

I held the accelerometer on the steering wheel. Note the difference in the y axis scale at 55.

If my differential is 2.82:1, I would expect to see different frequencies, correct? Specifically, at 60mph, I'd expect the driveshaft to spin at around 290rpm, and therefore see a major peak at around 5Hz.

Please check my math and let me know your thoughts. Do these data indicate anything other than a tire/wheel based imbalance?

Thanks!!

 

[rubberchicken]

Did you have the tires "road force" balanced ?

 

[Danno]

I had an uncorrectable vibration issue, turned out to be a bent rim.

 

[02SE]

I've seen plenty of new tires that had poor uniformity to the point a road force balancer with competent operator, couldn't make them ride smooth enough to be acceptable. Plenty of wheels with excessive run out too, which can also be determined on a road force balancer that is calibrated, and the operator knows what they are doing.

 

[JHZR2]

Originally Posted by rubberchicken
Did you have the tires "road force" balanced ?



I had whatever Costco did. Not all that confident in that.

My point of this thread however, is to see if these data can help shed insight into the actual cause. Since there's two schools of thought on the cause.

 

[rubberchicken]

Look on Tirerack.com and check for an installer that has "road force" balance machines- take it there and explain the problem. Also ask for printouts of before and after readings. Be prepared that road force balance costs more that regular balance, but if your chasing a vibration problem, then that is a good place to start. Search for Hunter GSP 9700 locations as well.

 

[rubberchicken]

Taking readings off of the steering wheel seems only applicable if you know the problem is on the front axle. Do you know the exact outer diameter of the tires, and does that match the 13.6 hz reading ?

 

[rubberchicken]

So, looking at Tirerack.com, your OEM size is 205-65 / 15, and the Bridgestone I selected shows 817 revs per mile. 817 / 60 = 13.61 which is spot on with your reading. I think your math is spot on, and seems to rule out differential and center drive shaft, and rule in a possible tire-wheel problem. But it rules in anything turning at wheel rpm, so I guess wheel bearings cannot be ruled out, but seems unlikely. Does it have independent rear suspension ? I know many dealers have special equipment for vibration analysis, so perhaps some local shops as well.

 

[rubberchicken]

Check out this iPhone app- is this what you have ? https://www.youtube.com/watch?v=s1cr3QaOzGY

The NVH App , looks interesting.

 

[Ducked]

What's are the X-axis units and scale?

 

[JHZR2]

Nvh app is like $400. This I'm doing my own math.

 

[carviewsonic]

I have had to get my wheels balanced at a second shop, because the balancer at the first shop was out of whack. Twice in the past three years now. Frustrating. It sometimes takes persistence and a careful process of elimination to find a vibration.

 

[ZeeOSix]

Need to check wheel and tire run-out. Your wheels are probably OK if there was no vibration with the old tires.

Last time I bought a set of new tires, I had a steering wheel wobble/oscillation. After driving home from work one day, as soon as I got home I jacked up the front end and rotated both front wheels and used a block of wood moved up close to the tire tread to look for radial run-out. The front right tire was not perfectly round - I could definitely see a "high spot" in the tire. It would balance out fine, but when going down the road it would cause an oscillation in the steering wheel. Went to the tire shop and they put a new tire on and balanced it, and it was smooth as silk after that.

 

[MParr]

Does this vibration come on at a certain engine rpm? If it does, it's not the tires. Engine rpm related vibration will be related to an out of balance torque converter or drive shaft/u-joint problems. Have you changed the pinion angle?

 

[JHZR2]

Originally Posted by Ducked
What's are the X-axis units and scale?


Frequency in Hertz. Very faint on this free app. Look at the top.

 

[JHZR2]

Originally Posted by MParr
Does this vibration come on at a certain engine rpm? If it does, it's not the tires. Engine rpm related vibration will be related to an out of balance torque converter or drive shaft/u-joint problems. Have you changed the pinion angle?



No. Verified That by driving the car in a lower gear to get the engine speed up to a value typical of when I see it on the highway. The vibrations come on around 55 and get progressively worse up to about 75. Driving slower with the same engine speed does not create any vibration.

My major concern was if there was some sort of a driveshaft issue that I could not see from looking and pulling on the components.

It seems to me that the observed frequency would indicate that it can only be tires or earrings or the rear axle shafts. Something that turns at the same rate as the tires.

 

[millerbl00]

Costco couldn't balance my new tires. Get them balanced somewhere else...

 

[Driz]

That's my thought. You can't alwat see the bend . Have them toss the bare rims on their machine and spin it up.

 

[Trav]

I am not familiar with the app so I don't really know much about it. What I do know is if the vibration did not change with new tires at the same speed you may be dealing with a damaged rim (which they should had have seen balancing it) or a drive shaft of some sort.
IIRC this car uses 2X Guibo and one U joint with a center carrier bearing and rear half shafts. I would start by pulling the drive shaft and checking the center bearing and for a tight U joint then have the balance checked by a drive shaft shop.

That car has been around a very long time before you got it so god knows what repairs have been done in its history, the F/R shafts may be 180 off at the bearing.
If that okay you will need to pull the rear CV axles and check for a tight inner joint, its not a bad idea to re-lube and reboot them anyway at that age.
Put 200lb of pebble stone bags in the trunk (its no problem it has a 4 body trunk) and see if the vibration changes at all if so its probably a short axle.

 

[Nick1994]

I started thinking. I'd put the back of the car up on jack stands and put it in drive, see if it vibrates.

My Jeep has one heck of a vibration on the freeway that could be duplicated on jack stands. Got it mostly sorted out, was a combination of bad motor mounts and a transmission mount.