T1 Vibration Help, Please

wmarlexingtonsc · Post by **wmarlexingtonsc** » Sun Dec 10, 2023 3:56 am

Hello everyone,
This is my first time using the NVH software from Pico and it feels like I have chosen kind of a doozy of a project to start out on. I am mainly looking for guidance or direction on how to test this vehicle and reach a resolution on a vibration that occurs at 64-68 mph. The vehicle is a 1996 Suburban with steel wheels, 3.73 gears, and 255/70r15 tires. It is Rear Wheel Drive. The steering wheel does not appear to visually shake when the vibration is occurring, suggesting to me a problem with the rear. The vibration is somewhat intermittent. Not every time you hit the 64-68 mph range do you feel it, but very often you do. Sometimes it feels like you CAN get the vibration to go away by shifting to Neutral and maintaining speed while the problem is occurring, sometimes it feels like you CANNOT get the vibration to go away by shifting into neutral.

The customer has been dealing with the vibration for years. It has been present before and after new tires being installed. When the car most recently came into us for sorting out this vibration, there was some play in some suspension/steering components. So, we replaced the idler arm, repacked the front wheel bearings, replaced a worn out transmission mount, and balanced all the tires. We also aligned the vehicle. All this eliminated the play in the front end, but did not correct the vibration. I ran some tests on the car with a newly purchased NVH kit on the seat rail, and was able to see that a T 1 tire vibration was the worst offender on the radial axis. I have included a NVH file of this, titled "suburban seat rail." I checked the radial runout on all of the tires at the center of the tread using a dial indicator. I found .030-.050" of radial runout from each of them when mounted on the car. The numbers when the tires & wheels were mounted on a balance machine and checked for radial runout at the center of the tread were all within .005-010" of the measured radial runout on the vehicle. I thought my job was complete at that point and it meant that there must be some kind of road force issue with the tires, despite the customer's insistence that he did not believe the tires to be the issue. Not having access to a road force machine, I thought I should at least eliminate the question about the tires and install a new set. I told the customer that I would be responsible for paying for the tires if it did not fix the issue. It did not fix the issue.

My next plan of attack, which in hindsight should have been performed before replacing the tires, was to try to isolate the vibration to a particular wheel by removing the accelerometer from the seat rail and attempting to put BOTH accelerometers on one particular axle at a time (for example, one sensor on the LF and one on the RF, then another run with one sensor on the LR and another on the RR). When I first tried this, I thought there was a home run when I found the RR to have over 1g of vibration (!!!). (See attached file "threelronerr"--the title of which denotes that the three channel accelerometer was on the left rear, while the one channel (radial) accelerometer was on the right rear wheel.) None of the other wheels were even close to that amount. I rechecked the radial runout on that one wheel and still found it to be within .050" when the wheel/tire was mounted to the car. The wheel/tire was still balanced properly. So, I rotated the RR to the RF, expecting to see a shaking steering wheel and over 1g of vibration on the RF when I hooked the scope up again. Unfortunately, that was not the case. The RF and RR had 200-300 mgs while the vibration was occurring, but now the LR had 500-800 mgs, the highest offender. See attached file "9yellowLRthreechannelRR," which denotes a capture on 12/9/2023 with the single channel accelerometer (yellow bar graph) on the LR and the three channel accelerometer on the RR. I also took a capture of the LF and RF, but the laptop battery died before I could save it. I will try to post a capture of that within the next few days. When the vibration was occurring, the RF had 50-150 mgs, while the LF had 200-300 mgs, both well below the current readings of the LR.

I am filled with questions, most immediately: why did the vibration get worse on the LR when simply rotating the RF and RR tires?

Perhaps it's easier if I ask more pointed questions about my methodology above and where holes may exist that allowed me to overlook some crucial piece of evidence.

1) is my methodology for NVH correct? I started with the seat frame reading to identify the worst offending axis of vibration and order of vibration. Here, it was the Y/radial axis, T1. That in my mind allowed me to then hook up the fourth channel connected to only the Y axis and start moving the accelerometer to different corners of the car. If I had 4 accelerometers, I could have hooked them each up to the y/radial axis only and stuck them in all 4 corners of the vehicle, utilizing all four channels of my 4425a scope. Does this all seem sound for diagnosis of T1 vibrations using the Pico NVH? Additionally, am I OK to focus on the radial axis alone and measure runout only on things that could affect that axis, for example, tire radial runout, rim radial runout, axle flange radial runout, axle shaft radial runout (anything else?).

2) Am I missing something when measuring radial runout on a tire/wheel combination when mounted on the vehicle? Should I take radial runouts, not only on the center tread, but also on the inner and outer treads on the tire? To me, the fact that radial runout when measured at the center tread of the tire was within the specification (i.e., less than .060"), meant that I could eliminate a number of things by that method, but did I infer too much? I thought that if that measurement was OK, then I could infer other things, like that the wheel/rim was OK, and that the axle flange runout was OK. If rim runout or axle flange runout was excessive, I thought that would show up on my radial runout measurement of the tire/wheel combo when mounted up on the vehicle. Was I wrong to infer that much? Should my next steps be to measure Left Rear radial runout on the rim when dismounted? Axle flange radial runout? Axle shaft runout? Ring gear radial runout? Should I do all this EVEN THOUGH the runout on the tire/wheel combo when mounted to the vehicle was in specification?

3) mounting the accelerometers. Please see attached pictures to see where I am mounting the accelerometers on the front (control arms) and the rear (axle shaft tube). Are these locations OK, or should I seek others for more accurate results? On a side note, can the accelerometer be mounted upside down, as long as the bolt hole is facing forward? What is the best way/ materials used to mount these accelerometers to pieces on the vehicle that are not magnetic, e.g., aluminum control arms.

4) What mgs would be considered acceptable for an older vehicle like this when analyzing vibrations. I am sure this thing was not built to the same exacting NVH standards as modern cars. I think I saw in a video that anything above 20 mgs was worrisome. Is that applicable to an older vehicle like this, or is that asking for too much as a realistic goal.

Thanks to anyone that can help me sort out this vehicle. I feel like it will be a wonderful learning experience, if I can follow it through to a resolution.
Josh

wmarlexingtonsc · Post by **wmarlexingtonsc** » Tue Dec 12, 2023 5:38 pm

Update:

I wanted to attach two more files from some work I did yesterday on the car. We checked the static balance on the rear brake drums and found them to be out of balance by 6.00 ounces each (!!!). The new brake drums measured less than .75 ounces on a static balance test. We replaced the drums and test drove the car again. At first the vibration seemed better, but after driving more, it seemed just as bad. I have attached the latest files from the drive after replacing the rear brake drums.

"12rearthreerronelr"--denotes the capture was taken on the 12th. The two sensors were on the radial axis channel on the rear of the vehicle. The three channel sensor was on the RR. The one channel sensor was on the LR.

"12frontthreerfonelf"--denotes the capture was taken on the 12th. The two sensors were on the radial axis channel on the front of the vehicle. The three channel sensor was on the RF. The one channel sensor was on the LF.

I am thinking now, since the radial runout of the tires is less than .060" when mounted on the car, and the wheels are balanced, that this T1 vibration, which is the worst at the LR, is coming from the axle or differential area. One sensor on the diff. case and one out at the wheel perhaps? Would that help matters? I will continue the investigation. If anyone has any advice, it is appreciated. Thank you!

Post by **Steve Smith** » Wed Dec 13, 2023 6:33 am

Hello and thank you for the post. Sorry for my late reply

I will respond ASAP but a little consumed at present

Just quickly however, could you over inflate your tyres deliberately for another road test and see how the T1 is influenced

I.e., if they run at 35 PSI (and the tyre rating will allow) can you co to 70 PSI

Could you also have a look here topic23332.html

I hope this helps in the interim and I will respond ASAP

Take care......Steve

wmarlexingtonsc · Post by **wmarlexingtonsc** » Thu Dec 14, 2023 3:05 am

update 2:

I acquired two more accelerometers to allow for testing of all four wheels at once. I really want to make this NVH method work for me when diagnosing vehicles, but I know the learning curve is steep. Before I hooked those additional sensors up and took captures, I did try hooking up one accelerometer on the differential housing and one on the LR of the vehicle. The vibration was much worse at the LR axle tube (200-300 mg) than it was at the differential housing itself (50-100 mg). Not sure if that means anything, but there it is.

After that, I hooked up the two new accelerometers, for a total of four, and took two captures. The first capture is with all the tires at 51 psi (the max allowable on the sidewall of the tires). The second capture is with all the tires at 36 psi (what I was testing at before). The sensor orientation is as follows: A=LF, B=RF, C=RR, D=LR. All sensors are hooked up into the radial/y axis.

A couple observations:
1) I was wondering if the road that I was testing on may have been playing a factor, and it seems to have been. The earlier captures in the earlier posts were taken while cruising on the interstate, a rougher surface around here, but easier to get up to 65-70 mph without getting a ticket. However, these latest captures were taken on a smoother stretch of road nearer to the shop. The results do look substantially different. Instead of rear wheels running at 500mg-1g, you can see no more than 300-400 mg on this smoother stretch of road. I will test only exclusively on this road from now on. I see the importance of a controlled patch of road for each test again and again. I am learning.

2) Overall, the mg numbers were less with MORE air in the tires. Furthermore, the main offender with more air in the tires seems to me to be the RR. The RR wheel does have a slight visual dip in the rim. I assume the rim to be OK, since the radial runout when measured at the center of the tread while the tire/wheel is mounted on the vehicle is around .050", which I think is within the acceptable limit of .060". Is it worth dismounting this tire and measuring the radial runout on the rim regardless? Additionally, of further interest is that the two rear tires seem to be more evenly matched on mg levels when LESS air is in the tires.

I hope some of this is helpful to you, Steve. Thank you for taking the time to respond to my post. I very much enjoy watching your videos and reading your articles online.
Josh

wmarlexingtonsc · Post by **wmarlexingtonsc** » Thu Dec 14, 2023 10:17 am

A further thought: the front two accelerometers are mounted on the front control arms and are completely flat with 100% contact on the arms. The two rear accelerometers, however, are mounted on the rear axle tube/housing, which is round. The rear accelerometers make probably 90% contact on the arms, but there is a slight amount of contact that is not made due to the roundness of the axle tube/housing. There is a slight amount of wiggle back and forth on the rear sensors. Perhaps I will run a new test and find an area that is perfectly flat to mount the sensors to, so that NO movement is possible. I know there is a vibration; I feel it, but perhaps the results are skewed due to the way I have the rear sensors mounted. I will take another capture today, if I can.

PicoKev · Post by **PicoKev** » Fri Dec 15, 2023 8:47 am

I use some round steel punch blanks (think of coins of differentthat sizes or washers without a hole!) I liberated from a scrap skip at a metal factory a good few years ago.

To mount the accelerometers on a curved surface I simply use a fast epoxy to bond a blank in the orientation I want and stick the accelerometer on and "Job's a good 'un" as we say over here. No need to remove them as you may need to use them again and the are not causing any harm.

I use the punch blanks because I have them and they cost me nothing but a nice thick washer will do just as well.

Regards,
Kev.

wmarlexingtonsc · Post by **wmarlexingtonsc** » Fri Dec 15, 2023 10:59 am

Thank you for the tip. I will try this!

wmarlexingtonsc · Post by **wmarlexingtonsc** » Sun Dec 17, 2023 10:43 pm

I just wanted to give an update on this. I found a flatter spot on the rear axle tube to mount the rear accelerometers. The data shows T1 still as the main offender, still in the rear of the vehicle. I took one rear wheel off that has a visual dip in the outer area to measure run out on the rim itself, thinking that my logic was flawed about no more measurements being necessary if the wheel/tire combo radial runout is NOT excessive when mounted on the vehicle. The rim showed signs of pitting from rust in spots. On these spots, radial runout was nearly .050", which is in excess of what I believe to be the allowable limit of .040". Therefore, this answers one of my questions above about whether it is necessary to continue measuring run out on different areas, even if you have already confirmed that wheel/tire runout is in specification. Yes, you may yet find something.

Unfortunately however, I tried rotating the tires around one more time (X-pattern rotation) and taking another capture. Again, the main offending mgs were on the rear of the vehicle. To me, this confirms that, even though the wheels may be CONTRIBUTING to the overall vibration, they are not the MAIN offending component that I am hunting. To me, via process of elimination, this must be a problem with something inside the rear diff/tube assembly. Either a ring and pinion issue or a bent axle. Somebody stop me if I'm wrong. I have entertained ideas about the shocks or springs possibly being bad, but how would that affect the data in PICO NVH software? Would it show up as a T-1? Technically, it is not a part rotating at the same speed as the tires, so I would think the answer there is no. But am I right to assume that? How would a bad leaf spring present itself in NVH data? As some unidentified frequency level?

As a final note, sometimes it looks like there is a P1 vibration that is not insubstantial, though it isn't as great as the T1 vibration. The driveshaft does have some visual marks on it, as if it was in a vice at some time. I wonder if I might not be chasing multiple issues here between the worn/rusted rims, something in the rear end causing the T1 vibration, and a driveshaft issue. It is still my thought process that the T1 vibration in the rear, which does not move around when rims/tires are moved, is priority #1. After that, the P1 vibration.

The attached file is after I rotated the tires around recently. I was trying to maintain 65 mph.

Post by **Steve Smith** » Tue Dec 19, 2023 9:53 pm

Hello and thank you for the posts, sorry for my late reply

I will chisel way at each post and hope to answer as many questions as I can starting with the first post on Dec 10th

It does sound like we not only have imbalance to consider but also the influence of “load” upon the vibration. Tyres like joints and shafts can be affected adversely by load.

From your initial assessment I would go along with suspecting a rear zone imbalance based on no vibration felt through the steering

Regarding the tyres you have replaced, are you confident in their quality/branding as we know not all tyres are made even? The temptation to save on tyre costs will always manifest a reason for their “good value” and typically this is high speed stability

Your initial preparation with this vehicle is awesome, in that you rectified suspension and steering component wear and included wheel alignment (Was the alignment actioned after the new tyres were installed?)

With the above in mind, we can pursue the vibration with confidence given the fundamentals have been checked and rectified where applicable

Please bear in mind that Road Force Balancing remains a valid option with this case given we are pursuing a T1

The results you obtained regarding radial run out focused on the tyre tread

Could you take a look here at this case study where the alloy wheels proved to be the offenders which in turn affects the tyre run out (Both lateral and Radial)

https://www.picoauto.com/library/case-s ... way-speeds

There are videos embedded in the above study, but here is the left hand front wheel radial run out procedure

We not only have to consider the radial and lateral run out, but the degrees of wheel rotation from min to max deviation

For example, in the above video we saw a huge radial deviation of 1.45 mm in approx. 30 degrees of wheel rotation and therefore the effect on the tyre (seated on the deformed rim) will contribute to imbalance

Please bear in mind the case study above utilizes Run Flat Low-Profile tyres and so less forgiving when it comes to radial run out. I am assuming your vehicle will have softer, standard profile tyres which will provide an increased level of tolerance to rim deviation?

A word to the wise about radial and lateral run out measurements, please do not overlook the influence of the hub flange and hub “center” in relation to the rim

Moving onto your methodology:

“I started with the seat frame reading to identify the worst offending axis of vibration and order of vibration. Here, it was the Y/radial axis, T1. That in my mind allowed me to then hook up the fourth channel connected to only the Y axis and start moving the accelerometer to different corners of the car.”

This approach is bang on the money and exactly as we have discussed here:

“Am I missing something when measuring radial runout on a tire/wheel combination when mounted on the vehicle?”

Hopefully we have covered this in the above BMW case study but please remember that T1 indicates one disturbance per revolution of the road wheel. (This can be multiple components from the differential to the road wheel rotating at the same frequency as the wheel/tyre)

“3) mounting the accelerometers. Please see attached pictures to see where I am mounting the accelerometers on the front (control arms) and the rear (axle shaft tube). Are these locations OK, or should I seek others for more accurate results? On a side note, can the accelerometer be mounted upside down, as long as the bolt hole is facing forward? What is the best way/ materials used to mount these accelerometers to pieces on the vehicle that are not magnetic, e.g., aluminum control arms.”

When comparing accelerometers against one another, as best you can we need them mounted securely to components of equal mass and location

For example, caliper mounting bolts are perfect or something similar. We must keep variables to a minimum and there must be no “tip” or “rock” in the accelerometer when mounted

When measuring just the Y axis, the accelerometer can be mounted upside down as the vertical orientation has not changed

Alternative mounting methods can be applied by utilizing the thread in the accelerometer (i.e. stud mount) G-clamp mounting (via the magnet base) or in the case of aluminum , you can bond a washer to the relevant suspension component to which the magnet will mount

"4) What mgs would be considered acceptable for an older vehicle like this when analyzing vibrations. I am sure this thing was not built to the same exacting NVH standards as modern cars. I think I saw in a video that anything above 20 mgs was worrisome. Is that applicable to an older vehicle like this, or is that asking for too much as a realistic goal."

A great question and one which can only be answered by back-to-back testing against a known good as VM’s do not typically quote these values

With a low-profile tyre on a sports chassis 15 mg will be felt in the cabin but 15 mg in your vehicle may not be detectable as everything is relative to vehicle intended use & design.

This is where we need a RWL (reference waveform library) for NVH. If anyone out there has a good example for such a vehicle it would great to see it posted here?

Moving onto the captures from the 10th of Dec and the 1 g of vibration that appeared to shift with the location of the accelerometer on channel D

Here we have a measurement error and I am so sorry this was not made clear in the software

Referring to the image below, where we have added an accelerometer to channel D we have chosen the TA148/TA366 (SUM Interface) which has a single BNC output

: 1

In the above configuration we are not comparing apples with apples

We have a 3-axis accelerometer connected to channels ABC where we have found the vertical axis (Y) to be the offender

When using the XYZ output of the TA259/TA366 (XYZ) there is no alteration to the voltages, i.e. whatever the input voltage of the TA143 accelerometer (into the TA259/TA366 XYZ) the output is the same to the scope. The scale factor when choosing XYZ = 108 mV/g (1 V = 9.259 g)

When using a TA143 3-axis accelerometer with a TA148/TA366 (SUM) interface each axis (XYZ) is added together and divided by 3 to derive the SUM average

As a result, the scale factor differs when choosing SUM = 36 mV/g [one third of 108 mV] (1 V = 27.7 g)

This why the high amplitude followed channel D accelerometer when testing the RL and RR

The SUM output is useful when comparing SUM outputs with SUM outputs and not SUM outputs with a Single axis (Y in this case)

I will add a new feature request to introduce a warning for the above during the set-up of NVH

I will follow up with more feedback on your other posts ASAP

Thank you for taking the time to share this journey which will help others

Take care…….Steve

wmarlexingtonsc · Post by **wmarlexingtonsc** » Wed Dec 20, 2023 12:41 am

Steve,
This was all super helpful. Thank you so much for taking the time to reply in such detail! One follow up question that I do have: you say that a brake caliper bolt is a good place to mount an accelerometer, but I have trouble visualizing how that works if the y axis needs to be vertical. It seems to me that the brake caliper bolt as a mount would make the accelerometer sit horizontal. What am I missing here about the brake caliper bolt being an ideal spot for an accelerometer?
Josh

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