Wrench589

Lately, my Yukon XL is behaving slightly different. Not as bad as it used to be. The low speed drone over rough surfaces is the same - BAD! But, the buffeting seems to come and go. I almost have convinced myself I can detect it coming and going with varying the torque load in the driveline (driveshaft). Almost seems like it goes away when the load is neutral (ie coasting). But, slipping the transmission into neutral does not eliminate the buffeting, so that would kill that theory. Still, I do hear it coming and going. Ultimately, I think any vibrations felt by the body are generating the buffeting. Seems like those vibration sources can be tire/wheel, roiled road, driveshaft balance, u-joints, ring and pinion, etc. it will be impossible to eliminate all these. The solution is to de-tune the body/frame assembly. That includes even if it's the rear hatch, and it sound like FogDucker might be on to something. Question, FogDucker.... Did you do any adjustments to the stops at the bottom of the hatch? Did you notice there is a second set of stops above the lower ones, and I'm wondering how critical it is that the two are adjusted correctly relative to each other and the body seal itself. One way to test the amount of seal is to put a strip of paper between it and the hatch, close the hatch, and try to pull it out. And, test all the way around. Haven't had a chance to check mine but I will one of these days!

I did some rough calculations of the Helmholz frequency and it's not close to the frequency that I have measured the buffeting to be at. I measured a sound pressure spike between 25 and 30 hz, while the Helmholz frequency for the size of the Yukon XL cabin size is over 40 hz as I recall (been about a year since I did it!). And FYI, the non-XL would be even higher frequency, being shorter. A Helmholz quarter wave resonance would be completely independent from any flexing of the roof skin or moving of the hatch. A quarter wave resonance is a standing wave inside a tube and does not need the tube walls to be flexing to have the standing wave. To help visualize, think about blowing across a glass bottle top. The sound you hear is the standing wave of the air inside the bottle, but the glass isn't "flexing" much, is it? Not to say that a quarter wave resonance couldn't excite a flimsy surface of the container it's inside of. But that would be a result of the standing wave, not the cause of the standing wave. That said, the buffeting is definitely a result of something flexing/vibrating. And that something is likely a very large surface.... Roof, floor, firewall, rear lift gate, rear quarter panels. Something like that. As for the roof skin being attached to the bows or not.... From what I saw of the cross bows, they are very flimsy looking. And, the spacing between them is quite large. So, I would suspect that if the roof is the issue, then attached or not there will be a problem. Maybe the severity will vary, but if don't think the bows are sufficient to do the needed job.

An out of balance brake rotor does not explain why the vibration seems to come and go, even at a given speed. Some folks report the vibration is gone for a day of two, then returns. Some folks report the vehicle was fine for several thousand miles when they first got it, then the vibration developed. Any out of balance will show up all the time, even if only at a given speed. In fact, pure mass unbalance will always be present, regardless of speed, and show up as a simple multiple of speed. The forces generated will vary with the square of the speed, so at low speeds the forces will be small, and likely not felt by the occupants. As the speed increases, so does the frequency and more importantly, so does the amplitude of the forces generated. With simple mass unbalance, this will continue as speed continues. If there is some kind of resonance, then things can get complicated. A small amount of unbalance, even so small it won't be felt at any speed,can set off a resonance at the natural frequency of whatever it is that is going to resonant. That only happens at, or near, the natural frequency. And the resulting forces can be very, very high. Usually,when the speed is further increased, the resonance condition subsides and things get smooth again. That is how you can tell the difference between a resonance and a simple mass unbalance. Of course, it can get even more complicated as the mass unbalance can have a harmonic component that can excite the resonance multiple times on the way up in speed. And, there can be multiple natural frequencies (modes) that can get resonant at different speeds. So, what could be a series of multiple resonances could appear to be simple mass unbalance. Only a highly trained vibration analyst would likely diagnosis such a condition and I doubt the average technician at a car dealership has that kind of training. So, we appear to be stuck with GM swapping out tires, over and over. Waste of time..... Go for it. Have your rotors removed and balanced at a driveline shop. I'm sure any driveline shop that does driveshafts can do rotors. If not, take them to an industrial machinery repair shop that has a dynamic balancing machine. They will usually have a wide selection of arbors and mandrels to balance a whole host of rotating parts and I'm sure a brake rotor would be no problem to do.

Don't forget, many tires will have been ground at the tire factory to get them to pass their "Tire Uniformity" limits, which is, of course, the road force variation. If you grind them round, then they may be way out on tire uniformity. Now, this does bring up and interesting question. Imagine the following.... A perfectly round wheel, mounted perfectly concentric to the axis of rotation (the axle). Now we install a tire that has been ground to compensate for some non-uniformity in the road force variation as it is rolled through the loaded test drum at the tire factory. In theory, should ride well in our perfect wheel setup. But, I still struggle win the flat spot that has been ground on the tread. I realize the road force variation measurement is supposed to measure the resulting ride, but does it really? As an experiment, it would be interesting to take a truck that vibrates real bad, and have the tires balanced on a machine that measures the runout with laser, instead of the road force method. I think Bosch is one manufacturer that makes such a balancer. Anyway, would be interesting to see what happens if the mechanical runout is eliminated only, instead of the whole road force thing. I guess that is exactly what you are talking about. Go for it!! Can't wait to hear the result.

Weird. Early on, I disabled the active suspension (shocks) by pulling the fuse to that system and it did not help the ride, the drone, or the buffeting. The ride did get worse in that the vehicle wallowed thru dips and around corners! Might have to try thus again. How dis you disable just the front shocks?

Yes, I have noticed that. I'm actually starting to wonder if there is something going on in the front end of these. There are a couple of design "features" GM added to these vehicles. There are braces from the centre of the fender to the cowl that pervious pickups didn't have. And, there is some kind of strut under the dash that ties the firewall to the dash somehow (read that one in the marketing literature). I wonder what would happen if the bolts were loosened for a test drive?

Do you have any more info on this? Is this what GM is saying the root cause is? This sound a lot like the "frame beaming" I read about on an older generation of GM trucks. There was a TSB on fixing it. Well, not he frame itself, but new cab mounts to reduce how much transmitted to the passengers. Sound like a very similar thing.

Very likely it had flat spots. In fact, GM is pretty adamant that the dealers drive the vehicle at least 15 miles before doing any Roadforce Balancing to get rid of any flat spots. And, your experience would be consistent with that. What is not clear, is why these vehicles are so bloody sensitive to flat spots. I can leave any of my vehicles sit for weeks on end and when I drive away, zero vibration right from the get go. So why are these GM/Chevys so fricken sensitive. It seems like if the wheels and tires are not 100% perfect! get some kind of vibration (or buffeting, in the case of the SUVs). GM can blame the tires all they want, but there is. I other vehicle on the market today that is that sensitive, not even in the rest of the GM brand! So unless GM plans to design and manufacture the perfect tire for us, it appears these vehicles will always be a problem. FYI, I've notice the buffeting on our Yukon is much worse the first 5 or 10 miles, then gets a little better. So, likely the flT spots on the tire making it worse. One of these days I'm going to jack up a wheel aft it's been sitting all night and measure how much of a flat spot is there. I bet it's very minimal.

The math is essentially calculating the equivalent weight/mass unbalance that produces a certain force, in lbs., at a certain speed. I used 110 kph. Basically calculated based F=ma using angular velocity to determine the centripidal acceleration. Have to remember that the weight you see on the wheels is not there to correct for Road Force Variation, even when it's been on a Hunter Road Force machine. The Hunter machine uses "Force Matching" to position the tires on the wheels in the most optimal combination so any runout of the wheels can be set opposite to annoys RFV in the tire and cancel each other out. After that is done, then the weight balancing is done, essentially in the traditional process. The Hunter GSP 9700 does not add weight to correct for RFV. There are limitations to how much Force Matching can be done. For example, if the wheels are perfect, then you are stuck with the tires as they are. There is no correction. To do the best job possible, supposed to start with bare wheels and measure the runouts. Then mount the tires and take he RFV readings. Then the computer will calculate the optimal arrangement of which tire on which wheel and in what orientation. Many many hours of work. According to Hunter, recently they are finding a lot of trouble with getting accurate and repeatable RFV readings because of improper bead seating. Apparently, the technology in the beads and wheel seat has changed and it's more and more tricky to get the bead to seat properly. And, can't see it from the outside. Supposed to use lots and list of lube and even over pressure to get the beads set properly, then "drive gingerly" for the first 500 miles to prevent slippage! Yes, that is what Hunter is saying. Hunt even says can deflate the tire then overinflated to attempt to correct the bead (up,to max pressure, of course). So, I wonder is some of these changing RF results on tires is this going on? The other interesting thing Hunter says is that the results of the RF readings should not be used to reject a tire! That's interesting. Last thing...several GM Bulletins, as well as Hunetr, clearly say that the tires must be driven for at least 15 miles just prior to doing RF balancing to remove any flat spots and warm the tire up. I wonder how many dealers do that, especially when they get a brand new replacement tire in! ( I realize a new tire won't have a flat spot, but certainly won't be "warmed" up.

I believe numerous owners have had their trucks diagnosed by the dealer using the Pico vibration analyzer, which will correlate the vibration frequencies to tire or driveline speed. Results - don't have. Are the dealers qualified to read and interpret the data? Don't know. It is interesting that the vibration severity in the SUVs appears to be nowhere near the pickups. Now, the rear suspensions are completely different between the two. But, the other big difference is one piece body on the SUV vs separate cab and box. Thus, I would expect the SUV to add some overall stiffness that the pickup wouldn't have. I recall years ago the Ford F150 has issue with the rear vibrating like crazy. Owners report they could see the box jiggling like crazy. Has anyone done anything like that to see what the box is doing relative to the cab? Maybe a Gopro? Of course, it is likely that the body is taking the brunt of the frame flex on the SUV and hence the buffeting and pressure issue we are experiencing with the SUV. Less vibration, but the buffeting is brutal. Has anyone wondered how much the Road Force number really are? To put it in perspective, I did a calculation to take the Road Force force and compare to an equivalent amount of mass unbalance at 110 kph, on my 22" wheels and tires. Here's what I got: 35lb RF = 1.6 oz (original GM spec for trucks with LT tires) 24lb RF = 1.12 oz (original GM spec for trucks with P metric) 18lb Rf = 0.84 oz (original GM spec for cars) 15lb RF= 0.7 oz (revised GM spec for trucks and SUVs with vibration and drone/pressure issues) And, this is total mass unbalance. Often, this is split between inside and outside bead of the rim. So, for arguments sake, think about half and half.... So that looks like a 0.35 oz weight on the inside and another on the outside of the wheel. That's not a lot. Noticeable, yes. But not a huge amount, for sure. Puts it in perspective, I guess. For what it's worth.

That definitely sucks. It's weird that some roofs have the problem of the roof pulling away at the sunroof and others don't. GM must really have to quality control over that part of the fabrication and assembly. Atrocious!

Has anyone wondered how much the Road Force number really are? To put it in perspective, I did a calculation to take the Road Force force and compare to an equivalent amount of mass unbalance at 110 kph, on my 22" wheels and tires. Here's what I got: 35lb RF = 1.6 oz (original GM spec for trucks with LT tires) 24lb RF = 1.12 oz (original GM spec for trucks with P metric) 18lb Rf = 0.84 oz (original GM spec for cars) 15lb RF= 0.7 oz (revised GM spec for trucks and SUVs with vibration and drone/pressure issues) And, this is total mass unbalance. Often, this is split between inside and outside bead of the rim. So, for arguments sake, think about half and half.... So that looks like a 0.35 oz weight on the inside and another on the outside of the wheel. That's not a lot. Noticeable, yes. But not a huge amount, for sure. Puts it in perspective, I guess. For what it's worth.

GMC....Generally More Crap!

Frank, yea I would expect colder to be worse. Colder oil in the shocks. Colder body mount rubber. Colder suspension rubber bushings. Colder tire rubber. I will admit it was Avery short trip. On,y about 5 miles round trip. But, usual the worst buffeting is the first 5 miles, so it worth noting. A word about tire rubber. I happened to have a meeting with a professor at the local university engineering department a few month ago. His specialty is polymers. We were meeting on something related to my work but in the conversation he made a comment about tires, since of course all modern tires are synthetic rubber. I took the opportunity to derail the meeting for 5 minutes to talk about tires and turns out one of his grad students now works for Goodyear at their research centre! Anyway, bottom like is the tire industry has changed the way they make synthetic tire rubber over the last 5-10 yrs. it has to do with the way the cross linking is done on a molecular level. Now, the new rubber tends to be stiffer but, is affected less by temp changes. So hard to say what "stiffer" means - at what temp? I suspect the new polymer also wears better? Anyway, was interesting if nothing else. And by the way, the new process is cheaper and faster!!

Mine has the stock exhaust - no Borla - and has the vibration and buffeting/pressure issue. Interestingly, I has to run to the store the morning and the vehicle basically exhibiting not noticeable buffeting. The vibration was there, but possibly less - hard to compare to last time I drove it a few days ago. But for sure the buffeting was almost non existent. Just an extremely slight shudder once in a while, times with the vibration. So why? Don't know. I do know that it's a bit cold here right now. The vehicle is parked in an insulated garage but the heat is turned off. Outside temp is -15C. I also noted the tire pressures are a bit down, to 30 psi, per the TPS. Could be helping? Maybe. I never did play around with tire pressure in the summer. I think the normal pressure was around 33-35 psi, which isn't that much different, but maybe with such a low profile tire, make a big difference?? (22")

I'm started to question this whole "Road Force" thing. Not the concept, as I completely understand it. But, it seems that it doesn't matter what brand or model of tire, they are continuely changing. One minute a tire is 13#, then all of a sudden it's 33#. If that s truly happening, what's the point of RF balancing in the first place. If the tires are not repeatable, and for whatever reason they are changing their Road Force Variation that much, then the entire notion becomes pointless, and the only solution is a vehicle that isn't so sensitive to RFV. Something just doesn't add up here.

My roof jumps like crazy, too. And mine buffets like crazy, too. But, for what it's worth, I've done the same check on numerous other SUVs that don't buffet and they all bounce as well, including an '09 Explorer, a '10 Explorer, and an '09 Trailblazer. The Trailblazer was the worst by far! Way worse than even my Yukon. And, quiet as can be inside. So, not sure the "roof bounce" test alone is the indicator.

Stumbled across this about older GM pickups. http://www.justanswer.com/chevy/29lyw-silverado-that-issuing-vibration-cab-mount-supresses.html "Frame beaming".... I'm thinking that is the term GM used to describe the frame actually bending in one of its natural mode shapes. Now, be aware that everything flexes to a certain degree. The question is, will it result in a "problem". What I find incredibly interesting is that frame beaming has happened in the past and thus, can be added to the list of possible root causes here. And if it is the frame, then we are all screwed. Cause I'm sure there is NO practical fix. Makes one wonder how this could have every got through design and testing. Well, don't forget. A lot of vehicles get designed and tested on the computer these days. Those images you see on the TV ads of a vehicle up on some kind of road simulator or shaker are just that - images. In order to cut costs and reduce cycle time, all manufacturers take shortcuts. And computer simulations cut time. But, they don't always give the whole answer. All I'm saying is it's totally possible such a fundamental screwup could get through and end up in the production vehicles. And as that post says, any vibrations make it worse, so GM will be chasing all those vibrations. Hence, all the Roadforce Balancing and the like. And, all to no avail. This goes back to a post I made many months ago, sharing a Technical Paper in found on this exact topic and the research the author had done. Looking more and more like a possibility, even if it is pure speculation at this point.

Thanks for the description, very good. I think I have a pretty good idea what it looks like. Boy, disappointing with all that Dynamat it didn't completely solve things. Although, if it improved the buffeting, might be a sign it's on the right track. So, did you remove the headliner yourself, or did you do it when the dealer was doing the TSB?

That's a lot of ifs to go right. I'll take a proper torque wrench over those lazy-man extensions any day, thank- you.

That's my point. The normal road force variation measured and reported by the Hunter machine is only a Radial Force Variation measurement. Any lateral forces would not be included in that measurement. And so a wheel tire could look fine but put it on the vehicle, and the lateral runout and/or lateral force variation would cause LATERAL shaking of the suspension. And, on a vehicle with shear type body mounts, that could be a significant problem. Taking the Lateral Force measurement is an extra step, in an extra menu option on the Hunter machine. I doubt most operators do it. One guy I had balancing some tires for a car one time didn't even know the machine had that option, when it asked him about it! But hey, if it's mindless, then never mind! I'll keep my mindless suggestions to myself.

Just had a thought. Has anyone considered lubrication of the u-joints as a possible issue? I'm sure they are not greaseable, so not sure what one would do to inspect or do something about. I've actually never had apart a driveshaft with the new style "sealed for life" u-joints, but wouldn't it be a kicker if the supplier screwed up?

Has anyone had their wheel/tires Lateral Force measured on the Hunter when they did the Roadforce balance? So far, I don't recall anyone talking about that. It's an optional feature in the Hunter GSP 9700. No idea if GM's version of the Hunter (GM buts their own branded version of the GSP9700) has that feature, or if GM is using if. I highly doubt it because the GM specs and Bulletins don't say anything about it at all. I am thinking about taking mind to an independent shop and having the Radial Force Variation and Lateral Force measured and if they are high, go back to the dealer.

Any chance you have some photos of the Dynamat you did?

I couldn't agree more. Using high strength steel is done to get the weight down for the same resultant strength. Unfortunately, that messes with the mass/stiffness ratio, which when dealing with NVH is a big, big deal. Mass absorbs energy. Mass also affects the natural frequency of everything, which ultimately affects resonances. And like you said, it's caught GM off guard. I've been doing some research into automotive acoustic treatments and dampening. There is a fairly extensive industry out there that serves the OEMs for various products and technologies. Some of these technologies are quite "space age". For example, honeycomb type plastic structures filled with special acoustic foam to provide structural stiffness without the weight. I have to wonder if GM cheaped out and didn't use any of these technologies when making this vehicle lighter, and hence the result. I was always wondering why other brands were making their vehicles lighter and apparently not having these issues, but I can imagine a Mercedes, for example, is full of the latest high tech stuff. GM, having just come out of near bankruptcy, was likely trying the cut as many financial corners as possible and I wouldn't doubt the engineers there wanted to use more technology but were told to forget it. Go cheap and dirty. A example is have alook at the inside of the rear wheel well. There is one or two little strips of some kind of sound damping foam sprayed on there, but looks pretty skimpy. An another example - the inner wheels wells front and back are not metal any more. It's some kind of felt-like material. Obviously to absorb road noise. But, is that parts of making the whole vehicle flimsy? Who knows. All I know is they messed something up. And they likey know exactly what it is, but it's totally impossible to rectify without major design change. And that just isn't going to happen. At least not on existing vehicles already out there. And from the looks of it, they aren't going to revise the deign for new ones. My guess is they will accelerate the next platform release and relegate this one to type boneyard!!