Vibration/Wobble at high elevations + acceleration

[DriverX]

Hello,

I am stumped. Since February I have been having a vibration/wobble when accelerating or let's say going up hill at high speeds, but only at higher elevations. And even then, it takes a while to show itself, but once it develops it continues at about a 100% success rate. Then a few weeks at home at low elevations and I just can't reproduce it.

It feels similar to a wheel weight balancing issue, where the front right passenger can feel it strongly and I can feel it in the steering, but no one else in the car really notices. It typically starts 2-3 days into road trips to the mountains or high desert and continues until we get back down to near sea level.

It is felt only when pushing the gas to accelerate, or go up hill at higher speeds, say 40-70. It is not felt at lower speeds, or at high speeds while foot is off the pedal.

Since then, we have tried rotating the wheels a couple of times, and have also replaced the engine and transfer case mounts. But it continues.

I've checked the wheels for play but don't find much. No cracked CVs or grease spray or anything obvious like that.

Any ideas?

Thank you!

[andrewwynn]

Do these tests:

1) gentle braking from 50-40 mph.

2) on the center lane of 3-lane highway (or equivalent so not leaning due to crown), make a gente s curve. You can stay in your lane.

On 1, if the test catches the problem you'll feel a crazy amount of steering shake.

On 2, you hold the wheel very gently (think pinch finger and thumb). If the test catches the problem, you'll get 2-3 gentle wiggles in the wheel when you cross past 0° on the steering wheel (feels like it's fighting you to go back the other way a couple gente tugs).

If either of those happen it's almost positively the rear control arms. (I've never figured out how to tell if it's just one I always replace both).

As to why at high elevation probably very similar to why mine that need replacing badly stopped being a problem. In my case i believe enough rust built up in the joint to make it tight.

In your case I'm thinking air bubbles in the grease change the dynamics of the joint as they change with air pressure.

(This is just educated guess).

[Purplefade]

Andrew is probably going in the right direction but I have to ask anyway - you figure the higher the altitude the lower the air pressure, could it be an issue of the trans fluid being on the threshold of low and the decreasing pressure dropping overall pressure in the valve body causing a “studder” - strange you’d feel that in the wheel though... or, possibly exacerbating a very minor vibration by allowing the tires to expand ever so slightly (captive air wouldn’t decrease, it would put more pressure on the inside sidewalk being outside air pressure would be lower) possibly high lighting uneven tire wear? Which might also explain why it goes away as you drop in altitude.




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[EODguy]

Speed bump test for control arm wear.

With just enough speed to go over a speed bump so it feels harsh but doesn't lose contact with the road on the way down... If you hear a clunk on the down it's the shorter arm if you hear a clunk on the way starting over the speed bump it's the longer arm.

Granted if it's one you will replace both anyway....

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[oldskewel]

Originally Posted by Purplefade

Andrew is probably going in the right direction but I have to ask anyway - you figure the higher the altitude the lower the air pressure, could it be an issue of the trans fluid being on the threshold of low and the decreasing pressure dropping overall pressure in the valve body causing a “studder” - strange you’d feel that in the wheel though... or, possibly exacerbating a very minor vibration by allowing the tires to expand ever so slightly (captive air wouldn’t decrease, it would put more pressure on the inside sidewalk being outside air pressure would be lower) possibly high lighting uneven tire wear? Which might also explain why it goes away as you drop in altitude.

Fascinating question / situation. I'll definitely be following to see what this turns out to be. Not sure if I should post these foolish ideas, but ...

I was thinking along the lines of this suggetion from @Purplefade, where the actual altitude and drop in atmospheric pressure was the driving factor. My crazy thinking had been that if the CV boots are tightly sealed, then a ~3 psi drop in ambient air pressure, while the air inside the boots remains sealed, could cause several pounds of force displacing the CV joint slightly. Perhaps checking for CV axle motion at low and high altitude would help. Changes due to temperature rise will similarly affect things.

The other thing, much simpler, related to ambient atmospheric pressure is the tires. Seems very odd that things might be so sensitive to an effective change of a few psi. Simple test would be to let some air out of the tires when at altitude.

Any other variations with night / day, road surface, or is it really just altitude? BTW, what altitude changes are we talking about here?

I don't know what to make of it, but it seems to be an important clue that the problem has some sort of hysteresis, so once it appears it stays until going back down to low altitude. I.e., rather than passing through a phase as altitude increases, where the problem comes and goes.

[andrewwynn]

The fact that my control arms cured themselves is very perplexing and I plan to saw them in half at the b-ball joint doors post mortem when I finally change them. I just had to swap a tire that had its red neck wear bars (fabric belts) showing, so maybe the ball shifted enough to cause the wear.

[Purplefade]

I am again agreeing with Andrew - I honestly don’t see how just a few psi could/would cause such a situation unless there was an underlining “culprit”, maybe that few psi develops a small bubble in the side wall, older tires, weak sidewalk cords or something along those lines that once the issue sets in, it stays until you go back down the mountain so to speak.

@Andrew - is it possible that it may be the transfer case? Something along the lines of the pressure applied to the clutches changes due to altitude? I could actually see the clamping force being affected by just a few pounds.

I grasping at straws here... and anxious to see what this comes down to.


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[andrewwynn]

Elevation change is a couple psi at best. The main effect is boiling temps are changed but should not affect much in the car.

@ OP: are you sure it has to do with elevation? Is it possible just a particular patch of road causes it and the elevation is coincidental?

Why I ask is that when control rod joint fails the very best feedback is that on certain parts of certain roads you will get steering wheel shake out of the blue. When wife’s went there was a particular part of I-94 for about 1/2 mile caused wheel shake 100% of the time and was the only time.

About a month after that started then finally the 50-40 gentle braking it kicked in.

[oldskewel]

I did not doubt OP when he said it was due to high elevation, strange as it may seem. Makes for a much more interesting problem.

I spend a lot of time at Lake Tahoe, which is about 6200 feet above sea level, 3 psi less atmospheric pressure than at sea level, and water boils 12F lower than it does here at home (2F per 1000 ft is a good rule of thumb), so my coffee brews a little colder up there. Of course not knowing what altitudes are involved for OP, ...

[Maruzo]

What about the steering wheel u-joint?

I've done a complete front suspension refresh except the u-joint, which isn't really part of the suspension setup, but it does affect the steering wheel.

I do get slight steering wheel vibration around 65~75 highway speeds.