It happened twice and both times on rainy days.

[SlickGT1]

Not the shocks.

It is your bushings. You cannot visually check them. You need to let the air out of bags, and then the tension will come off the bushings.

Follow me here.
You have good tires. In the dry, you are ok because the tires are gripping great and keeping your suspension tight by not slipping around on the road. When it gets wet, the traction is reduced, and now your tires follow the most grip, which is the imperfections in the road. Your control arm bushing are most likely shot. The thumping is the road throwing your rear end around. You are driving a 5000lb SAV, so when the traction is great, your bushing stay tight.

Try this, next time it rains, and it starts the thumping, floor it. The sudden tension from the drive line will make the thumping stop.

[TwinsPoppa]

Originally Posted by SlickGT1

Not the shocks.

It is your bushings. You cannot visually check them. You need to let the air out of bags, and then the tension will come off the bushings.

Follow me here.
You have good tires. In the dry, you are ok because the tires are gripping great and keeping your suspension tight by not slipping around on the road. When it gets wet, the traction is reduced, and now your tires follow the most grip, which is the imperfections in the road. Your control arm bushing are most likely shot. The thumping is the road throwing your rear end around. You are driving a 5000lb SAV, so when the traction is great, your bushing stay tight.

Try this, next time it rains, and it starts the thumping, floor it. The sudden tension from the drive line will make the thumping stop.

I think this is more plausible than bad shocks. I would think if you have bad shocks, they would be bad all the time.

I guess a simple way to confirm your theory is just hose down your shocks and take it for a test drive. What do you think?

[nebilex]

Originally Posted by TwinsPoppa

I think this is more plausible than bad shocks. I would think if you have bad shocks, they would be bad all the time.

I guess a simple way to confirm your theory is just hose down your shocks and take it for a test drive. What do you think?

Before you hose down your shocks though, rock the back of your car (up and down motion)and see if it bounces or not (it's not supposed to bounce) and since you only see this issue when wet, probably nothing will happen. Then hose the shocks down and bounce the rear again and see what happens. This may or may not prove anything but it's worth a try.

[mrathell]

I'll let the air out of the airbages and check the rear bushings and linkage.

The reason I was heading in the rear shocks direction was because I saw it as two seperate issues:

1) the alignment
2) the shocks

The issue has been inconsistant. The only thing that is consistant is the fact that it only happens on rainy days:

- 2 of the rainy days the banging sound began right away. (the rain was more of a short sprinkle)
- the 3rd rainy day that banging sound didnt begin until 10 miles into the trip. (this was a constant heavy down pour)
- on the third heavy rainy day the banging would come and go. One mile it sounds like the rear end is about to fall off and the next mile no banging and no bumps felt.


I was thinking the same as Twinpoppa about the shocks, that if bad bushings was causing it, wouldn't it be constant?

I'm quite sure SlickGT1 is correct and I will take his suggestions for troubleshooting it.

[mrathell]

BTW: What's the best way to let air out of the rear air bags?

and thanks a million for your help

[SlickGT1]

Originally Posted by mrathell

I'll let the air out of the airbages and check the rear bushings and linkage.

The reason I was heading in the rear shocks direction was because I saw it as two seperate issues:

1) the alignment
2) the shocks

The issue has been inconsistant. The only thing that is consistant is the fact that it only happens on rainy days:

- 2 of the rainy days the banging sound began right away. (the rain was more of a short sprinkle)
- the 3rd rainy day that banging sound didnt begin until 10 miles into the trip. (this was a constant heavy down pour)
- on the third heavy rainy day the banging would come and go. One mile it sounds like the rear end is about to fall off and the next mile no banging and no bumps felt.


I was thinking the same as Twinpoppa about the shocks, that if bad bushings was causing it, wouldn't it be constant?

I'm quite sure SlickGT1 is correct and I will take his suggestions for troubleshooting it.

Actually Twins said if it was bad shocks, it would be all the time. Bushings would not be consistent. There was actually a thread about this a while ago. Someone else had this issue. I just don't remember how to search it.

You should also factor in how long it has been raining and at what volume when applying your inconsistency theory. When it just starts raining, the road is most slippery, as the oils from the road surface get lifted by the water. If it rains for a longer period, that same oil had a chance to get washed off the road.

So in rain that just started, you should get the issue right away. Constant rain, the issue could be sporadic. In heavy rain, you have to consider hydroplane effects, so it could also be inconsistent, and dependent on road drainage characteristics.

I do know know of any confirmed way of letting air out of the springs other than pulling the fuses for the compressor, and loosening the air line on the spring itself.

[TerminatorX5]

Originally Posted by SlickGT1

You should also factor in how long it has been raining and at what volume when applying your inconsistency theory. When it just starts raining, the road is most slippery, as the oils from the road surface get lifted by the water. If it rains for a longer period, that same oil had a chance to get washed off the road.

So in rain that just started, you should get the issue right away. Constant rain, the issue could be sporadic. In heavy rain, you have to consider hydroplane effects, so it could also be inconsistent, and dependent on road drainage characteristics.

Dennis,
you just took the words from my mouth - the light drizzle is more dangerous as people don't take it seiously enough...

The "ski effect" = subframe bushings... Consider the polyurethane ones, people had some nice reviews about them...

[mrathell]

thanks guys, what you are saying makes perfect sense

Would this be correct:


SlickGT1 is recommending me to take a close look at the control arm bushings as a possible culprit.

and TerminatorX5 believes the rear Subframe bushings may be cause.


Of course I understand it could be both.

I was considering chaning the subframe bushings myself, but the more times I get under there, the more they look pretty dam good.
I'm under the impression (based off of the visua condition) the previous owner had the changed back in Mayish of this year when he had the driver side air bag and control unit changed.

[TerminatorX5]

the OE bushings should last upto 70K, even though some can drag longer and some will be gone before... regular bushes run for about $70-90 a pop, and I saw KO Performance selling them for about $25-30 a pop - so, there might be a spectrum in quality... the Poly bushes can run for $100-120 a bush...
the symptoms of the butt skiing, are consistent with the bad bushings. if the bushes are good but the symptoms are still there - then it could be another component of the suspension...

On a separate thought, check the threads on GUIBO and see if those symptoms are similar to yours - just to eliminate the guibo from this equation...

[SlickGT1]

Could be both sub-frame and control arm. I am thinking controls arms because of the random handling. Sub frame, with all attached components would tend to swing you in one direction, and lock itself into that direction. Control arms will tend to throw you all over the place when they are worn. Each side will try to find its own path.