watt's linkage? What's the lateral shake...

Hello,
I have an 04 town car with 44K miles and there's objectionable lateral motion of the rear of the car when going over bumps. There was absolutely no lateral motion when I first got the car used at
36K miles. Even just going around a multilevel parking garage, when the car encounters a small bump in the floor at each turn of the garage (where it sharply transitions from level to inclined), the rear shakes sideways. The lateral shake is like an undamped spring that lasts for a few oscillations. This never happened before. It used to be solid with no lateral shake over bumps. The ride is less stable than it used to be over mildly bumpy roads. On the highway it affects straight line stability slightly, but it doesn't seem like trailing arms and dogtracking.
The Watt's linkage is supposed to be the only thing restricting the rear axle to vertical motion only. I had it put on the alignment rack to look at the bushings but it was too difficult to shake the rear of the car sideways to see if there was any excessive movement.
When I described the problem to the service advisor at the dealer by mentioning the watt's linkage, he said "what is that?". Then he went on to talk about trailing arms (maybe?), air bags etc.. I then said I was willing to experiment so he suggested I go to the parts counter and get an invoice quote on the parts and he'd figure the labor. The guy at the counter never heard the term "watt's linkage". Couldn't find it in their database. I had to go home and print out diagrams and parts list from alldata and bring it to him, and only then did he have a clue. Then when told I want it replaced, the service advisor refused to do it. I then called customer relations and they called the advisor. The advisor then said they'd install it but that he insisted that it won't solve the problem. I thought it was already understood that I wanted to experiment by spending $400 and see what happens; it's not covered in the premiumCare service plan. If I'm wrong and the problem doesn't go away then that's the process of elimination. I'll have other things looked at.
What else might I look for concerning this problem? I had the shocks replaced with KYB gas-a-just but of course they only damp vertical motion.
Thanks in advance for your help
Ben
--
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
BR wrote:

How old are your tires?
Rob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

43rd week of 06. Looks like plenty of tread. They are Michelin energy LX4. I notice that one of the rear tires leaks but no detection under water, so it's probably due to flexing. It does wiggle more sideways when it's low. I think I'll try temporarily inflating them a few pounds over just to see. Supposed to be at 35 psi for the rear. You suspect the sidewalls weakening so suddenly? I bought the car used in Feb this year.
Ben
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
BR wrote:

My fault, my question was not worded well. How many miles on the tires, when did you put them on?
But you are going in the right direction with the sidewall issue. Not sudden, or likely, but you may have a broken belt.
If the Watts linkage was to blame, you would get some obvious play in the affected / problematic joints with the car on the lift. They certainly could be disassembled quickly to inspect them without replacing them.
Also, has anything else changed? Trunk load? Air suspension service or adjustment? Does it occur regardless of fuel level?
Why did you replace the shocks so early in the car's life?
Rob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload


I didn't put them on. They were already on when I bought it. They have 7/32 or more according to the dealer report card.

Wouldn't a broken belt cause vibration? I just tried inflating the rear tires 2 pounds over. It did reduce the sideways wiggle by a noticable amount. I will have to study it some more. Perhaps the watt's linkage is OK but those tires would have to be crap to have such poor sidewall support.

They can't disassemble it unless all new fasteners including the stud that goes into the differential case are replaced.

The trunk as been filled with stuff and a full tank of gas but it doesn't seem to affect it much. The air suspension is OK. The ride hight must be spec as it was just aligned after new struts were installed, and headlight adjustment (vert only) was done and I think the dealer would want to check rear height.

It had 43K miles. Big-O said they should probably be replaced at 50K. The left front strut was weak so I just replaced all of them. I thought the KYB so-called high pressure gas type was going to offer more damping, but it seems they're valved much like stock.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Don't have an answer , theres a drawing at this link for ayone interested.
http://en.wikipedia.org/wiki/Watt 's_linkage
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Try rotating the tires front to rear. See if it changed anything. Tires can do some strnage things. And just for the record I have had bad luck with KYB right out of the box. They dampen like a worn out good shock if you ask me.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Thanks for the tip. Will have big-o do that monday. I've never heard of tire sidewalls loosing lateral support. It would be understandable with old tires. The tires were made in late 06. There's 1/4" of tread. Could I have punished them by driving the car thru turns too aggressively?
Yes, the KYB so-called high pressure gas shocks didn't impress. They seem slightly better than what was in the car before one of the front struts wore out. The urethane strut mounts and shock bushings increased the ride harshness a little, but can't detect how they improve anything.
Ben
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.