Checking my brain logic

TollKeeper

Original poster
Supporting Donor
Member
Dec 3, 2011
8,199
Brighton, CO
So back in 2020, I replaced the front Bilstein struts on my Envoy with Carquest Strut 18-813331..


Ever since, every time I hit a medium to large bump, pothole, etc, the truck bottoms out. I went thru the whole gambit of replacing every front end component: control arms, control arm brackets, inner/outer tie rods, upper control arms. But the problem persists. I had basically forgotten about the issue, having had a replacement truck. The XUV turned into the weekend truck with the wife and kids, and the Saab (previously the Cadi) turned into the daily driver that I could not have the kids booster seat in, and baby seat.

I drove the XUV this past weekend, and it quickly started annoying the hell out of me..

@flyboy2610 's thread got me to finally figure out, I THINK, whats wrong.

Advance shows that Carquest part #18-813331 as a direct replacement, for the 4.2 SWB/LWB/XUV, and 5.3 SWB/LWB/XUV. To me, obviously this is a problem. The XUV/LWB is 1200+lbs heavier than the SWB. I know my XUV is 5980lbs!

So my theory here.. The CarQuest Part #18-813331 is a SWB/LWB strut, with a SWB Spring. It simply doesnt have the spring rate for the heavier truck..

Does my math make sense?
 

flyboy2610

Member
Aug 24, 2021
500
Lincoln, Ne.
I understand where you're coming from! Here's a chart from MOOG about their coil springs:

If enter my '04 EXT, it recommends the 81114 spring. If I enter a SWB '04 TB, it recommends the 81110 spring. The only difference between them is that the 81114 is about 1" longer than the 81110, so the preload is greater. The spring rate is the same at 375 lbs./inch.
I'm thinking the struts I bought last summer were the same way yours are: sold for an EXT with a SWB spring. I bought them from 1Aauto.com.
 

Mike534x

Member
Apr 9, 2012
998
Sounds like a possibility! I've always seen warnings here, and elsewhere on how crappy the preassembled struts usually are. Your issue sounds like the one I had with my prior 05 TB Extended with the 5.3, I put on KYB Excel-G strut assemblies, and after they settled in they would bottom out depending on the road imperfection. Otherwise, they were pretty smooth like stock. I tried chasing down what could've been the culprit, but it never occurred to me that the wrong type of spring could've been used.
 

Mooseman

Moderator
Dec 4, 2011
25,671
Ottawa, ON
Could be both. haven't heard too many good comments about the pre-assembled struts, even Monroes. Most say it's too soft and that may be contributing to the bottoming.

How does your Envoy's ride height look like? Is it OK or low? If the height is OK, I think it's the strut.

As far as the spring rate/weight is concerned, if you think about it, most of the extra weight in an LWB/XUV is in the back. However, I think it's incorrect for them to assume that the 5.3 weighs as much as the 4.2. IIRC there is at least a 150lbs difference so that's extra weight right on the front wheels. The ride height may be the same however the spring rate or weight carrying capability may be too low.
 

TollKeeper

Original poster
Supporting Donor
Member
Dec 3, 2011
8,199
Brighton, CO
Finally getting back to this...

Could be both. haven't heard too many good comments about the pre-assembled struts, even Monroes. Most say it's too soft and that may be contributing to the bottoming.

How does your Envoy's ride height look like? Is it OK or low? If the height is OK, I think it's the strut.

As far as the spring rate/weight is concerned, if you think about it, most of the extra weight in an LWB/XUV is in the back. However, I think it's incorrect for them to assume that the 5.3 weighs as much as the 4.2. IIRC there is at least a 150lbs difference so that's extra weight right on the front wheels. The ride height may be the same however the spring rate or weight carrying capability may be too low.
I didnt realize it, until last night, but it was actually about 1-2 inchs LOWER!

Last night I put my OE springs, with new Bilstein struts I was finally able to source from Amazon. FYI - Not a bad price at all for NEW Bilstein struts and its a 2 pack.

Edit: And on to your second thing.. Not entirely correct. Because of the axle being moved 13 inchs back, this also shifts some weight forward to the steer axle. My Envoy is nearly a perfect balance of weight, 3000 front/3000 rear. My Saab is not.. I dont remember the weight, but I do remember it was 750 lbs difference front to rear, plus the truck being over all lighter because of the shorter WB and over length of the truck.

The truck I immediately noticed was taller, like 2-3 inchs! Once I took it our for a short drive, they settled down to 1.5-2 inchs taller.

For curiosity sake, I looked at the spring diameter of the "uni-strut" and the OE springs. The size of the spring was visually different. I dont have a micrometer, but I would hazard a guess of 1/8-1/4 inch in diameter. Thats pretty huge in the world of springs!

Been driving it around some today, and the ride is substantially better!

Now if I could just figure out this damn vibration the truck has at 57mph+. Making it so I cant trust the truck as my weekend driver, cause I dont want something to explode/break!

Minor annoyance was the upper strut bearing plates did not survive, and had to buy new ones.

I paid $187 bucks for the strut assemblies from AAP
I paid $254 bucks for the Bilstein's and upper strut mounts

Was it worth the extra 67 bucks for the OE quality ride.. HELL YES!
 
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