A few weeks ago; I had my Z51 shocks replaced by base shocks, to get a better ride on the bombed-out roads of Illinois. Here's the thread on that:
http://forums.corvetteforum.com/c6-c...uspension.html

As mentiond in that thread, the base shocks lowered the car by 5/16" in front and 4/16" in the rear; using multiple careful measurements. Since we use the car as a DD and road-tripper, I don't want it to be lower.

Yesterday I had the dealership adjust the ride height by using the GM measuring tool and also by verifying with marks I had made on tape on the fenders. According to the Service Manual, each turn on the adjusting bolts should raise the car by 2mm, so 4 turns = 8mm = 5/16". And by using full turns on the bolts, the rubber pads will have the same "set" angle on the springs, no need for them to settle.

After 4 turns and a short drive, the car was sitting so high it looked like a monster truck. The tech thought maybe he'd gotten something lodged under the rubber pads, so he temporarily backed 'em off and used the air hose in there. No change. So he backed off 2 turns on the bolts, still too high. He backed off one more turn, so now the bolts were only 1 turn higher than starting. Drove it for several miles and let it sit overnight. That seemed about right.

I took it another 5 miles home today and carefully measured the fender marks, the car is now 6/16" higher in the front and 5/16" higher in the rear, which means it is currently 1/16" higher, front and rear, than before I changed the shocks. That's close enough for me.

But, why is the effect of turning the bolts so much different from what the manual says? We both checked again, the Service Manual says that 1 turn on the bolts = 2mm change in height. But it seems like 1 turn is at least 8mm, that's a big difference. Especially since the set angle on the rubber pads has not changed, so they shouldn't need to settle in.

In one sense, I don't really care. But, I want to get an alignment done soon, and if the car is going to settle by another 6mm, then I should wait. But how long to wait? I'd think that 24 hours + 10 miles of driving, should be enough. Do I wait another day, or week, or month? Drive it a lot and hope I don't mess up my tires?

BTW, I believe CF member "Vet" experienced the same issue of 1 turn = way too much, but I never heard if his car settled more over time. And no one else I have seen has mentioned the problem.

Any experience out there?

 

Good questions, Jim......I'm thinking..... I don't think it's a month's worth of waiting and driving, but probably a bit more than you have on it now. I'd suggest not a super-bumpy road, but one of the less-than-glass-smooth roads around you would be a good one to take up and down a bit, say, 20 miles of driving. That's an additional 20 from what you've already done.

And take a log of where it is now, or tomorrow morning and see where it is (if it's changed at all) after that next 20 miles. Then, I'd say you're ready for your alignment.

I do need mine raised a bit, too.

 

Just an observation, Jim quoted the manual as saying 1 turn equals 2mm. American car sold to primarily and american audience and we refer to the metric system. I know we are the only ones that use our system, but still.......

 

Bill-

Good points about "exercising" the suspension, and keeping a log.

Thanks.

 

Update-
Today we put another 70 miles on the car, including some rather rough and undulating back roads. Preliminary measurements suggest it dropped another 2/16". I'll wait until everything cools off, then get more precise numbers.

 

Jim,

You must have changed more than the shocks. New bushings maybe? I swapped out my Z51 shocks for base shocks which look identical but have different part numbers, and used all my original hardware so just the shocks were changed. I also did careful measurements before and after (put about 20 miles on the car for the after measurements). The measurements were exactly the same.

 

When the shocks were changed, I did replace those rubber washers at the top. GM calls them "insulators", is that what you mean? If the original insulators were somewhat compressed, then the new ones should actually increase the ride height if they did anything. However, the shock should still allow the spring to settle to whatever position it wants, in theory you could remove the shocks and the ride height wouldn't change. But for most people, replacing the Z51 shocks with something else does lower the car slightly. It sure would be nice to know why the operative word is "most" instead of "all".

The bigger issue seems to be that it takes more than a day and a dozen miles, for the system to settle to its new ride height after an adjustment on the bolts.

 

 

After 3 days and 120 miles, the suspension has settled about 3/16" at each corner.

 

My garage has a lip/bump that exercises the suspension each time I pull in, so I can take measurements, drive around the block and get the same measurements; repeatedly. That's good.

But what's not good is that the measurements seem to change depending on how warm the car is. I didn't think that ~3 psi more in the tires, after driving, would make any difference. But i took the numbers this morning, drove 5 miles into town at ~40 mph, stayed for a while, and when I got home the car was about 2/16" higher.

Since I can't recall how warm the car was when I did the original measurements, that is perplexing.

Now I'm trying to decide whether to trust the GM Trim Height tool and try to set by that, or raise it one more turn (2 turns total), or just leave it alone.

Decisions, decisions...

 

Part of the problem may be where you are taking the measurements. As far as I know GM doesn't measure from the ground to the fender (or any exterior body part). GM measurements are at their specific suspension points.

1mm movement at the bolt (a foot from the pivot point?) could equal 3mm at the outside of the tire (3 feet from the pivot point?).

 

Unless you're running coil over shocks the ride height should not change no matter which shocks are installed or no shocks at all. If there is a difference then that would mean that the shock was bottomed out and actually lifting the car which is the job of the springs and not the shocks. It would also explain why the ride was so rough as there was no shock action only the spring resistence.

 

are you saying he has damaged or non-working springs?

Jim, is the ride height difference both front and rear? if it is, somehow I'd doubt that there is spring damage. jm guesswork tho.

 

I was checking other posts before I wrote this. The shocks do not support the car. The cross leaf springs support the car. The shocks control the bound and rebound of the springs, thereby controlling the ride of the car. With your ride height changing so much I would suggest other factors are involved. 1/16 of an inch is insignificant. A few gallons of gas can cause that.

 

No, I was just trying to clarifying that simply changing the shocks would not affect the ride height.

If the ride height had changed, OP stated car now sat lower, it would seem to indicate that the shocks that were pulled out were too long and acting like a spacer for higher ride height. Also that the ride quality would be harsh because there was no shock action because they were already fully compressed.

 

OK, now I understand what you were saying "Byron." But, those were stock shocks that came with the car from the factory. All Jim did was replace them with a different model of brand new stock shocks.

And while I agree with "Rusty B." about what controls ride height, I do know for a fact that different shocks have/produce different ride heights, i.e., Koni FSD shocks tend to drop ride height up front by anywhere from 1/4"-1/2"+. Very possible that those Konis are a different length, end-to-end tho. Of that I am not certain.

 

My son installed an Edelbrock E-Force in my C6, Z51. From the seat of my pants, I thought the car was lower in the front with the added weight. We did not take before & after measurements. Then I read a CF thread on Trim height adjustment, went to the service manual and read up on it. We raised the car one turn, 2mm, and I thought it was significantly higher...possibly back to the height it was prior to the Edelbrock install. I looked on Ebay and found an inexpensive used special tool for measuring the trim height adjustment. While waiting for the tool to arrive we installed a Johnny O'Connell Suspension package. When we got the special trim height measuring tool, we used it and found the suspension was within tolerance. So we left it as is....car is very planted and does not seem any lower than prior to mod installs.

 

Some explanations:

Changing shocks should not change the ride height, but putting softer shocks in a Z51 car (like I did) has been reported to lower the car about 1/4" or more as reported by several other members here. That's what I experienced also. The best theory is that because the Z51 shocks have very strong gas pressure and it takes a lot of force to compress them for installation, they do in fact slightly support the car. I am confident that my measurements before and after the shock change were accurate (same car/tire/garage temperaturre, multiple measurements). Installing the base shocks lowered my car 5/16" in front (both sides) and 4/16" in back (both sides), measured to a mark above the wheel wells.

The original Z51 shocks were definitely allowing the suspension to move properly, just too stiff for the crummy roads in Illinois.
Springs were inspected also- no damage.

The Service Manual wants us to use the J 42854 Trim Height Measurement Gage, and i got one. But the tech who used it said there is enough slop in the tool attachments that he's not fully confident in the numbers, he said it's a good "ballpark" measurement. I've been using marks on the fenders because that allows me to easily check the difference from before. But the differences due to temperature (I think) are confusing that situation.

GM says that one turn of the adjustment bolts changes the trim height by 2 mm, but haljensen pointed out that's not the same as ride height measured at the fender; nice catch! Looking at the distances involved, it should be closer than I've been getting but i didn't measure the relationships.

 

Tomorrow it goes back up on the rack. If we can get more precise measurements when using the trim height tool, we'll go with that.

If not, I'll have him raise it one more turn (two turns total) and drive it a bit, then get an alignment. We ain't building a swiss watch!

 

I have to stand corrected here. I was not aware that the Z51 shocks were "gas" shocks. You're right, the pressure that it takes to compress them would in fact cause the vehicle to be lifted slightly. My comments were with regard to non-pressurized or non-gas shocks. The gas shock is normally pressurized with nitrogen gas to reduce internal foaming and cavitation of the hydraulic fluid used for dampening.