Gearhead Jim

EDIT:
My long post is scaring people away. Cliffsnotes version:
Using the GM measuring tool, my 2009 Coupe measures too low. Has anyone else measured theirs with the GM gage, to see if it meets the factory specs?


Our 2009 Z51 Coupe has about 36k miles, no mods except as noted in this thread. No accidents or other unusual history.

Due to the ever-worsening roads of Illinois, last month I had the Z51 shocks replaced by base model shocks. Because several other people had said replacing the Z51 shocks lowered their car (the best guess is reduced gas pressure in softer shocks), I made detailed, repeated measurements and marked a point on the fender above each wheel well.

After the change, more detailed/repeated measurements showed my car had dropped by 5/16" in the front and 4/16" in the rear.

The dealer has adjusted each spring bolt by 2 turns. The car looks to be about equal to before the shock change, and about equal to other stock C6 Coupes. Each turn raises the bolt by 2mm, but because the bolt is well inboard there's almost a 2 to 1 ratio; 2mm at the bolt is about 4mm on the outside edge of the fender after driving several hundred miles to settle the suspension. So, the fenders have been raised about 8mm (5/16"). Everything should be perfect.

But...

GM does not measure height at the fenders or anywhere on the body, probably because that can be altered by production tolerances in the body, and even worn tires will be 1/4" lower than new tires.

Instead, GM specifies a "Trim Height" that is the difference between the bottom of the ball joint and front/center of the lower control arm mounting bolt. On the rear of the car, you could simply use a level and metric ruler. The front control arms have a "bulge" that prevents using a level, instead GM makes a "J 42854 Trim Height Measurement Gage" with a magnet for the ball joint, a clip that positions onto the bolt, a sliding ruler, and a leveling bubble. Something like $275 list, but I got a used one for $37 on fleabay. I verified the straightness, bubble calibration, etc; all is good.

Today we put the car on an alignment rack (gotta be level to use the tool) and checked the rack level, then did the typical bounce & measure thing 3 times on each corner. The measurements were pretty consistant, but not in spec.

The rear trim height is supposed to be 115mm, I'm at 111mm left and 112mm right. I'm surprised it's low, but let's measure the front, which is supposed to be 48mm. Wow, I'm at 39mm left and 42mm right. That's at least 6mm low. Yes, there is a 6.4mm tolerance, but this doesn't seem right.

If I raise the trim height by 4mm in the rear and 6-9mm in the front, that's going to raise the fenders (remember that 2 to 1 ratio) by an additional 8mm and 16mm, which just doesn't sound right, and would probably make my Corvette look like a monster truck.

Has anyone else actually compared their trim height to the Service manual specs, using the GM tool correctly?
I'm beginning to suspect that the cars settle significantly after leaving the factory, and virtually everyone is "too low".

Gearhead Jim

An additional thought-
If the cars are set up for correct ride height at the factory, but then settle 1/4" or so during their first few hundred miles (like mine did after adjusting the bolts); that might help explain why so many Corvettes are found to be out of alignment.

cclive

I must say that after looking at the trim height specs and drawing in the manual, it is too complicated to check without an alignment rack and the tool that you mention. Even then, it's complicated....so I have nothing brilliant to add other than my front air dam is 2.5 inches from the ground in the center, which seems like it is on the low side of normal.

Gearhead Jim

Chris-

The center of my air dam is 3 1/4" above my smooth concrete garage floor.
That's with almost new tires.

Any suspension mods, different shocks, etc; on your car?

cclive

Yes, I have Bilsteins. Also Z51 bars, though that shouldn't make any diff. Same here, smooth concrete floor...almost level. 11K miles on PS A/S+ tires stock sizes.

Gearhead Jim

If you went from base shocks to Bilsteins, that shouldn't lower the car. Neither should the Z51 bars. Should have lots of tread left on those Michelins, and I know from your posts that the pressures are correct.

You're not storing bricks in your car?
Holding the ruler upside down?


Even more than before.

cclive

That's why I said that I had nothing brilliant to add,,,LOL...I wish that they had come up with an easier spec to use...

AORoads

So, let me jump in here, Jim, since I, too, having nothing to add.... I think you are correct in the ride height changing from leaving the factory. I know that from when I first got the car, there were places/roads where I didn't bottom out. Then, later on, I did. And I didn't gain a lotta weight, either.

Even tho the car may look like "monster-truck-world", I'm still thinking about having it raised a bit more. I am really low-down.

BTW, I presume you saw Paul's post (talon90) re the new alignment machine at the factory for the '12s which will supposedly make the cars better/closer to spec.

Kensmith

Mine is 2 1/4" with good tires that are about 30% worn. 7/32" tread and tires come with 10/32".

Gearhead Jim

I've almost given up on believing the numbers from the GM tool, now I've started a thread about actual ride height, as measured to air dam and fenders, on members' cars here. Please post your numbers there when you can get them. Thanks!

http://forums.corvetteforum.com/c6-c...oupe-vert.html

Vet

Here's a pic of my car after I raised it about 5/8" above the original height as it came from the factory. To me, at a quick glance, the car still appears totally stock and does not look goofy. Of course, if you stare at the fender well gaps a bit too long it starts to look a bit "high". But, with two passengers and a load of luggage, the car does drop a little bit.

I'm totally happy with my raised suspension. It did require an alignment. Car handles and rides beautifully, as good as before, even when pushing the envelope in the twisties and when hitting triple digits etc.

In my personal opinion, this is about as high as you'd want to go. The tech at the alignment shop even told me that in the rear, I was just about at the very end of the adjustment window (on the camber), if I had raised the car any higher in the rear it may have been impossible to align it without modification.

Jim, when I get a chance I will measure my car from the air dam and fender wells as you requested. Not that you plan on doing the "monster truck" mod, but it might be an interesting point of reference anyway. But, I do think that my car was a bit on the low side originally, so my 5/8" raise from originally might actually be only a 3/8" raise from official factory specs.

All I can say is that I'm extremely happy with my raised suspension. The car is still dangerously low for many real-world situations in my opinion, but that extra mere 5/8" of clearance seems to really make a difference. Before the raise I was scraping all kinds of things in my local area, assorted parking lot entrances, dips in the road, my own driveway entrance, etc. After the raise, no more scraping. Not that I care about scraping the air dam since it is designed to scrape, but... it's interesting how a mere 5/8" raise can make a difference between scraping a lot of things and now just about not scraping any of those things.

Again, in terms of the ride, handling and overall driving experience with this car, the suspension raise did not negatively affect anything. Car is excellent in every way. No adverse effects whatsoever. The only "negative", if you wish to call it a negative, is the extra bit of "unsightly" gap between the tops of the tires and fender wells. Doesn't bother me one bit, but I know a lot of guys on the forum would rather rub glass in their eyes than see a "gap".

By the way, these pics DO tend to exaggerate the "gap" a bit since I was down low with the camera. Looks less obvious when you're standing. And once again, two passengers plus luggage make the car drop a bit anyway. Don't remember how much fuel was in the car when these shots were taken, possibly 1/2 - 3/4 tank. I usually never let it go much under 1/2 tank.

Monster truck...?

Gearhead Jim

Vet-

Good post and nice pics, thanks!
I think mine looks about like yours in the back, slightly lower in the front. I think...

My additional height above stock (if any additional actually happened) does not seem to have effected my handling either. I know that a designer would walk over his own mother to lower the CG by 1/2", but if you're not a hardcore racer, I wonder if it makes any real difference.