MisterMidlifeCrisis

Jim, one other thing that occurred to me was when I was lowering the car, I noticed how many threads there were below the spring. I think when I first got in there, it must have been six or seven full threads that were visible.

Raining cats and dogs right now, and I was going to put on a Z06 intake. If I get it all done, I'll snap some pics tomorrow.

MisterMidlifeCrisis

Here's a picture of the current ride height. Measured at 27 1/8 both fronts, 28 1/4 both rears. Air dam is at 3 3/8 inches.



The garage that is open is where the Vette parks. I'd love to go lower, but I have to get in and out of that garage on a daily basis, and at the current height I can do so without any scraping. Not sure I could go much lower without at least rubbing the air dam.

Still looks high, doesn't it. . .

Gearhead Jim

To my eye, your car looks more "normal" than high. But that's just me.

Now that my latest adjustment has settled partially, my car is 27 1/16" in the front and 28 1/2' in back. So I'm about equal to you in front, 1/4" higher in the back. Mine might settle another 1/16" to 1/8" or so over the next 200 miles.

Keep in mind that my "trim height" measurements are still lower than the GM 2009 specs. If I translate the trim height specs to fender heights, my car should be about 3/8" higher in front, and about 1/16" higher in back.
Since the tolerance in trim height translates to about +/- 1/2" at the fenders, I'm within the GM tolerance. If my car was a 2012, my rear would be slightly out of tolerance low. That's bizzare.

Today our club had a car show and there were 11 other "narrow body" C6's there. Using the crude "how many fingers fit between the tire top and fender" test, my car was higher than any other C6 there. A couple looked to be deliberately lowered, the others were about 1/8" to 1/2" lower than mine. That's a very rough measurement, but it looks like virtually all the cars with some mileage are out of tolerance, low.

Is it possible that the specs in the Service Manual are meant for a new car, and GM knows that they will settle another 1/4" or more after some use? That shouldn't be true, this is a "Service Manual" and not a "Manufacturing Manual." But sometimes they just get it wrong.

MisterMidlifeCrisis

Jim, how hard is it to use the tool? Is it something you could use to measure a car while it's on the ground? I'm curious how much variation there is in body panel fit in relation to those hard suspension measurement points.

It seems like around here, the majority of Corvettes look like they've been lowered, which makes me feel as though mines is on stilts.

Boomer111

Looks about a 1/4 in. high all around. Just a educated guesstimate.

Gearhead Jim

For a picture of the tool, Google J 42854. List price is around $275, I bought a used one for $37 on Fleabay. The tool could possibly get damaged, I checked the straightness, measuring ruler, and bubble accuracy on mine before use.

You could probably use just a long level and a ruler for the rear suspension, but the front ball joints have a curve in them that seems to prevent using a simple level.

The car must be up on a level alignment rack, full fuel but no cargo, doors, etc closed. We used two people- one to hold the magnet against the ball joint and read the bubble, the other to hold the clip on the suspension bolt and slowly adjust the ruler. You bounce the car upward three times and take readings, then bounce it downward three times and take readings. Average them. Look up the specs for your year in the Service Manual and discover that your car is too low. Then start a thread about it on Corvette Forum.

So, this is not something you can do in the driveway. Your dealer might have the tool, but will they take the time and trouble to use it properly?

I think the most practical thing to do is use my numbers that compare official "trim height' to actual fender height. You'll need to adjust the numbers for tire tread depth, and take several sets of measurements in "level" garages to get the best understanding. And of course, the GS/Z06/ZR1 have different fenders, so those people have to work out their own system.

In about a week, after I've got enough miles to be sure mine has settled completely, I'll post my "final" version of "trim height compared to fender height".

MisterMidlifeCrisis

Earlier today, I slowly backed the Corvette out of its parking spot onto the street while my son was watching to see where and when the air dam was closest to scraping. I got out and had him get in the car, and measured 1/8" of clearance to the middle section of the air dam. I've got the NCM license plate mount so it basically turns that middle section into a rigid air dam, and I can't afford to scrape that section hard against the ground or the NCM mount would probably break the entire center section.

Since my plate mount is most likely illegal anyway (I'm just hoping for a break in case I get stopped), I'm really considering taking some tin snips to the bottom half inch of the plate. Then, I'd drop it down that last quarter inch. . .

MisterMidlifeCrisis

Jim, I'm all ears waiting to hear what you find. Thanks for all of the information you've provided.

Fly'n Family

Front Dr: 26" Front Pax: 26"
Rear Dr: 27.5" Rear Pax: 27.5"
Air Dam: 2.525"
All Michelin P/S RF's with less than 500 miles
08' C6Z51 using WCC lowering bolts and slightly increasing the rake

***Edit*** just noticed the OP was asking stock......sorry



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Gearhead Jim

Is it possible to raise your plate in the mount a little, or raise the mount itself?

Here in the distant suburbs of Chicago, I've been without a front plate for 12 years and only got stopped once (he let me off, we worked together).

But in Chicago itself, they'll ticket you for no front plate even if the car is parked on the street.

Different attitudes in different places.

MisterMidlifeCrisis

The NCM mount is basically a long, narrow piece of hard rubber that mounts to existing holes used, I believe, by the center section spoiler. The mount itself has holes for mounting the plate. Because there's not a lot of room under there and the mount is narrow, there was no way to adjust the height of it or the plate mounting to it.

I've been to a number of vehicle gatherings and front plates are pretty rare among that crowd. Still, I'd rather have a front plate on in some way rather than not so I can at least say "I tried" if I were to get pulled over.

I've got a day off tomorrow. I'm going to trim the plate and drop my car down a quarter inch more in front. Even these small ride height changes make a noticeable change in how much roll I get during cornering. It seems that adding rake by lowering just the front has taken out some of the understeer in the base/F55 setup, and it's nice to know this is an additional way I can fine tune the handling. I just wish it didn't cost $100 every time I fiddled with the ride height by paying for an alignment.

Gearhead Jim

Based on my experience with raising the car, it takes at least a week and 100 miles for the suspension to settle after an adjustment, maybe 200 miles. Lowering might be different, but since you must relieve the load on the spring to lower, it could well be the same issue. So I'd suggest waiting a bit for the alignment, you won't cause much tire wear in 200 miles.

To lower by 1/4" (after settling as mentioned above) should take, according to my calculations, 1.5 turns on the bolts. Please let us know how it all works out for you.

cclive

'07 Vert with Bilsteins and bars. Front 26 1/2, Rear 27 3/4, air dam 2 3/4.

MisterMidlifeCrisis

Hmm, I just did about a turn and a quarter on each of the fronts, and about a full turn on the rears. I am positive I did it the correct direction (counterclockwise). I then went out and drove about twenty miles, over speedbumps and lumpy pavement to get some suspension movement, and my ride heights are exactly the same in front, and a hair higher in back. Hmmm!

I've drive it around a bit more this coming week, but the first time I turned the bolts, the ride height dropped within just a couple of miles. Your ride height didn't fully settle for a couple of hundred miles? But even though the measured height didn't change, the steering feels different as if the alignment changed. There's seems to be a bit stronger self-centering (I would have thought lowering the front would reduce caster and decrease self-centering) and it seems a bit more darty over one-wheel bumps.

I also took a peek at the NCM mount. It actual does look like I can adjust it a bit. The top of the mount lines up with the top of the center section dam. I could remove the mount and redrill the mounting holes to give me the extra half inch or so that I'm wanting, but then the plate would be WAY up under there. I'll have to fiddle with it some more.

Gearhead Jim

Just another mystery.
Seriously, I don't know why the amount of "settle time" you experience has changed. Probably something to do with how quickly the rubber pads on the bolts slide into their new position, but???

The last adjustment on my car was up 1 turn on each bolt, which should raise the car about 3/16" in each corner. It actually raised the car by over 3/8", after 6 days and 115 miles the raise has decreased only a little to 3/8" or slightly less.

Gonna take it on a 400 mile trip with some energetic driving this weekend, then get an alignment next week.

MisterMidlifeCrisis

Some interesting findings. Because my ride height hadn't changed, I gave the bolts a few more turns up front and drove around, measured the fronts at about 26 1/2 inches, while the rears were at 28 1/4 inches. So, it did finally drop and I think it's a combination of trying to get full suspension excursions with both one wheel at a time and with both wheels at a time.

But since my ride height changes were up front only, I would not have expected the rear suspension alignment to have been changed, yet with the increased rake, there is a noticeable increase in the tendency for my Corvette to tuck in hard when letting up on the throttle at high rpms and high cornering loads. I thought that early Z51's were known to be a bit tail happy and had more rake - I wonder if this is the cause.

So, I raised it back up to 27 1/8 up front, 28 1/4 in the rear, and it's more stable.

Anyone have track experience to verify that the amount of rake makes a difference in lift throttle behavior?

Gearhead Jim

We had a 2006 Z51 at factory height (whatever that was), and now the 2009 Z51 which was at factory height (whatever that was, but officially different from 2006) until I changed the shocks last month.

Now that you mention it, the 2006 did seem a bit more tail-happy; but I was autocrossing that car and have not done so with the 2009, so comparisons are difficult. I have a big parking lot where I can do some handling tests before anyone gets annoyed but autocross-to-autocross would be a better comparison. One thing is for sure, replacing the Supercar tires with Michelin PS2 runflats has made the car much more predictable at the limits.

I have not been able to notice any change in the 2009 handling as I have changed the ride height and rake. Even replacing the Z51 shocks with base shocks made a significant ride improvement over of nasty frost heaves in Illinois, but I can barely notice the handling change even in my parking lot.

I would not make a good suspension test driver.

AORoads

maybe it's the car itself and not the driver that makes for a good suspension tester, Jim. I know I drove some familiar roads around me when I first got the car. Some I'd scrape the dam at the speeds I was driving (slow, usually in curves), and some I wouldn't. later on, most of the familiar low spots I'd scrape the dam. why, I don't know, but I'll chalk it up to this thread: the ride height changes independent of owner input. now, w. the ncm license plate holder, just about anything close to being a dip in the road is a scraping sound. oh well....

Gearhead Jim

This "settlement" issue is getting confusing. The most recent adjustment on my car was 1 turn up at each corner, done on 9-15. Initially, that raised the car up about 3/8" at each corner. After 158 miles/9 days, the rise had decreased to 3/16" front and 2/16" rear.

We took it on a road trip this weekend, including some hard driving on some twisty roads. When I measured the height this morning at 650 miles/11 days, the nujbers are:

Front L 26 14/16"
Front R 26 15/16"

Rear L 28 4/16"
Rear R 28 4/16"

Air dam 3 2/8"

EDIT:
On 11-04-2011 I re-measured the car. It's got about another thousand miles since the measurements listed above. Also, I replaced the summer Michelin PS2 runflats with Michelin A/S runflats which spec out as having the same diameter, like they should. My A/S have about 2/32" more tread than my PS2, so that should raise the car about 1/16". Instead, it went up more than that. New numbers:

Front L 27 2/16" (about 3/16+" higher)
Front R 27 2/16"

Rear L 28 9/16" (about 5/16" higher)
Rear R 28 9/16"

There are obviously some variables in here...

You will notice that compared to my height in post #1, the recent adjustment raised the car only 0-1 16" in front, 1/16" in back. Air dam is unchanged. Go figure.

I'm being careful with the measurements, including waiting overnight so everything has cooled off before measuring both before and after.

It does appear that my car takes more than 150 miles to completely settle, and that's assuming that my current measurements won't settle even more over the weeks ahead. If nothing else, I've learned that my car needs a lot of miles to settle in after any adjustment.

At this point, I'm going to say "Screw It" and just drive the car, no more messing around with the ride height. It handles well by my standards, and I am scraping the air dam a bit less than before starting the project. Alignment scheduled for next week.

EDIT (old): Just drove about 10 miles on a local trip so everything in the car was warmed up but not "hot". Pulled into an angled driveway where I always scraped before, nothing this time. Measured the car again when i got home, it got about 1/2 of 1/16" higher (1/32").

cclive

Good experiment, and I agree with your final conclusion above!

It definitely appears that earlier C6s were lower than later ones...interesting...