Gearhead Jim

I've been trying to figure out if my 2009 Z51 Coupe is too high or too low.

My measurements were taken with the usual "stuff" in the car and a full tank, but no people or luggage. Tires 30 psi, almost new tread. Doors, trunk, etc closed. Drove around the block and parked on my smooth/level concrete garage floor.

My air dam measurement is taken from the center of the dam to the floor.

My wheel well measurements are taken from the floor to the underside of the wheel well arch. I place a yardstick with a bubble flat against the side of the tire and flat against the side of the fender, move it back & forth so it goes thru the center of the wheel centercap and the bubble shows level. Bend over and eyeball the underside of the arch.

My numbers:
Air Dam 3 1/4"
Front Wells both 26 14/16"
Rear Wells both 28 3/16"

If you have a stock Coupe or Convertible, please post your own numbers when you can get the proper conditions.
Please indicate if your car is a "narrow body", or GS/Z06.
Thanks.

Gearhead Jim

Here's my story of trying to use the official GM tool to measure my height:

http://forums.corvetteforum.com/c6-c...de-height.html

Here's my story of what I changed on my car and need to measure my height:
http://forums.corvetteforum.com/c6-c...uspension.html

Turbo6TA

Your numbers:

Air Dam ............3 ...1/4"
Front Wells .....26 .14/16"
Rear Wells ......28 ...3/16"



My numbers: (2011 GS)

Air Dam ..........Didn't Measure
Front Wells .....26 .15/16"
Rear Wells ......28 ...5/16"

Gearhead Jim

Hmmm, I should have asked for numbers for "narrow body" cars-

The different fenders on the GS may prevent a good comparison to mine, but no reason why you GS folks can't compare to each other.


I'll edit the original post to ask for which model everyone has.

Bill Suttie

Jim Let me begin by saying that you are going to drive yourself nuts with Z & D trim heights and their relationship to front air dam and wheel opening dimensions. My front tires average 5/32 tread depth and my rears are new at 11/32 tread depth. When I had the rears installed, I had the dealer use the gage to set the trim height before doing a full alignment. Rear D hgt 4.8" desired LRr 4.79" RRr 4.83" Front Z hgt 1.81" desired
LFr 1.81" RFr 1.805" These are average numbers after adjusting spring bolts and exerising the suspension. When this was done 4k miles ago the CL measurement to ground for the front air dam was 3 5/8"
Wheel opening to ground dimensions:
Front left 26 7/16" right 26 13/16"
Rear left 27 15/16" right 28 2/16"
What this proves is with the chassis set up level the body may or may not be square to the world. Since one can not see both sides of the car at the same time, it does not mean anything. My front wheel opening and air dam dimensions to ground today are
3/16" less.

gilbybarr

Lets see a pic from the side view.....like below. Oh....mine is the base suspension.

Gearhead Jim

Bill-

I notice your car is a 2006, and the trim specs have changed for our 2009. No one knows why for sure, best guest is simply for appearance.

My service manual has the specs in mm, so here's a trim height comparison:

Front: 2006 = 46mm, 2009 = 48mm, 2012 = 48mm

Rear: 2006 = 121mm, 2009 = 115mm, 2012 = 122mm

Edit- 2012 specs added

Although your car was set to the 2006 specs which are a little lower in front and a little higher in the rear, it looks like my car is higher at both ends if we subtract the 3/16" that your car has settled. Of course, my tires happen to be all new.

And despite my car being higher that yours, using the GM trim height gage shows my car to be slightly lower than spec in the rear and more lower in front. The dealership tech had measured it before and got similar numbers.

I've about given up getting any useful numbers from the GM tool, just gonna see if my current numbers are in the ballpark.

Gearhead Jim

One more thought-

Although my front trim heights differ by 3mm from left to right, repeated measurements show the fenders to be perfectly level.

Is that just a coincidence, or is it possible the factory adjusted the bolts so the body was level instead of setting equal trim height L & R?

You can make an argument that being aerodynamically level is more important than having equal trim height. Or you could say that being perfectly level either on trim height or on aero, doesn't make any difference for small variations.

Kensmith

Stock 2005 vert with factory Z51 suspension except I switched the Z51 shocks for base shocks which did not change ride height at all.

Front air dam 2 1/4"
front wheel opening L 26 1/4", R 26 1/4"
rear wheel opening L 27 15/16", r 27 7/8"
front tires 18" wheels and tires 70% tread
rear tires 19" wheels and tires 60% tread

Gearhead Jim

Thanks!

Vet

Are the 2009 and later wheels / tires the same exact total diameter as the pre-2009 wheels / tires?

Otherwise, it would appear that they've changed the "rake" for 2009 (overall angle of car, front to rear). My guess would be that if they actually bothered to officially change the specs, it might be more of a technical / aerodynamic issue than an appearance issue. They may have determined that they could reduce the drag coefficient by a tiny fraction by slightly altering the rake or something... perhaps.

Actually... a light bulb just went off... I think I know the answer! When you lower the rear, you actually increase ground clearance at the front air dam (being that the air dam is ahead of the front wheels). The 2009 change lowers the rear and raises the front. This should effectively create a noticeable increase in ground clearance at the air dam without altering the appearance of the car too much. GM is probably tired of hearing complaints about scraping air dams!

Measuring at the fenders is really a rough guide at best. The body panels on each and every car will be in a slightly different position just due to adjustments / tolerances. If I remember correctly, when the bottom edges of the frame of my car are at almost equal ground clearance left to right, my front fender well to ground measurements differ by at least 3/8" left to right. This shows that the fenders are not perfectly "level" which each other with respect to the frame.... which would be considered normal. The point is, don't place too much weight on the fender well to ground measurements. If anything, perhaps measure from the bottom edge of the frame on each side, to ground. Even this may not be a "perfect" way to do it, but... just mentioning. This is why GM instructs to use the method in the manual, which in theory, should be most accurate.

Another thing, some folks, especially heavier folks, prefer a little extra trim height on the driver side of the car to compensate for their weight. Even myself, the larger percentage of time I drive my C6 I am alone... and I'm not too far under 200 pounds (yeah, I need to get to the gym!). I can't remember if I took measurements now, it was a while ago, but I think the left side of the car may drop 1/8" - 1/4" when you place a 200+ pound weight in the front seat. If I remember correctly, I did set my left side maybe 1/8" higher than the right to compensate.

Finally, while this is all interesting, 1/8" this way or that way I don't think will make any difference in the real world... unless you're racing regularly at over 120mph. At any given time, due to the degree of change in weight distribution, both front to back and left to right, due to changing fuel level, addition of luggage and whether or not you have a passenger etc, the car is always existing easily 1/4" or more away from it's "ideal" position. Point is, there is an acceptable "window" ultimately, I don't think there is reason to be concerned if the trim height is out of spec by a very small amount.

Remember too, GM claims / advertises that the C6 coupe can do 186mph (at least that's what they said in 2006 when I bought my car, not sure about now). So, for this claim to be legally sound, I'd imagine that the manual specs / instructions are designed to make sure that the car CAN safely travel at 186mph. At 186mph I'd think that suspension adjustments / rake etc ARE quite important. Since most of us will likely never go over 120mph, any degree of precision designed for 186mph travel is simply overkill for those of us who use the car on the street and run under 100mph almost 100% of the time.

Just thoughts.

AORoads

Good thoughts and insights as usual, Dave. (fr. one of the first to raise his C6)

Bill Suttie

Jim Like I wrote in post #5 you are going to drive yourself nuts worrying about trim heights. VET in post #11 gave a plausable answer for your question about the F & R trim height changes between 2006 and 2009.

TMyers

Jim, narrow body, F55. Never been lowered but I do have poly bushings.

Air Dam ............2...1/2"
Front Wells .....26...3/8"
Rear Wells ......27...1/2"

JoesC5

2009 Z06 on nearly new tires.

Front 26.5"
Rear 27.6", but I have about 50 pounds of extra weight in the rear of the car. Remove that and the rear would probably raise a 1/16".

Gearhead Jim

Thanks to all for the numbers and comments.

When driving around yesterday and today, great Midwest Corvette weather, we only saw three Corvettes for the entire weekend. I presumed that was because everyone else was at home measuring their fender heights for me, maybe I was wrong.


Seriously, more numbers is better numbers. If able, please measure and post your results. Thanks.

Gearhead Jim

Thanks again for taking the time to comment.

Wheels/tires are supposed to be identical diameter for all years of narrow body C6's. I've got Michelin PS2 runflats on mine, there might be a slight difference from one brand to another but I expect it is very minor.
EDIT: Tire Rack specs pages show the same diameter for Goodyear Supercars and Michelins PS2 ZP, both front and rear. The "revs per mile" are 809 for the front Michelins vs 813 for the Supercars, only about 1/2 of 1% difference. Rears are even closer. Thanks for making me look at that.

The rake change could indeed be for aero, or appearance, or to reduce scraping. Or maybe all three. We need to start waterboarding those GM engineers.

It would also be interesting to hear what they would say about side-to-side variations in trim heights vs body height. I'm pleased that my fender heights happen to be so close, I'm going to stay with equal body height unless the engineers say otherwise. At 150+, I suspect aero symetry is more important than suspension symetry. I don't go that fast very often, but...

Thinking about adjusting trim height to account for the driver's weight, I considered that. But it seems to me that corner-weighting the car for best handling (by a very tiny margin) actually does the reverse- the heavier corner(s) get their ride height reduced a little so that more weight will go into the other lightly loaded wheel(s), producing closer to equal weight on all four corners. As I understand it, a corner-weighted car would probably ride a little lower on the driver side even before the driver gets in. Or maybe I've got that wrong, can someone educate me?

As I said once before, this isn't a Swiss watch. But I want to get my ride heights as close to optimum as possible, just gotta figure out what "optimum" is. Then, set the ride height and forget it for the life of the car. Get an alignment once per year. Go drive.

Gearhead Jim

Just measured a friend's car-

2005 convertible, base suspension, no mods or accidents, ~20k miles, original tires ~50% tread remaining:

Air Dam 2 5/8"

LF arch 26 7/16"
RF arch 26 8/16"

LR arch 27 15/16"
RR arch 28 1/16"

Vet

Ok, just took some quick measurements.

2006 Coupe, Z51, manual, 100% stock including all suspension parts, wheels and tires, except trim height was purposely raised a bit for increased ground clearance.

Tires about halfway worn, car on pretty level concrete garage floor. Forgot to check fuel level, but likely about 3/4 tank.

Front left: 27 1/4"
Front right: 27 1/16"

Rear left: 28 11/16"
Rear right: 28 11/16"

Air dam: 3 1/16"

Comments:

My front left is 3/16" higher than my front right mainly due to tolerances in fender position, not because the actual left side trim height was specifically adjusted 3/16" higher. I don't have my original notes handy and I was not able to measure from under the frame to double check. But per memory, I did note that my front fenders were not perfectly level with each other with respect to the frame, which is not a problem of course.

Also, now spending a lot of time with this new raised height, I am feeling that the rear is a tad too high with respect to the front, and I think this is why my front air dam clearance is not impressive. I remember being a little concerned about the rake, I tried to keep it stock but then dialed in a tad bit of extra rake for good measure by raising the rear a tad more, a little extra rake is better than too little rake at high speeds. But I think I went a tiny bit too far. When I get a chance I think I will lower my rears by a small bit, maybe 1/8" or so. I'm getting nit-picky here. Regardless, car has been handling beautifully with zero adverse effects since the raise and alignment was done, thousands of miles ago.

I'm noting that an average rear measurement is hovering around the 28" mark. So my rears are 11/16" above average, generally speaking. Jim, at 28 3/16" you're a bit on the high side it would seem... good!

Seems that the rough average front measurement is maybe 26.5 - 26.75"... so I'm say maybe about 1/2" higher in general here. Jim, seems you're towards the high end compared to the others, at 26 7/8"... again, good!

My measurements in general tend to make sense here since, per my memory, my overall average raise was roughly 5/8" over my original position. And I'm noting here, compared to the other measurements posted in this thread, that on average, I'm about 1/2" higher in the front and say 5/8" higher in the rear. This also tends to show that my rear may indeed be a bit "extra" high with respect to my front as I am suspecting.

But Jim, even though your front and rear wheel well heights are a little less than mine, your air dam clearance is a bit greater than mine. Interesting. I wonder if this might also be a tolerance / adjustment issue of the dam or where it attaches to. I doubt that the air dam itself has changed from 2006 to 2009.

I'm thinking too that if my rake wasn't "excessive", my air dam might be up by another 1/16" or so.

Even a concrete garage floor isn't dead perfect and it's not like we're using precision instruments to take these measurements, so I'll be expecting an error margin of at least 1/16" anyway.

If we really wanted to go nuts we could all measure the clearance under the front cross member (under engine) or measure under the frame rails right behind the front wheels and at other points, to really see what's going on. But ultimately not really necessary.

Anyway, Jim, looking at the assorted measurements throughout this thread and looking at my "raised" measurements, your measurements look darned good! You're pretty much at the high-end of average, but yet not as high as me. I guess I could be considered the "obscenely high limit", so as long as you're less than me, but a bit greater than most others, to me this means that you're in a "perfect" spot... generally speaking. If I were you I'd be very happy! I like your measurements a lot.

All interesting stuff!

Boomer111

08 1LT 3/4 tank tire maybe 60 % left.

Koni shocks installed.

front 26 6/16 26 7/16

rear 27 15/16 27 13/16

To lazy to get on ground and measure air dam while parked in garage.


Mine was lowered and then dealer put it back to book specs. Koni installed after.