C3 bent frame ?? question

Hi,
Have a urgent question about the C3 frame. I have a 79 here which has the complete right side (were the 4 bodymounts are) sitting on average 1/2"
higher than the left side. Cross measuring from one bodymount to the other suggest the frame is still very much straight. I had the frame jacked up evenly (with carpentersbubble) both on the front and back outer tips (measurements taken from a workshop manual/engine, transmission, rear axle, tires, still on it)
There is no evidence of an accident on this car. I checked it already for nicks, welding etc, but have found nothing. Welds are still looking normal on this car. Only engine crossmember and rear axle crossmember has got damage from a jack.
Could this be normal ? Seems to much difference between the two sides to me. There were also a lott more shims on the right than on the left side. Shims seem to be original (already pretty much rusted).
Thanks for any input.
YT
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YT wrote:

saying the body (and frame) "leans" to the left side when the frame isn't propped up straight, or that the frame is bent, or that the bodymounts on one side aren't in the right position, or what? What does "on average 1/2" higher" mean... random, higher in back than front, or what? You also state that there were "a lot more shims" on the body mounts of the same side that's high. Either that's a "duuhh!" or you got me all confused.
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Wayne,
I'm going to try and give you a better discription : This is what I did. I had a sheet here that gave the height of the frame bodymounts (top of these mounts) from a certain datum line both on the front and back tips of the frame ends + for each bodymount. Since the datum line in this drawing was under the car (where my floor is now) and it is difficult to measure the top of the mounts from this line, I used a datum line above the car. Recalculated the difference in heigth between the front and back tips of the frame and adjusted according it. Frame hangs flat + level (hanging on the front and back tips of the frame) Then I used a tube filled with water and measured the heigt difference for the bodymounts (all 8 of them) from this datum line What I got was (difference in height left to right) : - difference on bodymount nr 1-2 : 0,51" - difference on bodymount 2-3 : 0,55 " -difference on bodymount 5-6 : 0,28" - difference on bodymount 7-8 : 0,20"
So as far as I can see the frame has got the correct height on the tips both front and back and thus is level, but the middle of the right side rail is sitting high compared to the left side. This would suggest a front end damage on the right side, but I cannot find any trace of this.
When the body is on it, there is a difference, meaning that it leans about 0,50 " (measured to the lower siderail of the birdcage) to the left.
Concerning the bodyshims : for an example : there were about 4-6 shims on the right side (nr 4-6) and 3-3 on the left side (nr 3-5). You would expect them to be less on the right since this side is higher...
You're not the only one puzzled by this.
Thanks again Wayne.
Yves
Yves

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What condition does the driver's seat look like? How many miles are on the car and how much use do you see on the interior - like the brake and accelerator pedals?
I'm heading off in the direction that a very heavy driver used the car on a regular basis and the left side springs are more tired than the right. I've seen it with other cars, so I guess it could be similar on a Corvette.
What you're saying is that when the car sits on a, for all intents and purposes, "level" surface, the top of the arches of the wheel openings differ from side to side by 1/2"?
Here's waving to ya - \||||
Owen ___
'67BB & '72BB
-- not affiliated with JLA forum in any way -- alt.autos.corvette is original posting -- ___
"To know the world intimately is the beginning of caring." -- Ann Hayman Zwinger
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Car is pretty clean inside and out. Body is not on it right now. It had some fairly good use on it, but of coarse it is 25 years old.
Thanks
Yves
wrote:

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Barking Rats wrote:

No, it appears he's saying the frame is out of spec, because when the reference points at the front & rear of the frame, right and left, are all leveled, the passenger compartment body mounts on the right side are higher than those on the left side. Sounds like he needs to have a good frame shop check it out and bend it back into proper shape, if the distortion is noticeable when the body is on.
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If the body sits level does it really matter? He indicated something on the order of twice the number of shims on the "high" side - maybe the mounts in the body were fabricated a tad out of specs and that's why the factory shimmed it noticeably more than the left.
Here's waving to ya - \||||
Owen ___
'67BB & '72BB
-- not affiliated with JLA forum in any way -- alt.autos.corvette is original posting -- ___
"To know the world intimately is the beginning of caring." -- Ann Hayman Zwinger
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Check it out and fix it now, with either proper body mounts or frame straightening whatever the best frame guy around tells you to do. It's always a good idea to start with a strait foundation
Kickstart
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Hi guys,
I did some additional measuring and comparing of these values again.
As far as I can see it now, the left front of the siderail (where the bumperbrackets mount and front suspension mounts) seems some half inch higher than left (when I have the bodymount positions levelled out to the right side). This could in my opinion explain for the fact, when levelled at the front and back, the frame would try to sit lower on the left side (in the middle where the mounts are). What do you all think ?
Of coarse I can have this corrected on a bench (pulling the front left leg about 1/2" down), but since there is already some extensive shimming on this frame (shimming which was still like it came from the factory), the question arises, if these frames had this kind of deviation from the 'correct' values when new. I read info on the net that suggests that the factory shimmed the frame because the frames where already out of specs when built and the fact that it were not the body's that needed shimming... (where did they make the frames anyway, this kind of deviation considered...China ?? I always thought that frames should be as close to specs as possible) An accident should of coarse also be considered (hit front wheel ?), but I didn't find any trace on the front suspension points either. I would think that the lower suspension arms or spring towers would tear off or something. All welds are still looking normal.
Thanks
Yves

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Hmmmmmmm. Seems I read a loooong time ago it was due to the body molding process that the shimming took up the variations in mounts on the body assembly.
Here's waving to ya - \||||
Owen ___
'67BB & '72BB
-- not affiliated with JLA forum in any way -- alt.autos.corvette is original posting -- ___
"To know the world intimately is the beginning of caring." -- Ann Hayman Zwinger
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First a disclaimer: my engineering experience is in aviation, not automaking.
In general steel can be formed a lot more accurately than fiberglass, and that was even truer twenty years ago. If the variation was present at the factory, it's much more likely in the body than the frame. The builder would shim the body so the doors and other stuff lined up, not so the body would lie smack dab on the frame.
The fundamental problem here, when check dimensions don't check - be it on a pranged airplane or a car under repair - is determining what's really bent. The physical features designated as dimensional references might have moved. It can be most trying, even for an expert, to interpret measurements of a damaged structure and arrive at a really correct model of what happened to it. Lots of people, having taken the position of designated reference features as gospel, have twisted up structures something awful trying to make the wrong things line up.
There's more dimensional variation in manufacturing that you might suspect. When something like a perimeter frame for a car is fabricated it's mostly welded with the parts clamped in a jig, and the jig is assumed accurate. (They do get checked periodically, at least in the aviation world.) But tooling wears, clamps wear, components have dimensional variations, all sorts of odd things happen. The two "identical" 747's Boeing fitted out as Air Force Ones were - are - eleven inches different in length, with nothing anywhere being out of spec. The tooling for a frame would be designed to hold the IMPORTANT dimensions closely and let any variation be taken up in other areas - hence the presence of shims and adjusters and such. It's not really needful for all of a car's frame to be spot-on accurate, although I grant it's certainly aesthetic.
I guess what I'm driving at is (a) be very, very careful determining what's really out of position relative to what, (b) be careful also in determining what ought to be fixed. If everything fit on the car as the frame was built, moving things around will not necessarily help.
-- Vandervecken
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