I'm about to have subframe connectors welded to my 68 Camaro with an LSX. I don't know whether the frame has been in an accident, whether the front and rear frames are square with themselves and/or each other. Car seemed to drive straight and now I put in solid body mounts (haven't tested it at speed).

If the car runs straight at speed and aligns fine, how big of a deal is it to make sure the frame is perfectly square? I tried measuring it but I am not quite sure that my measurements are accurate. I went to a collision shop and they told me 8-12 hours of labor just to measure the frame. I promptly left. I assume welding SFCs will make the front frame impossible to move relative to the rear. Do you guys think it will be ok to just weld up the SFCs and go, or should I make sure the frame is 100% straight first?

 

There should be two 5/8" holes next to the body mount at the firewall. Supposedly they should line up before you tighten the sub to the body. If those are aligned and your thrust angle is correct you should be ok to weld them on. Your alignment shop will tell you whether or not your thrust angle is good. The thrust angle, just in case you are not familiar, the thrust angle of a car reflects the angle of its rear tires in relation to its center line. A thrust angle alignment corrects the problem by realigning the rear axle to be perpendicular with the center line and parallel with the front axle. Have you ever seen a vehicle riding down the road that looks like a dog trotting? Where the front tires and back tires do not track in the same path, thats when the thrust angle is out of whack. You can also criss cross some measurements from the front of the sub to the body (firewall) to see if it is square. The same can be done for the rearend. You can criss cross from the rearend to the subframe as well. The longer distance you measure the more accurate your measurement will be.

 

See if this helps ?

 

Doug G you are the man! I never knew that bit of information existed. Awesome!!!

 

Does the car need to be perfectly level to make the frame measurements? I imagine that the way to do this is to jack the car up off of the suspension, but I'm sure jackstands would be uneven with each other to some degree. Wouldn't this effect measurements?

 

I wouldn't think that being level would be the issue, but rather that the car is 'seated' uniformly on the suspension. For this, the car should be 'as is' on wheels, on the pavement, since this is the way you'd drive it.
On jackstands, the sheetmetal holding the frames would bend, giving erroneous readings.
Any thrust alignment I've had done was with wheels and car on the alignment ramps (on the pavement).
My 2 cents.

 

There is a simple way to do it. Take some fishing line and thumb tacks and put the line in the center of the rear wheel and tack it to the front in the same place. If your rear is straight with the front you will see a toe in angle just like the front. This also tells you if your rear end is out. You will be able to tell right away. A dog legged rear end will show up on both sides. This can happen when guys get lazy and put new springs in the back without removing the rear shocks. The rears are also set to automatically straighten themselves because the rear leaf in front is adjustable.

 

As long as side to side is the same and corner to corner is the same....should be good to go

 

The bottom of the rear frame at the front master gauge hole on the passenger side is 5 5/8" off the ground on the driver's side, and 5 3/4" to 5 7/8" to passenger side (the difference could be human error).

The front of the front subframe is 10 5/8 off the ground on the passenger side, and about that on the driver's side.

The alignment holes seem to be dead on just by eyeballing it and sticking my pinky in there.
It looks like the frame is bent or warped in the front driver's side thus making front gauge hole not a reliable (is it datum?) point. So I went by one of the other holes in the side of the frame, and just hung a string from the edge of that hole. Those were the front of the subframe data points, or point "F".
The rear subframe, or the middle of the car was measured with a bent metal hanger to try to get the middle of the hole as close as possible. That is the middle data point, or point "M"
The rear frame rail was measured in the center of the master gauge hole in the front bottom of frame rail. They is my rear data point, or point "R".

In these measurements, "P" will connote passenger, "D" driver sides.


Measurements are as follows:
RP to MD = 79 5/8
RD to MP = 79 13/16
MP to FD = 50 1/2
MD to FP = 50 5/8
RD to MD = 69 5/8
RP to MP = 69 7/16
MP to MD = 44 3/4
FP to FD = 38 5/16
MP to FD = 28 7/8
MD to FP = 29 1/16

So basically, the lines from the master gauge hole of the rear frame to the opposite center of the master gauge hole outboard of the firewall body mount are 3/16" different. Is that too much?

It appears that the front subframe isn't 100% square - the lines from the master gauge hole to the front of the frame are 1/8" different. Is that OK?

Strangely, the lines from the rear frame master gauge hole to the sub frame master gauge hole out board of the body mount on the same side are about 3/8" different, and the same for the subframe master gauge to the front. Is that a problem?


What about the differences in ride heights?


I tried my best to measure, but it is extremely difficult to get it 100% perfect. These numbers are best estimates, but they look decent.
This was done with the suspension fully loaded, and the car on the ground. Is my frame OK?