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I'm getting ready to do a complete rebuild on my 1981 front end and saw a lot of threads discussing "craddle sag". It sounds like this occurs on more big blocks than small blocks due to the weight difference and got me thinking I should measure mine before ordering parts. Currently the car does not pull in either direction and tire wear is even and not noticeable at all, but the bushings are in need of replacement.
I first looked at the number of shims and found the following: on the drivers side front bolt has 1/32", rear bolt 1/8", and on the passenger side front bolt spacers were two, a 1/32" and 1/18", rear bolt 1/8" and 3/32. The camber angle on both front tires measures 0 degree with a level.
Next I took a measurement for any possible cradle sag, what I found is mine measures right at the factory specs in-between the shock towers at the point where the front bolt of the upper a-arm passes thru the tower to be exactly 26 3/8" with the car on the ground. After 104,000 miles I thought I would find the frame sagging a bit, but must be lucky or the PO repaired at some point in it's life. The car had none to very little surface rust on the frame when I purchased it and always garaged kept before I cleaned it up, primed and painted 2 1/2 years ago.
Looks like I will not need any frame straightening or offset a-arm shafts.
Unless your car has been driven extremely aggressively (think Dukes of Hazzard) or has substantial rust in the frame, I wouldn't expect to see very much "sag" in the front suspension at all. Your measurements seem to verify that.
What you may be referring to as "sag" may in fact be what a normal C2 - C3 frame experiences in the front suspension which is flex. As the suspension travels up and down over irregularities in the road, high speed turns and reversals, the frame will flex where the frame rails are welded to the front crossmember. As the pick-up points of the upper control arms move in and out, even a little bit, they will alter the alignment specs of the front suspension which will affect how the car handles. The more aggressively the car is driven, the more the specs will vary.
If you do a search on "spreader bars" on this forum you will get quite a few hits, many of them involving recent discussions on the various methods folks have used to address this. From those bars supplied by various vendors as well as many that were self-made by Forum members.
I added one to my C3 convertible back in the early 90's while autocrossing it regularly and very quickly noticed a difference.
Unless your car has been driven extremely aggressively (think Dukes of Hazzard) or has substantial rust in the frame, I wouldn't expect to see very much "sag" in the front suspension at all. Your measurements seem to verify that.
What you may be referring to as "sag" may in fact be what a normal C2 - C3 frame experiences in the front suspension which is flex. As the suspension travels up and down over irregularities in the road, high speed turns and reversals, the frame will flex where the frame rails are welded to the front crossmember. As the pick-up points of the upper control arms move in and out, even a little bit, they will alter the alignment specs of the front suspension which will affect how the car handles. The more aggressively the car is driven, the more the specs will vary.
If you do a search on "spreader bars" on this forum you will get quite a few hits, many of them involving recent discussions on the various methods folks have used to address this. From those bars supplied by various vendors as well as many that were self-made by Forum members.
I added one to my C3 convertible back in the early 90's while autocrossing it regularly and very quickly noticed a difference.
Good luck... GUSTO
Thanks for the great explanation GUSTO and the links.
Unless your car has been driven extremely aggressively (think Dukes of Hazzard) or has substantial rust in the frame, I wouldn't expect to see very much "sag" in the front suspension at all. Your measurements seem to verify that.
What you may be referring to as "sag" may in fact be what a normal C2 - C3 frame experiences in the front suspension which is flex. As the suspension travels up and down over irregularities in the road, high speed turns and reversals, the frame will flex where the frame rails are welded to the front crossmember. As the pick-up points of the upper control arms move in and out, even a little bit, they will alter the alignment specs of the front suspension which will affect how the car handles. The more aggressively the car is driven, the more the specs will vary.
If you do a search on "spreader bars" on this forum you will get quite a few hits, many of them involving recent discussions on the various methods folks have used to address this. From those bars supplied by various vendors as well as many that were self-made by Forum members.
I added one to my C3 convertible back in the early 90's while autocrossing it regularly and very quickly noticed a difference.
Good luck... GUSTO
Gusto Nailed it!!!!
The issue is frame flex, not frame sag................
Every C3 whether a cruiser or racer should have a front spreader bar...it just tightens up the whole front end....speedirect if you have an OEM factory mechanical fan or VBP's if electric fans ^^
07-17-2017, 01:44 PM
Front End pre-rebuild observations
After 104,000 miles I thought I would find the frame sagging a bit, but must be lucky or the PO repaired at some point in it's life. The car had none to very little surface rust on the frame when I purchased it and always garaged kept before I cleaned it up, primed and painted 2 1/2 years ago. 
