Spring-back after straightening chassis?

florio · 11-10-2015, 03:41 AM

Hi, I'm looking for some help from body experts: I started working on a '72 911 Targa that I got with the floor already removed by the PO. Trouble is, after all those years the body flexed and there was a 1'' gap when fitting the Targa top. So, I built a small jig using square tubing, bolted it to the floor, and then "pulled" the chassis onto it so that all the measuring points (front suspension, torsion tube, engine mounts) were factory spec. Then I banged on it a bit and welded a cage inside to keep everything in place (and it still is). In addition to this sag, the chassis also had some twist in it. The sag did not require much force to pull out, but the twist was more resistant and a little +- 3 mm could still be there.

I'm now almost ready to weld the floor pans back in, but I have a horrible thought: I'm worried that when I remove the steel reinforcements (after welding all the floor, rockers, and rear panels), the chassis will spring back and make all this work useless. Does anybody have any experience or ideas on this?

Thanks for any help, I'm really worried!

Dario

PS, restoration thread with jig photos are here:

That's it! my first 911 restoration thread- '72 911 S back from the grave (maybe)

962porsche · 11-11-2015, 02:43 AM

all the work should be done with the car bolted to the jigs/fixtures once all the panels are replaced you then should be able to unbolt it from the fixtures and the car still should be as it was when you had it on the bench .

you stated you measured all the suspension points but how about the upper body measurement ?
are they all with in the 2.5mm + and - specs ?

florio · 11-11-2015, 04:16 AM

I measured the door openings and targa top (front to back and diagonals), and they were all right on spec. That's when I welded 1" square tubing inside the frame and across the targa opening to hold everything in place. When I unbolted it from the jig, the chassis did not budge and remained in place.

The chassis is now on a rotisserie. I'm worried that when I finish welding the floor, rockers, etc, and cut out the jig, the car will spring back. Does that sound stupid?

Thanks for your help, I really appreciate any advice.

962porsche · 11-12-2015, 07:20 AM

I personally never put a car on rotisseries . we do all the work on the celette bench or build table .
with the old framed chassis cars you can get away with rotisseries but IMO not with a unibodied car . your only holding the car for each end and they were never designed that way nor were they built that way . all the car has to do is flex more them the 2.5MM and it now out of spec .
the reason Porsche made there door gaps larger on the top to the QTR's is because cars do flex even when they are all together and 100% finished .

florio · 11-12-2015, 10:37 AM

You think the rotisserie is more of a problem than the chassis springing back after it's welded back together? I could support the center if it looks like it's flexing, although with all the bracing I put in, it should be pretty solid. I'll take some measurements before I weld the pan back in.

When you bend back the chassis, do you bend farther than required so that it just rests on the fixing points, or do you pull until it lines up and then bolt it down?

962porsche · 11-12-2015, 04:04 PM

I have 3 rotisseries and just never use them for car bodies . the last time we used one for a car body was about a year ago and it was only mounted on it to sand blast the hole car inside and out and the underside also . after that we removed it and put it on the celette bench to do the repair work .
I do use the for chassis but only for sand blasting and painting chassis go on the build table for repair and build work .
if you have a old framed car like 57 chevy that's one thing cars like that use shims to get panel gaps so if it's not right it's not a big deal just add or remove a shim . this is not the case for a unibodied car . you have 2.5MM to play with and that's it .
do people use rotisseries with out any problems ? YES but I have seen many with big problems from people using them .

florio · 11-13-2015, 02:09 AM

Ok, thanks. I guess that means I have two potential problems to deal with.

- I will follow your advice and put the car back on the jig to weld in the panels (floor, pan, rockers and rear QP)

-What about the issue with the chassis springing back? Do you think I need to worry about the fact that I did not over correct the twist and sag before welding the support frame inside the car?

Thanks, I appreciate your help!

962porsche · 11-13-2015, 03:09 AM

not if you have it bolted , clamped or even weld to your frame bench / build table . it would have no reason to go back to were it was .

florio · 11-13-2015, 05:35 AM

Right! Thanks!

I'll be back to share if there were any problems. While searching, I read in another post you were writing a book on this, did it ever come out?

962porsche · 11-13-2015, 08:22 AM

no I have it finished but that is were I stopped .
i was thinking i would finish it when i closed my body shop . but i started building race cars in my airbush shop dkbautosports.com to keep my self busy and one thing turned into another and my part time ride in a Porsche RSR cup car turned into the test driver and chief engineer on the team . so I'm back with little to no time for my self between building cars , engineering work and going to the track twice a week . at some point i will finish it .

florio · 11-13-2015, 12:36 PM

Now that sounds fantastic, I'm really happy to hear that you're having a great time. I'm sure many of us out here would love to read what you wrote, but at least we have you coming back to help us out. That's really good of you, thanks and good luck!

962porsche · 11-13-2015, 01:26 PM

thanks