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Posted by: 57lincolnman Aug 17 2013, 08:41 PM

My 75 has a dealer installed a/c unit. The lower pan in the front trunk was cut out years ago by the dealer. Has anyone tried to fabricate cross bracing to tighten the front end? I think the structural integrity of the unibody was compromised by this "dealer authorized" surgical procedure. Interested in your thoughts. Thanks.

Posted by: messix Aug 17 2013, 08:50 PM

if it's just the sheet metal behind the front part of the trunk after about 3" and along the sides 2" from the front sub frame box it should be ok.

looks bad but the strength is in the box sections

Posted by: 57lincolnman Aug 17 2013, 08:57 PM

Yes. Your dimensions accurately describe my set-up. The cut out begins about 3" from the front and I have about 2" to the box frames on each side. Thanks for your help.

Posted by: 57lincolnman Aug 17 2013, 08:59 PM

I suppose what I lost in the transaction was the toe hook (mounted in that section of the panel removed) and drain tubes.

Posted by: messix Aug 17 2013, 09:22 PM

there are more effective tow hooks that bolt on to the front torsion bar mount. search for it in member vendors I think some one here makes them.

Posted by: SirAndy Aug 17 2013, 09:42 PM

I dis-agree ...

I think any cutting that close to the front A-Arm mounts has to have a negative impact.

Posted by: messix Aug 17 2013, 09:56 PM

I think the only forces put on that section is horizontal axis locating the front of the A-arm, most all loaded force is placed on the back mount at the cross member.

if there is a lot cut out of the front vertical panel at the nose I would worry more about that!

Posted by: FourBlades Aug 17 2013, 10:01 PM

Restoration Design just started making a perfect panel to fix this if you want to.

They totally rock when it comes to Porsche repair panels.

John

Posted by: messix Aug 17 2013, 10:05 PM

unless he wants to keep the a/c if it is working....

Posted by: FourBlades Aug 17 2013, 10:08 PM

Doh! I did not catch that part..

John

Posted by: messix Aug 17 2013, 10:24 PM

I just went out to put my '75 back in the garage and looked at it, and it looks like there is a lot of "bracing" on the front of the later cars. if it's just a street car I don't think its worth worrying about.

if it keeps nagging at you weld in some 3/4 square tube along the inside edge.

Posted by: 914Sixer Aug 18 2013, 06:44 AM

Yes, 75-76 do have a little more structure to them due to the heavy bumpers. The front bumper brace runs almost the full length of the wheel well whereas the 73-74 only goes half the distance. The U shaped brackets had some strength too. The 70-72 do not have this extra bracing. Early cars are the ones I would be concerned about.

Posted by: jimkelly Aug 18 2013, 06:57 AM

comparision


Attached thumbnail(s)

Posted by: Racer Chris Aug 18 2013, 07:38 AM

Posted by: rhodyguy Aug 18 2013, 08:27 AM

cracks can result that run towards the front control arm mounting points. plus there are bunch of holes for the hoses that would have to be welded up and in most cases the pass side engine shelve gets butchered. as noted installs vary. somewhat clean to out right dog crap.

Posted by: 57lincolnman Aug 18 2013, 10:24 AM

I have invested quite a bit to have working a/c, so I want to keep that. I've thought about welding in some "x" cross-bracing just above a/c unit to get some rigidity back. With a uni-body it seems to me that this floor pan acts as a diaphragm to resist some torsional forces between the box beams.

Posted by: zambezi Aug 18 2013, 11:54 AM

Heres what I did. Not sure if it added any structural rigidity, but it should not let cracks form and travel.

Posted by: Racer Chris Aug 18 2013, 04:44 PM

THe biggest loads at the front mounts are twisting forces from stock rubber bushings or tight polygraphite bushings binding. There are no loads transfered from the torsion bars through the front mounts, and the side loading from cornering is much smaller at the front than at the rear.

Posted by: Larry Hubby Aug 18 2013, 06:01 PM

If you've ever looked at the floor of the front trunk on a 911, you'd worry less. 911's have a hole, albeit with a flanged edge, that covers practically the entire area between the two A-arm mounts.

Posted by: sean_v8_914 Aug 18 2013, 07:58 PM

if anyone has evidence of stresses in this area, please post pictures. i have not seen much movement up there. the rear is a different story. the

Posted by: Krieger Aug 18 2013, 08:25 PM

When I started to take my 75, that had A/C, apart in 1991 I had to repair a tear/ crack on the frame. It was near where the front A-arm bolts up on the driver side. I'm pretty sure the car was not an autocross/ TT car. The crack was about 1.5"

Posted by: Racer Chris Aug 19 2013, 07:37 AM

This car had the full trunk floor intact.
Cracks occur right around the a-arm mounts as a result of tight bushings and rough roads.

Posted by: Eric_Shea Aug 19 2013, 08:56 AM

I've seen pictures of a bad bushing literally ripping the front mount off the pan.

Posted by: sean_v8_914 Aug 19 2013, 10:27 AM

right along th spot weld line