Adding Subframe Connectors - Why not lift them to reverse unibody sag?

oz · Nov 30, 2010

I am getting ready to weld on a set of subframe connectors on my '69 Fastback and I've read that the car has to be sitting on the wheels, axle or lower control arms to keep from flexing the body.

My car has small cracks in the paint at the top of the A-Pillar and on the rear valence panel between the tail lights - both signs of excess body flex.

My thinking is that if one were to place the subframe connectors into place and then put some upward force on them (unload the unibody and suspension by a few hundred pounds) prior to welding them so that some of the 40+ years of body sag may be reversed. Why would you want to leave the sag in the unibody and then lock it in with subframes? Wouldn't it be better to reverse the sag and pretension the subframe connectors?

As a mechanical engineer, this makes sense but I keep reading that I shouldn't do this... can someone please explain why?

tx65coupe · Nov 30, 2010

The subframe connectors are to reinforce the car, so they should be installed with the car in a relaxed state and not with stress in one direction or the other. I would not do what you saying.

Loup-garou · Nov 30, 2010

I've read that the subframe connectors are best installed when the car's weight is on its wheels too - but then I see lots of full "rotisserie" restorations that include subframe connector installation when all of the drivetrain components are out of the car and NO weight is on the wheels. The latter method sounds sort of like what you are proposing, so others have had some success with it. Just make sure the basic chassis dimensions are correct before welding them in.

I suppose it all depends on how much structural work the car really needs, and how much of it you are willing to take apart.

blown65 · Nov 30, 2010

I did some tweaking on our 68 Camaro when I installed them. Right or wrong it worked for me. The driver side door was to the point of not opening easily. Car has had a rough life from 20 yrs of drag racing abuse.

Doors open pretty good now. My 65 though, I welded in place with it as it would go down the track.

rbohm · Dec 1, 2010

the thing you want to avoid is setting a twist in the body. if the doors open and close well now with the car on the ground, and you raise the car off the ground and install subframe connectors, what can happen is the body might twist as you raise it, and the subframe connectors will set that twist, and you will have to realign all the sheet metal gaps. the reason that subframe connectors installed when the car is on a rotisserie not having a problem is that there very little weight to cause a twist in the body, and when you put the car back together you are adjusting the gaps anyway.

blown65 · Dec 1, 2010

Yup that's kinda what I was trying to get at. If everything is right you should just install them with it on the ground IMO.

HDTony · Dec 1, 2010

blown65 said:
. My 65 though, I welded in place with it as it would go down the track.

On 2wheels? Lol!

phutch11 · Dec 1, 2010

Oz - as a fellow ME, I say trust your instincts.

My car had similar paint cracks, and had a huge bulge in the tranny tunnel from compressive forces that I believe were generated by a rear end collision when the car had almost no original floor left. (The PO had actually used liquid nails to glue down sheet metal from a washing machine over the rusted out floor panels...)

At any rate, I did exactly what you're contemplating. I intentionally "reverse" loaded the unibody by putting a long 2x4 down the middle of the transmission tunnel and then jacked the car up by the 2x4. I let the car sit in that state for probably 48 hours to relieve whatever stress I could. Then I burnt in the subframe connectors.

In my situation there was really no other choice, so I did it and it worked out well. I will, however, say that 5 years out from the restoration that my paint cracks are back. I believe that these cracks are more than likely caused by the torsional stresses in the car, which subframes will not address.

Good luck...

2+2GT · Dec 2, 2010

I think you give subframe connectors too much credit. A pair of tubes not much bigger or stiffer (if any, considering current trends in big diameters) than your exhaust system won't add that much structure. Adding front torque boxes to an early car will probably do more. Remember, the rocker panel already is a subframe connector, the front torque box just finishes the job, as Ford finally did in 1968. Coupe and fastback bodies don't sag so much as they flex. Most of us, me included, use bigger tires, stiffer shocks, and fatter sway bars than the designers specified. All that force has to go somewhere, and it does it through the body, since the Mustang has no true frame. The boxy front end virtually demands an export brace and Monte Carlo bar, anyone who hasn't added them, at least the brace, is a fool, and nothing you do under the car means anything.

And add-on subframe connectors are a marginal upgrade, at best. If you really want to stiffen the car, in addition to front torque boxes, you should add the convertible inner rockers. Think of them as "subframe connectors from hell", because not only do they connect far more thoroughly to the car, they are exponentially stiffer than any subframe connecting tube. You could beat a subframe connector to tinfoil using an inner rocker as a club. If you put a subframe tube on a pair of blocks, and jump on it, it's junk. Try that with an inner rocker, you'll hurt your feet.

Of course, you'd have to use convertible carpet and kick panels, but so what?

blown65 · Dec 2, 2010

You can say that, but it made a huge difference in our Camaro. BTW, the ones I have are much thicker than a pair of exhaust pipes.

oz · Dec 2, 2010

Thanks for all the replies.

My doors seem to line up pretty well and I did add upgraded export bars and a monte carlo bar to my car.

2+2GT, Your comments are well taken. I realize that torsional forces in the body are probably what is tearing up the paint and that if I want to stop that I'll have to build some sort of ladder structure under the car that cross braces the subframe.
The paint on the car is still good enough that I don't think tearing up the rockers to install the convertible pieces is a good idea right now either.

I've read that adding a steel plate across the span behind the rear seat helps with twist - in addition to stopping fuel from getting into the passenger compartment during a rear end crash. i may get a sheet of 3/32" or 1/8" and install it too.

I've also got a scary 'pop' that comes and goes in the rear left near the wheel. I'm going to pull the interior and take a look to see if the spring mount is good and that torque box is intact. I know I need to add a patch panel where my rear shackle goes through the frame. The rest of the frame extension looks fine so I'm just planning to grind away the bad and scab on a small patch.

Thanks again.

2+2GT · Dec 2, 2010

oz said:
The paint on the car is still good enough that I don't think tearing up the rockers to install the convertible pieces is a good idea right now either.

I have seen many convertibles with good paint and bad rockers get repaired. The only damage to any visible paint is on the pinchweld at the bottom of the rocker. Since that should be blacked out on most Mustangs anyway, it's not a big deal.

HDTony · Dec 2, 2010

This thread is very interesting, can i get some more detail on some of these strengthening items? What is this torque box you are refering too 2+2b

phutch11 · Dec 2, 2010

You could beat a subframe connector to tinfoil using an inner rocker as a club. If you put a subframe tube on a pair of blocks, and jump on it, it's junk.

I wouldn't try that with mine. I custom made them from 2x2x14ga square tube. They are WAY overbuilt.

I think you give subframe connectors too much credit.

Agreed. If you want stiffness there are much better ways to get it. The addition of a bulkhead behind the rear seat is an excellent start.

rbohm · Dec 2, 2010

phutch11 said:
Agreed. If you want stiffness there are much better ways to get it. The addition of a bulkhead behind the rear seat is an excellent start.

while i agree, understand that subframe connectors are but a start on chassis stiffening.

68RCodeConv · Dec 3, 2010

When the inner and outer rockers were replaced on my 68 vert the guy doing it reinforced them internally with a lot more welds than just the spot welds. As a result it has the stiffest body of any old vert I have seen. I was jacking it up under the front frame rail and once I got it to a certain height the rear tire started coming off the ground. I was shocked. A normal vert is so flexible that if you park one of the front tires on a curb it can be difficult to open the doors.

brianj5600 · Dec 3, 2010

I think the attachment at the front is the downfall of most SFC. Mine go into the front frame rails. All the SFC on the market I have seen connect to the floor supports which seem to be the same metal the floor pan is made of.

Jester67 · Dec 7, 2010

I have the Maier Racing Subframe Connectors and they are VERY strong and attached to the front and rear torque boxes as well as the frame rails.
http://www.maierracing.com/pnms2201.html

oz · Dec 8, 2010

Yea, those Maier ones are better than what I got.
It appears that the ones I have butt up against the end of the front frame rail and then attach to the rear rail about even with the front spring attachment... I'm thinking I need to cut the cap off the rear of the front rail and then put in a piece to bridge the frame and the SFC... At the rear, I'll probably put attachment flanges on the sides of the SFC (1.5 x 1.5 or 2x2 tube) so that I can get more weld surface between them and the body.

MachOneMan · Dec 30, 2010

Could anybody post pictures of the Maier connectors installed? I used the conventional "generic" weld on subframe connectors on my 69 & was happy but have been looking into different ones for my 70. Anybody use the ones from Grab-A-Trac?

Adding Subframe Connectors - Why not lift them to reverse unibody sag?

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