So I got a 1976 fiat 124 that has 240 bhp thanks to a 13b rotary, up from the stock 80, so I figure some chassis stiffening is in order. Its a unibody rust free body and I already have this welded in half cage:
not my car^
sooooo, where and how do i weld in some more stuff to give this car some stiffness? I plan to do a strut bar in the rear (welded to the tower)
First thing is to do a baseline of where you are. Then make it better
Pick a jacking point up front and measure how high you have to go before the back comes up.
What you are about to find out is that a roof does a lot to stiffen a chassis. Convertibles rely on the sills and tunnel to do the rigidity thing across the open gap where the doors fit.
How much tubing are you willing to accept in the cockpit in the form of roll cage structure?
Pete
I have no Fiat 124 experience, but I'd look at adding something similar to a Miata butterfly brace under it. At the very least look at the box sections of the chassis to see if they're crushed form being used as jacking points. I would get some 18 gauge sheet folded up in a U-shape, then weld or bolt the bigger channels to the floor through the passenger compartment.
Lack of chassis stiffness is mostly due to the chassis structure having doors. Your half cage doesn't do much to address that. A bar connecting the rear shocks (no struts there) is just added weight. Unless you plan on fitting some large and sticky tires I suggest you just enjoy the car as it is with a consideration for upgraded brakes and some stiffer bushings in the rear trailing arms and Panhard rod.
I still plan to experiment with structural foam at some point. But, i agree with the sentiment that if you have no firsthand experience of what it is you're trying to 'fix', i would just wait and get that first so you don't waste energy or actually make the car worse.
Close the box on the trans tunnel (I love the fact that “butterfly brace” got mentioned here, that name was from a joke years ago). Close the box in the engine bay. Add a door bar. Seam weld.
Why has nobody mentioned seam welding yet?
In reply to _ :
Look above you. 
In reply to Appleseed :
Whu? Psh. Ffff.... it wasn't there when I posted, I swear it i really do!
What about something like this , adding the internal brace in the rockers , most cars just have a box section ,
this was a photo on the Volvo 1800 build
I have seam welded and caged and braced multiple cars.
Your car looks nice.
Add some bolt in door bars and a truangulated strut tower bar (front rails and firewall).
You'll get 90 percent of the stiffness you think you want without paint/bodywork/extensive work
You can add some stiffness with the aformentioned "butterfly brace" to the trans tunnel you can also add some by adding additional metal along the rocker boxes inside of the car.
Ford added these to the Fox Mustang convertables with a "L" shaped 16-14 gauge metal form over the inside surface of the rocker boxes. They just notched out for any braces already there on the stock structure. Once the carpet went in you'd barely notice any difference.
Last you can also add some metal along the pinch weld on the bottom of the rocker boxes. A simple length of 90 deg. angle steel welded to the underside (Floor) of the car and the pinch weld.
Although I will be running 225 sticky tires, in hindsight maybe stiffness aint to bad, as some of you have pointed out:
A dangerous picture I took a long while back (dont worry I put that jackstand back after the picture)
Notes in case they are relavant: 225/45 r17 Hankook Ventus R-s4 tires, all brand new suspension, and a big brake kit (picture taken before that stuff)
The half cage is the extent of cage work for now, and just in case somebody didn't see my little note, that first picture is not my car just another car with the same style cage. And it seems the consensus is seam welding, or its fine the way it is maybe, yes?
jimbbski said:
Ford added these to the Fox Mustang convertables with a "L" shaped 16-14 gauge metal form over the inside surface of the rocker boxes. They just notched out for any braces already there on the stock structure. Once the carpet went in you'd barely notice any difference..
When you say "inside" do you mean on the inside of the car so you could see them when you pull the carpet back ?
or "inside" the rocker so its hidden ?
THANKS
When I built the Molvo, I was worried about chassis rigidity. Roll cages are frowned upon in Canada if you want to drive on the road. So I built this to be inside the sills
I extended it into the front to tie into the shock tower.
My MG has massive extra supports welded down the frame rails. There’s no such thing as too much chassis rigidity unless you’re building an actual literal go kart.
californiamilleghia said:jimbbski said:Ford added these to the Fox Mustang convertables with a "L" shaped 16-14 gauge metal form over the inside surface of the rocker boxes. They just notched out for any braces already there on the stock structure. Once the carpet went in you'd barely notice any difference..When you say "inside" do you mean on the inside of the car so you could see them when you pull the carpet back ?
or "inside" the rocker so its hidden ?
THANKS
Inside of the car and not inside of the rocker as some have done and/or sufested to do. What you would be doing is just making the long hollow box that the rocker is a bit stiffer by increasing the thickness of the steel present and making it a bit wider. Both will help stifness and would not be very noticeable once the carpet covers it up.
Keith Tanner said:My MG has massive extra supports welded down the frame rails. There’s no such thing as too much chassis rigidity unless you’re building an actual literal go kart.
Now with visual aids! That’s the thin gauge stock rail on the left, the new one on the right. I don’t remember the exact wall thickness of the new bit, might be 0.125”. Welding that on was a challenge. The forum software may have chopped off the bottom, here’s the pic: http://slowcarfast.com/gallery/images/MG/A2CF1FAF-BB76-4F80-8716-5288F810ED72.jpeg
This bit runs from the center crossmember where the other one terminates and provides the lower rear suspension pickup points. It may be 1/4”.
There is definitely such a thing as too much chassis rigidity when you get a in wreck! But, in a car that old and that size... nevermind.
Vigo said:There is definitely such a thing as too much chassis rigidity when you get a in wreck! But, in a car that old and that size... nevermind.
Only if you make the whole thing too rigid. Look at a lot of newer cars in a crash, for example. The ends are very squishable and the middle section (where rigidity would matter most for performance anyway) is very not-squishable.
Some kind of torque boxes?
Useless image, but basically, box the rocker to the subframe. (unibody, right?)
I really want to foam fill the commute attack car frame rails. Some production E36 M3 like tge q45 had it from the factory. Sport compact car mag did Mike Kojima's z32 and said it was a revelation, like it had a cage. Allegedly if you use automotive specific foam it doesn't cause corrosion issues.
The ends are very squishable and the middle section (where rigidity would matter most for performance anyway) is very not-squishable.
I agree with it needing to be targeted. Preferably it would be targeted at a particular symptom. Making things stiff for no reason is probably a downgrade overall.
Vigo said:I agree with it needing to be targeted. Preferably it would be targeted at a particular symptom. Making things stiff for no reason is probably a downgrade overall.
Stiffening the crap out of the chassis might require some suspension re-tuning, I'd expect. But in theory at least, stiffer chassis means less movement you can't control, which means you can better control everything with the suspension and given good tuning, the end result should be better.
Carbon said:I really want to foam fill the commute attack car frame rails. Some production E36 M3 like tge q45 had it from the factory. Sport compact car mag did Mike Kojima's z32 and said it was a revelation, like it had a cage. Allegedly if you use automotive specific foam it doesn't cause corrosion issues.
Ask Alfa Romeo about that last bit :) Isn’t that why the Alfetta dissolved?
We foamed a couple of Miatas in our fleet back in that era. SCC may have overstated the result, but the car with the denser foam does feel like it’s on the rigid end of the Miata spectrum. The one with the lighter foam feels normal. Neither are as stiff as a caged car.
There no downside to a stiff chassis if you’re not having an accident. It gives the suspension a solid structure so it can do its job. Making the chassis into a big undamped spring is bad.
You'll need to log in to post.
