Been making more plans for beefing up the suspension/chassie on my barracuda and was thinking that the rear rarely get any attention and could use some modifications. I have seen folks box up the rear erea for added strength and reduce flex, but I was thinking of something different. Possibly a small cage that goes under the rails and helps perfect the gas tank. It would be good for flex and put some safety measures on an exposed erea. Any ideas or pics would be great.

Kind of did...

Pic #60 shows some of the completed solutions I devised. Other pics may show more info in the process.

https://board.moparts.org/ubbthreads/ubbthreads.php/topics/1553823/1.html

Close, but I was looking for something that would cover a stock gas tank and have enough strength to help with rear frame rail flex.

The Kit Car cage pretty much does what you are asking about, but from the top not the bottom.

I don't think I've seen anyone build a cage underneath with the idea of stiffening the rear suspension. Dunno if it's a case of not enough room to do it right or that cages in compression are easier/lighter to build than ones in tension.

The loads in a leaf spring car would be (Mostly) in the Z plane.

The chassis flex behind that z plane where the axle sits would be negligible (My thoughts, contrary opinions are always welcome)

Adding weight to these cars where it is needed to increase rigidity is good.


The first thing to stiffening the unibody would be to add stitch welds along all the structure panels. These cars are 40 to 50 years old, most were driven and the factory spot welds can use some help.

I had a discussion with the guys at US Car Tool a while back. One of the topics was how Ma Mopar stiffened up the Volare/Aspen cars with factory T tops.

The factory actually had two floor boards installed in these cars to increase rigidity for the missing roof sections.

The auto cross guys typically add braces front and rear spanning the front struts and the rear strut areas.

On your car the section under the package tray just behind the back seat is a great area to cross brace the frame rail to to inner wheel well and place a lateral under the package tray.

With this lateral under the package tray is how Tom Quad mounted the race belts in the Black N Blue Charger. Do some searching there are detailed pictures. Eye bolts were mounted protruding through the package tray and the belts were mounted to these via some kind of hardware.

PHJ426, do you have any drawings or pictures of what you are talking about under the package tray?

I suspect the OP is intent on doing something, however this is all based on the assumption that there is a lot of distortion rear? of the drivers seat, and it effects spring rates?, and optimum rear axle alignment during cornering, and this is not a wheel to wheel race car?. So all rear chassis input forces originate first thru the tires, and connect into the chassis at the front leaf spring hangers, the upper shock mounts, and the rear leaf hangers. The only significant sprung mass in this area besides the chassis is the fuel tank, and that is variable. The efficiency of any stiffening mods mainly depends on how an efficient design is incorporated to connect these 6 points, and then how they connect to the front suspension. The amount spring rate lost due to flexing can be compensated slightly by spring choice, shock settings, and or swaybar. The amount of rear live axle misalignment due to chassis flex I suspect is minimal. Chassis responsiveness for driver comfort is extremely subjective, but also very worthwhile. I don't see a lot of bang for buck stiffening the area around the fuel cell for handling purposes. Did I miss anything?

In my experience, a reduction in chassis compliance is always a benefit in any performance application.



I think this is quite underestimated, it's very difficult, if not completely possible, to tune a suspension to accompany chassis compliance. I would say that properly tuning a car for anyone here is challenging enough, adding a another variable that is not consistent isn't going to help things. Also, while steel does have excellent fatigue resistance, I'd rather not use my chassis as a point of wear at every turn, if I can help it.

Just short of a cage, I'm doing as much as I can to knock the compliance out of the chassis. With my car supported by the subframes and the front rail, the rear of the car is not as solid as I would like, and is all factory at this point. I do agree that there shouldn't be a significant amount of load in the chassis past the front hangers and most roll will be handled by the sway bar. Upgrading the crossmember behind the bumper mounts wouldn't be a terrible idea however.

For the OP, I mentioned earlier 6 points that loads are transferred into the chassis, it should also be noted, the two shock connections are mainly a vertical load input, if one discounts the fact most shocks are leaning slightly, the two rear leaf hangers have vertical and a lateral inputs, which because mopar springs are asymmetrical, these loads are always proportionally less then the front spring hangers, and then the front hangers induce, vertical, lateral, and longitudinal loads to the chassis. It should be obvious where most efforts would give the most result.

And yes I agree, chassis compliance has little upside, but always exists no matter what we do, including filling the car with solid concrete.

Dunno, polar moment of inertia is affected negatively if I understand where you are thinking of adding a cross member.

Not sure any stiffening benefits would outweigh the negative change in inertia.

I don't think we're talking about a signifiant amount of weight here. I'd be more worried about the fuel level and the sloshing of it than the ~4# added from a crossmember reinforcement. Or the 40# battery that is usually put in that same area, behind the axle.

I certainly agree it's not the best place to add weight, but on a nose heavy car, it could be worse.

Most that put the battery that far back aren't into handling, just straight line. If I was gonna relocate my battery it'd be pass side, just behind the seat or under it if I could.

In this thread there is a post from Tom Quad take at look at the package tray here and the mounts for the belts, there is a reinforcement underneath. Read his other posts on We ran hot laps, the Black N Blue Charger at Watkins Glen

https://board.moparts.org/ubbthreads/ubbt...tml#Post1183866

That's where mine is, but expecting to soon switch to a lightweight battery, makes one start to question the effort and the downside of the extra wiring/switch. Having a single exhaust also helps in lowering the battery thru the floor if muffler(s) are in stock location.

On this thinking, does a busted rear section of driveshaft always self disconnect and fall off, there is a front safety loop, but I am not excited about the DS flaying around and causing battery damage, especially a new light weight one (read expensive and maybe explosive?). I'm debating adding a rear loop, but this starting to get complicated.

Thanks for the information and ideas everyone. My soul intention was for safety in regards to pertecting the fuel tank and seeing if there was a way to strengthen the rear erea in the process. With is info, I might be able to design something that will work for my application.

Here's my fuel cell cage 1x1 square tube with 2x2 tubing supporting it laterally front and rear