master
Joined: Jan 2003
Posts: 5,395
Pikes Peak Country
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This is long, read at our own risk. Quote:
The first question should be IMO, could someone describe and BACK UP at least, with sound reasoning, what actual forces sub frame connectors resist? I am not questioning many many positive reports of their improvements in ride etc. So are they resisting compression as the rear leafs push forward upon acceleration, are they resisting twist individually, if just under tension we can just use a steel cable, are they preventing a bending moment as front of leaf pushes up upon acceleration with a bending moment provided by the front mass of the car, i.e. engine, etc, are they just straightening out the force path thru 4 right? angle turns from the front hanger to the doorsills back to the TB cross member, etc?.
If we knew what they are resisting, it would be a lot easier to design a solution. I suspect they are doing all the above, but some more than others, but which one? Say if twist was the main force, a large dia tube would be first choice, if bending moment, a tall I beam, yaw? bending, an I beam on its side, etc. My understanding is it needs to be the smallest and lightest shape/size that gets the job done. Anyone can oversize a solution, only a smart designer knows just how much is needed, if you way oversize, you look.....?
Years back member AAR? (patent attorney) had a lengthy thread on Sub-frame connectors, and main conclusion was the larger the crossection of the sub-frame, the stiffer, wall thickness provides a lot less stiffness then crossection increase gives. The installation advantages are numerous with the contour cut-out connectors, and installation ease IMO is their only advantage. Performance wise, compared to a full tube they leave a lot on the table. 
Once again, JCC performing the sanity check.
Going back to the muscle car era, from which most of our favorite rides have been derived, the predominate activity involving our automobiles was drag racing. Reading through the literature derived from the era, Mother Mopar, with support of her engineering staff, developed sub-frame connectors to support a car against high powered drag launches and to aid the stock uni-body against axial flexure. It seems 500 horses, drag slicks, a prepared surface, and high rpm launches would conspire over a period of time to bend a car in half. The super stiff front segment of SS spring packs would push up on the passenger compartment floors while pulling down on the shackle mounts. Since the only things holding the middle of the car intact are the door sills, roof, and floor plate, sub-frame connectors were developed to keep roof/quarter seams from splitting, quarter panels form buckling, and back lights from popping out.
In light of that activity, the best connector would be tall, probably bisecting the floor, and provide a great amount of bending resistance against the launch. Think of the floor joists in a house. They are stood on edge to resist deflectionand support weight. Drag race connectors should do the same thing. Of course, anything is better than nothing and even wide 1x2 connectors help out, as do contoured weld in connectors. Obviously 2x3, 3x3, or even 3x1 are better. Using the old bumper jack and open the door test, a simple before and after test will show improvements with the most basic of bolt in connectors installed. Are they optimal, no, but they are better than nothing.
But we aren't talking just drag racing since we have this new forum to play in and we need to consider alternate applications. Looking at research done by XV, DSE, Herb Adams, Julian's, Nascar and others, sub-frame connectors alone don't do a whole lot to increase torsional rigidity. Think of a ladder, there are very strong side rails and lots of cross members, but you can still twist it. So you need to think three dimensionally.
Which segues us into this;
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Aren't the factory rockers in effect thick gauge sorta-square tubes connecting the front and rear sub-frames? If so, what do aftermarket sub-frame connectors do that they don't? Just add strength to the connection because you now have four sub-frame connectors instead of two? Perform some other function?
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I thought the same for decades. The I started to notice that newer Mopars -- every B-van, Cherokee, Neon, Stratus, etc. had what was effectively "frame connectors" from the factory, which they started referring to as "uni-frame" design. So I give some simple 2X2" tubing a shot (A-body). Holy cow, the first block, I was fairly well freaked out. The difference in platform rigidity was instantly noticeable.
Rick E.
Actually, yes, you're are somewhat correct in your assumption. However, the rocker panels aren't thick gauge steel. They are the same flimsy stuff that the rest of the body panels are made of. Again, Mother Mopar's research in to this confirmed this and when the Kit car program was developed, the door sills were a dominate structural fixture. However, the also suggested gutting out the inside (floorboard area) and adding square tube bracing to it before tying it in with other structural components. The methods they suggested in this tie in were all bisecting angles that created a car full of triangles. In nature, the strongest inherent shape she has developed is a circle, or sphere. Since cars aren't made with spheres but rather intersecting lines, the next strongest shape, that is compatible with auto design, are triangles. To create rigidity in a car, you need to fill it with triangles. If you look at a traditional sub frame connector, they creates three rectangles out of the cars floor. Rectangles can bend and diamond out of shape. By contrast, triangles resist deformation. Additionally, as a car goes through a corner, it transfers weight diagonally and laterally. Again, triangles assist by providing rigidity where needed while transferring weight were necessary. Obviously this works much better if you think three dimensionally and start creating triangles through-out the car, much like what a current Nascar Cup car looks like. But not all of want to have a street car that looks like a race car. So we compromise and add them in select places without getting to intrusive. If you have ever viewed XV's before and after video of their chassis stiffening it, you'd be amazed what 5 simple pieces of steel can do to a cars foundation.
A big part of why sub-frame connectors works so well is because our cars are put together with spot welds. On top of that, our cars are old, which means those spot welds have had years of flex working against them as well as decades of elements attacking their structural integrity. Consider this, your clothes are stitched together to tighten up seams, close gaps, and provide a better fit. By contrast, our cars are like clothes that are held together with snaps or buttons. Sure they are all together, but they allow gaps and flex. Stitch your car together, just like your clothes, and you eliminate the flexure the requires sub frames to begin with. Julian's did this research on a 1st gen Mustang and through simple rocker sill reinforcement, seam welding, and select bracing, they were able to firm up the Mustang uni-body 70% over stock without installing additional sub frame connectors. SCCA has not allowed seam welding in all stock level competitors because it provides an unfair advantage in structural rigidity. They have only recently relented and allowed sub-frame connectors at all.
A couple of places to read a more in depth analysis of it can be found here:
http://www.pro-touring.com/showthread.ph...;highlight=sn65
http://sn65.com/Chassis%20Design%20101.htm
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