Yea Dean I know what it is, I designed it. Its a suspension bridge. May have been confusing but just in case it was, I didnt use that for modeling the car frame.

 

Since I have built many racecar frames in my lifetime, I'll give some imput on what I know and maybe this info will help with this debate.

I have looked at the two manufacturers designs above.

What this guy is arguing about retangular tubing has some merit to the designs of the subframe connectors that are offered by these two aftermarket companies. Square tubing is used on drag race frame designs in a ladderbar style application which is very rigid. The square tube subframe connectors shown above from UMI Performance do not have a ladderbar design so in fact that square tube design will have some twisting since there are no crossmember rung connecting one side to the other. Without a cage to box the lateral plane form flexing in a corner of the chassis, the square tube design will twist without square tube rungs.

Now to the problem with using square tube rungs on a ladderbar style design. It can not fit under a 3rd gen chassis due to ground clearance issues and the location of the driveshaft tunnel. Unless you can tie those aftermarket connectors into a 3 demensional cage assembly, they will twist just like he is expressing. In an application of this stlye without cross ladder rungs along its length, the round tube design is far superior without a cage. In both applications though, a cage will be the best chassis stiffener but generally not practical for a street car.

 

MAX,

How much cracking are we talking about ? The sheet metal where it is welded to the SFMs or is it working its way up the actual rocker panel ? Probably should stop drill those cracks.

 

dam i didn't know this post would turn into this lol. i just want some SSFCS fore my firebird. its an everyday car only flows on it thats it bone stock i have t tops and i like to run my car pretty hard around corners and on the freeway lol so i beat on it alot i just saw those and they looked good what would be the best ones for my car. links would help to i need links i have no idea where to buy whatever you recomend. thanks guys.

 

Its along a couple of the lengthwise welds (horizontal), has actually cracked the relatively thin rocker sheetmetal rather than the overly thick SSM's (SouthSide Machine subframe connector). I only noticed it because of the telltale hairline rust mark it left so I looked closer and sure enough... its cracked. So I have 2 cracks, about 1.5" long each.

Here's a link, I am not going to copy it all. There's a section on torsion for those not afraid of books or worms.

http://en.wikibooks.org/wiki/Solid_Mechanics

In order for torsion to occur in the connector itself, the tube itself needs to twist around its centerline (or thereabouts), one end relative to the other. If you were to assume that its connected at front and rear, worst case scenario is if the front twists clockwise, and the rear (like where it attaches, LCA bolt likely it looks like, like my SSM's do) twists counterclockwise. In the twisting case, there's no deflections horizontal, vertical, or any other direction, it only twists. In order for it to twist in that manner in the clockwise direction, it has to do the following, based on how my SSM's attach and likely how the Spohn's attach and probably the UMI:
Twist the rocker (since its physically attached to the SFC and is parallel)
Twist the frame rear rail (since its physically attached to the SFC and parallel)
Pull/stretch the floorplan
Push/compress the rear 1/4 and inner 1/4
It'd probably compress most if not all of the roof
Pull/stretch the opposite side rear 1/4
At some point the two push/pull would converge, likely the opposite side bend from the roof and 1/4, as a wild guess based on where these cars like to stress crack.
I'm ignoring a few things like the other frame rail, etc, but the principle is the same. In any case, either there is some major deflection in the rocker and rear frame rail caused by torsion, or the whole car is twisting about the SFC in order for significant torsion to occur IN the SFC. Two things would happen that would be obvious if this were true:
1. Damage at the rocker and/or front and rear frame rail, or crossmember, from trying to tear the rail or rocker from its welds or compression of the sheetmetal causing bends. Ever seen that happen? I've only heard of some fast drag cars pulling the LCA so hard it damages the frame, much like the brand F guys do much easier because brand F cant build a frame.

2. Visual deflection in the body of the car, if the whole body twisted. We're talking like 9 second drag car twist here, to induce any significant torsional force in the SFC worth talking about, and that would be twist around the SFC itself, and not the center of the car.

I've attached a picture of what torsion looks like. It requires a relative angular deflection along the part in question. Take an exaggerated example of a car, make it 5000 feet wide. Put a small round tube at the edge of that car. If you twist the car around its center 45 degrees, how much twist is in that tube? Not much. Move the tube to the middle of the car, repeat the process. Now how much twist is in that tube? A whole lot.

Look at it another way, if both right and left frame rails are in such heavy torsion, the car is trying to tear itself apart down the middle. If torsion is the issue at the rockers, you should be attaching the right to the left to prevent them from twisting.

Personally, I would be much more concerned about bending. Like the kind of bending that allows the rear frame rail to move away from the front when the rear goes over a bump. Same kind of movement that transfers load through the roof and cracks every hard driven or high mileage F-body hardtop right at the rear top of the side windows and the base of the windshield.

 

Spohn link is at the top of the board, one of the banners. Board sponsor. Support our sponsor.

 

You answered #1 above when you stated yourself that your square tube subframe connectors have cracked the welds from the floorpan along the span
Quote from you:
"Its along a couple of the lengthwise welds (horizontal), has actually cracked the relatively thin rocker sheetmetal rather than the overly thick SSM's (SouthSide Machine subframe connector). I only noticed it because of the telltale hairline rust mark it left so I looked closer and sure enough... its cracked. So I have 2 cracks, about 1.5" long each."

#2 you answered but were talking about how those cracks form from bumps. They do not form from bumps in vertical movement of the chassis, they form from 3 dimensional twisting of the chassis and the roof tearing as a result.
All this twisting happens when a 3rd gen has no subframe connectors. So it happens less with square tube connectors non ladder style connectors, and even less still with round tube non ladder style connectors.

You are basically answering you own examples guy, you almost have it right but just have the wrong culprit.

 

Glad you can do a failure analysis with nothing more than a rough description, however your conclusion is wrong as you dont have all the information and you have not seen the damage.

I see. I assume you can prove this is the only manner? I'm sure there's some twisting going on helping it along, but its not the only thing.

Take an aluminum can, cut halfway through the middle, then cut halfway down from the top and remove that piece. Bend it up and down a few million times, straight. The concentration of stress is highest at the angle point in the L, just like the roof of a thirdgen. A straight load can, and will, crack it there first, just as well as twisting. Thats part of the reason most buildings, especially here in Cali, are not L shaped. A seismic event will tear that L corner to shreds and needs lots of attention.

 

Is mad max a self discribed nickname, or a given one?

What ever floats your boat pal. Go build a better mousetrap. I'm not buying you product. If you are a structural engineer, then why dd your own car crack? Don't need to answer that cause the responses I have gotten on my few post tells me I am in the wrong website.

Carry on without me, pretend I was never here. I just have to now figure out how to delete my info off this registry so I stop getting these email notices.

 

I'm running the Kenny Brown "Y" Supersubs. They connect front to rear and tie into the trans tunnel on both sides (triangulated). They are tubular with tabs on each end, and weigh 12 pounds each (I think, not 100% sure its been a while). I'm not gonna claim they are the best or strongest, I have no way to know. All I can say is they work great. And I guess its good too because I dont have any graphs, or crappy MS Paint diagrams, or college engineering projects, or copied paragraphs from some unknown source to prove myself the authority on subframe strength discussion. hahahaha

Devine3rdgen, there is an option you can disable the e-mail notifications if you dont want them, I also dont like them.

 

Well if CrazyHawaiian won't say it, I will. KB's subs are hands down the best ones made for thirdgens. Go ahead, prove me wrong.

But seriously, if you are concerned with the absolute strongest design, you will not get a straight answer unless you buy every set and spend tons of money on testing. Just buy a set you like and can afford, install them and be happy.