(Un)Twisted- Making Subframe Connectors
Look at any race car and you will find one thing in common, a stiff chassis. Whether it is for the drag strip, circle track, or the rally course, a solid chassis is critical for putting power to the ground and controlling the handling of the suspension. Full-frame cars have the advantage here, while they may be heavier, the full-length frame adds torsional rigidity that helps plant the tires under heavy cornering and acceleration. The uni-body cars have a front and a rear subframe, which are welded to the floor pan of the body and act as the mounting points for the suspension. The problem with this design is the large section of thin, unsupported floor pan between the subframes.
On the starting line, loading the rear tires puts the rear suspension under tension. In a uni-body, the unsupported section of the chassis flexes upwards with a twist, towards the roof. When the brakes release, and the subsequent power increase, the floor pan will flex outward, to the ground. This negates the previous loading and inhibits the weight shift that helps the tires grip. This wastes a good portion of power that could be propelling the car down the track. Through the corners, the problem is compounded by the front subframe twist and flex, which can unload either tire, causing the car to lose control.
Adding a full-length frame to a uni-body car like the A-body is not only cost prohibitive, but kind of ridiculous to boot. All of the weight savings from the uni-body construction are lost and it just isn’t necessary. There are a couple of simple additions to the stock chassis to reduce the amount of chassis flex, and they look cool to boot. We are talking about subframe connectors and firewall support bars.
Both of these components work together to stiffen the chassis, you can do one or the other, but you only get half of the benefit. Here’s a brief overview of each-
Subframe connectors- Adding subframe connectors is one of the most popular uni-body chassis upgrades. By tying the front and rear subframes together, the section of unsupported floor pan is eliminated. This reduces the torsional flex, helping with weight transfer under initial acceleration, as well as both front and rear handling.
Firewall support bars- Sometimes referred to as down bars, these tubes run from the upper firewall to an area in front of the front suspension. Often, unibody cars utilize fender support bars that run from the center of the firewall to the tops of the inner fenders or shock\strut towers. The front subframe often extends less than a foot past the firewall, while the suspension sits a foot past the firewall in the other direction. This puts the front sheet metal under a fair amount of stress, both upward (like a banana) and inward (towards the top of the engine). Adding firewall support bars, the stress on the front sheet metal is reduced significantly. If your car has a roll cage, these bars can be tied together, making for an extremely strong chassis.
Our Scamp project will be the recipient of a 550+ horsepower 383, so we need the best chassis strength possible on our way to reach a 10.5 1\4-mile timeslip. While we could have bought a set of subframe connectors, for less than $20 bucks in steel we cut our own. The firewall support bars were a little different. We have a tubing bender, so we were able to bend our own, saving us some cash. Using 1 5\8” x .120 steel tubing, the JD Squared tubing bender simplified the process. Your local steel supply shop should have the necessary tools to custom bend this for you, even the local muffler shop can do it, it just might cost you a few bucks more. Before you have anything bent, you want to make a template, like we did.
Each of these components were welded in place. Some companies offer bolt-in subframe connectors, but this is akin to putting a band-aid on a severed limb, it might slow down the bleeding, but it just ain’t right. Other aftermarket offerings require cutting the floor pan, which really is not necessary. As we will show you, a quality job is in the technique.
1. We started with the subframe connectors. Since we were building our own, a pattern was needed. Using cardboard, the line of the floor pan was traced and trimmed out. It is important for the connectors to follow the floor pan as close as possible.
2. Next the template was transferred to the 2×3 14-gauge steel tubing we picked up. Make sure you leave a tab on the rear side to overlap the existing subframe. The front side sill sit flush with front subframe, forming a right angle.
3. We used our ESAB Handi-plasma 380 plasma torch to trim the tubing, but this could easily be done with a cut-off wheel, and it will likely come out a little cleaner, it just takes longer.
4. Then each piece of tubing was cleaned up with a grinder. This is important for the next step.
5. We took a piece of 2-inch wide 14-ga steel (we had this cut at the steel supply store) and bent it to match the profile of the floor pan. This will be added back to the tubing, in a process called boxing. Since we had to trim away a section of the tubing to match the floor, the tubing loses its strength. By adding this piece to the open section, the tubing becomes fully enclosed, getting all of its strength back.
6. The flat steel was then tack welded in place with our Millermatic 211 MIG welder. The steel is fairly thick at 14 gauge (.0747”), but warps easily under heavy heat like welding, so we stitch welded it.
7. Stitch welding is a process that uses a series of spot welds, spread out over the length of the entire weld. This reduces the heat forced into the part, allowing the metal cool quicker, lowering the potential for warpage. We place tack welds about every 2 inches.
8. Then we continued adding spot welds, in the same order, until the entire seam was welded.
9. Once the part had cooled, we dressed the welds with a grinder. This step is merely cosmetic, and we only ground enough to clean it up, excessive grinding weakens the overall weld.
10. We only cut enough of the tubing to clear the low section in the floor (extending about 18 inches past the rear subframe), leaving the tubing intact where possible. Note the 2-inch tab on the end, this part laps over the rear subframe for strength.
11. We hired a mobile media blaster to come out and knock the original undercoating off the chassis. While it is not necessary for every build, we needed a clean chassis. You must at least grind off the areas that you will be welding too.
12. Fits like a glove. The idea is to create an extension of the original subframe. Our boxed tubing will provide better support than the stock floor. We could have followed the entire floor pan and welded the entire floor to the connector, but that is unnecessary.
13. On the driver side, we had to clearance this marked section. The e-brake cable will be relocated.
14. The stitch method was employed once again to weld the connectors in place. Some readers might point out that the car is not sitting on the suspension, which loads the subframes, an important part of this process. This car is different in that the rear suspension is being converted to ladder bars, and the pick-up point is not even on the stock subframe anymore. If we were still using the stock suspension, we would have set the car on jackstands with the suspension loaded.
15. The first step of welding the connectors is now completed. Note the wide discoloration of the steel, this shows deep penetration of the weld, if you don’t get this coloring, you are not getting a good weld.
16. The front portion of the connector was stitch welded as well. Weld all the way around the connector, which can be really difficult for the top of the tube.
17. The completed job. See how the connector follows the floor pan? At the center of the connector, we welded the connector to the floor pan brace, which adds a little more strength. This chassis is almost ready to hook up.
18. We had already converted the front suspension to coil overs. The only problem with this is that the upper shock mount is held in place by three spot welds to the inner fender. The lower portion of the shock mount is part of the upper A-arm, so we just needed to address this flex point.
19. Using a piece of TIG welding wire, we made a pattern for the support tubes. The firewall section will be welded to the plate shown here (actually an old door hinge, hey, ‘rodding is all about recycling), with the lower section welding to a piece of angle steel to the top of the subframe. The best part of this component is in the middle. We are going to weld the tubing to the inner fender directly above the shock mount, eliminating the flex point.
20. Using our JD Squared tubing bender, we bent the tubing. The pattern was used as a guide for the bends. If you have a local shop do the bending, make sure you leave extra length on both ends for trimming. It won’t always come out just right.
21. To get the right angle for the ends, we used this angle tool from PipeMaster. This thing is invaluable for getting the cut just right.
22. We used a chopsaw, but this could be done with sawzall, it just takes a lot longer.
23. The same process was used for the front side. The angle steel was cut down on the chop saw as well.
24. The firewall end was tacked in place on the plate.
25. Then we took it out of the car and fully welded the tubing to the plate.
26. This requires two connectors, so we bent a second pipe to match. It doesn’t have to be perfect, but it needs to be pretty close.
27. Next, the lower mount was tacked up to the angle steel. This was removed from the car and fully welded as well.
28. We prepped the fender by grinding the paint away. We also did this to the subframes and the firewall areas.
29. While it would be nice to just bolt this sucker in, welding the plate to the firewall yields a better connection. Besides, you can’t gain access to this area of the firewall from inside the car without cutting anyway.
30. We welded the upper and lower plates to the car, then we laid down a nice bead to the fender well. This completes the chassis work for the Scamp. Combined with the 10-point cage, this is as close to a full tube chassis as you can get without actually having one.
31. Not only does it stiffen the chassis, it looks pretty sweet too. The motor will clear the bars easily, but the driver side may have an issue with the stock batter tray. We are using a trunk-mounted battery already, so no big deal, but if you insist on a front battery, then you will have to alter the design a little. The fender braces are history by the way, so toss them in the junk pile.
Sources-
JD Squared
Miller Electric Manufacturing Co.
Esab Cutting and Welding
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