Five of the nine transaxle mounting points connect to the bulkhead plate, a one inch thick piece of 6061 billet. The top bracket completed in a previous post connects to the top of the plate. The next step is to fabricate four tubes and associated brackets to connect the sides of the bulkhead plate to the top and bottom of the billet chassis pieces. This creates an “X” pattern that triangulates both sides of the transaxle and essentially cross braces the opposite corners of the trapezoid at the rear of the chassis.
The brackets that mount to side of the bulkhead plate are fabricated from three pieces of 1/8” 4130.
Transaxle bulkhead plate bracket. Misalignment washers are not shown.
The transaxle bulkhead plate bracket is finished. Note that the outer third is discolored from welding. Ignore the mixture of bolts which were used for mocking.
The larger challenge was designing brackets that attach to the top and bottom of the chassis billet pieces. Specifically, they need to be located aft of the billet pieces to clear the sway bar blades and to better align with the transaxle’s bulkhead plate. In addition, I plan to fabricate a 1” OD tube frame that supports the wing, exhaust, transaxle cooler, tail hinges, air jack connector and diffuser while also providing protection should I be rear ended. It makes sense to combine the brackets for the links with the mounting points for the rear frame. This was accomplished with a combination of 1/8” and 3/8” 4130. Both the upper and lower bracket utilize the stock suspension bracket’s mounting screws as well as the bolt for the control arm’s rod end. The upper bracket additionally utilizes the two screws that mount the rear shock mounting plate.
Multiple views of the left billet chassis piece and the upper and lower brackets. 3/16” 4130 plate (purple), 1/8” 4130 plate (green), 0.120” x 1” 4130 tube (orange). The orange tube shows the starting point of the the rear frame.
The “X” pattern triangulates that transaxle’s bulkhead plate between the top and bottom of the chassis billet pieces. This adds torsional rigidity.
Note: the brackets utilize the bolts in the control arm rod end, hex nuts are welded on the tubes and the large holes will be filled with the 1” OD tube frame.
Given that several parts of each bracket intersect at a 90-degree angle it made sense to use tabs and slots. Interestingly, the stock tail hinges which the lower brackets replace have two tabs and slots. Bob Wind forwarded me Experimental optimization of tab and slot plug welding method suitable for unique lightweight frame structures. It’s geared towards structures where the exposed ends of the tabs are plug welded without the need to fillet weld the edges of the parts. The primary reason that I used tabs and slots was to locate the parts and keep them from warping when welded. In any event, the biggest take away from the paper are some of the differences between aluminum and steel.
In both cases the tabs should be twice as long as the thickness of the material. Furthermore the inside corners of both the tabs and slots should be relived with a radius less than or equal to 0.5mm. This makes sense for several reasons; the mating corners won’t bind, there are no stress risers and I assume that it facilitates weld penetration. They further recommend tab spacing to be 30-100mm.
The primary difference between steel and aluminum is the optimal height of the tab (i.e., how far it sticks into or beyond the slot) as shown in the above image which was copied from the article. Steel tabs are strongest are when they are flush with the top of the slot. Grinding the weld flush for aesthetic reasons has little to no impact on strength. So the strongest looks the best — how often does that happen?
My slot geometry (left) and tab geometry (right)
Aluminum is a different animal. The strongest is when the tabs stick 1 mm beyond the top of the slots with a 45-degree chamfer which doesn’t look great. Grinding them flush reduces strength by approximately 30%. So if Bob’s tabs are aesthetically pleasing you’ll know that he chose beauty over science LOL
The image above shows my interpretation of how to relieve the corners. I created a reusable “block” in Solidworks for both a tab and a slot so that I can simply position the profile onto a solid and do an extruded cut wherever I need. If I update a block it will ripple to multiple tabs on multiple parts. I found that adding a 0.005” gap around the tab works well. In other words, I make the slot 0.010” longer and wider than the tab. My Wazer's kerf is too large to support the corner-relief geometry. A high-end water jet would probably work, but the 0.5 mm radius lends itself to laser cutting.
Stock hex nut (left) and machined nut (right)
The links are made from 4130 tube. Since they aren’t solid I couldn’t machine wrench flats into them so I welded hex nuts on them. While I could have used a nut whose ID fit over the tube, that would be bulky. Instead I drilled the interior of a smaller nut. This could have been easily done on a drill press or a mill, but I used a lathe because there is no need to line anything up. Just chuck it up and go.
The next step is to fabricate four dog-bone brackets that connect the billet chassis pieces to the bell housing.