Nocones - that has always been one of my dislikes about using a unequal length dual arm IRS on these cars. There seems to be no way around having "that" tube that is conveniently placed at about the same position as the passengers. I wanted to get the suspension mounted since I had two ideas for reinforcing but I wasn't sure which one would work.
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Right now I will be adding a few 0.75"x0.049" tubes where the red and the green lines are. As a bonus the differential mount should help to absorb some energy in the event of a rear impact. Even though it is 1/8" plate, it is still weaker than the tubes around it in compression.
Carguy - you would be amazed at what little tubes can transmit. I cringe a little when I see the 1/4" armor plating come out for these cars. There is always the chance it will totally fail in some spectacular fashion but the math looks hopeful.
There are two torques acting on the differential. The torque from the driveshaft tries to rotate the differential left or right. At the worst it will be the engine torque x first gear ratio x some shock load value. The second torque tries to rotate the diff's snout up and down. It is the previous torque times the rear end ratio - by default this will be much worse and is the reason people who have hard mounted the snouts of their Miata diffs have had the mounts tear out.
The torque you were questioning will only be about 150 ft-lbs x 3.14 x 1 (we'll assume no shock for now) or about 470 ft-lbs. The long "ears" actually help in this case since they provide a longer moment arm for the two mounts to react the load. In the end each mount will see about 235 lbs vertically.
When you factor in the 4.1 rear end ratio, this torque increases significantly to 1,930 ft-lbs - the snout mount isn't so lucky. Still though the snout mount only sees 1,000-2,000 lbs in the vertical direction and the ears see at most the same in the horizontal direction. The 3/4" tubes the ears attach to can withstand approximately 5,400 lbs axially and the smaller tubes can carry about 3,500 lbs.
The killer for the diff is really the shock loads. As long as there is enough rubber to isolate everything and all of the mounts are the similar stiffnesses, it seems like you are typically safe. Most production cars are rubber mount differentials to even weaker structures than we are and they do fine. The key is to not create a hard mount which will carry a disproportional amount of load.
Now watch my diff tear out of the frame on the first launch.....