[TRS]

The issue is the rpm combined with the length. Calculation wise a aluminium one piece shaft of the required length and max speed of a LS swap need to be 4" plus in diameter to not exceed critical rpm. This is an impossible size for the 86 chassis if you dont want to change the complete tunnel.

In my case I have a overall length of roughly 1250mm using a T56 Magnum F. I dont know the specs of your setup, meaning the length of your drive shaft, but I expect something in the same region.

Less driveshaft angle helps definately. 5° is pretty extreme. But the main issue is the difference between front and rear end. If both have the exact same angle and the pinions axels are in the same plane, the angle will cancel out each other. Nevertheless, for a high speed application a low angle is important. But 0° is also not optimal. Ideal would be 0.5° Front and - 0.5° in the rear. Both +0.5° would also be fine.

The main issue you have to solve is the weight of the tube material. Weight locted on the ends of the propshaft, meaning the joints, isnt that decisive for the stress of the tube.

The higher the weight of the tube, the higher centrifugal force will climb at high rpm. Imagine a point of imbalance in the middle of the tube. (you always will have such) By dynamicaly balancing the shaft on both ends, you can balance the entire shaft perfectly. But the unequal point in the center, spoken for this specific section only, is still there. If now rpm climbs very high, this will cause bending stress to the tube, even if the tube is balanced. While taking this load, the tube will slightly deform what in turn causes a even higher imbalance. Since the shaft was balanced at a lower rpm without deformation, now the entire system of the shaft is (under the dynamicaly load) out of balance. This effects will be stronger the heavier/more dense the tube material is.

This effect can be compensated by using a larger diameter to improve stiffness of the tube. But this has quiet some limitations, since the larger diameter will cause also higher loads with the same weight of a local imbalance due to higher g loads on the larger diameter of rotation.

The magic key is using a lighter (=less dense) material with same or even higher strength. This is one of the main reasons why aluminium and CF is often found in performance applications, meaning its not only a question of weight and inertia. One could say that those effects go hand in hand.

Now it might become obvious that a steel shaft is the wrong direction to go. If a steel shaft solves this kind of issue over an alliminium one it only proofs that the aluminium unit was of very bad quality. This doesnt necessarily means that it was bad balanced, but it means that it must have been pretty much out of balance right after welding/before the balancing process. Note: Even a dynamicaly and static perfect balanced shaft might run out of balance at rpm if it was not well manufactured!

In my case I changed to a 3.5" CF driveshaft and this solved the vibrition issues I had with my steel shaft before.

Also a possible option would be using a 2 piece design, since reduction of the length per section dramaticaly improves bending stiffness. But thats a little tricky for custom applications.