Quote:
Originally Posted by new2subaru
That’s quite possible and gives me more reason to yank them out and test it again. I just want to measure the pinion angles first.
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My research seems to point to a potential design flaw, and potentially why almost EVERY person with an AL driveshaft has vibration, which is based on critical speed and pinion angles. Any 1 piece driveshaft has a lower critical speed than a 2 piece of the same total length, plane and simple.
Change the pinion angles slightly with a 1 piece, and a 3" Aluminum driveshaft that is ~50" long doesn't seem to have a critical speed of more than the redline of our car, 7400 RPM, anymore. So no matter what you do, you will likely always exceed the critical speed (frequency) of the driveshaft, unless I'm missing something about the DSS construction, or you somehow balance it or change the frequency.
As I approach 6500-7000 RPM in 5th gear (1:1 ratio) that it vibrates. I believe this may be the critical speed of the DSS AL driveshaft. Others who feel the same vibration at the same speed also then makes sense. You just blast through it faster in lower gears, but in 5th, where you more slowly approach it, the vibration is felt. If you have stock cars, or interior, or rubber, you dampen it and don't feel it, but it's still there.
Carbon Fiber has a higher critical speed, but DSS used a smaller diameter version again, which lowers the critical speed. Case and point, the Verus CF driveshaft actually uses a 3.25" diameter. Now why do they use a .25" thicker diameter than DSS AL 3" version, even though CF already has a much higher critical speed for the same length/diameter?? It's already known that larger diameter also raises critical speed of a shaft.
I am pretty certain the answer is to ensure the critical speed for the length is HIGHER than the shaft speed it will see on this car. This is exactly why the OE unit is 2 piece. I think this is what they mean in their marketing about "frequency balanced". You can balance any driveshaft to any RPM, but EVERY driveshaft still has a critical speed (based on length, diameter, and construction material), in which vibration is induced and unavoidable, which is based on it's natural frequency. So they can also balance it at that frequency to further avoid vibration.
So unless the wall thickness of the 2.75" DSS CF driveshaft is crazy thick, I'm not even sure if it's critical speed is high enough. At this point, I'm heavily leaning toward the Verus one. Not to mention, the Verus is dry laid, which is also known to be more rigid. So they use a much bigger diameter, of the same CF, but also use a method which makes it more rigid. Very interesting. They question is, why?
I specifically asked Verus today if that was the reasoning for the larger diameter of their unit? I'm waiting for their email back, but I'm almost certain that is why.