[matt88]

Originally Posted by Icecreamtruk

Hello guys. I have an LS1 mated to an AR5 (with the fabbotfab kit) and Im experiencing some vibrations above 110mph or so. (track only car, dont tell me to drive below those speeds, dont be silly). Im using a 1 piece aluminum driveshaft with u-joints (both new).

At first I though the driveshaft wasnt balanced, so I ordered a second one and made it sure it was balanced. No change.

I have the truck transmission (for a 4x4) but without the transfer case behind it, so the driveshaft just slides into it. Since the the shaft has to go thru the transfer case (that is not there), its quite long, so we decided to cut it and make the driveshaft longer so the u-joint sit closer to the transmission (thinking that might be the problem. That also didnt do shit.

Then I looked into the driveshaft angles, so the transmission to driveshaft angle (basically the angle at the first u-joint) was 5degrees, and so was the second one (driveshaft to diff, so second u-joint). I read a bit everywhere that over 3 degrees you might have vibrations if you go over the critical rotation speed of the driveshaft. Using some online calculators mine is around 5000rmp, and it turns out 5000rpm is around 100mph or so, so it made sense. So we lowered the rear of the transmission by about .5" and raised the diff by about the same. Angles changed to 3.5 in the front and 2.2 in the rear. But there's still vibration. Hard to tell how much but it feels like less, but still there.

Another annoying point, is that when it vibrates, oil spews out the rear main seal on the transmission. Its not a ton, but enough to coat the underside of the car a bit and I need to clean it up after every session. I also cannot put on my flat floor and diffusor (who in their right mind would with oil leaking above it), and that is also an inconvenience.

Im all out of ideas, I turn to you guys, if you have any knowledge or experience in the matter, im all ears. Thanks!

Why not keep it simple (and relatively cheap). A 3 inch steel shaft with 1350 joints from Denny's for 700 bucks. From the issues I hear from aluminium and carbon shafts maybe its just easier to go back to boring old steel. I know it probably isn't as exotic and perhaps a tad heavier but if it solves the problem...
P.s. When I say boring old steel I mean chromoly, DOM etc.