Drivetrain vibration at high speeds
 

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!

Interesting that some other people have vibrations with one piece driveshafts. I reached out to Verus and do not make custom driveshafts sadly so getting a CF driveshaft out of them is out of the question. Any other supplier that could make a custom one (and be good, I've heard a bunch of horror stories about DSS stuff).

Maybe try PST? Their website shows custom carbon drive shafts.

My DSS aluminum shaft was so bad at about 105mph I couldn't see out of my rearview mirror, even after being rebalanced.

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.

CF DSS taken to low 130s tons of times zero NVH from driveshaft.

I also have the same DSS Aluminum shaft, finally tested it in 5th and 6th at 100-105mph (on a potholed hwy) and its fine. Just another datapoint really. I don't have any trans/diff bushings/inserts or engine mounts so its all stock with EBSC.

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.

My first diff had WL bushings on the outrigger and using a cusco diff brace with stock rear bushings. My current diff has stock bushings and I tried with and without diff brace, still did it. I am running solid subframe mounts so that may not be helping. I can still hear whine at high speed with the stock drive shaft but no vibration.

Well, I put the car on stands and ran it on 4th and 5th from low to high rpm to look at the driveshaft and found the following. While 4th gear was not vibration free, it was fairly low and I think "acceptable", so while the bar starts to vibrate a bit towards 6000rpm, its not mayor. But in 5th gear it started vibrating almost immediatly (starting at like 2k rpm, so around 3k driveshaft rpm), and there was an additional "whomp whomp" noise that was rythmic with rpm. So apparently I have two problems, a driveshaft that cant go much higher than 6k rpm, and a transmission that has some kind of problem in 5th gear, apparently

You are correct, my driveshaft is 1inch lower than yours. Seems like the information I found doing my own research confirms pretty much everything you said, so thats good. It seems I have a problem on my actual transmission as well so I'll fix that and then either go to CF bar, or go to a 2piece, costs will probably be the determining factor.