canbaufo

Well I tried it, no change at all. I put the pinion angle up 2.25* to offset the 3* downward angle of the trans output shaft. I reasoned that under light load like highway cruising the pinion may be up ~2.75* and the trans may only be down by ~2.75*, hence the .75* difference I left in the angles on purpose. With the pinion at that angle it was up about 2.0* relative to the DS (DS went uphill ~.25* from rear to front).

At this point I think it has been narrowed down to three things:

1. Trans output shaft is slightly warped

2. I am simply experiencing the limits of a steel DS (to be clear, it is rather subtle)

3. Potential issue with slightly wrong type of pinion seal and cap (whatever it is called) on outside shows a minor wobble when turning the drivetrain assembly.

I wish I could buy a cheap aluminum DS just to slap it on and see if it masks the issue well enough (like an OEM LS1 or 1LE), but due to having a 12-bolt I can't go that route. I could then sell it and have a custom aluminum shaft made by Denny's.

Back to the drawing board. Time to do another 180* turn of the DS independent of other changes and make sure it does not help. Then perhaps more sound deadener on the DS. Maybe try dial indicator idea on trans shaft (kind of want to rebuild rear of trans to upgrade to Viper shaft and .62 ratio 6th gear anyway). Pinion seal leaks slightly and we know it's not the exact seal for this 12-bolt ....what if it's in slightly crooked and that's the whole issue????

Arrrrghhhh ...this is such a PITA but at least it's not a major issue and just a minor vibration. I just want it fixed because I find it intolerable for long distances (like a 5 mile stretch of cruising at 90 MPH or the like).

Injuneer

Starting from the front of the pinion shaft you have the yoke, a dirt deflector, the pinion seal, outer bearing, crush sleeve, inner bearing, shim. Having the wrong seal won't cause any problem other than the leak.

Are you saying that you can see a visual wobble in the pinion shaft? If so, that's the place to start with the dial indicator. That would explain the problem.

canbaufo

I've now tried a touch of extra negative pinion angle, no help ...a little worse. It was set at -4* relative to the DS. In the meantime I've read some other things that are of interest. Some say the stock PHB was deliberately offset to reduce vibrations (I guess it needs to be offset for the DS and TQ Arm to be straight from front to rear), mine is an adjustable PHB and I have it set perfectly even (measured by rear quarter panels to wheels). Some say if the DS is a little short you will have the problem, I'm not sure how short is too short, but mine seems to go into the rear of the trans by plenty. I have also stumbled on driveshaft slip yoke harmonic dampeners and I'm definitely interested in rigging something up at the least.

Wow, is this one persistent problem! I have not yet checked the trans output shaft other than turning it by hand and observing it (seems straight as an arrow, but haven't done a dial indicator or other test). I am so ready to just throw an aluminum DS on there and hope for the best ...arrrrghhh!

Going to change the pinion angle back to the accepted optimal - ~1.75* since I know that is best and safest for rear U-joint and probably the best for minimal vibration ...even in this case. Going to paint the heck out of the DS too and do the dial indicator test or other on trans shaft. Also may try to rig up a harmonic dampener or try to find the exact correct sized rubber ring somewhere. I may take the car on a trip soon and I'm desperate to at least bandaide the issue for awhile.

Injuneer

Did you check the pinion shaft and it's visible "wobble", as I suggested?

I have my PHB set dead center - only way the 315 tires work. Does not cause a vibration.

Most high performance DS's are shorter, allowing for more clearance (up to 1") between the front of the slider and the trans output shaft seal, than the stock DS (1/2" clearance). I've run multiple "short" driveshafts and never had a vibration.

The stock steel DS has a harmonic damper on the slip yoke.

canbaufo

The only wobble I noticed was in the dust cover, not the actual pinion shaft. The dust cover just isn't perfectly even all the way around (some tiny variances in clearance between the edge of the cover and the pumpkin), I seriously doubt it's an issue.

The way you describe the DS clearance (with regard to length) has me thrown off a bit, can you expand on the clearance explanation?

I am not having any luck trying to find an aftermarket damper for the slip yoke. I can't help but think that some well placed rubber rings in a few places could really help. There's a rubber company locally that might have something with the right I.D. ...or I may be able to find something at a hardware store.

Injuneer

The distance between the rear axle and the tranny changes as the rear axle moves up and down. The driveshaft will also move forward and backward as the pinion shaft rotates upward and downward. The rear axle will move forward and backwards under accel and decel loads, as the bushings compress. That moves the slider in and out of the transmission tailshaft housing.

You need clearance, measured with the car at normal ride height, no torque applied to rear wheels, so that the front end of the slider can't hit the output shaft seal, or worse meet resistance from the bearing, or have the inside of the slider bottom out on the end of the output shaft. The factory DS has about 1/2" clearance.

If you had the car at normal ride height, unbolted the rear u-joint caps, and moved the DS forward until it stopped, it would move 1/2" forward.

When you get to higher HP applications, the movement of the driveshaft increases, and it is generally recommended that you have 0.75" - 1.00" clearance.

I went through all this with one of the engineers at ACPT when a carbon fiber DS failed with the car basically in limp mode, tuned just enough to drive it from the engine shop to the body shop. The front metal end of the shaft broke loose from the carbon fiber tube. The customer service dolt blamed me, and said I probably didn't have enough clearance on the slider, and it bottomed out. Fortunately, I had purchased the DS through a third party, and he was able to verify that the shaft had been ordered with 1" clearance specified. Eventually they let me talk to the designer, and we were able to reach the conclusion that it was probably a bad bond between the metal and CF parts, caused by inadequate cleanliness of the surfaces. I got my money back, and decided to stick with chrome moly in the future.

canbaufo

I've worked on the car a bit today and observed the clearance you're talking about before I got a chance to read this post, but it appears I approximated it the right way. I unbolted the rear U-joint caps and it moves forward about an inch, maybe a hair less ...so it should be ok for my current ~450 RWHP and I guess good for up to a max of ~550 that may be seen later on (sounds like you wouldn't go more than an inch anyway). Looking at the slider (as installed at ride height) you can see roughly 1" sticking out of the transmission seal.

I tested and analyzed things more thoroughly today with the wheels on solid blocks. I ran the engine in gear with the DS removed so I could observe the trans output shaft in motion. In 1st gear at idle (where I figure a wobble would be most easy to detect) I could not detect but the faintest of a wobble if any at all. What little I could "seem" to detect could have well been minor surface variances on the very end of the shaft. I observed it with a flashlight at sharp angles, carefully comparing it to the edge of the seal. If there's any wobble, it's really-really minor. I proceeded to check in 2nd gear at idle and finally 6th gear at idle. I did note that 6th gear made some "clickity-clackity" noises and I could feel some roughness in the shifter ****. The feeling in the **** was somewhat similar to what I feel through it during highway operation at speeds above 70 MPH. I think I have written the trans shaft off though, as I suspect the clickity-clackity noise is most likely due to the shaft being spun twice as fast as the flywheel (.50 6th gear ratio) with no load at all since the DS was removed. I would think this would exaggerate effects of any slop in the gear. I think the vibration in the shifter was probably the same source. Still, I can't help but suspect ...something, but I'm not sure what. I couldn't find any practical means to measure different points of the end of the trans shaft vs seal or similar...so I thought the visual wobble test would be as good as I could do. Of course the rear seal started to let some fluid out, especially in 6th gea, so I ended the test. Note that I get the high speed vibration in 5th gear as well (and I think I have observed it in 4th too but not 100% sure right now); another reason I doubt that the clacking sound and vibes felt through shifter **** in 6th gear during the test were indicative of anything.

I set the pinion angle as close to 1.75* down (relative to the DS) as I could reasonably estimate with my cheap angle finder. I did this with a tremendous amount of double checks (releasing parking brake and cycling suspension after each tq arm adjustment, among other things), so if I'm off from 1.75* down relative to DS it's not much. I am certain it's between 1.5 and 2.0 without any doubt. I also checked the actual pinion yoke vs the place in the tq arm where you can fit an angle finder, there is actually a variance of about .75*. I decided it is most accurate to take the measurement from the actual pinion yoke when I arrived at my 1.75* down adjustment. Finally, I even checked the pinion yoke itself for any play or variance in degrees when turning it in 90* increments. At each 90* turn I used the angle finder to confirm the exact same angle from top to bottom (using the amount of negative pinion angle to confirm that the pinion shaft is straight). There was no play in the pinion shaft and I believe the amount of slop in the ring gear and pinion gear is fine (like .25" turn of the shaft).

I will not be doing any more torque arm adjustments, as I feel the negative 1.75* is optimal and I have tried every pinion angle adjustment I can do for experimental purposes to rid the driveline of vibration.

I feel that I likely only have two things left to try before trying an aluminum shaft:

1. More sound deadening undercoating on the DS (very little on there now).
2. Mock up a rubber damper system for the DS and/or slip yoke.

canbaufo

Well after many hours of mocking up a carefully balanced damping system and spraying sound deadener on the DS, the vibration is worse. I don't know anything about balancing a DS, but I figured the circumference being even was the most important part ....wrong I suppose. I put a rubber damper on each end of the DS, both very exact in circumference all the way around and identical. I suppose for the DS to be properly balanced it needs to be the same circumference for the whole LENGTH of the shaft ...so this just couldn't work. There was simply no place to put a damper on the slip yoke and I could not find a dampered yoke on the internet. I will remove the dampers and evenly spray on one more can of sound deadener (the only thing that's ever helped before). This will serve to make the issue more tolerable until I have an aluminum DS built, or even better ...find a nice used one at the correct length. Once an aluminum shaft has been installed and the problem still remains I will rebuild the back of the trans and put in my preferred .62 6th gear. If the problem still remains after that, I may sell the car ...that's how sick of this I am.

canbaufo

I know it's not likely to help but I'm going to put some lightweight hose clamps on the rubber dampers (the dampers are rubber pipe connectors). Right now they are firmly slipped over carefully applied rolls of duck tape. The theory is that the relatively large rubber dampers are free to expand outward significantly at high DS speeds, throwing off their balance (become out of round with centrifugal force). This is a cakewalk to try so what the heck.

I just had a lightbulb go off with regard to some resonating vibrations that seem to come from the poly motor mounts during clutch engagement. I've noticed when I shim the transmission it seems to cause this. What just occurred to me is that this may put the motor mounts in a bind unless you loosen them and re-tighten, so I will probably try that too.

I wanted to take the car on a trip recently but refrained from it due to the vibrations. We used my wife's 2006 Hyundai Elantra and I easily cruised it at 90 MPH with absolutely zero vibration. Pathetic that I have a car capable of well over 170 MPH and can't even drive it above 80 MPH without annoying vibrations.

canbaufo

Just continuing to update here. I found one culprit last night that's definitely going to make some difference. When I loosened and retightened the motor mounts I noted that there was a bolt from the engine making contact with the front nut on the driver side mount. There is ridiculously little clearance (how about ZERO) between these bolts, requiring you to lift the engine some in order to get a socket or wrench on either one - God forbid. After some scratching of the head I managed to loosen the driver-side mount, reposition and space out the nut/bolt interference problem, and re-tighten. This was certainly a source of vibration, especially as noted during clutch engagement.

I made a nice and relatively soft rubber shim/spacer for the trans bushing in favor of a stack of large washers I was using and compressed it on install, maintaining the same trans output shaft angle. It spreads out nicely on compression and may be an improvement over a stack of steel washers.

Finally, likely an exercise in futility but easy none the less; I put hose clamps on the rubber dampers (rubber pipe connectors on each end of DS) ....with the fasteners opposing each other at 180* for balancing. The purpose of that is to reduce expansion of the dampers at high DS speeds and perhaps control centrifugal forces that throw them out of round / out of balance. lol, I will be shocked if this helps but it was worth an easy try.

I have other "theories" I may test. I wonder if the use of poly motor mounts and a rubber trans mount is a bad idea. The thought is that the poly bushings keep the engine in place and vibrations are actually transferred to the rubber trans mount since it can move more. Also, I may switch to a rubber torque arm mount. I think I would switch back to rubber motor mounts if it wasn't such a PITA to replace them (looks like a PITA on the driver-side anyway). Seems like it would be perfectly safe to use a rubber torque arm mount since it's encased in that clamshell (using a rubber trans mount on a car with this much power is admittedly questionable).

Injuneer

You do realize that the stock DS damper is similar to the crank damper - a heavy metal ring, supported by an annulus of hard rubber? Just a ring of rubber, even constrained by a band clamp, is not likely to provide any "damping" at all.

canbaufo

That's what I assume as well, it was an act of desperation really. I think it will be better to remove the "dampers" and lightly sand the DS evenly, then evenly spray on some more sound deadener style undercoating. Likely I will try a rubber torque arm bushing since that's easy as well. In the end, I'm betting (and hoping) a larger diameter aluminum shaft will remedy this well enough.

*** ...I was reading up on Jeeps yesterday and CV driveshafts were on topic. I wonder if that could possibly be an option for an F-body or if it would even help (haven't searched yet).

canbaufo

Here's a fantastic and intersting link (this website is problematic though, takes patience) about T-56 vibrations in 5th/6th gears at high speeds due to the 5/6 gear cluster having slop. This is due to not having a full spline on the shaft and the gear cluster's press-fit loosens over time. The gears get "clunky" as they can slide back and forth, causing vibration at high speed. I need to carefully test whether or not I get the vibrations in 4th gear, as the symptoms of the 5/6 cluster press fit failure seem very familiar:

Project "fix that damn 5th/6th gear high speed vibration". - LS1TECH