Sorry in advance for the long post. I have searched and searched and not found this described anywhere and trying to put all the relevant information in here...

About 6 months ago, I thought I had fixed my driveline vibrations when I found my transmission and rear axle angles were poor, and I got the vibration completely gone with a net pinion angle of zero - transmission output and rear axle input parallel. I have poly motor mounts and stock transmission mount, stock transmission, stock aluminum driveshaft, stock (barbie) 10-bolt, stock torque arm, rear LCA's relocated to address wheel hop, very not-stock 99LS1 in a 99 trans am. Engine has not been modified since the pinion angles were corrected this summer, so it was smooth with the engine as it sits currently.

When it started getting cold outside, I noticed the vibration randomly returned. sometimes there, sometimes gone. Here is a list of different scenarios as I started to find the pattern:

1. car is cold, engine is cold, first thing in the morning - vibrates badly in 5,6 gear until 5 miles of driving. Or if I take the car to 100+ in 6th and let it coast back down, it quits vibrating.
2. car is stored overnight in heated garage, first thing in the morning - no vibrations at all.
3. Car is driven and then parked at work. When I go home, same pattern. If it's a cold day, vibrates badly. If a warm day, silky smooth
4. If I power through in 3rd and 4th clear up to 130, no vibrations ever, until I drop back down to 80-90 mph, at which point it vibrates. If I take it that highh in 6th and coast back down, it doesn't vibrate.

Here's the kicker:
5. When I took the car on a 300 mile drive, I found that cruising at exactly 93 MPH, the driveline shudders for 8 seconds and then smooths out for 8 seconds. it does this repeatedly and consistently in all gears 3-6. While shuddering, it doesn't feel any different between gears. I timed it for stretches 20 miles at a time, going uphill, downhill, and level. If I slow down below 90 or speed up to 100, it goes back to being random. WTF???

The only time I ever had a car vibrate and smooth out with a set rhythm was a front wheel drive with two different size tires on the front end. I have already verified the tires match and have rotated them several times with no change in pattern.

I'm thinking that if I solve the 93 mph thing, I solve all of it. Has anyone ever heard of this before?

 

here's a similar thread from back in december

https://ls1tech.com/forums/general-m...nterstate.html

 

The thread basically points to a tire issue, which would make sense if I hadn't already rotated them to no avail.

 

there was a TSB on this some time ago and the Dealers would fix by giving you a new aluminum driveshaft
you dont say what yours is
also try setting the pinion angle down a little
Zero isnt really ideal
I have a manual that GM produced in the late 60s on Harmonics and its astounding how much really goes into the design of a vehicle to combat this

 

I'll give it a shot. My old angle was 3 degrees. Transmission was at 6 degrees down from front to rear, and axle was at 3 degrees front to rear, and it was WAY worse. When I first got them both at 4 degrees, it was smooth as butter - until the cold weather.

I have an aluminum driveshaft. I bought the car in 2010, so I don't know if the driveshaft is original or has been replaced.

more info - I tried slaloming (spelling?) the car at high speed with the clutch in so I could listen for driveline noises, and the right rear axle bearing appears to be making noise and fighting a left-ward slalom. Much less steering effort to go right vs left. It didn't vibrate any worse in either direction.

So, I went to a parking lot and did some turning radius tests, and the car turns tighter right than left by quite a bit. I'm starting to think that the engine mods took out the rear axle, and that's the vibration. Plausible?

 

SO, I put an angle finder on the axle tubes. I can't tell which one is level relative to the center section, but the left axle is level at 0 degrees and the right axle is showing 2 degrees side to side, so I guess the axle tube is bent where it meets the center housing. Since it feels like the right wheel is the one fighting me, I'm guessing that the left is correct.

Is 2 degrees enough to throw the axle out or put it in a bind?

 

Ok, I think I stumbled onto something. In the car, the u joints felt nice and tight. I dropped the drive shaft, and the rear u joint "clicks" into place at dead straight. Feels tight other than that. Swapped it out for a greasable from Napa, and the vibration is tremendously reduced. Still a bit there, so I'll do the front one as well.

Now, the other observation is if I turn the yoke by hand back and forth on the rear pinion, there is quite a lot of lash. Estimating 3-5 degrees of free rotation before the splines audibly engage. Is this normal? On the transmission end, there is little to no free play.

 

A transmission that is sloping 6 degrees down towards the rear, 4 degrees down even 2 degrees down is not a good transmission angle, most people will say the angle of the transmission must be exactly equal to the angle of the pinion so if the transmission is pointing 2 degrees down then the pinion must point 2 degrees, that is exactly wrong! and misunderstood the working angles must be minimal and equal, if you get under a factory FBODY and have a look at the angles you will see that the pinion is pointing up about 2 degrees and transmission is pointing up about 1 degree or is level, your going to think this is wrong well its not at that transmission angle if you work out the working angle between the transmission and shaft you will see that it is minimal and opposite to the working angle between the pinion and driveshaft my suggestion is find a bone stock FBody that doesn't vibrate with stock mounts and everything measure the transmission angle, driveshaft angle and pinion angle get your cat to have the same angles.

 

Those measurements were taken with the front wheels lifted so I could get under the car. What matters is the relative angles between the transmission yoke and the pinion yoke. According to driveshaft theory, the two should be parallel and perfectly phased so the vibrations at each end cancel each other out. Most people recommend a -1 or -2 angle so that when you throttle, the rear naturally rotates up, making the two parallel during acceleration, but slightly off while coasting. I have taken the relative pinion angles everywhere from -3 to +3 degrees and zero gives me the least vibration - exactly as predicted by 01SsRedA4.

I had previously followed advice I found searching from 01SSRedA4 to a T and it worked. I was vibration free for six months. Then it got cold, and the vibrations returned. If you check my most recent post you'll see that I found a flat spot in the rear u joint and replaced that u joint, resulting in substantial reduction in shudder.

Now to the question in my final post - is the rotational lash in the yoke normal, and can it induce the vibrations?

 

Rotational lash is normal most pinions have it, and the driveshaft theory is misunderstood a lot, a lot of people throw the driveshaft out of the equation ok let's say we go by this theory and throw the driveshaft angle out of the equation and just go with -3 degrees at the degrees at the transmission and plus 3 at the pinion but this time what happens to the working angle between the driveshaft and transmission? It increases significantly enough to cause a vibration and so does the pinion/ driveshaft working angle, trust me get underneath most cars and you will see that they don't go by this rule of the pinion angle must be equal and opposite to the transmission angle and it doesn't matter where the driveshaft sits, when you get underneath a factory car you might be amazed at what you see because of how strongly you believe in this theory, and you might think something is wrong here but it's not it's just that the factory understood and followed the rule correctly, the transmission/driveshaft working angle must be minimal and opposite to the driveshaft/ pinion working angle, as I have said earlier our cars come with the pinion pointing about 2 degrees upwards and the transmission pointing 1 or 2 degrees upwards at the rear, you might think this is wrong because they are not opposite but it's not because when you raise the transmission to that angle the whole driveshaft is raised and the working angle at the pinion becomes minimal and opposite to the working angle at the transmission which will also be minimal, I stand by my words give it a try get underneath a bone stock FBody measure the tranny/driveshaft/pinion angle and dial in the same number you won't lose anything by following a factory car, and with the temperature, the temperature could have an effect on the Ujoints if they are old and are working on high working angles.

 

I hear what you're saying. If I can find one that's stock I'll see if the owner will let me measure it. I can tell you if I try to put my transmission higher than 2 degrees downward, it lifts the car. Thanks for the answer on the yoke. That'll save me throwing another part at it

Also, I got a 40 mile trip on it, and the car now only vibrates from 65-80, which is far improved. That's from the u joint replacement on the rear.

 

I might stop by my friends shop today, I think they have a stock FBody in there I'll have a look at the numbers, can you give me the exact angles of your transmission, driveshaft and pinion, I will work out angles that will give you minimum and opposite working angles,.I'll even talk to Frank from The Driveshaft Shop, he is the guy that built me my driveshaft he is one of the best people in your country to talk too about this, if its possible can you send me a sketch with the angles.and which way the tranny, driveshaft and pinion is.angled so there won't be any confusion, send it to me on this email firehawk69@hotmail.com, I will work it out and we will solve it.

 

That would be great. After your post, I lifted all four wheels and put the car on level ground and remeasured everything. I'll get it sketched up and sent. I'm at work, so it'll be a bit later before I get the drawings emailed.

Also, to make sure we're on the same page, I did not use that flat spot on the transmission tailshaft to measure angles, because it is not perfectly parallel with the output shaft. I used angles from the machined surface ont he cylinder head and the damper on the crank to get the angles. Used the bearing caps on the yoke-side of the u-joint on the back with a socket as a spacer.

 

Sure my brother, the lower middle part of the T56 transmission is also a good it's like a fin that goes right through the centreline, measure there, measure at one of the Ujoints on the slip yoke, and measure at the harmonic balancer that should give you an accurate measurment, measure at the middle of the driveshaft and measure at one of the Ujoints on the pinion yoke and place the finder just on the edge to the side of the two torque arm bolts just as a reference.

 

I emailed the picture. Posted here as well. I got ambushed at work. Didn't even get back to my desk until 5pm pacific time. Sorry it was so late in the day



I have very little room to move the trans up and down or the motor. If I was to set the driveshaft up with both interior angles on the same side instead of opposing sides, I would need to set the rear axle closer to level. I'd never get the transmission going upward front to rear.

 

I think I need to start a new thread. I was afraid to do this out of fear of pulling a "knight-rider" impersonation in my garage, but I'm grasping at straws. I raised the car - front tires on ramps, rear axle on stands - and speed tested it.

In first through fourth, there is a minor amount of vibration at any speed. It doesn't change too much with speed, but it does change a little with speed. Now, put it in fifth, and it shakes the car, and the shakes become very speed sensitive. Put it in sixth and holy ****. Now, when the car shakes, the wheels don't appear to shake at all. The body of the car shakes, but the wheels spin true. So, I pulled the driveshaft and repeated the test, and it's butter smooth. No vibrations.

So now I'm thinking I had a whole collection of issues, which is why nothing by itself worked.

There was improvement replacing the rear U. I played with all kinds of angles and got my best results according to the drawing in the previous post. I need to try replacing the other U joint, since it does move very easy. Not tight like the new one in the rear.

I never felt like the vibes were gone in lower gears, but they are minimal at worst with the car raised, so I need to force balance the wheels. I bought new wheels and tires already trying to combat this.

If getting the wheels road force balanced gets rid of the vibes in lower gears, But not in 5,6, I'm guessing that's the fifth and sixth gear loose on the countershaft issue. I would feel more confident in that conclusion if it vibrated without the driveshaft, but with no load on the trans, it may not have the inertia to shake the car.

I still question the integrity of the rear axle, but I don't know how to isolate it to see if it is the rear. On the road, the vibes feel like they are coming up through the tail shaft.

I know that's alot of rambling, sorry about that

 

Sorry I didn't get back with the angles yet I'm still waiting on an answer from the driveshaft shop, look if you removed the driveshaft that really eliminates the engine and transmission, it will be more of a driveshaft pinion matter, 5th and 6th Gear just give you lower engine RPMs at the same driveshaft speed that is why the vibration will be more noticable in those gears because you have reduced the normal noise coming from the engines high rpms but are maintaining the same driveshaft speed so that is why it will be more noticable, hopefully I will get you the angles by today

 

I just noticed this detail you said you had front Ujoint play? Am I right even the slightest amount of play caused by Ujoints will result in high runout resulting in vibes especially noticeable in 5th and 6th Gear doesn't mean there is something wrong with 5th or 6th Gear and before you do any force balancing with the tires, with the car on stands remove the wheels and speed test see if the vibe is still there it will save you time and money, like I have said if you have noticeable play at the front of your driveshaft it will give you vibes, 1 mm of play is about 0.040 shaft runout, 2mm of play around 0.080 of runout, 3 mm around 0.120 of runout not in the tube itself but the path it will follow because of the play, get the picture, look it's good that you have pinpointed it down to the driveshaft/pinion, change the Ujoints, if its a steel driveshaft change over to aluminium and I will confirm the angles with you, and hopefully your vibe will be gone.

 

That's a good idea. i'll pull the rear wheels and try again. I still have the DS off to replace that front joint, which I'll be doing tonight.

The driveshaft is Aluminum, and it has a UAW (United Auto Workers) sticker on it, so I'm guessing it is still the original. The weights are welded on, so I'm pretty sure it's still balanced, but if the front U joint doesn't fix it, I'll take it to check the balance. It seems that when I do forum searches, most of the time, the DS is correctly balanced, and it is rarely the issue.

I also still question that rear yoke. if I drop the clutch in third and coast at 15 (which I used to be able to do with no issue), the rear makes a tremendous amount of noise as the yoke repeatedly slams home on those splines.

 

The rear end is a possibility, when you tested it without the driveshaft and you say that it was butter smooth, the only probabilities that are left in the equation are the driveshaft and rear end that's it nothing else, I have the exact same issue as you do its most noticeable in 5th and 6th Gear but when you remove the driveshaft there is absolutely no vibration in those gears, I went even further to pinpoint the exact location which is the pinion area, runout at the front and middle of the driveshaft is normal but at the rear is excessive, pinion yoke has been changed, angle is spot on yoke doesn't show any runout so This is where it starts to get confusing, after I did the horizontal alignment the vibes have significantly died down but are still there, now it's a matter of pinpointing what in the pinion area is causing this, and every time someone walks up to me and says I'm sure it's the tires I swear I feel like slapping the persons lips, I know it's not the tires because the rear is jacked up with the wheels removed and it's still doing it.