Vibration problems...

[cleetus]

A couple months ago I lost a transmission and driveshaft to a driveline failure. With the old transmission and DS, I was getting a bad vibration at 70 mph. The whole thing exploded when I mashed it into triple digits. I was hoping the vibration would even itself out or just go away :lol: Not quite.

New (used) transmission, converter, and a brand new driveshaft later, the vibration is STILL there at 70+. The shaft is from PST, which should rule it out as imbalanced.

The vibration is there while coasting in neutral at 70+, so it isn't an engine issue.

I measured my drive shaft angle at about 8*. I did not measure pinion angle, but wonder if a DS should be at 8*.

Any ideas where the problem is? I'm thinking it's it's an angle problem, but am using all stock parts. This whole fiasco started when I had the rear redone with new 4.11s. I may have always had the problem, but never noticed it with higher gears.

 

[cleetus]

Just bought an angle finder and found my rear is +4*.
The driveshaft is +7.5*
I was unable to get a proper measurement from the transmission, but it appears to be a degree or so off from the driveshaft.

So where do I go from here? Can I shim the trans mount to raise the driveshaft angle, which would inversely affect the rear pinion?

Weird how stock boxed arms can set the angle off.

 

[Doober]

It could be because the output shaft/pinion are at different angles. I think in an ideal situation they would be equal. To correct this you could probably shim the transmission mount, or use adjustable control arms on the rearend.

 

[TNTS79]

Your lowers could have warped when you boxed them?? Not likely but possible... Did they go back in easily or was it difficult? I would shoot for a few degrees of - or downward pinion angle when your cars sitting on the ground. As you accelerate the pinion is going to want to go up towards the floorpans of the car. Factory bushings that are past worn out will allow the rear to travel upward more than it should... Throw up some pictures and we can be of more help!

 

[cleetus]

I'll grab some pics tomorrow, thanks for the replies.

Anyhow, I shimmed the transmission mount with a 3/8" piece of metal I had handy. There is still a very slight vibration, but that may just be tires, as it lessened considerably.

The trans is now very tight to the floor pan however.

What I remember from installing the rear was the uppers did not want to cooperate too easily. Both the uppers and lowers are boxed, and had manufacturing tags on them. I'm assuming they were done by an factory. IIRC, I got those arms from a guy on the boards here. They appear to be stock bushings.

 

[Doober]

They were stamped at the factory, G-Bodies didn't get boxed arms, they're probably either aftermarket or welded. If the transmission's against the floorpan, then I'm guessing the problem lies more with the rear suspension.

Far as the vibration, once you've gotten the hang of tracking down vibrations, noises, etc., you'll be able to tell if it's high speed or lower speed. The driveshaft spins whatever the gear ratio is more than the wheels... in your case the driveshaft is spinning 4.11 times more than the tires. 26" tires spin about 775 turns per mile, with your gearing the driveshaft would spin about 3200rpms per mile.

Another way to think of it is the driveshaft spins as fast as the engine or faster (depending on if you have an o.d. trans or not).

 

[cleetus]

They only way to fix the rear pinion angle is with adjustable arms right?

 

[Doober]

Unless something happened to the stock rear arms. It's a stock 7.5 correct? If it is, maybe try a set of stock G-Body arms and see how that pans out.

 

[cleetus]

Sorry no pics yet.

It's an 8.5 from a GN. I've used it with the stock 3.42s with no problems whatsoever. Ever since the 4.10s went in, the vibration started.

I was running the boxed lowers and stock uppers for a while, and I THINK I changed out the stock uppers to the boxed ones when I had the rear out and the new gears put in.

It's possible the boxed uppers might have thrown me out of whack, but I already scrapped the stock uppers.

I eyeballed the trans and rear and it looks like the trans is now level compared to where it was prior to me shimming it. So far, the vibration is greatly lessened but I need to take it on a longer trip to tell for sure.

 

[maitstoy]

Sorry no pics yet.

It's an 8.5 from a GN. I've used it with the stock 3.42s with no problems whatsoever. Ever since the 4.10s went in, the vibration started.

I was running the boxed lowers and stock uppers for a while, and I THINK I changed out the stock uppers to the boxed ones when I had the rear out and the new gears put in.

It's possible the boxed uppers might have thrown me out of whack, but I already scrapped the stock uppers.

I eyeballed the trans and rear and it looks like the trans is now level compared to where it was prior to me shimming it. So far, the vibration is greatly lessened but I need to take it on a longer trip to tell for sure.

i'm having the same problems after lakewood lift bars were installed, but vibrates around 90mph