So while I was getting my Racetronix pump installed, I got my driveshaft looked at as I was getting some vibration staring around 80-85mph.

I was told it was out of phase and slightly warped so I got a new steel one built. I have a Moser 12 Bolt with an Eaton posi and 33 spline axles and needed to have the 1st DS shortened because it wasn't fitting up properly during the install and so it was probably then that the DS was rewelded a little off centre (phasing).

So I'm looking forward to a smooth ride after the install... and I get it up to that 80-85 mph mark and the car vibrates like crazy... a lot worse than before. Sounds like a deep grown and variable depending on speed. The vibration remains up to 115mph (the max I've taken it with that sound)

I was told the TQarm was adjusted slightly after the install... and that may be it. So I've tried making some adjustments, trying to get it to as close to 0 as possible (smoother for highway driving). The car started vibration at lower speeds ~75mph. I adjusted it to be more negative (looks like a "V") and the vibration moved up to that 80-85mph mark and stays there at higher speeds.

I have noticed that it is there ONLY during slight loads and heavy acceleration.... but not on decel.

My Questions... could a new driveshaft be causing this problem? Should I try turning it 180degrees... as I know sometimes it does the trick? Or could it be my rear end as I was told the change in Driveshaft, even if it is more balanced, may make the rear end vibration even worse?

Thanks a lot guys... I'd like to solve this problem before I hit at the track before the end of the season.

 

Who built your driveshaft?

 

Who built your DS and how fast was it balanced too?

Poorly built and/or low-speed balancing can both cause the issue that you're seeing.

 

Thanks for the response guys.

We tried to different sized driveshafts as the one I had was 1/4 inch shorter for when the Moser was installed.... and so I got one built from a local shop here. I then got a longer stock size one... and rechecked the balance of the new shorter one.

The same problem.

Now the interesting thing was this. When I tested the longer driveshaft... by taking it on the highway... it seemed to have solved the problem as most of the vibration went away. As I continued to drive at though, and continued to test it... the vibration came back.

It seems to only happen when the car is warm.

I also noticed this at the dragstrip. My first pass of the day... the vibration was there.... as I had just driven 2 hours to get to the track.

Each run afterwards... after long cool downs... and the vibration went away.

I've also noticed that my eaton posi has been making more "groaning" sounds as I turn corners... as it is getting louder and while initially you had to turn tight and slow when the car was warmed up... it is making it now, even when the turns are not as sharp... and the turns are faster. I put some Royal Purple synthetic in the rear end... and it seems to be working for those noises...

but I'm thinking a bearing or something is going in the rear end.... and causing this high speed vibration.

What are your thoughts guys... anyone with any experience with this?

I think in the spring, I'll rebuild the Eaton, slap in a new set of 3.73's and basically get the whole Moser rear end overhauled.

I hope it solves my problems.

 

Heat induced vibration.....? That is a weird one.

No offense.... But my local "truck shop" retubed and balanced a driveshaft for my third gen and it vibrated. They checked both my forth gen shafts and told me they were straight and balanced. They were wrong. I spent a lot of time and money chasing my tail on something I thought I had diagnosed.

Ask your driveshaft shop how fast their balance machine spins. If it doesn't go to 6500rpm.... Get a second opinion.

 

I agree with this, as well as measure your Driveshaft, and compare it to the measurements of the car; www.dennysdriveshaft.com has a great how-to for measuring properly for driveshafts. If they don't match, then that can be some of your issue as well.

It also could be a bad pinion bearing either front or rear, that would make it heat-induced; check for slop in the pinion yoke.

 

That's the thing... if it wasn't "heat induced", I'd check the Driveshafts out even more... but that the car makes more noise after some driving makes me think it is something else.

 

I'm not trying to be a dick..... DOES YOUR DRIVESHAFT SHOP BALANCE PAST 4000RPM?!?!?! YES or NO? Make a call, ask the question.

This might not be your current most pressing problem, but there could be relating factors if they did not high speed balance.

 

Sorry, I should have responded... they do high speed RPM tests as well. The shop that made my DS is one of the top in Toronto... anyone I know who got a DS made around here has gone through them. They do a lot for the big drag racers as well.