Heartland

Hi, new member but long time lurker here. I recently purchased a used MWC 9" and it came with a PST driveshaft. I'm shaking down some drive line issues. The drive shaft was balanced ahead of me purchasing it and when I installed it, I picked up some wicked vibrations at 70MPH. I took it to my local drive line shop and they said it is balanced. So I took it to 2nd drive line shop and they also agreed that it was balanced. Over the past few weeks I decided to put some hose clamps on the driveshaft to see if there would be any improvement to the vibes and to my surprise, the hose clamps (2) have really smoothed out the vibrations. I used the quarter turn clocking procedure where I installed a hose clamp then drove the car, then rotated the clamp 90° then drover the car again until I noticed improvement. While there is still a small amount of vibrations the ride it is much better now.

My questions:
Why does a balanced drive shaft vibrate and why does it go away when I install the hose clamps?
What could the actual issue be?

MidwestChassis

iTrader: (5)

Make sure your pinion angle is set correctly.

squarles67

Another vote for pinion angle

Heartland

Thank you for the suggestions. The trans and rear are both set at about the same working angle. I have adjusted drive line angles quite a bit when dialing in previous torque arm setups and am quite familiar with dialing it in to 0 vibs. I have a stock shaft outfitted with a conversion u-joint that does not vibrate at all with my current driveline angle. I do not like the idea of running the stock shaft as it's a little short and not up to the task of handling hard launches.

The thing that gets me is that the hose clamps have really helped alot. Any other ideas?

01ssreda4

iTrader: (96)

Man that does sound weird. When I added an adjustable panhard bar and centered the rear end, I started getting acceleration vibration. Moved the rear back over a little off center and it went back smooth. Maybe it was binding? Im not sure. Suspensions and vibes sure can be goofy sometimes.

Heartland

I was thinking that it may be resonance within the shaft and that the hose clamps were dampening the vibes but if I adjust the clamps 180° opposite of where they are located now, it vibrates substantially. The slip yoke is a Sonnax and it isn't any better or worse fitting that the factory piece. That said, there is a little play there. I installed a rockland gear output bushing and it had no impact.

Darth_V8r

iTrader: (4)

If you think resonance, is there by chance a carbon DS you can borrow from someone as a test?

Heartland

None that I know of. Is it bad form to permanently leave hose clamps on a drive shaft? Lol. Seems like a bad idea.

RockinWs6

Because something other than the drive shaft is out of balance. You are compensating with the weight of the clamp. You have a few choices but was the car ever smooth running? I'm guessing it was with the factory driveshaft? They check each car for vibration and move the driveshaft or change it accordingly till it meets a acceptable level. Also the weight of the factory DS with its built in damper on the yoke dampen most vibrations out. I suspect you are just feeling normal vibrations using a non dampened DS. If the clamps help I'd go with it all day long. Good Luck

01ssreda4

iTrader: (96)

I agree, clamps wont hurt anything.

shepss217

iTrader: (5)

Does your car vibrate at all if you rev it slightly while in neutral or park? Mine seems to around 2,500-3,000 so i’m not sure if that’s where the vibration is coming from or if its in the driveline somewhere

SteenH

If it's vibrates while not moving you can rule out the driveline.

shepss217

iTrader: (5)

I get what your saying but i also never felt the vibration until i switched in the 9” rear end along with the UMI Tunnel mounted torque arm. Even though it does vibrate while in park lol...like a freaking mystery

shepss217

iTrader: (5)

I thought it was because i discovered i had a cracked stock flexplate but after changing it i still had the vibration lol

pressureprows6

Go find the thread about the driveshaft yokes that go into the back of the trans...alot of driveshaft places use a cheap yoke that is to small OD wise and are somewhat universal causing problems....i fixed mine with a genuine strange yoke...cost some $$$ but fixed my issues...couldnt even see out the mirrors over 50mph...all from cheap *** parts.

shepss217

iTrader: (5)

Mine isn’t all that bad, doesn’t start vibrating till around 70 or so

blackbyrd

iTrader: (14)

did you tighten the PHB before setting it on the ground? You could have a preload problem shifting the rear end over.

hold up missed the torque arm item on the first read through, did you have the OE with the rubber mount on the tailshaft of the tranny? IF so its possile this was pre existing you are just feeling it now due to the added rigidity of the new setup?

shepss217

iTrader: (5)

No never vibrated until the rear end and torque arm swap. I am using a 1” longer driveshaft too and it seems to be the right length when sitting at rideheight. I was told the tunnel mounted torque arms tend to do this

Darth_V8r

iTrader: (4)

Can you "hear" the vibrations or only "feel" them? Reason I ask is I have seen and experienced rear end swaps that require hub centric spacers to kill vibrations. If this is the case you will feel it but not hear it. If it is the driveline it will be a drone that fills the car. 16 he vs 60 if that makes sense.

FirebirdStud

iTrader: (5)

Did you ever fix your vibration issues? How?