Hi to all,

First off, let me say that I have read lots of threads on drivetrain vibration on TGO and other forums. Sooooo much information. However I chased this for years to a point I was obsessing over it and unable to enjoy my car. So here's my story:

Firebird Formula, SP383 Stroker, FIRST TPI, Restomod
T56 after market
Custom 3" steel drive shaft, 1350 series
Poly trans mount with Spohn crossmember
Spohn adjustable torque arm
9" Moser rear end
Eaton Truetrac
3.89 Motive gear
Wilwood brakes
Eibach Pro Kit
Poly engine mounts
All parts were new

I have a drive on hoist that is leveled. Car is only for street use.

The vibration was 2 fold:

• A harmonic wave on 5th and 6th gear over 60 mph while cruising (classic!) - VIBRATION "A";
• A rough intense vibration (like worn out bearing) when coasting in neutral, foot on or off clutch, over 60mph - VIBRATION "B";
• There was also a nasty exhaust drone that would exacerbate everything and make it difficult to diagnose;
• Eliminated engine and clutch as source of vibration;
• Vibrations were speed related.

The obvious that did not work:

• Had the tires rebalanced many times;
• Checked driveline angles and tried +3* +2*,+1*, 0*, -1*, -2* and -3* operating pinion angle (means to me: difference between transmission angle and rear end angle). Did not make any difference;
• Switch drive shaft 180*;
• Aligning the rear end (adjustable LCA + Track bar);
• Drive shaft was fairly new and had been balanced;
• Rear-end was new, bearings in good shape;
• U-Joints were very solid.

The drama timeline:

• Measured driveline angles including compound angles. Found out, transmission u-joint was operating over 3* compound angle. Lifted transmission with shims as much as I could and could only get 3* of working compound angle;
• Laser measured drive shaft run out and it was within specs;
• Use dial indicator and measured slip yoke and rear yoke run out. All within specs;
• Tested car on hoist with suspension loaded, leveled, with back wheels on/off, with brake rotors on/off. Ruled out wheels and rotors;
• Used hose clamp trick with chalk to mark high spots on driveshaft. I was able to quiet down vibration "A" with hose clamps;
• Brought drive shaft for balancing. First shop said it was perfect but they did not check it with slip yoke attached;
• First shop indicated the slip yoke had been hacked. You could tell it had been cut by the original shop who had made the drive shaft;
• Tested car on hoist with suspension loaded, leveled, no back wheels. Removed slip yoke from drive shaft, installed it in transmission, and ran a test with no drive shaft. vibration "A" still there. Ran a test without slip yoke using a catch bag for oil spill. vibration "A" gone;
• Concluded slip-yoke was at fault;
• Ordered a new slip yoke and I installed it on the driveshaft. vibration "A" back with a vengeance. Hose clamps had been removed;
• Tested car on hoist with suspension loaded, leveled, no back wheels. Removed new slip yoke from drive shaft, installed it in transmission, and ran a test with no drive shaft. Vibration "A" still there. Ran a test without slip yoke using a catch bag for oil spill. Vibration "A" gone;
• I was dumbfounded;
• Took driveshaft to second shop. They installed the new slip yoke and rebalanced it;
• The rebalanced drive shaft quiet down vibration "A" just like the hose clamps had done;
• Directed my attention to the transmission and possible problem with loose over drive gears;
• Consulted with a reputable transmission shop. Explained at length problem and all testing done;
• Shop owner wanted me to do a test before bringing the transmission;
• Tested car on hoist with suspension loaded, leveled, no back wheels. Disconnected poly trans mount and had transmission held by brace, off the crossmember. Vibration "A" gone! However, terrible vibration in shifter;
• Tested with rubber trans mount, Vibrations still there;
• Brought transmission to shop;
• Shop opened the transmission and inspected everything. They had shafts laser measured. Nothing wrong with transmission!
• Had the transmission rebuilt with TheGearBox Transmission Stage 2 rebuilt;
• Installed transmission back, same vibrations;
• Vibration "B" had been there since the beginning, no improvement at all;
• NOW WHAT?

The never give-up timeline:

• Based on my testing, front tires were out of round. They had nasty flat spots from a few years back when it had been 8 months in storage. Balancing process could not take care of issue. So, got new tires on all 4. It did get rid of front vibration and made the car a bit smoother. It was obviously exacerbating the driveline vibration;
• Ordered a brand new drive shaft from The Driveshaft Shop, high speed balanced;
• Installed driveshaft, same vibrations;
• I noticed, with new driveshaft, vibration "B" would go away when foot on clutch coasting down hill (neutral) putting the transmission in gear 4th or 5th or 6th;
• Also noticed, vibration "B" would quiet down if I would barely engage the clutch while coasting downhill in gear;
• It seems like engaging the gears in neutral was making vibration "B" go away. Car was smooth as it had never been when Vibration "B" was gone;
• The above trick would only work between 65-80mph;
• Still had vibration "A"
• Had rechecked driveline angles probably 10 times during the whole saga. They were always ok with u-joints very close to 3* working angles and operating angle within 0* to -1*

Last resort:
Using UMI weight jacks, I decided to raise the front of the car (ride height) for the following reasons:

1. I was tired of hitting stuff on the roads;
2. Eibach Pro Kit did not lower front & back evenly because I had a Moser 9in and not the OEM 10 bolt;
3. I wanted to relax the transmission u-joint working angle of 3*;
4. I wanted to get more front A-Arm angle;
5. Passenger side ride height was 1/4in lower than driver's side.

The transmission u-joint working angle settled at 2.75* from 3*. This is a 2.35* angle plus 1.43* offset which compounds to 2.75*.

ALL, and I say ALL vibrations are gone.

So what ever you read or someone suggests checking, check it, recheck it and triple check it. Use accurate tools and make sure the surface is flat, specially the rear yoke.

The compound angle calculation is key. Only measuring the transmission angle is not enough. You have to factor in the driveline offset angle. I tried correcting the transmission angle by raising the transmission as much as I could but it must have been not enough. I was borderline 3*. By raising the front of the vehicle only 1/2in, it corrected the angle enough to fix vibration.

Hope this helps someone one day.

 

thanks for the write up. I worked a long time to find mine a few years back. Never solved it, but starting to drive the car more now and will use this to see what I can find. Thanks!

 

Wow, nice work. I bet there have been countless lowered cars with mysterious vibrations that never got solved.

But now I want to know if you shimmed the trans more, would it also be fixed?

 

Yep!

Forgot to mention that I could not raise the transmission anymore as it was touching the tunnel.

 

This is most certainly my issue. I made my trans crossmember for my swap, and there is lots of room above the trans. Thank you!!

 

No worries!

Hope you find the source of it.

 

No problem,

Hope it's the case

Peace

 

nice write up, I'm sure you will help a few people at least!

 

Thanks. At least put them on the right path.

 

Just a thought on vibration A... I used to get a vibration and wave in my 69 Camaro above 60 mph... After I had the engine rotating assembly balanced, it went away. Didn't change the damper... Just changed pistons and had them balance the assembly. Not sure what you did with your engine...

You would think it would always show up at that engine rpm... About 3000... But it only showed up at 60 mph+... I thought I had a tire balance issue.... Replaced rims, tires... Nothing worked... Until the engine work... Go figure...

 

Interesting! You never know eh?

Engine was brand new from GMP, SP383 crate engine. Both vibrations were present with the factory LB9 305 TPI. It started after the drive train swap. It got worse with the new engine because the new poly engine mounts raised the engine 1/2in increasing the transmission u-joint angle. So, I knew by processes of elimination that it was not the engine(s).

 

Damn...you're more patient than me.

 

"necessity is the mother of invention"

Had no choice to be patient... if you can call that being patient! It's more being forced to figure it out.
Peace

 

Just tested it again a few days ago. No vibration up to 140mph. Could not go faster, I ran out of highway lol.

 

Awesome car and great write up. Thanks for sharing.

 

Thank you kindly!

 

So the solution was to reduce the u-joint working angle to slightly under 3° (2.75° in your case) as anything more, even 3.0° on your car (and likely all third-gens) caused the vibrations?
I have a similar issue and will definitely check that horizontal offset!

 

So the solution was to reduce the u-joint working angle to slightly under 3° (2.75° in your case).

Yes, compound working angle.

as anything more, even 3.0° on your car (and likely all third-gens) caused the vibrations?

Yes, more than 3 degrees caused a vibration on my particular set-up. I can’t speak for all third gens. The important point here is to mesure compound angle. Most calculator online and tech articles leave the off set angle out of the equation. In my case that was important.
The vibration got worse when I changed the engine mount as it lifted the engine 1/2in accentuating the compound transmission working angle.

I have a similar issue and will definitely check that horizontal offset!

It’s worth checking for sure.

 

I would be very interested to know what technique you used to measure the side-to-side angle.

 

In summary:

Plumb bob tools
Mesure distance from u joint centres to rocker panel.
Calculate delta
Use Pythagorean Theorem and right angle triangle calculator

Have to remember, this can produce some variation. The idea is to factor in the off set angle. Make sure to centre rear-end first if adjustable.

 

Thanks.