Hey guys,

I have a 1985 305 TPI with a 700R4 with 93K on it and been chasing vibration issues on the freeway about 70mph. Already replaced the driveshaft with an aluminum one from Summit and it's a drivetrain improvement but hasn't taken away the issue. Also changed gear oil and nothing bad in the old fluid. Car sat for 25 years and I did replace rear calipers and pads and check the axle flanges for runout and they were fine. The car was missing the drivers rear wheel and I replaced with factory wheel as was suspicious of a bent axle.

Appreciate any help or guidance from everyone!

Update- Trans mount replaced, new tires and balanced them twice including a road force balance. Reindexed the driveshaft at the pinion 180 degrees. Still there

I’m thinking rear-wheel axle bearings next as GM shop manual lists this as a source of vibration?

 

Bump, can anyone help?

 

Are you positive it's a '70 mph' thing and not a rpm thing. Just saying it could be a problem forward of the driveshaft.
Maybe get the wheels SAFELY off the ground and have someone 'drive' it to see if there's anything abnormal. Not run the speedo up to 70 obviously, but maybe with the tires moving and it not having hwy speed noise, you'll be able to see or hear something you couldn't otherwise.

 

Wheel speed (wubba-wubba-wubba-wubba- ) or drive shaft speed (hhhmmmmmmmmmmm)?

Start with the basics. Nobody can help you if your post is that vague. Kinda like, "I don't like my car, sometimes it doesn't work right, what should I do to it?" Give us something to work with.

 

Wheel speed with a “wubba wubba” type vibration and when I throw it in neutral it still has the vibration, feel it in the seat and if I hold the hand brake handle it resonates… does that help @sofakingdom ?

Will throw the car on jack stands and have my wife “drive” so I can watch, I did this with a camera on the old driveshaft but couldn’t diagnose an issue, replaced the driveshaft anyway

frustrating trying to solve and appreciate the help and will try and replace anything at this point

 

Well, if it's wheel speed, that would explain why a drive shaft swap had no effect on it. Not saying "you shouldn't have swapped the drive shaft"; only, as far as the matter at hand, it was a left turn down a gravel road and through a fence and mowing the weeds and into the lake water.

Having your assistant bring the vehicle up to road speed while you watch is a good plan. Put the axle tubes - NOT any other part - up on the stands. You don't want your assistant to be at risk of physical injury.

Watch for wheel runout. If there is any, swap the wheels side-to-side. If the runout moves, it's the wheel; if it stays, it's the axle.

Could be a tire, too. You could always swap front-to-rear (if the offset issue allows it), at least temporarily, and see if it suddenly appears in the steering wheel. Usually tires though, you can just run your hands around them, and feel a lump from separated tread (the most common cause of such a thing), if there is one.

It's really not that hard. You can do this. You can figure it out.

 

Yeah tires are new Toyo and replaced as well, thanks for the side to side recommendation but front to rear is a no go with offsets on the wheels.

I see this as eliminating all the possibilities, the new aluminum driveshaft was a nice upgrade

Will update progress soon….

 

This is from the 1985 Camaro GM Shop Manual for Rear Axle that I referred to -

 

Went through GM vibration analysis school a few years back, there's a thing called "2nd order driveline vibration" which can be caused by u joint angles that are more than 1/2 a degree apart from each other (front u joint compared to rear u joint). I had an 87 350 formula (that got a 406 installed) and had this issue, but was only noticeable over 120 mph. Since I didn't really go over 120 that often, I didn't do much with it, but figured it came with lowering the vehicle almost 3". Since these vehicles have a "fixed" rear u joint angle (due to the "traction bar" bolted to the rear axle that goes all the way forward to the side of the trans tail), if the vehicle is lowered (or raised for that matter) it will change only the front u joint angle and not the rear. Your vehicle has to be on level ground at "ride height" (suspension sitting like the car is on the ground) (ie. 4 post drive on rack, or front tires on ramps with jack stands on the rear axle tubes, etc.) and measure the difference of angle between the front ujoint cap (one in the yoke of the driveshaft, the part that slides into the transmission) and the tube of driveshaft itself. This will be the front u joint angle. Now measure the difference of the rear u joint cap (the one in the differential yoke) and the driveshaft angle measured earlier, this will be the rear u joint angle. Now if the front joint angle and the rear joint angle are more than 1/2 degree different from each other, that can create a 2nd order vibration.
I use a cheap plastic angle finder with a dangling indicator and a socket that is proper size to fit onto the cap of the u joint, but I am old school and cheap. I am sure there are now digital angle finders available and I even think cell phone apps that can do the same thing. Just make sure you get on to the cap of the u joint, not the yoke.
The angles themselves aren't really important (until you get to 20 or 30 degrees or so, ie. lifted 4x4 trucks) , it's just the difference between them that is.
So, how to change the angles if needed?
This is an adjustable one I found on summit, but there are more availabe
https://www.summitracing.com/parts/b...t/model/camaro
It will let you "twist" the differential up or down to adjust the angle of the rear joint.
The front angle can be moved by using shims on the trans mount and/or crossmember.
So, without actually hearing/feeling your vibration, I can't say this is your problem. There is a very distinct difference about a 2nd order vibration and not many people even know it exists (even some "seasoned" mechanics), but this info and some measurements may turn up something for you. I have had, once before, an internal transmission part that was out of balance (made incorrectly) that caused kind of the same issue (found out by changing the whole transmission) which could also be a factor, but you could also have something such as a bent pinion gear or yoke ( those would be 1st order vibrations though) so it's hard to diagnose over the internet.
Keep in mind your driveshaft is turning 3 or more times the speed of your tires at any given road speed (actually based on your rear diff ratio), so for lack of a better description, a buzzing/humming speed type vibration would be driveline speed, and a bouncy (think Parkinson's hand shake speed)(sorry, that's a bad way to describe it, but can't think of anything else) vibration is usually tire/axle/brake rotor speed. I can't say I've ever had a bearing make a driveline vibration (bad pinion bearings make more of a noise than a vibration, at least to me)
The 2nd order vibration is actually 2 buzzing/humming vibrations that happen at different frequencies over time, (think of 2 different sine waves laid on top of each other, when they touch, that's the most noticeable part of the combined vibrations)
I actually have a copy of the VHS that was used in the class, it's very cool and informative. I should get it into a video file so it can be shared... just have to FIND the VHS player and hook it up to the computer some day....
... hope this helps