Vibration at speed

[1985olds]

Car in question is a 1985 olds cutlass with a 5.3 tremec magnum and 4.10 7.5 ten bolt and new 3.5 steel pst driveshaft. So last summer was the first summer with the new trans setup. I had a vibration at about 75. It wasn't steady, more of a surging vibration, so I figured it was u joint angles. The trans is 3 degrees down (pointing the slip yoke at the ground). The pinion is 5 degrees up (pointing at floor board) driveshaft angle is 5 degrees sloping down toward the rear. I even downloaded the tremec angle app. Could this be a motor mount issue? Any other ideas?

[andrewb70]

Car in question is a 1985 olds cutlass with a 5.3 tremec magnum and 4.10 7.5 ten bolt and new 3.5 steel pst driveshaft. So last summer was the first summer with the new trans setup. I had a vibration at about 75. It wasn't steady, more of a surging vibration, so I figured it was u joint angles. The trans is 3 degrees down (pointing the slip yoke at the ground). The pinion is 5 degrees up (pointing at floor board) driveshaft angle is 5 degrees sloping down toward the rear. I even downloaded the tremec angle app. Could this be a motor mount issue? Any other ideas?

Could be a lot of things! Having 4.10 gears really cranks up the drive shaft speed.

I would start by checking run-out and deflection starting from the slip yoke all the way back to the rear end. Also make sure that the pinion yoke tabs are not worn out and the U-joint fits snugly.

Andrew

[1985olds]

Thanks Andrew, I've just never felt a vibration like this. I figured based on how it was surging that it was u joint operating angle. Any other suggeations/ ideas are welcome

[dhutton]

What kind of trans mount are you using?

Don

[1985olds]

What kind of trans mount are you using?

Don

This summit brand rubber mount. I put it in New with the trans

https://www.summitracing.com/parts/s...1165/overview/

[andrewb70]

This summit brand rubber mount. I put it in New with the trans

https://www.summitracing.com/parts/s...1165/overview/

What swap components are you using?

Andrew

[1985olds]

What swap components are you using?

Andrew

Dirty dingo slider mounts and a homemade trans crossmember

[andrewb70]

Dirty dingo slider mounts and a homemade trans crossmember

Are the engine mounts rubber or poly?

Andrew

[1985olds]

Are the engine mounts rubber or poly?

Andrew

Old rubber now which is making me question them. I will be replacing them with poly inserts

[andrewb70]

Old rubber now which is making me question them. I will be replacing them with poly inserts

I suspect good, new rubber mounts will be smoother, but there is some debate about that. Plenty of people run poly mounts and don't seem to have vibration issues.

Andrew

[1985olds]

I suspect good, new rubber mounts will be smoother, but there is some debate about that. Plenty of people run poly mounts and don't seem to have vibration issues.

Andrew

I've heard that argument, but my friend who has a restoration/ performance shop said the poly makes a hug difference for the better. I can always swap them back out. Thank you for the heads up

[andrewb70]

I've heard that argument, but my friend who has a restoration/ performance shop said the poly makes a hug difference for the better. I can always swap them back out. Thank you for the heads up

As I said, there is debate about the use of poly mounts.

Andrew

[1985olds]

As I said, there is debate about the use of poly mounts.

Andrew

Got it, thank you

[ctcz28]

I've dealt with a pulsating vibration problems a couple of times with different transmission setups and each time it was driveline operating angles. Check out the document at the link provided below and work out the operating angles you have. If nothing else, this will confirm your measurements with the Tremec tool box angle finder. You can always make some adjustments to your pinion angle at the diff with some shims to see if changing the angles helps. Someone already suggested making sure the yokes are holding the U-joint cups tightly and there is no slop. I've had brand new yokes (no name junk from China) with such poor tolerances that the U-joint cups spun easily in the yoke and not on the needle bearings. Also check that you have not over-torqued the U-bolts holding the U-joints, 17 lb-inches max. (not lb-ft, lb-inches. Just flatten the lock washers plus 1/8 turn should work fine) If over-tightened, the U-joints will bind and this could cause vibration. Are you sure the driveshaft is properly balanced? I've had a new DS made only to find out that it was not properly balanced.

https://www.hurst-drivelines.com/fil...roc_111606.pdf

[1985olds]

I've dealt with a pulsating vibration problems a couple of times with different transmission setups and each time it was driveline operating angles. Check out the document at the link provided below and work out the operating angles you have. If nothing else, this will confirm your measurements with the Tremec tool box angle finder. You can always make some adjustments to your pinion angle at the diff with some shims to see if changing the angles helps. Someone already suggested making sure the yokes are holding the U-joint cups tightly and there is no slop. I've had brand new yokes (no name junk from China) with such poor tolerances that the U-joint cups spun easily in the yoke and not on the needle bearings. Also check that you have not over-torqued the U-bolts holding the U-joints, 17 lb-inches max. (not lb-ft, lb-inches. Just flatten the lock washers plus 1/8 turn should work fine) If over-tightened, the U-joints will bind and this could cause vibration. Are you sure the driveshaft is properly balanced? I've had a new DS made only to find out that it was not properly balanced.

https://www.hurst-drivelines.com/fil...roc_111606.pdf

I adjusted the pinion angle to 0 with adjustable upper control arms and left the trans where it is, we will see what happens if the weather ever breaks. Also with your suggestion, I verified the torque on the u joint bolts and checked the yoke (factory gm). Both seemed fine. I am going to trust the driveshaft is balanced for now unless I still have a vibration. Also I used the tremec app prior to adjustment, and it said my angles were good. We will see what happens. Thank you for the suggestions

[ctcz28]

From the data you've provided, I'm not sure your new adjustments will work. You don't state what the new driveshaft angle is. If the driveshaft angle remained the same (probably did not as you changed the pinion angle) the front operating angle is 2 deg and your new rear operating angle will be 0 deg. These should be equal, but opposite as described in the article to which I provided the link above. Let us know if your new adjustments work or not.

[1985olds]

Update....still have the same vibration, current angles are pinion up 3 trans down 3 and I don't remember the driveshaft, I believe it's 2.5 down. At 75 mph (2500rpm) its a surging vibration and gets worse with more speed.

[1985olds]

Would an uneven wheel base (1/2") difference cause a vibration?

[andrewb70]

Would an uneven wheel base (1/2") difference cause a vibration?

It can, because now the driveshaft is not square with the transmission output shaft, as viewed from the top (or bottom).

Andrew

[TheJDMan]

I purchased a brand new Strange driveshaft complete with 1350 U-joints and billet transmission yoke assembled. I fought a vibration for the first year or so until one day I noticed that I had play in the rear U-joint. Turned out that the rear U-joint had been installed incorrectly from Strange. I replaced the rear U-joint and problem was solved. Just don't assume everything is correct, check and double check everything.