[gkraushaar]

I have had a general car vibration from 45-50 mph only since I have owned it(2 yrs). Smooth before 45 and over 50 mph regardless of surface. Wheels are balanced, alignment done i can trust, drive shaft balanced done 2x by different shops, new tires, never been hit, has 2 sway bars, drums in back , disks in front. New bearings in the diff. (3.08)
The whole car shutters slightly, even when the clutch is in or out.
Places to look: check the angle of the 4 sp trans, the driveshaft, the diff angle? Needs to be 1.5-3 degrees? Could a bad control arm bushing affect it?
Any other ideas?
Thanks

 

[O52]

Check your motor and transmission mounts. Some replacement transmission mounts may be too tall throwing off your driveline angle

 

[ponchonlefty]

could it be the idle arm? other than that i agree the drive line angle may be the culprit.

 

[BearGFR]

.... the driveshaft, the diff angle? Needs to be 1.5-3 degrees? ....

Drive shaft - UJoint operating angle. The centerline of the transmission output shaft and the centerline of the pinion shaft in the rear end must be parallel with each other, within 1/2 a degree or less, and that means parallel in BOTH planes: "up and down" and "side to side".

Videos that do a good job of explaining things:
Theory:

Theory:

HowTo:

Important. Nearly every source of reference/instructional material you find on the "inter-tubes" talks only about the vertical plane - the "up and down" angle - and this is the adjustment that "everyone" focuses on. However, even if you have this alignment perfect, if the alignment is off in the horizontal plane (side to side) you'll still get a vibration.
In other words, if things are "off" like this:

The two centerlines do not have to be pointed "directly at" each other, but they do have to be parallel within 1/2 a degree.
If they're off, you either have to move the tail of the transmission "sideways" to correct it, or you have to change the alignment of the rear axle.
Changing the alignment of the rear axle is the easier option of the two IF you have adjustable LOWER control arms. Always make these kinds of changes with the lowers, not the uppers.

The difficult part of alignment in the horizontal plane is getting an accurate measurement because your "up/down" angle finder can't help you with this one. You have to get creative and do something like using a plumb bob to create reference points on the floor (or perhaps on your lift ramps if you have a 4-post lift) at "both ends" of the car, or maybe "creative" use of one or more laser pointers, or using your high school geometry figure construction techniques --- something to construct two lines, one at the front, one at the rear, that are exactly 90-degrees to the crank/transmission centerline and the pinion centerline, then measuring the distance between the lines, front to back, on both sides.
Something like this:

You want the measurements on both sides to be the same.
As you can tell, if you don't get the two lines at the front and rear of the car "right", your long measurements on the sides won't tell you the truth.

If you end up having to make changes in either plane, it can affect the alignment in the other plane so it's best to keep measuring and adjusting BOTH until you get both of them right.

Yeah - it's a painstaking process, but worth it.

Bear

 

[armyadarkness]

I have had a general car vibration from 45-50 mph only since I have owned it(2 yrs). Smooth before 45 and over 50 mph regardless of surface. Wheels are balanced, alignment done i can trust, drive shaft balanced done 2x by different shops, new tires, never been hit, has 2 sway bars, drums in back , disks in front. New bearings in the diff. (3.08)
The whole car shutters slightly, even when the clutch is in or out.
Places to look: check the angle of the 4 sp trans, the driveshaft, the diff angle? Needs to be 1.5-3 degrees? Could a bad control arm bushing affect it?
Any other ideas?
Thanks

There was a recent discussion on flywheels. Based on what I took away from it, you could easily have an isolated vibration from that. How old is your clutch/ flywheel and what did you install?

 

[gkraushaar]

There was a recent discussion on flywheels. Based on what I took away from it, you could easily have an isolated vibration from that. How old is your clutch/ flywheel and what did you install?

Unfortunately, I bought it like this and don't know what I have. Thanks for the effort

 

[gkraushaar]

Drive shaft - UJoint operating angle. The centerline of the transmission output shaft and the centerline of the pinion shaft in the rear end must be parallel with each other, within 1/2 a degree or less, and that means parallel in BOTH planes: "up and down" and "side to side".

Videos that do a good job of explaining things:
Theory:

Theory:

HowTo:

Important. Nearly every source of reference/instructional material you find on the "inter-tubes" talks only about the vertical plane - the "up and down" angle - and this is the adjustment that "everyone" focuses on. However, even if you have this alignment perfect, if the alignment is off in the horizontal plane (side to side) you'll still get a vibration.
In other words, if things are "off" like this:
View attachment 154718
The two centerlines do not have to be pointed "directly at" each other, but they do have to be parallel within 1/2 a degree.
If they're off, you either have to move the tail of the transmission "sideways" to correct it, or you have to change the alignment of the rear axle.
Changing the alignment of the rear axle is the easier option of the two IF you have adjustable LOWER control arms. Always make these kinds of changes with the lowers, not the uppers.

The difficult part of alignment in the horizontal plane is getting an accurate measurement because your "up/down" angle finder can't help you with this one. You have to get creative and do something like using a plumb bob to create reference points on the floor (or perhaps on your lift ramps if you have a 4-post lift) at "both ends" of the car, or maybe "creative" use of one or more laser pointers, or using your high school geometry figure construction techniques --- something to construct two lines, one at the front, one at the rear, that are exactly 90-degrees to the crank/transmission centerline and the pinion centerline, then measuring the distance between the lines, front to back, on both sides.
Something like this:
View attachment 154719
You want the measurements on both sides to be the same.
As you can tell, if you don't get the two lines at the front and rear of the car "right", your long measurements on the sides won't tell you the truth.

If you end up having to make changes in either plane, it can affect the alignment in the other plane so it's best to keep measuring and adjusting BOTH until you get both of them right.

Yeah - it's a painstaking process, but worth it.

Bear

Yes, that is the next area I'll focus on... challenging on the horizontal measurements.
Thanks for the ideas.

 

[ponchonlefty]

hope you get it sorted out. good luck.

 

[PontiacJim]

Yes, that is the next area I'll focus on... challenging on the horizontal measurements.
Thanks for the ideas.

Don't discount a bad/loose wheel bearing. Check your fronts for play. Rear wheel bearings are another sore spot that are overlooked. GTO's take a lot of abuse from all the beatings those rear bearings can get when smoking tires and jack rabbit starts.

 

[ponchonlefty]

i would check everything. before throwing money at it. shake the tie rods,idle arm,wheel bearings,check the drive shaft clips, check the rear trans bushing. make sure there is enough yoke in the trans. check to see if the yoke is worn. old cars may have or may not have the correct driveshaft.hope this helps.