Giubo Drive line vibration at 100MPH+

[crashmctavish]

Gents & Ladies:
My GTJ with TS is silky smooth at 80mph but starts to vibrate above 100.
Heavy duty doughnut fitted, new UJS & propshaft greased, wheels are balanced.
What is your experience, do the drive rubbers get excited at these speeds?
Anybody been up to 130 140? the motor on mine seems to have some left to go at 120.

Of course all of the above was completed on a track day.

[BigD]

Just a thought. Is your propshaft assembly one of the early types with the weaker centre mount arrangement?

[crashmctavish]

Good question: No its a 1976 car & I fitted a new heavy duty center bearing and support. Are you thinking the same as me that the drive shaft develops a whip at high speed?

[BigD]

Had the same problem on an old Jag with a similar propshaft arrangement. Had it rebalanced by someone who specialised in drivelines and made sure it went back on straight, i.e. a straight line along the centreline of the shaft from gearbox to diff. There was room for some adjustment in the lateral positioning of the centre bearing assembly.
Of course, tired doughnuts wouldn't be helping at all. Are all relevant parts in good order?

[BigD]

P.S. Used to have a '74 GTV 2litre that showed 120mph on the clock a few times and never experienced such vibrations.

[slyalfa]

There is a trick that Tom showed me years ago.
No mater what there seems to be a bit of out of balance left after a rebalance.
mark the shaft in to quarters on each end of each part.
run on jack stands. I removed the tires. but put on lug nuts tight to keep the disk in place(might need spacers if the threads stop short?).
run in 5th notice the vibration. now put on a hose clamp on one end say the front. Start on the first quarter mark run it in 5th is the better or worse. try all 4 settings find the best spot. then add a clamp to the next section and find the best spot. the small weight of the screw seems to be just right for the last bit of balance.

[genericwood]

We routinely run race 105's at up to 130mph or more. A properly maintained, balanced and aligned driveshaft should be fine at those speeds. Definitely check the alignment of the front section. Unlike a u-joint, you don't want any angle in the giubo. You can fine-tune the adjustment by spacing the transmission mount or intermediate bearing mount down with a washer or two. Also, make sure the tranny mount is installed properly and in good condition. Finally, make sure the driveshaft sections were reinstalled properly after the balancing. They can easily be put together 180 degrees off.

Erik

[crashmctavish]

Cheers Erik
Thats the answer I was looking for. 130 no problem.
You may be right about alignment, the TS motor & gearbox is pretty much where it landed on the mounts!
Will let you all know when I get to the bottom of it.
Cheers Keith