Vibrations at 80+

[OldZeuski]

It's definitely within spec. I need to actually measure my Ford and LMR shafts to see the difference. I had them side by side and it was obvious. I'm currently nursing a total knee replacement and plan on installing the TKX on Aug.1st. I'll measure the shafts then.

I’m curious, has your car had this similar vibration since you purchased it or did it develop over time?

 

[dsrtjeeper]

I’m curious, has your car had this similar vibration since you purchased it or did it develop over time?

It was bone stock when I bought it and it didn't vibrate. I installed 4.10 gears and it vibrated. This is a very common theme. The driveshaft picks up speed with the higher numerically gears. That's when any little issues rear their ugly heads. I even switched gear brands and that didn't help.

 

[OldZeuski]

It was bone stock when I bought it and it didn't vibrate. I installed 4.10 gears and it vibrated. This is a very common theme. The driveshaft picks up speed with the higher numerically gears. That's when any little issues rear their ugly heads. I even switched gear brands and that didn't help.

So you see it at 80 mph tho?

I’ve got 3.73. But 3k rpm in 5th is the same whether you’ve got 2.73 or 4.10 as far as the drive shaft is concerned. That said, 3k in 5th with 2.73 used to be like 120 mph or something.

Hmmmm……

 

[dsrtjeeper]

So you see it at 80 mph tho?

I’ve got 3.73. But 3k rpm in 5th is the same whether you’ve got 2.73 or 4.10 as far as the drive shaft is concerned. That said, 3k in 5th with 2.73 used to be like 120 mph or something.

Hmmmm……

For every revolution of the ring gear, the driveshaft spins 4 times with 4.10 gears. 3.73 gears would have less driveshaft rotations per one revolution of the ring gear. Stock 2.73 gears would have less driveshaft rotations. Right?

Yeah, mine starts vibrating around 80.

 

[OldZeuski]

For every revolution of the ring gear, the driveshaft spins 4 times with 4.10 gears. 3.73 gears would have less driveshaft rotations per one revolution of the ring gear. Stock 2.73 gears would have less driveshaft rotations. Right?

Yeah, mine starts vibrating around 80.

For a given speed yes, more rpm and therefore more driveshaft rotation is required for higher rear ratios.

I’m just thinking out loud.

 

[dsrtjeeper]

For a given speed yes, more rpm and therefore more driveshaft rotation is required for higher rear ratios.

I’m just thinking out loud.

The higher driveshaft speed seems to be key. That's why I suspect the driveshaft is whipping like it did on my '95.

 

[96blak54]

Help me get caught up here,

The previous driveshaft had the same vibration?

Is the new driveshaft using any driveshaft component from the last driveshaft?

 

[96blak54]

Is the trans mount strong?

Is all of the exhaust hangers good?

If you have exhaust dumps before the driveshaft, be sure the exhaust hangers are strong and at the exhaust end, like at the dumps. Crazy exhaust vibrations at speeds are a thing.

 

[OldZeuski]

Is the trans mount strong?

Is all of the exhaust hangers good?

If you have exhaust dumps before the driveshaft, be sure the exhaust hangers are strong and at the exhaust end, like at the dumps. Crazy exhaust vibrations at speeds are a thing.

From day 1 it had drive train issues. I eventually measured the runout on the drive shaft after I replaced all of the suspension, and found that it was bent (i forget how much, im not even sure it was out of spec to be honest). So I bought a new aluminum shaft. New yoke. New u joints. And when it still vibrated I had the shaft balanced. Then I had it balanced again at another shop because I wasn’t convinced.

New trans cross member mount (OE rubber), new motor mounts, rebuilt trans, new full exhaust with hangers.

I’ve been able to measure the vibration so I know for a fact it’s the drive shaft. But why? I have no idea.

 

[OldZeuski]

I had to downgrade the image quality but you can see a strong peak at 71hz. At the speed I was going that is Exactly the driveshaft rpm. The other peaks also match exactly the 5th gear ratio (0.68) and the rear tire rpm. So my data is good, and it points directly at the shaft.

I’ll gladly throw another drive shaft on here but it’s just hard to believe.