vibration dilemma

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wireweld

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When I accelerate up to about 3500 rpms it is smooth as glass, but after 3500 it starts to vibrate. At 4000 and up it is shaking the whole car and I could only take it to 4500 cause the vibration is too much. It vibrates on decel down to 3500 and then it is smooth again.
The driveshaft is 52.75 inches center to center, 3 inch steel tube, 1350 crosses. I have checked pinion angle and it is good.
Is the diameter of the tube too small and whipping at the high rpms?
I have no clue what to check.
This is on a 72 dart. 650 hp smallblock and a 904 and a 8 3/4 with 2.94 gears.
 
Welcome to my world. I have balanced tires, rebalanced my driveshaft, checked harmonic balacer, changed to electric fans, etc ... I am thinking I have a slightly bent axle. Uncommon in a Dana, but it does it. The decell part is weird. I can be doing 70 to 80 and it might be fairly smooth with only a light vibration and when I let off the gas it's like hitting a rumble strip until it slows down. At 80 it vibrates, 90 it shakes, 100 i'm getting worried about staying on the road, and 110 it is damn near undriveable. I am taking mine to Sharadon Performance to have them check it out and go through my rear end.
 
U-joints centered, fit correctly.
Proper slip yoke engagement in the trans (deep enough)
Drive shaft run out in the car.
drive shaft balance.
pinion angles.

According to the chart all other things correct a 53 in long 3" shaft should have a critical speed at or above 6000 RPM. BUT I have personally seen shafts with critical speeds lower than the chart depicts.

With those gears you must be going 120mph at 4500rmp.

If I had a 650hp motor I would seriously be rethinking the 3" shaft, and a drive shaft loop. Give Denny's drive shafts a call.
 
The shaft sounds as if the balancing isn't right, who did the shaft?

The 3" tube is good to up to 6 at the length you're at or another 500 more depending which wall tube it is.

See your local driveline shop and have it rebalanced
 
Supershafts said:
The shaft sounds as if the balancing isn't right, who did the shaft?

The 3" tube is good to up to 6 at the length you're at or another 500 more depending which wall tube it is.

See your local driveline shop and have it rebalanced
Click to expand...

I have the same vibrations. I suspected my driveshaft as well, even though I had it built and it had maybe 1500 miles on it. I had it checked and it was good. I agree it sounds like that, but in my case it was not.
 
Supershafts said:
The shaft sounds as if the balancing isn't right, who did the shaft?

The 3" tube is good to up to 6 at the length you're at or another 500 more depending which wall tube it is.

See your local driveline shop and have it rebalanced
Click to expand...

My local driveshaft place did the shaft. I have had it rebalanced. I don't think they can do high speed balancing. I am wondering if I should get a shaft that has been high speed balanced?
Will a bent axle shaft do this? The axle I am using are shortened and resplined c body axles.
 
The fact that it gets worse when you let off the gas tells that there is play in something.
If it was ONLY a balance problem it would be the same all the time.
 
TrailBeast said:
The fact that it gets worse when you let off the gas tells that there is play in something.
If it was ONLY a balance problem it would be the same all the time.
Click to expand...

The vibration doesn't get worse on decel it is not as severe as at 4500.
It is lessening as I decel until I get to 3500 and it goes away. Vibration is increasing as I accel past 3500.
 
Sorry about that, I misread it.
It does kinda sound like an off center axle, but also could be driveline deflection like mentioned.

wireweld said:
The vibration doesn't get worse on decel it is not as severe as at 4500.
It is lessening as I decel until I get to 3500 and it goes away. Vibration is increasing as I accel past 3500.
Click to expand...
 
I guess since it starts suddenly at 3500 rpm, some parts must be hitting a "natural frequency" where they want to oscillate. If so, it would have to be something with a springiness to it. Maybe the driveshaft could do that by the tube vibrating side-side, though I have never dealt with that. Of course, tires and rear suspension could also vibrate like that.
 
Could be as simple as a separated belt in a tire.


BillGrissom said:
I guess since it starts suddenly at 3500 rpm, some parts must be hitting a "natural frequency" where they want to oscillate. If so, it would have to be something with a springiness to it. Maybe the driveshaft could do that by the tube vibrating side-side, though I have never dealt with that. Of course, tires and rear suspension could also vibrate like that.
Click to expand...
 
I bought a new driveshaft and installed it. The vibration decreased but was still there. I then checked everything over and noticed that my fan clutch didn't look centered. I then used a dial calipers to center it and OMG did that make a difference. I have taken it to 5 grand and all is wonderful.
Who would of thought.
 
Not me. I thought fan clutches had a small hole to self-center on the water pump. Surprised you could even feel it in the cabin. Think of the stress that put on the water pump bearing. Good that didn't fail or your radiator would be toast, aka my 65 Dart when Ii bought it.
 
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