I'm getting a vibration in the car, 2500 rpm range, maybe around 60mph and up. 55 Victoria, 272, Mustang 3 spd, stock rear end 3.30. Been reading some articles on bad harmonics at a certain rpm range, and I'm getting close to that (2800), at 55 the car is ultra smooth, but 60+ and it gets very noticeable. Wheels and driveshaft have been checked for balance, spot on. My engine builder had me check for vibration in neutral, up to 3500 very smooth, does that rule out the harmonics issue, I'd like to have this solved before it gets put away for the winter. Thanx, Gary

       

Gary, 55 Vicky, St. Germain, Wisconsin

 

Although you don't say, is this a recent occurrence? The wheels may be balanced but are they round? Tires can develop problems resulting in radial run out for example. That's pretty easy and cheap to check. Turn each wheel by hand and measure or observe radial or lateral anomalies. Have you tried to run it up to say, 70 mph then kick it into neutral and coast? That might give a clue as to where to look. Any unusual sounds associated with the vibration? Kind of grasping at straws here- so many possible sources.

Have you checked for looseness at the rear transmission yoke? A worn bushing in the transmission tail shaft and/or a worn yoke can cause some out of balance in the drive train.

Lorena, Texas (South of Waco)

Did this start after recent transmission or engine work. If you had the driveshaft out try disconnecting it and rotating it 180 degrees and re-install. I've had a problem a couple of times in my years that this has fixed. My shaft guy says its like they are out of phase. He didn't know why it happens but it can show up on a brand new perfectly balanced shaft. Hope this helps. CHuck

Y's guys rule!
Looking for McCullouch VS57 brackets and parts. Also looking for 28 Chrysler series 72 parts. And early Hemi parts.

MoonShadow, 292 w/McCulloch, 28 Chrysler Roadster, 354 Hemi)
Manchester, New Hampshire

Had a 57 Custom that developed a vibration. Checked everything including the U-joints. Kept driving it and the vibration got worse and louder. It WAS the U-joint, but would lock in place when checking originally and only because it was so bad you could see the offset was it determined to be that. It was really hogged out, so it would seem to be easy to find, but was checked several times without identifying it.

The output of a U-joint (when operating an an angle) will have an RPM which increases and then decreases with respect to the input for each revolution. This causes a vibration frequency of 2 x RPM of the drive shaft. For a drive shaft with a U-joint on each end and out-of-phase with each other, this rotational unbalance will be absorbed by torsional deflection in the drive shaft tube. If they are in-phase, the rotational unbalance will be additive and you will feel it as a vibration at 2 x engine speed (when in 1:1 high gear). At 2800 engine RPM, this would be 5600 cycles/Min or 93 cycles/Sec. There would be some vibration at any engine speed, but the reason you feel it most at 2800 RPM is because it is in resonance (matches the natural frequency) of something else in the vehicle. Most other vibrations in a vehicle occur at 1 x engine speed or at wheel speed.

Very good explanation of the phaze but what about the 180degree rotation that helps with vibrations? I suppose manufacturing variations in the joints and couplers could account for it. Chuck

Y's guys rule!
Looking for McCullouch VS57 brackets and parts. Also looking for 28 Chrysler series 72 parts. And early Hemi parts.

MoonShadow, 292 w/McCulloch, 28 Chrysler Roadster, 354 Hemi)
Manchester, New Hampshire

Sorry, I should have worded it more clearly. The two U-joints being rotated 180 Deg from each other is what I was referring to when I said, "For a drive shaft with a U-joint on each end and out-of-phase with each other, this rotational unbalance will be absorbed by torsional deflection in the drive shaft tube."

I understood what you were saying. Its a different question. For example, I had a new drive shaft made for my Dodge van. When I first drove it it had a cyclic vibration almost all the time. I went back to the shop and they checked the shaft. The balance was dead on so the guy put it back in rotated 180 degrees from the previous installation. He had marked the carrier and shaft when we took them out. The vibration was gone! Thats what I was wondering about. Apparently this is common enough that the shop was familiar with it but couldn't explain why. Chuck

Y's guys rule!
Looking for McCullouch VS57 brackets and parts. Also looking for 28 Chrysler series 72 parts. And early Hemi parts.

MoonShadow, 292 w/McCulloch, 28 Chrysler Roadster, 354 Hemi)
Manchester, New Hampshire

Was the driveshaft modified for that three speed conversion? If so, then insure that the U-Joints were clocked or aligned with each other when any welding was performed. The balance of the shaft is yet another consideration.

Lorena, Texas (South of Waco)