Hey Guys...I have a driveline vibration for a couple years now, that won't go away, let me tell you what going on. 1938 Plymouth coupe, slant six, 904 tranny, Gear Vendors Overdrive,8.8 rear 3:73 gears. A couple of years ago I put in a overdrive system, that's when my problem started. I have had the drive shaft back 2 times and they insure me that's is not the shaft. I check and recheck pinion angle 5* up and engine angle 5* down. Before the overdrive unit I had NO vibration. now when the power is on or giving it the gas its ok, when foot is off the gas or on coast that's when the vibration starts. Any help would would be appreciated. Mike
Pinion bearings or just right axle wrap. Adding the OD shorts the drive shaft and that changed the working angle of the Ujoint.
Could be your front and rear angles no longer match, or both are excessive....for the reasons stated above. Measure the working angles of both joints and post them.
I'm not suggesting this, BUT, I had a driveshaft made for my 36, it was in 20 years, never out. For 20 years I've had a shake in the steering wheel at 60 mph, could never find it. After going all out, 4 wheel alignment, new torque thrusts and new tires, still there, everybody stumped. My buddy suggested lifting it and crawling under to see what we could see......bent driveshaft, ever so slight, barely visible, but bent right from the get go. At idle in gear, a pencil held close, very slight, but just enough.
What changed with the install of the overdrive ? Pinion angle? Trans to diff angle? Drive shaft to long/short? Universal joint phasing? Something in the od unit itself? Driveshaft bent?—- but with you getting this checked, I should hope it would have been seen in the balancing process.
Lots of info on line a bullit level, tape measure and angle finder are all you need to confirm it’s right or wrong
I have a gear vendor unit in my dually hauler. The rear bearing went out of it and displayed symptoms similar to yours.
You should draw a picture If your engine is down hill 5* and drive shaft is up hill 5* that’s gonna be 10 * working angle. The pinion needs to match that and you can expect to wear out ujoints and replace them frequently since 10* is beyond established limits
Exactly. Is the 5 degrees OP posted before or after OD unit. Well that naturally is still the same but!, u-joint angle did increase statically. Now under load, well the u-joint angle decreased and vibration went away.
Yes, it makes matters worse. The notion that the engine and transmission slope down to the rear, and that the pinion slops up to the front is pertinent to the pinion being below the transmission output shaft. The oft told 3º/3º mantra is shorthand. Those are the measured STATIC angles. Only the working angles really matter. The 3º shorthand references the ground. Since you are not turning the Earth, that angle doesn't matter. The angles between operational components are the only ones that matter. Those are the working angles. Make a shim that puts your engine and transmission combo level with the ground, with all of the weight on the suspension. Get leaf spring perch shims that put the pinion level, with all of the weight on the suspension. Report back.
Agree a picture would help. I don't think going up is an issue if the angles are the same for trans out and pinion in (ideally parallel) or as along as U-joint working angles are respected which I think is generally 5-7 degrees max. Assuming drive shaft was made shorter, the working angles would have increased. It might have increased beyond that of the U-joint specs. Also, is there room to allow the driveshaft to slip on the yoke as it works throughout the suspension travel range? If not, it could cause binding?
If you look at post #9, you will see what I mean. There are two 3º angles in that diagram. If you put on shorter tires in the front, and taller tires in the rear, neither of those two angles would still be 3º. Don't fall into the trap that this breaks the science here. By swapping those tires, you would be doing nothing to alter the working angles (cue the pedants arguing about static weight transfer, and spring loading). We use the 3º/3º shorthand to denote that the working angles need to be equal, measured at both ends of the driveshaft. Trying to adequately describe absolute values, and angle deviation, on top of working angles, slope, etc., leaves people spun. It is easier to use the "chassis level, trans 3º down to the rear, pinion 3º up to the front" mantra. Using the diagram in post #9, you will see that all three slopes (engine/trans, driveshaft, pinion) start high on the left, and slope down to the right. All of the slopes agree. You don't have that. Your engine/trans, per that diagram, are high on the left, and low on the right, as is your pinion. Your driveshaft is low on the left, and high on the right. Mitigate that, and you will be vibration free.
WAY exaggerated for emphasis. The shorter the driveshaft, the worse it will be. Where I have circled are the u-joints. The intersection of the lines that represent those components are the working angles. They need to be equal at both ends of the shaft, and as close to 0º as possible. All conventional cross-style u-joint driveshafts shake, even if you cannot feel it, as they do not operate at a constant speed, if all of the operating angles are not zero. The design of the joint cause the shaft to speed up, and slow down. By having equal working angles, the driveshaft vibrations are (mostly) cancelled out. I say mostly, because the farther the working angles are from zero, the worse the shake will be, and the shorter the joint lifespan will be. Initially, the rubber mounts on the engine and transmission, and the suspension mounts will fully absorb this vibration. As angles are made worse, the will not be able to fully absorb the vibration. It will be worse at certain speed windows. Given the cumulative resonant frequencies of an automotive chassis, it is not unusual for the vibration to arrive at a certain speed, and disappear at another.
Ok I also had a vibration after my build and have managed to get it to a very slight vibration, but I have the un-good option I. The picture and I cannot shim the rear of tranny up to correct the angles , what would you do in that case? Sent from my iPad using H.A.M.B.
To further obfuscate matters regarding working angles. We are using shorthand there too. The perfect working angle is no angle. That angle is not 0º. It is 180º, indicating a straight line. When we are talking about working angles, we are talking about degrees-of-deviation from 180º.
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