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My 1990 Lariat XLT had a vibration problem that was driving me crazy -- see string under "1987-1996 F150 ..." asking for help. I eventually solved it myself. I became convinced that it was in the driveshaft/rear axle area, because having the driveshaft trued and balanced changed it, but didn’t eliminate it. When a suggestion that I check the pinion nut torque didn’t help, I started some serious research. Here is what I found.
Ordinary universal joints, “Cardan joints”, are not really a perfect way to transfer torque through an angle. That is why Constant Velocity Joints had to be invented in order to make front wheel drive (steerable) acceptable. The name implies that there is something INCONSTANT about the velocities associated with Cardan universals -- right? Because a normal U-joint in attached to the driveshaft and pinion shaft at only 2 fixed points each, 180 degrees apart, each yoke point moves back and forth slightly in relation to the line of the back axles when the joint is articulated (bent) and rotated. That back and forth motion gets translated into a slight change in velocity and therefore a slight hump in the torque transfer, which of course gets exaggerated the more torque you apply. Result -- the torque through an angle on a Cardan joint pulses. This has been understood since the 17th century! The rolling ball geometry of CV joints eliminates this problem.
Most live axle rear ends are set up with the pinion shaft (out of the Diff) slightly more horizontal than the driveshaft. This is so that when the truck is carrying a heavy load (which is when you need to apply the most torque), the truck sinks down on its springs bringing the pinion shaft and the drive shaft MORE in line, so the U-joint doesn’t really have to do anything much. However, this means that when not loaded (which is the condition of your pickup most of the time) the pinion shaft and driveshaft ARE NOT LINED UP.
Got that? OK. Second big discovery! All live axle rear ends have a certain amount of flexibility in them, resulting from two things -- the springiness of the leaf springs, and the flexibility of the various bushings. This allows the entire axle and differential housing (which is all one unit) to rotate slightly around the axle bearings. Viewed from the point of view of the yoke on the diff pinion shaft, this translates as up and down oscillation (rapid motion clockwise/anti-clockwise of the housing, around the axles). An oscillation is a rapid back and forth motion.
Bet you are beginning to see the problem. All live axle rear end engineers face this treat! Most of the time it works out that the torque pulse doesn’t sufficiently match any possible oscillation rate of the housing to cause a problem. However, there is such a thing as a harmonic. A vibration input does not have to MATCH the rate at which the housing (or violin string, or whatever) wants to vibrate -- it can be a multiple or divisible of that. So there are a variety of “matches” possible. As your truck ages, the springs and bushings change somewhat in their stiffness and springiness, get more slack, and therefore are susceptible to a wider range of vibration rate inputs which might set up sympathetic vibrations, defeating the original engineer’s best efforts to damp them.
DAMPING. Now, there are three ways to fight this problem. What you have to do is change the rate at which your housing likes to oscillate around the axle bearings. Strategy #1 -- hook up a shock absorber between the housing and the truck frame to resist the rotary motion. The Explorers around the turn of the century (I think it was) use this method to damp out a nasty harmonic tendency. The second way is to attach a pendulum to the housing -- a rigid lever with a weight on the end of it. You point it in the general direction and plane of the driveshaft, so that for the housing to oscillate, it has to shake this pendulum up and down, which requires a lot of energy. You can flick a pencil back and forth between your fingers because it has little weight to it, but try that with a hammer! The result is that the housing can’t move fast enough -- you have drastically lowered the oscillation rate of the housing, so now it is unaffected by any harmonic rate of the torque pulse input. The third way is to use wedge shims between the shoe welded to the axle housing and the leaf spring that it sits on so as to straighten the angle between the pinion shaft and the driveshaft in the unloaded condition (least effective, because now the angle gets more acute in the other direction as you load the truck, probably creating a worse problem under load).
Soooo. There is a flat shelf cast into the diff housing of my Ford, with 3 bolt holes in it. Looks like an invitation to a pendulum damper. I happened to have a 16in. piece of 2” x 3” x 1/8” angle iron kicking around. I marked and drilled 3 appropriate holes in it and bolted it to the shelf, pointing along beside the driveshaft, but well clear of anything that suspension travel might bring it into contact with. I figured if it wasn’t heavy enough in itself, I would drill a hole in the forward end of it, and bolt on additional weight. However, the cure was immediate and spectacular -- vibration completely gone -- smooth as a baby’s ***! If your diff does not have that convenient mounting shelf, you can always fasten a hunk of angle iron to the housing anywhere convenient with a couple of spring shackle u-bolts.
I am willing to bet this will fix MOST rear end vibration problems in live axle trucks. The exact weight of the pendulum you use does not matter, because all you have to do is LOWER the oscillation rate significantly -- break the cycle. The secret is that rear ends WANT to vibrate because of the torque pulse in Cardan joints and the oscillation rates of the "springy" components. Its a natural tendency which has to be damped out one way or another. There is nothing "wrong" with your truck -- the damping components of your rear end (mostly bushings) simply have become a bit sloppy.
Let me know if this works for you by replying to this string.
I'm pretty sure my 1964 Meteor has a big nasty damper like the Galaxie.
These cars are long and low, the drive line is almost in a straight line, which is horrible for u-joints.
OK -- I get it. So the clue here is that the transmission tail shaft and the diff pinion shaft have to be parallel or you don't get the cancellation effect. What is probably happening on an older or worn truck is that fatigue in the bushings or changes in leaf springs (maybe slightly uneven fatigue?) are letting the axle housing rotate a bit and get the pinion shaft out of parallel. Wwhite says no need to reinvent the rear end damper weight. True, but if you don't have one, my hunk of angle iron was a quick fix. Hey -- it worked!
Thinking about it -- if your whole setup is getting a bit sloppy with age, the pinion shaft is probably moving in and out of parallel with your tail shaft as you apply torque, since the application of torque to the rear axles would tend to rotate the housing. The whole purpose is to rotate the wheels relative to the car, so, provided the wheels have traction, the wheels are trying to rotate the axle housing (and the whole car -- hence the possibility of a wheely). It's pretty obvious that putting your foot on and off the gas is going to upset that parallel setup unless everything is really stiff and firm. Hence if you have a vibration which changes with torque, it is due to rotation of the housing around the axle bearings, pulling the pinion shaft out of parallel. If the vibration changes only with speed, it's something else -- usually a balance problem. Application of a damper weight would be a sort of brute force cure. Ideally you should replace all the fatigued parts, but hey -- if it works, a damper could cost you virtually nothing.
I'm pretty sure my 1964 Meteor has a big nasty damper like the Galaxie.
These cars are long and low, the drive line is almost in a straight line, which is horrible for u-joints.
87-93 Mustangs had the damper in pic, on their 8.8's
I made one error here. The purpose of using the shims is not to aim the pinion shaft in line with the driveshaft, but in line with the tail shaft of the transmission. That way the fore and aft Cardon joint pulses stay in phase and cancel each other -- see response of wwhite. The problem with a older or worn truck is that the rear axle housing mountings -- spring shackle bushings etc. -- have become sloppy enough to allow the housing to rotate a bit around the axle bearings, pulling the pinion shaft out of line with tail shaft. Fooling around with wedges won't likely help much if this is the problem. A damper weight is likely to be more effective. In my case it worked.
That's only masking the problem, as you say the real problem is probably loose axle housing mountings;; spring shackle bushings etc. are the cause of the slop and cause of the vibration.
Not talking about going round and round -- just a slight motion clockwise and then counterclockwise, back and forth. True, the axles go round and round inside the housing, but think of the vehicle stopped with the nose up against a brick wall. You put it in gear, and the torque on the driveshaft cant turn the wheels, so it tries to turn the whole housing the other way. If you had someone step on and off the gas, and looked underneath, you would see the pinion shaft end of the diff housing move up and down a bit as it "rotates" around the axles. If you had enough power, and nothing broke, you could "rotate" the whole truck around the axles and pick the front right up off the ground. If the rear end bushings and stuff are getting fatigued and mushy, they can allow quite a bit of that wrong kind of "rotation", which can develop into an oscillation, or "hopping" of the pinion shaft. Putting a relatively heavy damper on it does mask the problem, true, but if there is still a potential oscillation there, and it is damped out so I can't feel it, who gives a damn? The vibration really is gone, even if the bushings are still less than optimal. It's an imperfect world we live in -- the trick is to "mask" the imperfections -- hey, I like that! Maybe I will adopt it as my motto.
Ya -- it's called axle wrap I guess. Sorry, I am new at this terminology. The idea must be that the housing tries to "wrap" itself around the axle -- rotate around the axle -- instead of the axle turning inside the housing. To every action there is an equal and opposite reaction. In order for parts of the truck to turn the wheels, something has to prevent the wheels from turning parts of the truck.
That’s why people used to put traction bars on cars.
As the front half of the leaf spring bends up from the torque the snubber contacts the spring eye. The spring loads and unloads since it isn’t supposed to be shaped like an “S” which can also cause wheel hop.
As was said the damper you made could have probably been sourced from a junkyard but there was reason that it didn’t have one from the factory. You should have determined and probably still should figure out what components are worn out allowing for the vibration.
Are you talking about axle wrap? 
