Visitor # 79454 since 28.AUG.2001
Have you noticed a vibration or rumbling noise when you are driving down the highway? If you have a lot of miles on your truck, have modified the suspension or drive train in any way, you may be experiencing driveline vibration.
From what I've researched, ideally you want the two ends of a double-ujoint drive shaft within 1-2 degrees of each other for maximum u-joint life and minimum vibration. This is actually the operating angle (under load) and not the angle of the drive shaft to the u-joints themselves (that has its own limit).
Since the rear pinion moves up under acceleration (unless you have anti-wrap control on the axle) ideally you set up the static pinion angle to be 1-2° below the transfer case output flange angle. This way, as the pinion twists up, it comes into a good alignment with the transfer case. Typically, this figure won't be listed in any service manual, but I did find a reference from Ford that lists a static angle of difference of 1.7°.
In my case, I had installed a 1.5" longer shackle and a 3° shim to compensate for the extra tilt of the shackle. I never measured the angles at the time. Later, I did measure and found even with 3°, I was still 1° above the transfer case and I needed to add another 2-3° to get me passed zero and into the desired range. So I have to conclude that originally my pinion angle was off with the stock shackle as well. Driving experience also confirmed this, I had drive line vibration under load (pinion tips up), but it would go away under coasting conditions (pinion tips down).
So my point is to measure what you have now and see if its OK and how much will it change with a longer shackle.
I recently installed a CV-style rear drive shaft and had to tip the pinion up to point directly at the transfer case, so there a longer shackle works to an advantage. I calculated I would need an 8° shim with my 7" (3.5" longer than stock) shackle, but after installing everything, found my pinion was about 2° high, so I made a custom 5° shim to place it 1° below the transfer case line. If you need a custom axle shim, I may be able to help you out.
The universal (or u-) joint is considered to be one of the oldest of all flexible couplings. It is commonly known for its use on automobiles and trucks. A universal joint in its simplest form consists of two shaft yokes at right angles to each other and a four point cross which connects the yokes. The cross rides inside the bearing cap assemblies, which are pressed into the yoke eyes. One of the problems inherent in the design of a u-joint is that the angular velocities of the components vary over a single rotation.
The universal joint was actually invented around 300 B.C. by the ancient Greeks. It was later re-invented in the 16th century by the Italian physicist Geronimo Cardano who used it as a mounting gimbel for holding a ship's compass horizontal in rough seas. Finally, it was re-re-invented in the 17th century by the English mathematician Robert Hooke who used it in its common form for transmitting torque. If that name sounds familiar, that is because he also is famous for Hooke's Law, which states that the stress in a spring is proportional to the strain (i.e. the spring rate). Where would 4-wheeling be without springs and u-joints? And I guess Hooke fooled around with a microscope looking at plant cells or something. So for some reason, the term "Cardan Joint" has stuck and is used interchangeably for a universal joint.
And note, it is "Cardan", not "Cardigan" which is alternately a sweater or a Swedish singing group :)
CV (or Constant Velocity) joints are a class of joint which are designed to eliminate the variation in angular velocity that plagues u-joints, thus they are given the name Constant Velocity. The simplest CV joint is simply two u-joints connected end to end, usually the center section is called an H-yoke because of its shape. In this manner, the angular velocity variations of one joint are canceled by the joint on the other end. Since there are two joints, the operating angle capacity of the double cardan joint is twice that of a single cardan joint.
More complicated CV joints utilize a multi-ball bearing assembly that rides inside a cup-shaped housing that allow the center section to rotate in a different orientation than the outside part. Further variations on CVs allow for "plunge" or in and out travel of the center section relative to the outer section. A combination of the two is often used in FWD applications, where a plunge-type CV is used on the transaxle and a ball bearing CV is used on the outside. The plunge capability allows the drive axle to lengthen and shorten as the suspension travels. The outer CV can handle greater angles to allow for both steering and suspension travel.
A CV joint often requires special lubrication, usually an EP moly grease that is very sticky. On exposed CVs, a flexible boot contains the grease, on internal CVs, like the enclosed Birfield-type joint on 4WD front axles, the grease is packed around the joint inside the steering knuckle.
Single Cardan is a term for a driveshaft with one universal joint at each end of the assembly. So actually there are two single cardan joints in a single cardan drive shaft. Here's an animation of a single-cardan driveshaft as it rotates.
Double Cardan is a term used when describing a one piece driveshaft with three (or more) universal joints. What a double cardan will do, is split a universal joint operating angle into two separate angles that are exactly one half of the original angle. Normally a Double-Cardan (a.k.a. Constant Velocity or CV) style driveshaft is used in applications where it is not possible or practical to properly align the ends of a driveshaft for a single-cardan setup. Examples include where the operating angle would be too great over a single cardan joint (see below) a double-cardan allows the operating angle to be split across the two halves of the joint. It is also possible to use two CV joints on a driveshaft which is commonly used where it is not possible to align either end of the driveshaft, such as when both vertical and horizontal mis-alignment occur, or when mis-matched operating angles are present, such as in front wheel drive vehicles, where both up and down motion is present from the suspension travel as well as rotation about a vertical axis due to steering action. Drawbacks of multiple CV joints are their higher cost and complexity as compared to u-joints, their extra length and weight, and their decreased maximum operating angle limitations.
This is the angle formed between the two yokes connected by a cross and bearings. It may be a simple or compound angle, depending on the geometry of the driveshaft. While u-joints can operate at fairly high angles (usually up to 30°), the speed at which the shaft moves provides a practical limit to the angle as follows:
| SHAFT RPM | OPERATING ANGLE |
| 5000 | 3.25° |
| 4500 | 3.67° |
| 4000 | 4.25° |
| 3500 | 5.00° |
| 3000 | 5.83° |
| 2500 | 7.00° |
| 2000 | 8.67° |
| 1500 | 11.5° |
This table is based upon the joint at rated load and life. Going above the rated load or angle will decrease the u-joints life. As a general rule of thumb, for each doubling of the operating angle, RPM, or load, the lifetime of the joint is decreased by half. Rated lifetimes are on the order of 3000 hours.
In the typical off-road vehicle, a suspension lift is done to increase clearance and allow larger tires to be installed. To compensate for the larger diameter, lower gears are installed in the axles. Lets see what this does for the drive shaft, the lift increases the angle of the shaft and the lower gears means the shaft has to spin faster for a given axle speed, both things are working in the wrong direction on this chart. No wonder, driveshaft problems are common in vehicles modified for off-road use. And what speed does the driveshaft operate at? Actualy, it runs faster than you think. Given a vehicle with a modest 0.85:1 overdrive gear in the transmission, the driveshaft is running nearly 18% faster than the engine is turning in top gear. So if the engine is turning 3000 RPM on the highway, the driveshaft is spiining over 3500 RPM.
Aside from u-joint lifetime, you also may be concerned with vibration-free operation at high speeds, at least for a street-driven vehicle. The maximum operating angle a given driveshaft can run depends on a variety of factors and is hard to give an exact number or formula to determine if a given setup will run smoothly or not. Some factors that come into play are the shaft RPM, the length and the tubing thickness. For example, a longer shaft can "soak up" more vibration than a shorter one and a slower moving shaft will vibrate less than a faster moving one. So, for an example. on my '85 4Runner I found that with the stock rear driveshaft running at about a 12° operating angle (approx. 4" lift, 50" long shaft), I had smooth operation. But when the shaft was shortened about 6" (due to adding a 2nd transfer case), the operating angle hit about 15° and I found it no longer operated smoothly, even with ideal u-joint alignment and a professional balance job on the shaft. So by converting the top u-joint to a CV joint and re-aligning the pinion angle, the shaft did run smooth again.
A frequently asked question is about driveshafts and angles and so forth, is "How much shim do I need for X" of lift" or "Is Y° shim too much?". Well, there really is no general answer to these general questions, rather the right answer is what works for that particular situation. For example, assuming that the driveshaft is aligned properly in a vehicle with stock suspension, if it is lifted with a block or spring lift, then everything should still be lined up, at least with a single-cardan driveshaft. It's like a parallelogram, the angles change, but the sides remain parallel. So why do some lift kit makers include shims with their kits? (Likely because they don't know what type of vehicle the lift will be installed on, so they supply parts that may or may not be needed in all applications). So the correct answer for how much to shim an axle to correct the driveshaft angles depends on how far off the angle is to begin with.
So, how do you go about measuring drivelines and angles, etc.? At first glance it seems kind of difficult, but I have some easy techniques that make the job very easy. How you measure the angles depends on the type of driveshaft you have:
Pardon the crude ASCII art that is supposed to show a typical driveshaft:
FT
-|\
-| \
FB\ \
\ \
\ \
\ \RT
\ |-
\|-
RB
The idea is to measure (FrontTop -> RearTop) and (FrontBottom -> RearBottom)
If (FT-RT) is equal to (FB-RB) then the angles are parallel
Ideally, (FB-RB) should be a bit longer than (FT-RT) at rest
So, why do the u-joint operating angles need to be the same on both ends of the driveshaft? To understand this requirement, you need to see how a u-joint operates as it rotates. For an easy case, assume no operating angle, that is 0 degrees, between two shafts connected by a u-joint. As one shaft rotates through 360 degrees, so does the other shaft, in exact unison, so at 0 degrees, there is no issue.
However, lets angle the two shafts to say 45 degrees. Now, look at the "cross" of the u-joint as it rotates. When the driving side of the cross is horizontal, it's ends are moving at the same speed as the yoke on the driving shaft. However, the driven side of the u-joint is 90 degrees offset from the driving side, but since the u-joint cross is rigid, all 4 ends are moving the same angular velocity, i.e. that of the driving shaft. However, since there is that 45 degree angle between the two shafts, the cross is also angled 45 degrees, meaning the effective length of that side is equal to the sin(45) times it's actual length or 71%. But, since it is moving at same angular velocity, the surface speed; which is equal to the angular velocity times the radius (or length); is now 71% of the speed of the driving shaft; i.e. the driven shaft is turning momentarily at 71% the speed of the driving shaft! Now, turn the driving shaft 90 degrees farther in it's rotation. Now the driving side of the cross is at 45 degrees, so it's effective length is now 71% and the driven side is 100%. Assuming the driving shaft speed is constant, then this means the driven shaft speed is now 1.00/0.71 or 1.41 times (or 141%) faster than the driven shaft! So, if you have the driving shaft turning at say 1000 RPM, the driven shaft will vary from 710 up to 1410 RPM as it rotates, averaging to 1000 RPM. This is what causes a driveshaft to vibrate.
So, how can such a setup ever work in the real world? As it turns out, if you stick another u-joint on the other end of the shaft and line it up in phase with the first one and keep the angles identical, these rotational speed changes nearly cancel each other out. While the driving u-joint is speeding up the driveshaft, the driven u-joint at the other end is slowing down what it is hooked to (usually the pinion on the differential). And while the driving u-joint is speeding up the driveshaft, the driven u-joint is slowing down the pinion. All this results in the pinion end of the shaft being driven and almost exactly the same speed as the transmision/transfer case end of the shaft.
I say "almost" becasuse the two u-joints do not even perfectly cancel each other out (except at 0 degree operating angle). The smaller the operating angle, the better the cancellation is, the greater the operating angle, the less the cancellation is. Also, if the angles on both u-joints are not the same, the cancellation is less good and if the two u-joints are not properly phased to each other, the cancellation is worse yet. In fact, if you were to go to the extreme and set the u-joints up 90 degrees apart from each other, not only would there be no cancellation but they would in fact compound the rotational vibration, the first joint would induce it's component, then the second joint would take that and multiply it by it's own factor depending on the angle. So, in the above case of a 45 degree operating angle, the driven joint would be running from about 50% to 200% of the speed of the driving joint, or from 500 RPM up to 2000 RPM for a 1000 RPM input. You can imagine what that would feel like driving down the road, say at an engine RPM the should give a 30 MPH speed, the tires would be turning anywhere from 15 MPH up to 60 MPH as they turned one revolution!
And if you don't understand the above (or believe it), have a gander at this animation and watch the center shaft speed up and slow down as it rotates:
 |
 |
| Pinion Angle: 68° | Driveshaft Angle: 23° |
3
/ 65 Inv Sin and see the answer is 2.7 degrees. In this
case, a 3 degree shim is about what is needed to correct the angle
change brought about by the 3" longer than stock shackle.
For setting the drive shaft length, measure it from flange to flange at rest. You should allow at least 1.25" of compression on the rear shaft and maybe a bit more in front (1.5"-2" - assuming spring shackles in back) to allow for the suspension compression. Then, be sure you have enough spline travel at full droop. If the existing spline length is not long enough (sometime a problem in the front drive shaft) a long travel spline shaft may be needed.
So, why must you run the u-joint at 0 degrees with a CV joint on the other end. See the discussion above and realize that the only time a single u-joint can operate smoothly is at a 0 degree operating angle. At any non-zero angle, the u-joint will induce a rotational vibration in whatever it is hooked to. This is not desirable, so the u-joint MUST be at 0 degrees operating angle in a CV- or double-cardan type shaft.
Phasing is a term that describes the alignment of the single-cardan joints on opposite ends of the drive shaft. As discussed above, a single-cardan (or u-) joint does not rotate at a constant velocity if the operating angle is non-zero. The drive shaft speeds up and slows down slightly as it rotates due to the nature of the joint. One way to reduce this is to make sure the joints at each end of the drive shaft are aligned properly. If the yokes on each end of the shaft line up with each other, as seen in the figure below:
Then the affect will be that the two joints will tend to cancel out the speed variations from each other. In most 4x4 applications, the drive shaft will have a slip yoke in the middle to allow for changes in length. If the shaft is ever taken apart, it is important to get it re-aligned properly when it is re-assembled. One way to do this is to mark both sides of the slip yoke. However, you should check that the joints really do align properly, don't assume they are. The reason for the phasing is that the speed variation of the joint is related to its operating angle and its angle of rotation. In order to get the most effective cancellation, the joint yokes *must* be aligned exactly with each other and the operating angles must be identical. Any variation in either angle will show up as uncancelled vibration. While unequal operating angles result in a vibration that increases with shaft RPM, phasing problems may be felt at lower RPMs and higher loads, like when accelerating from a stop.
Most driveshafts will have some sort of alignment mark stamped or painted on to indicate the proper orentation of the slip yoke. If there is none, they try lining up the u-joint end caps as close as possible. One trick that can sometimes help with phasing is to spin half of the driveshaft 180 degrees before re-installing it to see if this makes any difference. Often one orientation may balance out better than the other. Once you find the proper alignment, paint a mark on both sides of the slip yoke so that you can get it back together correctly next time.
For a double cardan driveshaft, phasing is not an issue, although you may want to try and line up the bearing caps anyway.
Most likely, if you've read this far (or even searched for this page) you may have a problem with driveline vibration.
If you suspect vibration in the rear driveshaft, one way to isolate the cause of the problem is to remove the rear shaft, lock in the front hubs and test drive in 4WD, assuming your transfer case and 4WD system allow this mode of operation. If the vibrations remain, you've just eliminated the rear shaft as the cause of the problem, its likley to be a bad bearing, bent axle, out of round (or balance) wheel/tire, or something in the engine or tranny. If the vibrations go away with the rear shaft removal, then its something in the rear drivetrain that is the cause, the transfer case output, rear shaft (and center bering if present), the single and/or double cardan joints, the pinion bearing and rear differential could all be the cause.
If so, you probably want to fix it. How to fix it depends somewhat on what led to the problem in the first place.
So, assuming there is no physical damage or worn out parts, and you simply have an angularity problem, there are a number of ways to fix it. Basically, you want to correct the angles. How you do that depends on a number of factors:
If you have a multi-link suspension, perhaps with coil springs, there are a few options. If the links are adjustable, you should be able to correct the angles with the adjustments. If no adjustments are provided, then you'll either have to get an adjustable link or relocate the suspension brackets on the axle.
If you have a leaf-spring suspension, then there are more options available. Among the options are shims, rotated spring perches, longer or shorter spring shackles, or driveline changes. Below is a table of common lifts and driveline affects:
| Type of Lift / Driveshaft & Location |
Single Cardan Rear |
Single Cardan Frnt/Fwd(1) |
Single Cardan Frnt/Rev(2) |
Double Cardan Rear |
Double Cardan Frnt/Fwd(1) |
Double Cardan Frnt/Rev(2) |
| Spring | None | None | None | Tilt UP | Tilt UP | Tilt UP |
| Block | None | None | None | Tilt UP | Tilt UP | Tilt UP |
| Shackle | Tilt DOWN | Tilt DOWN | Tilt UP | None(3) | None(3) | Tilt UP |
Notes:
Installing a shim between the axle and spring is the easiest way to correct the driveshaft angle (here's a convenient on-line source for custom-built axle shims). But which way does the shim go in to fix the problem? It depends on the spring and axle configuration, namely Spring-Over-Axle (SOA) or Spring-Under-Axle (SUA). The following table summarizes the direction of pinion tilt vs. axle configuration. and which way the "fat end" of the shim faces:
| Tilt/Config | Front/SUA | Front/SOA | Rear/SUA | Rear/SOA |
| UP | Backward | Forward | Forward | Backward |
| DOWN | Forward | Backward | Backward | Forward |
Its best to visualize the spring as fixed flat surface under the vehicle. Then the shim will sit between the spring (top or bottom) and the axle, which then must rotate up or down to align the spring perch of the axle with the angle of the shim.
[Last updated: 14.April.2008]
