Driveline vibrations: causes and cures
One of the most common complaints I hear involve driveline vibrations, whether it's the result of a suspension lift, transmission/engine swap, or a custom rearend. All these modifications can, and most likely will, cause some form of driveline misalignment.
My first engine swap was a Buick V-6 in a CJ-5 back in 1964. I ended up with a very short driveshaft (I was also using a Studebaker overdrive behind the transfer case) of about 10 inches or so, that not only was at a very steep angle, but was offset to one side. Talk about vibration! It was a new set of U-joints every 5,000 miles! Being young and dumb, I simply lived with it-but there's no reason why you have to.
I could wow you with figures that state maximum operational angularity per given shaft speed. But most likely, you're not going to take heed and will exceed them anyway because you have no other choice. The important thing is to cut out that vibration as much as possible. Not only does it shorten U-joint life, but the vibration can be transmitted to the bearings in the transfer case and the differential, causing early failure to these components as well. Three factors cause driveshaft vibration: balance, improper U-joint phasing, and angularity of operation. When the U-joints run at different angles, they're operating at different speeds.
Hold on, now, I know what you're going to say: Since both are welded to the same shaft, they have to turn the same speed. Yes, that's true. They're turning the same rpm, but at different variable rates. Because of their operational angularity, each U-joint operates at an uneven speed. That is, it slows down, then speeds up as it rotates. When the joint at one end of the shaft is in, let's say it follows a "different rotational pattern'' than the U-joint at the other end, and vibration results. The trick is to get them both to turn in a synchronized pattern.
Remember, I originally said there are two causes. Well, the first is that they must be in phase. That is, if you lay the driveshafts on a flat surface, both U-joints must be exactly in line-and I do mean exactly. Often, a person will disassemble the slip joint and then incorrectly align the splines, I've even seen factory-built drivelines that were improperly matched due to misalignment that occurred during the welding process.
The second cause is that the angle of the transfer case doesn't match the angle of the pinion shaft in the rearend. It's quite common for suspension kits to contain the spacers needed to drop down the transfer case to shorten the distance to the rearend and to lessen the severity of the operating angle. However, if this is done to extremes, it will turn the front driveshaft yoke up, causing a terrible misalignment and front shaft vibration.
The correct way to solve the problem is by cutting the spring pads away from the axle tube, rotating the axle and re-welding the pads in the correct position. It's possible, and a lot less work (and provided in most suspension kits), to use tapered shims between the spring pads and the springs to get a proper or improved angle. Sometimes, on short-wheelbase vehicles (a CJ-5, for instance), it's nearly impossible to obtain equal U-joint angles without putting the U-joints in a bind during suspension travel. While it's not correct engineering, I've found that you can turn the axle and place the pinion shaft in a straight line with the driveshaft. You need to set up the vehicle with a full load as you intend to drive it. In fact, it's better to be a degree or so downward as the housing rotates upward under acceleration. It's not "correct,'' but it may be the only way you're going to get enough driveshaft travel without binding. I hate to admit it, but this is how my own CJ-5 is set up, even with its high-pinion Dana 60 rear axle.
Remember, I said rear axle. Trying to shim the front axle to obtain proper driveline angle will also cause a loss of caster and, most likely, all sorts of handling problems.
Another solution to driveshaft angularity problems is to add what's called a "constant velocity'' (CV) joint in place of the standard joint. This is actually a double U-joint with a connecting ball between them. The design helps to cancel out the vibration. In fact, it's standard practice to use these in a lot of OEM applications-especially on the front axle; the factory uses them in cases where the angle was incorrect to start with due to the complete engine, trans and transfer case being dropped to correct the rear shaft angle.
There can be another cause to driveline vibrations: The shaft can be bent from some type of trail damage or dents, or out of balance due to improper construction. In this case, you'll usually end up taking the shaft to a professional to have the problem corrected. Yes, I've corrected out-of-balance shafts by using hose clamps around the tube. The heavier weight of the clamp's screw acts as a balance-or should I say, "counter balance weight.'' The big problem here is that t's a real "trial and error'' situation that takes a lot of time to correct.