One could say that an exercise in futility is trying to solve a problem when you don't know what the problem is. You can't solve a vibration problem until you know the frequency of the vibration. A driveshaft balance issue produces vibration at a frequency equal to the speed of the driveshaft. A driveshaft alignment issue produces higher than normal vibration at 2X driveshaft speed. To your hands or a well calibrated tail bone, these frequencies may feel about the same. But, you can't solve a balance problem with alignment and vice versa. So, you either need to find someone locally that has access to a vibration instrument that can measure frequency to determine the cause of the vibration or you can do a little trial and error testing to get a better handle on the forcing frequency that is actually causing your problem.
To prove or disprove driveshaft balance problem, you can force an unbalance condition through the creative use of hose clamps. Take two or three (or more) hose clamps and mount them on the front of the driveshaft with all the heavy spots in line and go for a drive. Compare the vibration you felt before at 70 (your initial complaint) to what you feel now. If the vibration is exactly the same, add some more weight until you can feel the unbalance. If the frequency created by the unbalance is lower than what you are trying to fix, then it is likely that you have a driveshaft alignment problem. If the vibration frequency is the same but worse then you have a driveshaft balance problem. Not all balance machines are equal and not all people who perform balancing have the same skills so "balanced" shaft from some shops can be pretty mediocre. For fun, you can move the clamps to the back of the shaft near the rear u-joint to see what produces a better reaction. Don't forget to take the clamps off when you are finished.
For alignment, there are some good suggestions. Add shims or put in a 5 degree shim and force the pinion nose up. If the problem gets worse or better the more you know that it is likely alignment related. You may never get it perfectly aligned but you can often reach a compromise. Having passengers in the back seats or luggage in the trunk can throw it off so go for an alignment that is best for most driving. As suggested, compare the vibration at the same speed while going up a hill, down a hill and in neutral. This can also help to determine whether the nose needs to come up or down.
The factory weights/masses suspended from rear axles, transmissions, etc., are often tuned absorbers or dampers that are designed to minimize a problem in a particular speed range. They may or may not help in a car that did not originally have one but that is sometimes pure luck. They are effective when there is a natural frequency of the axle or engine/transmission that is excited at a particular speed or rpm. These natural frequencies are a complex combination of mass and stiffness and changing components such as engine and transmission mounts can render them ineffective or make a problem worse at a different speed. When testing a new car, a manufacturer may find that right at 65 mph there is a vibration that is felt in the steering wheel or shifter, for example. Using a designed tuned absorber, the problem is effectively killed at that speed. But one of these absorbers can not correct for misalignment or unbalance. They are only effective when everything else is good.
The 28 or 50 oz-in crankshaft unbalance won't make any difference if you are fighting an alignment problem or a driveshaft balance problem. If they viration problem at 70 mph occurs in 3rd, 4th, and 5th gear, then the engine RPM is eliminated and engine balance is not the problem.
Get some tape-on weights and repeat for wheels/axles. Force a large unbalance in one of your front, and then rear wheels and compare what you feel at 70 mph. Since the wheels are turning at a much lower rotating speed (RPM) than the driveshaft or engine, the vibration will be at a much lower frequency. When can't measure frequency directly with a tool, use a few simple tests in a process of elimination.
There are some apps that allow you to use the accelerometers in your phone or tablet to measure vibration and frequency. The upper frequency limit of these is about 50 Hz or 3000 CPM (cycles per minute) which is not high enough to measure driveline vibration problems. Head on out to Denver and I'll measure your vibration and frequency. Only takes a few minutes with the right tool and with a little experience.
There are some other conditions that could be causing your vibration but start with the simple stuff. For example, a simple resonance of your exhaust system structure can produce a speed sensitivity. However, you mentioned that problem sets in at about 70 and gets worse. If it actually is worse at 75 and gone at 80 resonance is likely, If it gets steadily worse, then balance and alignment are more likely. You can also get into issues with 1/2 speed criticals of your driveshaft but the fact that you have tried several different driveshaft designs would tend to eliminate that.
Please excuse any typos. as jet lag has firmly settled in. Just landed in Indonesia a few hours ago to teach, of all things, a vibration course this week.
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Steve-G wrote:
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