Here's what I did to try to figure out what was causing the problem. First I put the car up on jackstands and ran it through the gears. The vibration was very noticeable even at fairly low speeds (40-50 mph). I took the rear wheels off and tried it again-same thing. That means that I've eliminated the front and rear wheels/tires, and the rearend (12-bolt) is the same one I was using previously, which didn't have this problem. I also noticed that the vibration becomes worse the faster I go, and that if I get the car up to speed and put it in neutral and let the engine idle, the vibration is still there. That indicates that the problem isn't engine-related, and that "most likely" the tranny is fine as well.
At this point I pulled the driveshaft and took it to the shop that originally chopped it and asked that they check the balance. When I picked it up later that day, the guy told me that sure enough, the balance was off by quite a bit, but that the shaft was good to go now. With that bit of encouraging news, I put it back in the car but still found the same problem. I took the driveshaft out and asked the driveline shop to check it one more time. Again they said the balance was off and that they'd corrected it. I reinstalled it again and still had the vibration problem.
This is where I am today. Although the driveline shop says the shaft is good, I question whether or not they can tell the difference. What I need to know is what I should do next. Should I have a totally new shaft built by a larger driveline shop, or is there some additional troubleshooting that I can do to help determine what's causing the problem?Brian BowlinAustin, TX
For starters, Brian, you should probably find another driveline shop. It's quite possible that the driveshaft isn't to blame, and we'll get into more on that next, but the shop shouldn't be finding a different balance every time you have them test it. That would seem to indicate that either their equipment is faulty, or that they simply don't know how to use it correctly. Either way, a second opinion might be beneficial at this point.
However, it sounds like the real problem may not be with the shaft, but with the operating angles of its universal joints. Since you've performed a complete engine/trans swap, and you're using a transmission that's longer than the original, it's very likely that your driveshaft working angles have been altered in the process.
The basic rule of driveshaft angles is that they must be equal but opposite. That may sound confusing, but what it means is that the operating angle between the driveshaft and transmission-usually measured on the bottom side of the shaft-should be the same as the angle between the driveshaft and the rearend yoke, which would then be measured on the top-side of the shaft. A magnetic angle-finder tool is useful in measuring the angles, and many chassis shops will measure the angles relative to the ground and then do the math to calculate what the angles are relative to each other.
Another rule of thumb for passenger cars is to keep the operating angles at less than 4 degrees, or the pulsations that occur when the U-joints rotate will become too pronounced, which will feel like a vibration. The number of degrees allowable is actually dependent on driveshaft rpm; one source we referenced showed that for 5,000 rpm of driveshaft rpm, 3.25 degrees of driveshaft angle were permissible. Keep in mind, however, that with overdrive, driveshaft speed is always higher than engine speed. With that six-speed, you probably have 0.50:1 in Sixth gear, which would put driveshaft rpm at double the engine rpm in Sixth. This is one of the reasons that overdrive transmissions seem to "amplify" driveline balance and alignment issues.