Hey ed, looks like I spoke too soon on the driveshaft problem.
Sorry to hear that Ian. I was hoping from your last post that the new DS had taken care of that.
The shaft Tom made for me has certainly improved the problem but I still have a vibe (annoying vibe) between 85-95 kph. Its more noticeable on deceleration or coasting & produces a hum as well. I didnt notice it before as I hadnt really taken it for a long run only a short one.
So now I am back to chasing the problem. Can you elaborate on your suggestion above to rotate the front diff. When you say 2.3 degrees I am assuming you mean rotate the diff upwards 2.3 degrees. How did you come to that figure?
I took your numbers for pinion angle on the front diff, drive shaft and so on and did the math to figure out how much you'd have to rotate the front differential to get the pinion to point directly inline with the drive shaft.
... because I had to clock the transfer case upwards a bit to clear the front torsion bar, that has got the front yoke pointing upwards at 5.5 deg. It then meets a downward pointing driveshaft of 3.4 degs, so that operating angle is 8.9 degrees.
But, now I can't figure out how I came up with that number either!
A perfectly horizontal pinion is 0 degrees. Rotated down would be negative and rotated up from 0 would be positive.
The 8.9 degree operating angle doesn't make any difference if you're running a CV at the t-case. And that's where you want the CV, BTW.
The front drive shaft is at -3.4 degrees. As I pointed out before, the CV/U-joint drive shaft needs the pinion-u-joint-drive shaft in line. The diff would have to rotate the pinion enough to make that happen.
I don't know what the front differential pinion angle is now, but it you assume it is 0 degrees, you'd need to rotate the pinion up 3.4 degrees to get it in line with the drive shaft.
If the pinion is +1 degree, then you'd need to rotate it +2.4 degrees - the difference between 3.4 degrees and 1 degree.
+2.4 = 3.4 - 1
Now, this is just the static math. Usually there is a degree 'off-set' figured in, to take into account torque rotation under power. When you apply force to a differential it tries to rotate one direction or the other depending on whether the gear box is in forward or reverse which changes pinion angle.
There isn't a hard and fast rule here for how much (in degrees, or fractions of a degree) you add or subtract because it totally depends on how much stuff rotates when the vehicle is in motion. For instance, I get a lot of pinion shift in the back axle because I have a heavy vehicle and run leaf springs with big tires and a LOT of gearing. The amount of pinion shift would be much less with a link suspension where differential rotation is strictly controled. So, you can't just say, "Do the math and add 1 degree and you're good."
Obviously, you don't have front housing rotation because that's IFS. You DO, however, have engine/transmission/t-case rotation. This is kind of a sucky configuration from a math standpoint because normally the differential would rotate up when the t-case rotated up as gas was applied and things would kind of stay in the same line, but you have one moving point and one static point.
I know how to calculate this, but I don't know how to get the data. What you need to know is how much the DS angle increases when the vehicle is under steady state cruise, with the vehicle at its average loaded weight and the tire pressure set where you normally run it (with the tires you normally run). On mine that would be, say, 115 KPH. About 70 MPH. That's where mine spends 95% of its time on the highway and where the most gas pedal is applied to maintain that speed.
So, if the t-cases rotates up and increases the DS angle from -3.4 degrees to -4 degrees, again assuming the pinion is at +1 degrees already ....
+3 = 4 - 1
The front differential would have to be rotated +3 degrees.
The techs can't guess this accurately, and they have a LOT more experience than I do so I'm not even gonna try giving you a number. I drive a M/T so I COULD measure the rotation using the M/T gear shift lever, figure the distance from the drive train center line to the top of the gear shift, measure DS center line to the center line of the t-case output and get the front DS angle increase that way. Being that we run the same DS assembly the static math would be relatively accurate - but all this depends on how much flex and opposing force is involved and that would be where the math totally falls apart.
We're talking about very small numbers so if the drive train is pretty stable and you don't have much movement of the t-case, you MAY well be able to do the simple math - line the DS and pinion straight up - and call it a day. Maybe.
I think if I were doing this I'd put your vehicle on a pad, put a jack under the output end of the t-case and jack it up 1". Simulate t-case rotation. Measure the front DS angle. I would guess that the DS angle @ 1" would be what you'd want to use as your DS angle for the math to figure out how much the differential needs to be rotated. But, if you think the t-case shifs up more than 1" - say, 1.5" - then jack it up that far. You have a one piece drive shaft and that's pretty long so the change in DS angle shouldn't be that much (in degrees).
I am now wondering if I have something out of alignment in the front end, maybe bad bearing or cv??
My plan would be to swap over the entire front end with one in my spare car that I know is good. It has to be the entire front end as you cant mix & match cv shafts between 97 & 99 as they had different lengths. Plus I need to redo the ball joints as well as all the bushes as I have a bad squeek & groan from the left side. If I have everything out , I can move the diff upwards & re weld the brackets, but need more info on this, thanks
Maybe. I wouldn't rule it out, but what you've described sounds like drive line. You could pull the front DS and try it again, but the last time you said that when you did that the vibration went away.