As some of you know, I recently damaged my rear driveshaft while wheeling. I decided to raise the rear pinion, putting my driveshaft in a 'broken back' configuration- both u-joints angling down from the front of the truck to the rear axle, instead of the t-case output shaft and pinion shaft being parallel as is normal. This would raise the pinion end of the driveshaft, hopefully keeping it off the rocks.
I have Slee's 6" lift, which comes with adjustable rear upper control arms, making this mod fairly easy.
I first removed the damaged driveshaft and had it re-tubed with a .156" wall tube, and balanced. The stock(?) driveshaft was .107" wall. I then measured between the t-case flange and the rear pinion flange and cut a piece of 1/2" square tube slightly longer than that measurement at 39". I placed this straight edge to span across the top of both flanges, simulating the driveshaft but easier to measure angles from. I used an angle finder similar to this one http://images.rockler.com/rockler/images/36915-01-500.jpg to find the relative angles between the flanges and the straight edge. I then cranked the upper control arm adjusters (lengthening the arms) to raise the pinion until I got matching angles at both flanges. This took some trial and error, as both angles change as you raise the pinion. I then dialed the pinion down just a bit to compensate for the fact that it would normally raise under power due to axle wrap. Then I installed the driveshaft and test drove it on both city streets and on the highway up to 80 MPH. I got no vibrations or noise at full throttle, cruising, off the gas or engine braking.
My rear t-case output shaft is at 4 degrees from level, the driveshaft is at 10 degrees, and the rear pinion shaft is at 15 degrees, indicating that I have a 6 degree angle in the front u-joint and 5 degrees in the rear u-joint. I estimate them to equal out under power- pure theory here. I have no idea if the 15 degrees at the rear pinion is going to be a problem- ideally it was at 4 degrees originally with parallel t-case and pinion shafts. If you see me post in the future for help on replacing pinion bearings, you'll know I had issues. I will be looking into an oil slinger setup (that Tools R Us mentioned is in the front diff, and I'm hoping can be fitted into the rear) if the bearing goes out or I get around to re-gearing, whichever comes first. By my calculations, the u-joint angles are less than they were with the 6" lift (approximately 7 or 8 degrees), but my wife's stock 80 seems to have even less of an angle at the u-joints at around 3 degrees. Even 8 degrees is no where near the operating limit, so all that is probably not relevant.
I figure I gained about 2-3 inches of clearance at the rear pinion flange.
I have no idea if anyone else has done this, if so please post up. The driveshaft guy agreed that this was the way to go for rock crawling rigs- apparently he's seen a few trashed driveshafts- and indicated that it was a regular practice on 4 wheelers. He didn't think it would be a problem. I asked him if the u-joint yokes should be in-phase or out of phase and he said that the way I had them was the correct orientation- what I assume is in-phase, with the yokes in line with each other. (I have a hard time remembering which is which, and I don't know the complete history of my rig so I don't know if the driveshaft was factory correct when I got it).
-Spike
I have Slee's 6" lift, which comes with adjustable rear upper control arms, making this mod fairly easy.
I first removed the damaged driveshaft and had it re-tubed with a .156" wall tube, and balanced. The stock(?) driveshaft was .107" wall. I then measured between the t-case flange and the rear pinion flange and cut a piece of 1/2" square tube slightly longer than that measurement at 39". I placed this straight edge to span across the top of both flanges, simulating the driveshaft but easier to measure angles from. I used an angle finder similar to this one http://images.rockler.com/rockler/images/36915-01-500.jpg to find the relative angles between the flanges and the straight edge. I then cranked the upper control arm adjusters (lengthening the arms) to raise the pinion until I got matching angles at both flanges. This took some trial and error, as both angles change as you raise the pinion. I then dialed the pinion down just a bit to compensate for the fact that it would normally raise under power due to axle wrap. Then I installed the driveshaft and test drove it on both city streets and on the highway up to 80 MPH. I got no vibrations or noise at full throttle, cruising, off the gas or engine braking.
My rear t-case output shaft is at 4 degrees from level, the driveshaft is at 10 degrees, and the rear pinion shaft is at 15 degrees, indicating that I have a 6 degree angle in the front u-joint and 5 degrees in the rear u-joint. I estimate them to equal out under power- pure theory here. I have no idea if the 15 degrees at the rear pinion is going to be a problem- ideally it was at 4 degrees originally with parallel t-case and pinion shafts. If you see me post in the future for help on replacing pinion bearings, you'll know I had issues. I will be looking into an oil slinger setup (that Tools R Us mentioned is in the front diff, and I'm hoping can be fitted into the rear) if the bearing goes out or I get around to re-gearing, whichever comes first. By my calculations, the u-joint angles are less than they were with the 6" lift (approximately 7 or 8 degrees), but my wife's stock 80 seems to have even less of an angle at the u-joints at around 3 degrees. Even 8 degrees is no where near the operating limit, so all that is probably not relevant.
I figure I gained about 2-3 inches of clearance at the rear pinion flange.
I have no idea if anyone else has done this, if so please post up. The driveshaft guy agreed that this was the way to go for rock crawling rigs- apparently he's seen a few trashed driveshafts- and indicated that it was a regular practice on 4 wheelers. He didn't think it would be a problem. I asked him if the u-joint yokes should be in-phase or out of phase and he said that the way I had them was the correct orientation- what I assume is in-phase, with the yokes in line with each other. (I have a hard time remembering which is which, and I don't know the complete history of my rig so I don't know if the driveshaft was factory correct when I got it).
-Spike
