It doesn't. Offset is offset, whether it's horizontal, vertical or diagonal. The U-joint just knows that there is some angle between it's input and output. Any angle will cause wear, even angles that are below the max design spec of the joint. We just try to minimize it as best we can to prolong the life of the parts. When we start putting extreme angles in the driveline, we follow certain rules. But ultimately, we accept that the parts are going to wear faster or break easier because of the stresses we have introduced.
Careful
@ericbphoto, no offset is actually worse than a little. Without any offset, the needles within the u-joint don't move and as a result the lubrication (grease) will breakdown, then the needles will fail and with them the u-joint. BTW, your video is very good at explaining the issue. If the front (transmission/transfer case) and back connection (axle) are not parallel both vertical and horizontal*, you will get vibration as the driveshaft attempts to speed up and slow down every revolution**.
@superj, On our drag car, we were looking for equal length axles, so when the last yellow light lit and our driver dropped the hammer, both axles twisted the same amount and the car launched straight as the green came on (similar to the torque steer issue in previous post). This actually results in the pinion being offset about 3/4" as the pinion doesn't line up perfectly with the center of the differential (Ford 9" axle).
@scotts90ranger, Explorer axles have the exact same offset as Ranger axles, both the drivers and passengers axles are longer by 3/8" - that's why we can swap them in so nicely. On other hand, Bronco II only has longer axles on passenger side, so axles doesn't swap in as nice. Most Mustang axles of same time period as our Rangers are equal length to ensure the car accelerate straight, so they wouldn't swap in either (but 2 driver's Ranger axles was simplest way to get 5 bolt rear axle in 'stang).
While you have the results correct, the passenger side tends to spin more because:
It often has several hundred pounds less weight (no driver or tank of fuel)
The shorter/stiffer axle, means the tire is starting to turn while the longer shaft on the driver's side is still winding up.
Lastly, the driveshaft is providing a rotational torque, so the the driveshaft applies 300 ft-lbs. to pinion, the drivers axle being 30" long, the wheel gets pushed down 120 lbs. force, but the passenger side at only 27" long gets lifted with 133 lbs. force.
With lower weight, delay and more force, the passenger side spins 1st.
@Shran, for the original Jeep/early CJs, the front and rear differentials were lined up, so that if you missed the obstacle (rock) with the front differential, 99% of the time, you missed it with the rear. It also saves a gear in the Dana 18 transfer case - that matters when you are building hundreds of thousands. It is offset to passenger as driver's tend to preserve themselves at expense of someone in the other front seat - passenger always has option to vacate, driver not so much.
Unfortunately, it isn't as efficient to have 3 gears constantly under load to power the rear axle, so the later Dana 20/300 & np 231, run power out center to rear axle, leaving only the front axle offset, as it is difficult move the engine/transmission far enough (and to do so causes other weight issues). Power (drivetrain loses) only need be applied when 4x4 needed.
*It is possible to offset both the front and rear connections by exactly the same amount and get good results - technically that is what a dual cardan u-joint aka CV joint is doing. But it is very difficult to achieve in practice - Ford does it with 2 piece driveshafts in early SuperCab Rangers, and lots of members of this forum have had vibration issues after modifications (either lifting or lowering).
*Note: lighter driveshafts (Aluminum, Carbon fiber) have less mass to accelerate/decelerate, so you "feel" the vibration less.
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