May I add....the 90 degrees only apply when the joints flex on the same plane. If they do not flex on the same plane, there must be an offset angle. That means the joints are still in phase, but not rotated at 90 degrees just at some some other angle (in fact "in phase" means: the driving yoke of the U-Joint which provides power is rotated 90 degrees to the driving yoke of the second U-Joint at the end of the shaft which delivers power to the driven yoke of the differential. So it is vice versa to the terminology @Zimm used) You see that most of the time at the front shaft and there is a reason for that.
In phase, the power delivery side (the driving yoke on the right) is rotated 90 degrees to the driving yoke on the left at the end of the shaft):
Still in phase, but with an offset angle due to the U-Joints flexing on different planes (90 degree rotation: dotted line, actual rotation: solid line).
If you look at modern 4x4 cars more closely you see that the engine and gearbox ar not leveld but the gearbox is pointing slightly down at the rear while pointing up at the front. That is because the rear prop shaft is longer and has more mass and tends more to swing. The goal is to keep this long shaft as quite an smooth as possible and thats why they try to get a straight line between the gearbox output shaft and the rear differential. Especially with non-permanent 4x4s as the rear shaft is always transfering power. Just think about Pick-Up trucks.
Remember, that the U-Joint pairs eliminate the oscillation of the U-Joint turns out, but only at both ends of the shaft. The shaft is still oscillilating and therefore its mass must be accelerated and deccelerated 4 times for each turn. That eats up power and leads to swing and vibration. BTW, a double U-Joint is nothing else than a very short drive shaft.
The disadvantage of the front outputshaft pointing up is, that it is increasing the U-Joint angle. In addition, especially when the differential is located in the middle of the axle you have an offset. That leads to the U-Joints not flexing on the same level so an offset angle must be calculated in the U-Joint phase.
There are many examples in the market. I know it from some Ford trucks, the Range Rover Classic and P38 and the Defender Td4 110.
Example of driveshafts from a Land Rover 101 Forward Control (front shaft, in phase with an offset angle) an a 90ty Defender Td5 (rear shaft, in phase without an offset angle):
You can read my article on this. There are some pictures explaining the level thing (and you can see my 90ty from below ;-) :
Alles über Kardangelenke
Überall dort wo es gilt eine Drehung um einen Winkel herum zu übertragen kommen zwei Arten spezieller Gelenke zum Einsatz. Entweder es ist ein Kardangelenk oder ein Homokinet. In diesem Artikel geht es um das Kardangelenk, seine Eigenschaften und den wesentlichen Unterschied zum Homokineten.
matsch--und--piste-de.translate.goog
Hopefully Google got the translation right.
AWo
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