Does anybody know why Volvo went with a two piece drive shaft? It seems like an unecessary complication - which is unusual for Volvo. Why not just a single driveshaft with two u-joints instead of the two shaft, 3 u-joints, and bearing? I am bringing this up, because I am going to have to have some drive line work done, and all these extra parts and labor is going to cost me.
Tom

The phase of the driveshaft is still important and having three joints does not affect the accel/decel properties whatsoever. It still occurs. In fact it is potentially worse with an odd number of joints and the only thing preventing that in the Volvo is the fact that the "odd" joint does not actually flex out of the drive-shaft line. Yet another reason why a failing center bearing or support is felt as such an amplified vibration.

As alluded to, many cars with both long travel suspensions and "solid" driveshafts exist. They have a splined segment at one end of the shaft to take up length changes as the suspension travels.

I think Volvo did it for safety reasons. Two shorter shafts with a hinge in the middle can flex on impact unlike a one-piece unit. This will help the engine/tranny to be pushed under the car instead of potentially jack-knifeing it into the passenger compartment (although I've never heard of such a thing I suppose it could happen). Incidentally (or perhaps not so), two shorter shafts can use thinner tubing and not collapse under the torque loading... Of course having extra flanges and u-joints kills any advantage.

There is a performance inprovement as well. The shorter shaft is less unsprung weight for the rear springs and shocks to deal with.

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Mike!

I am curious about the drive train work you are having done, and why it is needed. If it is just u-joints it shouldn't cost more than about $30 for parts and I can't imagine the labor would be more than about an hour or hour and a half. On the other hand, if you are somewhat handy and have a decent vice, replacing u-joints is pretty simple to do.

Don't most 'merican cars with one-piece drive-shaft have slip joints IN THE BACK OF THE (automatic, heh) TRANSMISSION?
Just a guess.
Shayne

If the shaft is at any sort of an angle, with two U-joints the shaft has to accelerate and decelerate every 180 degrees of rotation. With three joints, it doesn't. It's smoother and under much less stress.

the 120 has a lot of suspension travel in the back. The gives it better road holding abilities, especially on rough roads (when the 120 were designed, 90% of Sweden's roads were unpaved). A one peice shaft would severely limit the suspension travel, especially given that there is little distance between the end of the transmission and differential flange. If you think about it, trucks get away with a one peice as the movement of the suspension has a longer tangent relative to the end of the transmission, u-joints range of motion doesn't allow then to rotate freely past a certain point, hence the 3 of them.

I think the tunnel would have to be larger to give enough room for the complete range of movement of the rear suspension.
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I'm JohnMc, and I approved this message.

That slip joint (splined) has got to mean that some of that 'range of motion'is front to back and a one piece drive shaft wouldn't allow for that movement. The rear axle going up and down is actually an arc, albeit a short one which means that it moves closer and further away from the rear of the transmission (fixed) i.e. the distance chnages between the flange of the tranny and the flange of the differential. does this make sense?

You'd still have to have a spline slip joint in a 'single piece' drive shaft. You could just lose the center support bearing and the center u-joint. But you'd still have to study the range of movement on the rear axle a connected to the back of the trans vs. connected to the support bearing. For some reason I think that the shaft might be more likely to hit the top of the back end of the tunnel when the rear suspension is bottomed out.
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I'm JohnMc, and I approved this message.

the extra u-joint will cost 10$ more. Not that much..
-GIl