Drive shaft - why did they bother?

[georgem]

Hi,

I was disconnecting the tail shaft the other day, ready to pull the engine and gearbox when the thought struck me - our cars have a fixed gearbox - like most cars, and a diff that doesn`t move too much, unlike an old school live axle, yet why do our cars have a flexible disc, a short shaft to a bearing then another shaft with uni joints either end. The distance from the gearbox to the diff isn`t excessive therefore why couldn`t MB have run, as millions of cars have (and currently do) a single shaft with a uni joint at either end?

I`d be keen to understand their logic.

Cheers

George

[Shvegel]

One possible explanation is that Mercedes was an early proponent of crumple zones that allow the car to crush in an accident in a controlled way. The idea is to design the area in front of the firewall to crush which will absorb the impact and help the passenger compartment remain intact. This also serves to soften the blow so the accident will be more survivable.

Now imagine that the car is hit straight on the front of the engine and the driveshaft is solid. all the force applied will transfer to the driveshaft and directly to the rear axle housing which tends to really impair the crumple zone's ability to do it's job. Also  offsetting the center bearing slightly to one side or the other allows you to control which direction the driveshaft will "hinge" in the event of an accident.

[Raymond]

I wonder if it wasn't also a production consideration.  If the back section of the shaft is the same on several different models of sedan and coupe, then you can make them by the thousands over several years and have one less expensive inventory item.  The smaller front section can be made to fit the specific chassis/transmission combination. 

The flex disc is to cushion the shifts and in-gear decelerations to provide a more genteel ride.