THREE Hooke's joints in a driveshaft?

["John D. Fox" <jdfox@xxxxxxxxxxxxxxxxx>]

I have been enjoying our digest discussion of driveshafts, torque tubes,
guibo's etc but I was very surprised by John H's comments about a
production car which used three Hooke's joints ( simple U-joints) to
connect engine and transaxle using a two piece drive shaft. I'm quite
surprised to hear this description, and amazed nobody has commented. I
think it must have been two Hooke's joints, plus some other sort of Dzeus
or other constant velocity joint. Here's why.

A simple 1 part driveshaft, with two Hooke's type U-joints, is often used
to connect two elements ( such as transmission to differential in an
automotive drivetrain). In such a simple case, the two joints must be
phased 90 degrees from each other ( the two driven crosspieces and yokes),
to allow the engine end to turn at the same constant angular velocity as
the driven differential input shaft. Because of the crosspieces, if there
is an angle of the center shaft with respect to the output shaft of the
engine, the driveshaft does not turn with a constant angular velocity - it
speeds up and slows down twice per revolution. In fact, if you want to
have the engine angular velocity, and the differential angular velocity to
be the same, without this twice per revolution modulation, the engine
output crankshaft and the input shaft of the differential must always be
parallel. That is part of the design of the engine mounting, and the
suspension which controls the motion of the rear axle - to keep these
shafts parallel as the rear axle moves up and down. The distance can vary
( that's what the spline is for) but the two simple U-joints mean the
driveshaft itself speeds up and slows down relative to a constant engine
angular velocity. If these two shafts ever are not parallel, then the two
ends do not turn at the same instantaneaous angular velocity - you would
be constantly accelerating and decellerating with each revolution of the
engine.

More complex drivelines, such as the two piece driveshafts used in the
105/115 series Alfa, use the Guibo-type front coupling with the two
Hooke's joints for this reason - the front coupling must be of constant
velocity type - as the two following Hooke's joints, and the section of
driveshaft between them, act just as the simple 1 piece driveshaft with
two joints - this rear driveshaft speeds up and slows down twice
per revolution, though the input and output shafts can turn with constant
angular velocity if the geometry keeps the axis of the differential drive,
and the axis of the first driveshaft, parallel. And that's what the engine
mount geometry and rear suspension geometry try to do.

Different drivelines, such as front wheel drive drivelines, must use more
complex constant velocity joints, rather than simple Hooke's joints,
because as you steer and go over bumps you cannot enforce the geometry of
the two joints - so the use of the more complex CV joints.

The later alfa transaxle drivelines use the three flex couplings to allow
the engine and transaxle to be soft mounted, and they do move around
relative to each other. The Guibo style of coupling allows the motion, and
doesn't have the same restriction on the parallelism of the engine
crankshaft and transaxle input shafts as the simpler 1 part driveshaft.
There also is the impact absorbing component of this style of joint
compared to more rigid mechanical assemblies.

I agree with John H. that driving style might have a lot to do with
longevity of these joints - I tore the front coupling on my milano verde
apart at a driving school doing some fun full-throttle stuff.

Those who complain about these flex couplings might consider the
(original) Lotus Elan, which uses 6 of them in the drive to the rear axle,
and the two driveshafts to each rear wheel. The british trade name is
rotoflex.

So I think John's reference to a drivetrain with THREE Hooke's joints
might really be two Hooke's joints and some sort of CV joint.

                                                      John Fox
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