Re: Driveshaft vibration

["Peter L. Krause" <pkrause@domain.elided>]

Bert, Derek, et al

The driveshaft vibration in the 116/119 chassis cars is as old as the design
itself. As the person that hosts the Mid Atlantic AR Club Tech sessions
(raises and lowers the lift), it is easy to ascertain the condition of the
driveshaft doughnuts, center support and the shape of the shaft by visual
inspection. The first action that ought to be taken is what Fred said,
loosen the slip yoke and fire it up, shut it down and snug up the pinch
bolt.

The problem Bert is having was also experienced by Kent Decker at the
Savannah track event this March. The reason why a shaft might develop a
vibration that checks out ok before and after the onset of the vibration can
be deterioration of the spherical bearing that centers the forward part of
the rear half in the front and in the spherical bearing that centers the
rear of the rear half in the rear doughnut. These spherical bearings could
develop clearance when "exercised" hard. Without periodic replacement, or at
least lubrication, the bearings develop enough play so that the shafts don't
operate concentrically with one another or index properly at the rear center
line of the yoke. The doughnuts are NOT designed to do anything other than
absorb the rotational "shock" of accelerating and engine braking. They are
certainly not designed to center the halves with one another or with either
end.

A shaft doesn't just "go out of balance." For years I've been reading drivel
about differing weights and arrangements of washers, bolts and nuts (no
difference in the part numbers) and the idea of "balancing" the driveshaft
with the addition of strategically placed hose clamps (?!?) This is simply
hogwash. The reason why 99% of the GTV-6 driveshafts wobble is because of
catastrophic failure of the front doughnut, causing an incredible loading in
one outward direction at the time of failure and bending the nose of the
front of the driveshaft where the spherical bearing locates the bushing that
goes inside the front flywheel. This causes the shaft to lose concentric
location with the other rotating components and causes a colloidal motion
and preload that simulates an imbalance at particular resonant frequencies
(rpm). The only way to fix this is to repair the nose of the shaft in a
lathe and replace anything that would allow slop between the front flywheel
and front bushing, the center bearing (not the support bearing) and the rear
bearing that goes into the front of the rear doughnut.

These cars, like us, are getting older...

-Peter Krause

www.krauseandengland.com

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