augi@cbmvax.cbm.commodore.com (Joe Augenbraun) writes:
> Mechanical:
>
> When I drive the car at or above about 60 mph, there is a rumbly
> vibration from the back of the car. I was told that it probably
> was halfshaft U-joints, but I checked them and they seem fine.
> It doesn't feel like out of balance tires, and doesn't make nasty
> wheel bearing type noises. The only thing I can think of is that
> the fact that the transmission mounts are bad (the transmission is
> sitting an inch and half low) is causing this problem. I'm
> replacing those anyway, but are there any other ideas?
>
> Possibly related is the fact that the car judders when releasing
> the clutch and starting from a standstill. The last car I had
> that did this did it because the rear seal was leaking, and oil
> would get on the clutch. I suspect this is the case here, but is
> there anything else to be aware of as a possibility?
As has been said before in this forum: "Kind of hard to tell from here, ..."
One thing I have found about drivetrain noises is that sometimes inspection
shows parts to be fine, but replacement is still required to solve the
problem. Anyway, with that disclaimer out of the way (;-), there are a
couple of hints at a possibility from above:
> (the transmission is sitting an inch and half low)
and:
> Possibly related is the fact that the car judders when releasing
> the clutch and starting from a standstill.
You're right, this can be caused by an oil contaminated clutch disk (or if
you have an older car with a solid clutch disk), but I wonder if it might
be "launch stutter"?? The main difference between this and "clutch judder"
is that "clutch judder" tends to be most noticeable with a mild start, and
is much less noticable under hard acceleration, whereas "launch stutter" is
just the opposite (most noticable under heavy load).
Okay, what causes "launch stutter"? Why, I'm glad you asked. ;-)
As I'm sure you know, a universal joint has a non-linear (sinusoidal)
angular velocity whenever the shafts are not perfectly lined up. This
non-linearity is exactly balanced by a second universal joint, *provided
the final shaft is perfectly parallel to the first shaft*. Now, I'm not
totally familiar with a Spit driveline, but I believe that, with your
transmission 1 1/2" low, it's quite possible that this geometry is not
being maintained for the driveshaft?
I'd say, fix your transmission mounts before you do anything else.
Joe Bob sez: "Check it out!"
Pat Vilbrandt John Fluke Mfg. Co., Inc. Everett, Washington USA
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