Philippe;
15 to 20 MPH is a rather low speed for a non-acceleration induced shudder.
I wonder if you have a pressure bulge on a tire, or perhaps a bent wheel.
Can you arrange for another driver to ride alongside your car and inspect
all 4 wheels as you drive along? He will have to also drive behind you to
try to see the inner surface of each tire as it rolls along. 10 MPH is
fine, you just want it to rotate with weight on it. Jacking up the car for
inspection would show a bent wheel, but might not show a failing tire, as it
might bulge only with weight on it.
If this cause is eliminated, and you want to pursue the driveshaft angle
question, remember that (unless the car has CV joints), there will always be
some "lumpiness" to rotary motion unless the driveshaft is perfectly aligned
with the engine/transmission centerline and with the differential. By this
I mean not only left/right and up/down alignment, but that the "pointing
angle" of the centerline of both the differential and the
engine/transmission assembly is exactly coincident. In other words, if you
could imagine a beam of light held precisely by the pinion bearing of the
differential, it must exactly coincide with a beam of light held precisely
by the rear bearing of the transmission. The result of this precise
alignment is that the u-joint always transmits force at zero angle. Of
course this situation can only be arranged for one condition of weight in
the car, so you have to set this up with the car sitting level on its
wheels. The place to shim the rear axle, if necessary, is where the axle
housing is bolted to the springs by the U-bolts. Usually there is enough
play in the locating stud that you can loosen the u-bolts and rotate the
nose of the differential up or down as required to exactly aim it at the
rear of the transmission without using shims. If the springs have been
replace without checking this angle, it is almost surely incorrect.
In the US, there are a few professional driveline service companies that are
expert at finding this sort of problem; I've used "Driveline Specialists"
for this. I don't know what you might find in France, you're on your own
there. Of course, these same professionals will check your driveshaft for
straightness (even a slight bend will cause it to "whip" like a jumping
rope) or out of balance, but these problems usually show up at much higher
speeds - 65 MPH is typical.
Since your car has a center bearing support, you have exactly the same
problem squared - as you also have to have the center bearing exactly on
this same imaginary line. That is probably the first thing to check, as
these are easily disturbed when people work on the car. Try stretching a
string tautly between the rear of the transmission and the differential,
being careful to locate the string on the exact centerline of both
assemblies. You'll have to do this twice, once on the left/right
centerline, and once on the up/down centerline, since the driveshaft etc.
will prevent you from running the string on both centers at once. Then you
can squat down and eyeball to see if the string exactly splits the
driveshaft and center bearing assembly - if not, loosen the bolts on the
center bearing support and adjust it to coincide with the string path. The
car really has to be up in the air to do this, but you must have the weight
on the wheels, so you need to find a lift that picks the car up by the
wheels - over here we call that a "muffler shop hoist".
I hope this is clear enough, if not, please bug me and I'll try harder.
Dick Benjamin
----- Original Message -----
From: Philippe COURANT <accf_clb@xxxxxxxxxxxxxxxx>
To: <mailing-list@xxxxxxxxxxxxxxxx>
Sent: Wednesday, May 01, 2002 1:41 PM
Subject: IML: shudder on a '57
> Today i was at a meeting with my Imperial; good test because i
> drive 150 miles (3/4 on highway where i sustained a 70 mph
> average speed).
> I noticed some shudder or vibration of the car at a specific
> speed: betwwen 40 and 45 mph ther's a vibration (even if i
> release the accelerator). Above or under the car drives well.
> i've seen in the manual that a propeller shaft shudder could
> arise at some speed ( 15 to 20 mph). The adjustement isn't very
> clear in the manual: measure a "working angle" then shim the axle
> (but where ? behind the frame bracket struts or at the middle
> joint center)
>
> --
> Philippe COURANT (Pau, France)
> Imperial 57 Crown convertible
> Buick 58 Roadmaster sedan
>
> - American Car Club de France (ACCF) : http://www.accf.com
>
> - Chrysler Imperial France : http://www.ifrance.com/c-i-f
>
> - Cadillac " Standard of Excellence " :
> http://www.ifrance.com/accf-cad
>
> - SportsCars : http://www.ifrance.com/accf-sprtcar
>
>
>
>
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