This one takes a while to go through, because there are many causes
for vibrations. Let's do the little stuff first.
Tires and wheels: common to have tires out of balance because of flat
spots, etc. Symptoms are a vibration that comes on at a constant speed,
increases, decreases and goes away. Your steering wheel will shake in unison
with this kind of vibe if it comes from the front tires. The empty seat
next to you will do the same if it's the rear tires. You need to have them
spun balanced. Watch if possible so you can judge whether the wheels may
be out of round, or the tires 'hop' or 'wobble' on the balancer. Could
be a tire with a bad spot in it, or it could be that they aren't mounted
properly. One thing to try if the tires 'hop' or 'wobble' and the rim is
OK: it's called 'match mounting', and aligns the heavy spot of the tire
with the light spot on the wheel. Can be done with a certain kind of Coates
balancer at a shop that specializes in sports cars, etc. Not at Wal-Mart
or Mr. Tire.
Next, brakes dragging or rotors that are warped. Swishing sound that
starts just after you begin moving and increases to a kind of pinging sound
with speed. Steering wheel will pull a bit in unison with the noise if
it's the front brakes. Can be cured in a couple of ways: loosening and
retorquing the wheels *may* be all that's needed. Or, having the rotors
turned and new pads, then applying proper torque.
Next, driveline. This one is a bit tougher. The wheels feel fine, the
car drives well, but at a certain speed, you start to feel a pounding through
the seat of your pants. There's a sound also like something hitting something
else, but not a pinging - more like a moan. You might think that the problem
is in the rear of the car, but if you put your hand on the driveshaft tunnel,
you can feel the vibrations. Steering wheel is not affected either. This
is pretty common, starting around 65-70 mph, and going up from there, in
cars that have about 60k on them. This vibration generally comes from the
driveshaft: it indicates that the U-joints have started to fail or that
the center support bearing has started to wear out or both. You can replace
the center support bearing, but Nissan says the U-joints aren't replaceable.
They are, but it takes a certain level of shop to do this. Because of its
design, some folks replace their two piece unit with a one piece one that's
made up custom.... Nissan's price on a replacement driveshaft is over $600,
while custom one-piece units can be had for less than $300 from an outfit
in Texas....
Other sources that are even less likely relate to the driveline also.
If the driveshaft has been a problem for quite a while, it may have damaged
the rear bearings in the transmission.... NOT a good sign! Also, the power
halfshafts out of the differential have a CV joint at the inner and outer
ends. These can wear out, but do so much later in a car's life than the
driveshaft U-joints and the center support bearing.
Tom B(WI)
Drive the car at the speed at which it is vibrating and shift the trans
into neutral. This eliminates the driveshaft and/or center support bearing
as a possible vibration source. If the car is still vibrating, then the
source is the wheels and/or tires being out of balance. I believe that
since the engine will not be putting force on the driveshaft when in neutral,
a bad center support bearing will be more noticeable...the contrast of
no twisting force with the application of twisting force right after should
be pretty noticeable...
Chris (Miami)
