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RE: bearings or bad tire?

I originally wrote:
>Bad news, is that while the new tires and allignment make things
>considerably quieter, I have discovered a "throbbing" vibration which
>at about 65mph (which is 2800RPM in 4th on the auto-tranny).
>Some additional diagnostics:
>a) 2800rpm in 3rd (about 50mph) has similar noise but much less noticable
>and a different frequency.
>b) below 65 (say 60 mph) there is no noticable noise. Above 65, the ambiant
>road noise drowns out the throb...
>c) 65 mph in 3rd (about 4000 rpm) results in too much engine noise, so
>may not be present
>d) no shake in the steering wheel

Doyt Echelberger replied

Eric...You didn't mention air pressure. I'd consider testing the effect of
running at the upper end of the tire pressure recommendations, like around
34 pounds cold.  Then I drop the pressure 2 pounds at a time and  test into
the lower safe ranges, like about 26 pounds cold. This would give me data
that would help me understand what I was dealing with.

At all the pressures, I'd do figure 8's in an open parking lot, to see if I
could reveal any rumbling that might indicate faulty wheel bearings.  I
always do the air pressure running test anyhow, when I get new tires. Tells
me where they run best.


To which I have this reply...


Since I'm not hearing any unusal noises below freeway speeds, your
suggestions to do figure 8's seem off the mark. None the less, I can try to
see if altering pressure has an apprciable effect on the throbbing.