RE: bearings or bad tire?

["Eric Billing" <eric.billing@metaphasetech.com>]

I originally wrote:
>Bad news, is that while the new tires and allignment make things
>considerably quieter, I have discovered a "throbbing" vibration which
occurs
>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
throb
>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...

Doyt,

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.

Thanks,

Eric