My assessment of why the problem occurred could well be wrong. Perhaps 
if I describe it a better explanation can be found.

The vehicle in question was a 1996 Volvo 850R high boost turbo that I 
purchased new. The tires were 17" low profile, and I don't recall the 
brand name that was installed as the OEM.

In normal city traffic, AM radio reception was fine, as it was stop and 
go traffic with lots of application of the brakes and never hitting a 
very high speed. When cruising along the interstate I noticed the 
background noise would increase slowly, over the space of a few minutes. 
The background noise could best be described as broadband hash, no 
discernible tonal frequency to it.  It was strong enough to cover up 
stations that were at a moderate distance. I would guess something like 
20 dB desensitization but I did not make formal desense measurements.

One of my thoughts was alternator noise, so to test that I put the 
transmission into neutral and turned the engine off and coasted down the 
interstate at 60 MPH for a brief distance. No change in the noise level. 
Another test for alternator-related noise I conducted was to put the  
transmission into neutral and vary the engine RPM to see if the noise 
level or sound varied with the RPM. I could detect no change. Turning 
the headlights on/off and other accessories on/off made no difference.

When driving on wet pavement, no noise. This was only a dry pavement 
issue. On many trips (I was working in Northern NJ on a long term 
consulting contract at the time, so I was driving from the DC area to NJ 
several times a month) I would go from rain to clear skies and the noise 
always acted the same -- it would start to build up as the road and the 
car dried out. Or, in the other direction, it would quickly cease when 
the rain started or I drove onto wet pavement.

Touching the brake pedal cleared the noise up for a few minutes, but it 
would gradually build back up again until I touched the brake pedal 
again or it rained or I ran into wet pavement.

I found that spraying the tires with the cling-free did greatly reduce 
the noise level for a day or two and then it returned.

With absolutely no other changes in the automobile conditions, I had all 
four tires replaced when the tread had worn down. The new tires were a 
different make than the OEM tires. With the new tires the noise had 
vanished and never returned.

During the time the Volvo had  the original tires, I had them rotated a 
couple times so they had been removed and reinstalled on different 
wheels. No change in the noise level.

The only explanation that I could come up with that matched my 
observations was the the car body was being electrostatically charged, 
and the noise was  caused by discharges across small gaps and that the 
original tires had better insulating properties than the replacements so 
that the static charge did not have a chance to build up to the level 
that it would produce noise, and that the wheel bearings were acting as 
an insulator that was bridged when I touched the brake pedal. (I imagine 
the brake pads had copper woven into them. Or, they might have even been 
sintered metallic pads.)

So there it is. This was 100% repeatable under the conditions I 
described. I no longer have the Volvo so I can't conduct new tests, but 
I have a very good recollection of the problem and the tests I made at 
the time to pin it down.

Jack



Olin Lathrop wrote:
> Jack Smith wrote:
>   
>> Not so. It's well known phenomenon called  "wheel static" or tire
>> static. I  believe the actual insulating junction might be the front
>> wheel bearings that ride on a thin film of grease and do not have
>> metal-to-metal contact whilst the wheel is spinning.
>>     
>
> Hmm, sounds a bit implausible that significant static can be built up
> accross a thin layer of grease with moving metal parts on both sides, but
> maybe.  However then you said:
>
>   
>> I observed this consistently for two years until the new tires were
>> installed,
>>     
>
> Then why would new tires fix the problem if the static buildup is in the
> bearing, not accross the tire insulating the chassis from the ground?
>
>
> ******************************************************************
> Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
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>   
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