On several occasions, when driving behind either an ambulance or fire
truck, I have noticed that they typically have several chains hanging
off of their rear axles.  I can not, for the life of me, figure out
what the chains must be for and I've become obsessed with finding out
what their use is.

Howdy ryanhan-ga,

As a dispatcher for our local volunteer fire department, I know that clear
radio communications is very important.  That is the reason that some fire
trucks and ambulances have "drop" or "drag" chains on them.

From the Wilson Antenna web site.
http://www.wilsonantenna.com/generalq.htm

"... I have a high background or static noise in my radio, but when I go
through rain the noise disappears.
...
Radial tires are the source of this problem. In some of the radial tires
used today, a static charge builds up within the tires. Driving on a part
of pavement that is wet causes this static to discharge. The most common
solution is to install a drop chain on the rear axle."

Some people disagree that it does that much good any more, so it might be
from force of habit these days.

This ham radio message base on lists.contesting.com provides some thoughts
on this.
http://lists.contesting.com/pipermail/towertalk/2004-March/062805.html

"No, actually the straps did the job of bonding between the car's body/frame
and the ground and really did work until the tire manufacturers learned to
add conductive agents to tire rubber to make them conductive."

If you need any clarification, please feel free to ask.


Search strategy:  Personal knowledge.

Google search on: car OR truck static radio chain OR strap
://www.google.com/search?q=car+OR+truck+static+radio+chain+OR+strap

Looking Forward, denco-ga - Google Answers Researcher

denco-ga,

Great answer!  This has been driving me crazy, and now I finally know!

I had wondered the same thing. Thanks so much for the answer, Denco!

My pleasure, Pink!

I have noticed that large trucks also will have these chains as well, so I
imagine that it is the larger amount of metal in trucks, ambulances and fire
trucks that causes a higher level of sensitivity.

Emergency vehicles also have to transmit and receive on a broader range of
frequencies than most vehicles, again, probably exposing them to a broader
range of "noise" on their radios.

Looking Forward, denco-ga - Google Answers Researcher

I had always understood that the chains were there to avoid build up
of static electricity  - possible sparking -  as a safety factor. 
This was before mobile radio communication was so common.  Certainly
for ambulances this would seem of sensible precaution in light of the
presence of oxygen bottles, electronic equipment ....  The firetrucks
may have oxy-acetylene cutting equipment on board.

Much thanks for the kind comment, 5 star rating and generous tip, ryanhan-ga.

My pleasure!

Looking Forward, denco-ga - Google Answers Researcher

Any static charge that might have built up is dissipated almost immediately
when the vehicles stop, myoarin-ga.  It is the "rolling" noise that needs to
be eliminated or kept to a minimum.  For instance, I have seen lots of trucks
that carry oxyacetylene supplies with no drop chains.  Good thought though.

Looking Forward, denco-ga - Google Answers Researcher

I have another comment about your question. I was a paramedic for 12
years and have been a ham radio operator for 30 years, don't recall
ever having chains on any of my ambulances for improved radio
communications. What you may be seeing are automatic tire chains for
ice and snow. They're very common on emergency vehicles in colder
areas of the country. When they are not in operation you can see them
hanging near the rear axle. From one manufacturers website, here's how
they work:

An electric switch mounted in the cab provides 12 volts to an air
solenoid mounted on the vehicle's frame rail. Compressed air to the
solenoid is supplied from either the vehicle's onboard air system or a
12-volt compressed air kit.

When the dashboard switch is activated, the solenoid opens allowing
compressed air to enter the air chamber and lower the chainwheel so it
contacts the inside of the tire. The friction between the tire and the
rubber-covered chainwheel causes the chainwheel to rotate, creating
enough centrifugal force to flail the chains out in front of the tire.
(The principle of the system is similar to a small generator driven by
a bicycle tire to operate a headlight.)

Six lengths of chain spaced at 60-degree intervals on the chainwheel
ensure that there are always two (2) chains between the tire and road
surface whether you are accelerating, braking or are in a wheel lockup
condition. The traction from the chainwheel is obtained in forward OR
reverse.

When the dashboard switch is turned off, the solenoid exhausts the air
provided to the chain units and return springs in the air chambers
bring the chainwheels back to their resting position.

Do a Google search for automatic tire chains and you can find some
pics that will illustrate better how these work. Just from the
description it's kind of hard to visualize.