Since the chains I’ve seen on school buses don’t touch the ground (they hang several inches above), I tend to discount the electrical theory. And since they are, in fact, automatic tire chains, I’m inclined to believe the traction theory.
That doesn’t mean that fuel trucks don’t use chains for grounding to avoid nasty sparks.
I’ve seen chains on trucks and buses (unlike some of the above, I see less now than I used to) and these were NOT the traction devices that are available. There were usually only one or two chains hanging down in the back of the vehicle. Some semi’s had them connected to the frame near the “fifth wheel” or whatever the connection device is called.
A couple sites:
http://www.gi.alaska.edu/ScienceForum/ASF7/717.html
and
http://www.uspto.gov/web/offices/ac/ido/oeip/taf/def/361.htm
I don’t think it has anything to do with the earth’s magnetic filed. Instead (and this is a SWAG), I believe static build-up is the fault of three things:
The wind creates a build up of static charge on the car when you’re driving. The rubber tires have extremely high resistance, and thus do not discharge the car to earth ground. Dry air keeps the electrons from bypassing the tires. Again, just a SWAG.
You’re right about that. A loop of wire travelling through a magnetic field will, indeed, induce current on the loop. But a lump of metal travelling through a magnetic field does not build up static charge. Well, there will be a tiny amount of positive charge on one end of the lump and negative charge on the other, but it’s maintained by the movement and will disappear once the movement stops.
The school buses in our district are all equipped with these chains. My nephew is a mechanic for the district and confirmed that these really are self deploying tractions devices. The chains are mounted to a wheel driven device that can be lowered with air pressure. A small rubber wheel is pushed against the inside of the rear inner tires of the bus. This causes it to spin and the chains that are attached to it are thrown one by one under the front of the tires. I have been seriously thinking about checking to see if I can have them installed on my motorhome. With the limited clearance and fiberglass body I would rather not run regular chains. The real beauty of these is that if a chain breaks it simply falls off and does no damage to the vehicle. Grounding straps on the other hand are single in design and hang from the body of the vehicle and not the axle.
12 year old zombie thread, but still…
Here is a write-up on the Onspot automatic chain system.
Aircraft do build up major static electricity.
They have static discharge wicks along trailing edges & other ways to keep it dissipated.
Many have a stiff piece of small cable that will drag on the ground attached to the landing gear.
I remember from years ago that almost all fuel tankers had chains for static discharge, then they went to stiff graphite straps.
Also, there has been cases of people lighting off the fumes of refueling their cars because of undissipated or build up of static electricity from the fuel flowing through the hose.
Oldest ugly website ever? 12+ years old, and it doesn’t look a day over 18. Probably completely uncha(i)nged since 2001.
website has no traction.
Zombie thread about chains, … on Halloween.
