Anyone have the answer? I see them hanging from a near the rear axle of school buses and they jingle a bit as they occasionally bounce along the ground. All I could think was grounding…but why would that be?
Someone will be along shortly with a technical answer, but you are right in that they are for grounding the vehicle. Many large trucks have them as well.
I can’t tell you how glad I am to see this question here. I’ve seen the same thing on fire engines and have always wondered this myself.
*Thought I’d add the fire engine part to possibly trigger someone’s memory… I assume it’s the same with fire trucks as it is with school busses.
They’re on combines, too. My family owns a John Deere 4400 combine, and there’s a chain on the back axle. I was told that it was for grounding the vehicle while on the road. Sounds reasonable, I guess.
-Brianjedi
I’ve never seen them on school busses, but I’ve seen them very, very, very often on fire engines and ambulances.
The chains are self deploying tire chains, made by a company called OnSpot. The chains are hanging from a rubber-edged wheel at the end of an arm. When you want to use the chains, the arm is moved so that the rubber edge of the wheel hits the inside edge of the tire. This contact starts the wheel spinning, flinging the chains outward. As they rotate past the tire, they’re flung underneath, acting as ordinary tire chains. Once you are out of the slippery area, push a button, and the chains go away.
There’s a much better description here: http://www.onspot.com
For traction then, not grounding, right, Jeremy?
So the Onspot thing is a totally new idea to me.
But l have seen cars & commercial vehicles in the Uk with a black strip coming down from the bumper/fender whats that for?
Nah…these aren’t for traction, though that sounds like a good idea posted back there.
These chains are just hanging, constantly. I saw them yesterday and theres certainly no snow on the ground. And they don’t spin. They just dangle and jingle.
So if they ARE for grounding the vehicle, why? My car doesn’t have them!
I was told a few years ago that the black strip on the car bumper was for grounding, to eliminate electrical build up in the cars body, thus curing travel sickness. I know that when i get out of my car sometimes, i get a zap when i touch the door, so i am thinking that the chains on fire,ambo vehicles etc. is to stop them from blowing up while refueling. Just a thought…
IIRC:
Metal objects (that can conduct electricty) moving through a static magnetic field produce an electric current or electric potential. This is the basis for generators: spinning coils of wire near several fixed magnets produces electricity.
The same thing happens with vehicles: your car (a metal object) moves through the Earth’s magnetic field and produces a charge. But since it’s riding on rubber insulating tires, the charge builds up. It discharges: 1) as you drive if it’s raining, as the water will conduct the charge over your tires’ surfaces, 2) slowly upon stopping (the tires aren’t 100% insulating), 3) or if you get out of your car by placing a foot down before using the door to pull yourself out, you’ll get a static shock on your hand.
Bigger vehicles will produce more of a charge, one that can’t be dismissed as an annoying shock. (Bus passengers would get pissed getting knocked on their feet, and fuel trucks and sparks don’t go together. 
) Hence, they drag a chain to constantly drain the electric charge as it is produced.
How come this appears to be fairly recent and not universal? My school bus years ago didn’t have chains hanging down and we certainly never got electrocuted getting off the bus. Are people jumping on the bandwagon a little bit?
This would apply only if the metal object were part of a complete circuit. A ferrous mass that passes through a magnetizing field developes the potential to force electrical current through a circuit if one exists. If no circuit exists, the object retains a magnetic charge (if the field was relatively unchanging, like that of the Earth as opposed to that of a 60 cycle AC). In order for an object to actually build up and retain a static electrical charge, the object must possess electrical capacitance.
Virtually all things have some amount of capacitance (that’s how static electric shocks are possible) but the amount is so small as to be negligible. Even electrical components manufactured specifically to have capacitance do not commonly store a quality of charge that could be considered dangerous to humans.
To wit: The capacitance of a parallel plate capacitor =
[sup][sym]e[/sym][/sup]0×A
d
Where A is the mutually common facing area of the conducting plates and d is the distance between the plates. From this we see that effective capacitance diminishes as the conductive plates move farther apart, or are aligned in such a way that they no longer face each other along parallel planes.
The amount (or Quality) of charge a capacitor can store is determined by its value of capacitance and the applied voltage (in this case, whatever tiny potential the vehicle’s body will posess by moving through the Earth’s magnetic field).
Q = CV
Where Q is the charge in coulombs, C is the capacitance in farads and V is the voltage in volts. It is worth noting that manufactured capacitors are almost always made with values in the [sym]m[/sym]F (microfarad) range. I would insist that unintended, (or stray as it is usually called by engineers) capacitance typically appears in the pF range, possibly lower.
I only bring all of the boring technical details into the discussion to show that multiplying the appropriate powers of ten for pico (Farads) and milli (volts) will result in a quality of charge in the microcoulombs range under the best possible circumstances.
Supposing the “big metal object moving through the Earth’s magnetic field” theory was accurate, how do we drain what surely must be an enormous charge away from airplanes, which are huge metal objects that rocket through that field for hours without touching any grounded objects?
I agree, and I would be very concerned to see a chain hanging from a gasoline tanker, this would surely generate some sparks as it rubs against the pavement, no?
If there is no better explanation, I’m apt to say the chains are there for superstitious reasons, or out of paranoid fear.
Um… No… The ARE for traction. They are self deploying tire chains, just like KCB615 said. Go take a peek under one of those buses.
While the self-deploying chains clearly exist, they are not used on any of the busses here in the Boston area. The chains here are for grounding.
They may be more common for the same reason people are saying they get more shocks from their cars. Tire compounds have changed over the years, and today’s tires build up more static charge. I don’t have the cite, but I remember the Car Talk guys talking about it and they had the data to back them up. I’ll check out their website.
Yeah, Click & Clack have discussed it. I’m too lazy today :), but try searching under ‘low rolling resistance tires’, and I think you’ll find some columns about this. Basically, the tires used today to improve gas mileage by decreasing rolling resistance are also much better electrical insulators than the tires that were in common use twenty or thirty years ago. This is actually a bigger problem for heavier vehicles, but if you’re having problems with static discharge when you exit your car, there are discharge strips you can buy to serve the same purpose as the chains you’ve seen on trucks and buses.
Gotcha. I stand corrected.
I’m apt to agree with Attrayant. I could see grounding being useful for ambulances or something but I wouldn’t be suprised if other vehicles have picked them up just out of superstition or paranoia… Sorta like lightning rods. And the psycho anti-static policies we have at work!
I think the chains/rubber strips are used to easily check the air pressure of the tires and/or shocks. If they are touching the ground then the air pressure or suspension needs to be checked.
I don’t think the use of chains to ground moving vehicles is superstitious. I know that I often get a zap when I get out of my truck. I suspect that the dryness of the air in Colorado makes static build up easier.
I do remember hearing about how tire material affects this process. I seem to recall that modern tires were actually better than older tires because they were less effective insulators. I think rolling tires make the vehicle behave a bit like a Van De Graf generator, slowly building a charge in the body.
Here’s a tire tidbit I heard from my brother, a patent attorney, who heard it from a chemist. Apparently modern tires are made out of long polymers, and as the tires heat up through use the various molecules are forming new bonds and ever-longer chains. He claimed that over time that a tire will essentially become one giant molecule.
After looking at this I may have to apologize to KCB615 for my disbelief. Seeing how they connect, they very well may be the chains I’ve seen. I didn’t imagine they’d hit the ground when not deployed though. Now I’m torn between grounding and OnSpot!