Why do some trailers have a drum attached.
November 15, 2011 10:45 AM   Subscribe

Why do some semi-trailers have a plastic barrel attached to a frame structure underneath the deck and behind the rear axle? They are on their sides, (GIS fails finding an example.) They look like they may be used to hold water and provide weight for braking.
posted by Keith Talent to Travel & Transportation (10 answers total) 1 user marked this as a favorite
I'm not sure what you mean. Like at the very rear of the trailer? If so, that's probably there to prevent underride crashes.
posted by sanka at 10:56 AM on November 15, 2011

Response by poster: Yes, at the very rear. And that crash theory seems plausible.
posted by Keith Talent at 10:58 AM on November 15, 2011

It sounds like a counter-weight for balance and to keep excess weight off the tongue of the trailer. Perhaps something about the load being carried prevented required it to be positioned forward on the trailer.
posted by exogenous at 11:11 AM on November 15, 2011

I don't know.... A 55-gallon barrel of water weighs ~ 450 lbs. Google says an empty semi trailer is ~13,000lbs, depending on how it's built. It seems to me that putting 500 pounds that close to the rear axle (i.e., on a very short lever) is not going to appreciably affect the balance of a 13,000 pound rig. It also seems unlikely that a mere plastic barrel can keep something heavy and fast-moving from going beneath the trailer.

I vote that they're actually being used to hold something for the purpose of having it.
posted by richyoung at 11:43 AM on November 15, 2011

It wouldn't prevent something from going underneath, it'd mitigate damage from something going underneath.
posted by cmoj at 11:44 AM on November 15, 2011

Seems unlikely that a plastic barrel could contain enough anything to take any appreciable load off the tongue of a semi trailer.

I'd think:
Make the trailer more visible to tailgaters?

Provide a buffer to prevent banging something important on a low loading dock edge?

Protect important parts from low-speed rear-end collisions?

Convenient place to carry a spare barrel?
posted by chazlarson at 11:47 AM on November 15, 2011

I believe they are used as a place to store tie down straps. This was on a flatbed, correct? Not a dry van?
posted by cosmicbandito at 11:59 AM on November 15, 2011 [1 favorite]

Is it to hold the chemical that is required by law to be mixed in with diesel fuel in newer engines to reduce the particulate matter in the exhaust?
posted by roboton666 at 8:58 PM on November 15, 2011

Maybe it contains salt/sand to use for traction in icy weather?
posted by dreaming in stereo at 12:06 AM on November 16, 2011

I think you may be referring to the air reservoir for the brakes or rear suspension for an air ride trailer.

Some trailers have airbags as springs to provide a smoother ride for delicate freight. These airbags are adjustable, and have a reservoir to hold air. Most of the suggestions here are just plain wrong: Flatbed trailers usually have welded square toolboxes on the sides for tarps and straps, etc.

Trailer tongue weight is adjusted by sliding the tandems back and forth under the trailer. Take a look at any 53 foot trailer and you'll see a spring arrangement and if you look more closely, a small lever that unlocks the tandems and lets them slide along a track on the bottom of the trailer. This helps distribute the loaded weight of the trailer by changing the 'pivot point' for the weight and balance.

Preventing undderruns is the job of the ICC bar.

I suspect what the poster is seeing is the fuel tank for the refrigeration unit on a reefer.

Trailers for hauling refrigerated or temperature-sensitive goods have a massive diesel-powered refrigeration unit mounted on the front of the trailer. This must have it's own fuel tank, since refrigeration must be maintained on the load even if there is no tractor attached.
posted by pjern at 8:35 AM on November 16, 2011

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