Something I find wonderful about our comments is that in addition to learning something from the article, you will almost learn something from the vast cauldron of knowledge from our readers. Sometimes, you’ll learn something new about a topic you didn’t even know you were curious about. Take the semi-trailers that are hauled by semi-tractors. Have you ever wondered why they’re called a “semi trailer?”
Today, the excellent Huibert Mees published a ridiculously deep explainer about why trailers end up swaying out of control, causing a whole mess and maybe going viral thanks to someone inevitably filming it all. It’s a great explainer and you should totally read it.
Andrew Wyman asked:
That was a great write-up Huibert.
I’m wondering if we have any long haul truckers in the comments, and if so, do they teach this kind of information to them since it is something that directly impacts their driving
My Goat Ate My Homework responded:
If you look at the location of the wheels on a tractor-trailer rig the wheels are so far back on the trailer it would be nearly impossible to have the center of mass behind the tires. So I’m not sure it really impacts their driving. They carry much more trailer weight on their rear drive axles (tongue weight if they had a tongue) and are designed to do so.
They do have to make sure that their weight is distributed to ensure that an individual axle is not overloaded. There are axle limits in most states. They can adjust the distribution by sliding the rear tire carrier on the trailer fore and aft to change the balance.
They also have spike breaks to help straighten the trailer out if needed.
Now, it’s time to learn something new, courtesy of Rust Buckets:
That’s actually why it’s called a semi trailer: a regular light trailer to tow behind your car has the axle roughly in the center of the trailer and the trailer carries 85% or more of its own weight. A semi trailer has the axle way at the back and carries more like 50% of its own weight, the other half resting on the truck. The trailer carries half of its own weight, giving you the prefix “semi-“.
So far as I can tell, Rust Buckets is correct! Trucking company Schneider mirrors what Rust Buckets says above, explaining that semi-trailers have rear axles, but no front axle. Full trailers can fully support themselves with both front and rear axles. Semi-trailers also use a fifth-wheel connection to reach the semi-truck. Ah, and semi-truck? That’s reportedly shorthand for semi-trailer truck.
Some of you come for jokes. Don’t worry, I have one for you! Red Bull wants you to pay $6,500 for a super fancy e-scooter. Lewin thinks something that looks this cool should go 60 mph. Robert L sees a business opportunity:
A 60mph scooter would be a godsend to the mortuary industry.
Have a great evening, everyone!
Top graphic image: Freightliner
Just to clarify, the weights allowed per axle are set by the individual states. Typically, the limit per axle is about 40,000 lbs. Sliding the wheels and the fifth wheel (which connects the trailer to the cab) can cause thes weights to vary. Depending on the load, this can cause the fifth wheel to be ahead of the center of the tractor wheels or behind them. Likewise, the placement of the trailer wheels will also alter the placement of the load. There are also rules about the distance between the tractor wheels and trailer wheels.Sometimes, a couple of inches makes a big difference between being legal and not legal.
Awesome. I’ve now been schooled in the seminary.
In the image with the trailer skirts with zoomy wind lines, are those for areodynamics? Always assumed so, but it seemed weird to me that they curve inside as you get closer to the cab.
THis is so air is directed to the sides of tires. ANd like the picture shows invisible air flow lines create a skirt around side of the tires without needing physical skirt around them
There’s also a device that can allow a “semi trailer” to transition into a “full trailer”, it’s called a “converter dolly”…
In an only slightly different universe, perhaps one in which Jason Torchinsky followed his term as Secretary of Education by a stint as the senior executive in charge of ABC children’s programming, that explanation of how the term “semi” (pronounced [ˈsɛmaɪ], y’all non-native English speakers who’ve never watched an episode of The Dukes of Hazzard, not [ˈsɛmi] like one of them half-a-house things) came to be would have made a compelling episode of Schoolhouse Rock.
Rust Buckets is a little wrong, it’s not axle placement, but as you mentioned a full trailer has axles front and rear (think hay wagon) a semi trailer just has rear axles. Source: CDL holder and previously an R&D engineer for a company that made trailer towing equipment.
Since we’re talking about semi trailer trivia and the mortuary industry in the same article I must bring up underride guards.
The steel bars that hang from the backs of large trucks to prevent a passenger vehicle from going underneath it the in a crash.
Those are colloquially known as “Mansfield bars”. Many of you may not remember Jayne Mansfield, but most of you probably know of her daughter Mariska Hargitay. Mariska was in the back seat of her mother’s car when, in a dense fog, her mother piloted her 1966 Buick Electra directly under a stopped semi trailer. Jayne was killed instantly.
(Cue the black and grey German rainbow with somber “the more you know” music)
Cool.
(it was a 1966 BUICK ELECTRA 225 CONVERTIBLE)
In the movie The Seven Ups, when Buddy is chasing the two criminals down the freeway outside New York City, at the end of the chase his Pontiac Ventura crashes into a trailer without the Mansfield bars. The end result is a makeshift guillotine that damn near kills him in the story, and even scared the shit out of the stunt driver who performed it during filming. The entire sequence was an allusion to how horrific Jayne’s death was.
And yet we rail against government mandated safety requirements.
That’s only because many of them offer far less safety than requirement.
The shit those old stunt drivers used to put themselves through.
For a single take?!
Macabre brush with celebrity:
My grandparents lived in Mobile, Alabama, and my grandfather was a construction foreman. Around that time, he had a long-term job in New Orleans, and was regularly driving back and forth between the two cities. He was one of the first people to discover the scene – he came up on the crash before the police or paramedics got there. It was only later that he found out that a famous person had been killed in that crash.
He didn’t see any of the blood or gore, or Jane Mansfield’s head lying in the roadway (an urban legend), but he did confirm that it was one nasty, nasty crash.
Of course we know of Mariska. She was in Ghoulies.
And The Perfect Weapon. I remember thinking Jeff Speakman was going to become a huge star like Chuck Norris after that move came out. Without visitng IMDB, I can’t even name another movie he was in.
A buddy of mine came across a crash where the woman driving was almost unrecognizable from the chest up after running under a semi-trailer in fog. Her baby was unharmed in the back seat, though the rear facing car seat was pushed firmly against the back of the back seat. Would have gone further if it hadn’t impacted the rear tires. It’s something that still haunts him today.
The Mansfield bars have gotten better over the years in the US, but we are still far behind what is mandated in some EU countries. I’m not saying we should retrofit existing trailers, but new manufactured trailers should be and can be made safer.
Just saw 2017ish 4 door F150 after it rear ended semi-trailer on 30MPH road. THe top of the cab was bent and folded into the bed. The semi-trailer’s end was where F150s bed start.
“you will almost learn something”
Did you mean semi-learn? 😉
Darn it! “… almost always learn something…”
But now it looks silly so we have to keep it. 🙂