I got stopped at a CSX line today and watched the intermodal train pass.
The train contained quite a few “Spine Cars”. I am not going to describe these very well, but …
The spine car held one trailer. Where the landing gear of the trailer would be resting on the car, there was nothing. The trailer was held in place by a built in “5th Wheel”. The rear tires of course were on the floor of the car.
These didnt look real secure. I know that spine cars have been operating for years and they would not be out there if they were unsafe…but still, it appears the dynamic action (lateral) of the car would cause the trailers to tip over, particularly with the higher center of gravity.
I know there are a lot of engineering type folks on this forum and quite often you get talking about the physics of cars, motion, etc.
Can you explain these cars and how they work? I know the lighter weight of the car, due to the lessor amount of steel is a key factor in the efficiency of the cars, but have there been issues with these cars?
As a former OTR driver the only attachment point for a trailer to the tractor besides the air hoses is the kingpin. That pin is around 2 inches thick and made of hardened steel. The king pin has a grove cut into it were the locking jaws of the 5th wheel actually wrap around the pin. they are actually more stable hooked to a 5th wheel than when tehy are standing alone.
In general I would say that the ability of the trailer to take torsion, a twisting motion along the axis, is what keeps the whole system stable. That being said there should not be a lot of torsion acting on the trailer body, other than from dynamic loads, as it would indicate a misloaded trailer. The trailer is basically designed to use three suspension points.
Look at it from this point.
Does it tip over when the 18 wheeler is doing 70mph on the freeway and takes a turn?
So the 45 to 60 mph you will see on the railroad is less lateral forces than the trailer sees in it’s normal travel mode.
Add in the super elevated curves on the intermodal routes trains use…
Think about this…take a good look at an tri level Autorack…they look like they should fall over…running on tiny wheels, way too long and way too tall…
I am in agreement that it works, otherwise the cars would not be in use.
My point is that it doesnt look stable.
I understand how a trailer is attached to a tractor, having worked in trucking for 13 years before my present career…it just doesnt look right!
No doubt the center of gravity is not as high as it appears, it must be an optical illusion, or perhaps I had way too much time on my hands as that long intermodal passed me at 10mph yesterday.
Actually, about 1/2 mle away from me, yes! There’s an awful sharp curve on I-43/94 on the south end of Milwaukee, and tractor-trailers regularly jacknife or “miss” the curve there. Causes quite a mess, ties up traffic, and takes out a nearby exit sign! One of the more interesting accidents there involved a container of pickles in glass jars…
There’s probably a lot more weight in those spine cars than you would think, and usually the heaviest weight in the trailer would be at the bottom, so good ol’ gravity rules in this case. But remember the “good ol’ days”, when every TOFC was tied down to the flatcars with a few chains? Good thing intermodal technology has advanced a bit…
If I remember correctly the limit for CG is 96" for railcars such as intermodal spine cars. If you multiply the sum of the center of gravity of the trailer on wheels and the distance it is above the rails by the weight of a fully loaded, 65,000# trailer and then add the weight of the railcar multiplied by its center of gravity above the rails and divide by the total weight you should have 96" or less. Basically, the railcar weight allows the system to work. The only hope to drop the weight of a spine car below a certain point is to drop the height of the trailer above the rails.
They only reason it does not look stable is you do not see the wheels of a truck under it. It is no more or no less supported by the spine cars fifth wheel than it would be by a tractors fifth wheel. Unless its pin fails it is not going anywhere. The spine car weighs alot more than a tractor does. The trailer weighs much less than most freight cars loads. [2c] As always ENJOY