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Behaviour of semi trailers in tight sag curves

Behaviour of semi trailers in tight sag curves

Behaviour of semi trailers in tight sag curves

I'm a civil engineer, but this issue may be relevant to automotive engineering, so I want to hear your opinion on it.

I have to design a road for a large semi-trailer. In my simulations, the back end of the semi-trailer always snags the ground in sags.

There are 4 rear axles and no front axles. In the simulations, as the truck moves into the sag, more and more load is put on the rear most axle till it's completely inside the chassis. Is this even possible?

What would be a realistic distribution of load in this case.

RE: Behaviour of semi trailers in tight sag curves

Not statically determinate.

Combined suspension and tire spring rates, unequal compression as the roadway drops out from underneath the more forward axles, whether the axles are all independent of each other or if any two are in tandem (and which ones), maybe even bending deflection of the trailer frame, severity and extent of the 'sag', etc.


RE: Behaviour of semi trailers in tight sag curves

Big gun carriages and super-heavy lowboys typically put the rear axles on a bogie that connects to the trailer via a hinge, and in the case of four rear axles, might have two sub-bogies, which sort of implies the presence of two husky goosenecks at the ends of the load platform.

... but I don't do that stuff for a living.
You should probably be talking with people who do.
Let's hope that some of them show up here.
... or you can call some trailer builders on the phone, and social engineer your way to the people who know this stuff.

Mike Halloran
Pembroke Pines, FL, USA

RE: Behaviour of semi trailers in tight sag curves

Wait - you're designing the road, or the trailer, or both? Is this to handle a particular semi-trailer?
Or are you looking to accommodate a range of commercial vehicles?

Jay Maechtlen

RE: Behaviour of semi trailers in tight sag curves

I have to design testing ramp for military vehicles, but they use a civilian low loader for their tank transporters.

You can see the exact configuration here: http://www.defence.gov.au/jlc/Documents/Cleaning%2...

Most other vehicles that will drive on this road are obviously of military spec and have no problem with any conceivable road geometry.

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