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I'm working on a project that requires me to design an overload control centre where overloaded trucks are allowed to redistribute or shed some of it's cargo in order to continue to travel on the highway.

As we are pressed for space I would like to make the design as economical as possible and maybe force the vehicles in a certain flow direction in order to ensure the longevity of the driving surface. But before I can do this I need to understand what kind of maneuvers are possible for the vehicles.

I am aware of sources that that has done that, e.g. AASHTO. There are also various plugins that exist for different CAD applications, some of which are free as well.

With this myriad of information, I still cannot find any information on how to determine this myself. It is trivial to do this for a car, but the moment the vehicle is articulated my attempts don't match what the sources tell me.

Can someone please explain to me how to determine the turning profile of any vehicle, given that I have the necessary information? My main concern is: How does the trailer react when a force is applied that is not perpendicular to its axle?

I know that I need certain information such as:

  • The maximum angle that the front wheels can turn,
  • The distances of the axles from each other
  • Where the trailer pivot is in relation to the truck and trailer, etc.
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This is not a very easy topic. You are going to want to figure this stuff out for yourself and then do some testing to verify your design assumptions work the way you think they will. For the max steering angle you should be able to get that from different tractor manufacturers.

Frankly if it were me, I'd be grabbing a tape measure, a set of old skool trammel points

trammel points

a digital camera, a pair of coveralls, a bright safety vest and a pair of safety glasses and a clipboard and head over to your favorite fleet or construction truck yard.

The safety gear says you're serious and professional about your business.

Trammel points make it MUCH easier to determine distance between two points in 3D. I use them a lot to create after the fact accurate 3D CAD models of car and motorcycle layouts for custom parts. They would be invaluable for modeling a truck 3D steering geometry. Generally I use Solidworks for the CAD modeling.

Tell the manager what you want to do, ask permission. It really HELPS to bring along a dozen donuts and coffee before you arrive.

You can easily measure most geometry on a stationary truck (engine off) and recalculate everything you will need to know. The only thing that might be tough is to figure out maximum steering motion. On many trucks you can figure that out from grease tracks & wear marks, but not always. Often the steering on large trucks has physical stops that mark the ends of steering travel.

I will say (and you're going to laugh at this) a nice piece of analysis on steering systems is something I found by Peter Verdone on skateboard truck analysis. His explanation of different steering axes really helped me understand how this stuff works. Check out this link. When you get into the details, its not as easy as you think it's going to be. And a tractor trailer will have even more complications. Excel spreadsheets are your friend.

Note too, there are a whole bunch of different configurations out there. Probably best to grab two or three and use them as examples.

Good luck with the project.

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    Thanks. This is a great answer, and probably the best way to approach the problem. I am looking for a less empirical approach though, that delves more into the math and geometry and gives results based on a set of input parameters. – ChP Apr 30 '16 at 12:19
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    Too many degrees of freedom. You are going to find many different steering designs out there, never mind the wheelbase length variations. But you could assume both front tires create a perfect Ackerman linkage and move on. Monte Carlo other dimensions (wheel base lengths, trailer lengths) and then proceed to Peter Verdone Skateboard steering analysis. Have fun with it. – zipzit Apr 30 '16 at 17:41
  • @Charl if you want to communicate more I've done a bunch of mechanism analysis and automotive design engineering work. Click on my id to get my email address. – zipzit Apr 30 '16 at 17:56
  • Oops. Wrong link sent above for the mechanism analysis. It should have been this one. – zipzit Apr 30 '16 at 21:51
  • This is a great answer +1 – DucatiKiller May 2 '16 at 22:59
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If you take the length of the semi trailer from rear axle to towing pivot, that's technically your minimum turning radius. A driver who can get his tractor unit 90degrees to his trailer could make the turn. But it would be hard on the surface and his equipment. Why not design a straight through layout? Truckers hate tight maneuvers in big rigs.

  • A straight through layout would definitely be ideal but unfortunately our space constraints don't allow for that. Figuring out the turning profiles is so we can see where to reinforce the paving and what space we have for auxiliary equipment. – ChP May 4 '16 at 10:56

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