Toy Heavy Duty Lowboy: question

Some small outfit is offering plans for a toy (model?) heavy duty lowboy. You can see the project at: http://www.woodcollectortoys.com / (I have nothing to do with this toy designer!)
But on a forum based woodworking site, http://www.woodworkingcrafts.com/forums / two question have come up:
1. What is the name of the part of the tractor trailer setup that adds extra axles and wheels between the neck of the lowboy, and the 5th wheel of the truck's tractor? It looks like 8 extra tires to assist weight distribution.
2. What is the purpose of the two wheels and an axle at the top of the detachable neck?
--My wild guess has to do with low ground clearance of the lowboy, and maybe the two wheels would act like a sacrificial tires to keep the lowboy's neck from getting bent or damaged by uneven ground at the job site, or they could be just spare tires for the rig. (Wow, wouldn't it be a Bit** if at a job site the neck got damaged and the load couldn't be unloaded.)
Hopefully someone here was once a heavy duty rig driver.
Thanks In Advance.
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Phil Again, Have never been a heavy duty rig driver, but that lowboy looks like the kind that comes apart at the front end so that the excavator would drive off forward. After the lowboy is separated, the two wheels and axle that you refer to are likely attached to the lower end of the neck of the lowboy (which is now completely separate from the rest of the lowboy) to allow the tractor to pull the neck away from the rest of the lowboy prior to unloading. Kerry
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There may be other local terms, but in the northwest this extra unit is called a jeep. Jeeps can be designed with single, tandem or less comonly tridem axles, they normally have four tires per axle but extra heavy units may have eight tires per axle. The model unit is not exactly acurate as the jeep 5th wheel should be quite a bit forward of the axles in order to transfer a portion of the trailer load to the suspension of the tractor. This requires that the goosneck of the trailer have an extension to position the coupler pin farther forward to increase the clearance for the rear tires of the jeep. These gooseneck extensions are usually pinned on and can be folded up and back when not in use but some goosenecks have movable 5th wheel coupler pins.
See http://www.aspentrailer.com/ for many examples of custom heavy haul solutions.

Some very long goosenecks have liftable, free steering air suspension axles that can be used instead of a jeep to add load capacity. I have never seen these types of lift axles to be carried as high as on the model but YMMV. Gooseneck lift axles are not nearly as effective as jeeps as they usually only have two tires.
The added tires and axles are needed to spread heavy loads over more pavement and to reduce point loads by spreading the load over more length when crossing bridges. Different geographic configurations are needed because jurisdictions use non standard specifications when designing roadbeds and bridges and because of seasonal variations of roadbed capacity, especially spring thaw which lowers pavement capacity and requires more axles depending on season. Highway engineers use a 'bridge formula' to determine allowable loads and axle spacing, in general the longer the better.
When these heavy haul units are travelling empty the extra axles and jeeps are removed to reduce tire wear, fuel consumption and vehicle size.

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WOW,
That is just the answer I was looking for. Clear and well written. Thanks.
Phil
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