"Phil Again" wrote in message
...
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.

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.

2. What is the purpose of the two wheels and an axle at the top of the
detachable neck?

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.

--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.