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Last Changed 8/12/2012 |
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In Medium Duty Truck the case was made for a Medium Duty Truck (MDT) over a Dually. Now
the case will be built for our decision for a Heavy Duty Truck (HDT) Conversion. |
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| Class |
Weight Range
GVWR |
Typical |
Manufacturer |
| 1 |
0 - 6000 |
small 1/2 ton pickup |
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| 2 |
6,001 - 10,000 |
1/2 & 3/4 ton pickup |
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| 3 |
10,001 - 14,000 |
1 ton dually pickup |
Chevrolet = Cnnnn |
| 4 |
14,001 - 16,000 |
MDT, F450, C4500, Ram 4500 |
Dodge = Ram nnnn |
| 5 |
16,001 - 19,500 |
MDT, F550, C5500, Ram 5500 |
Ford = Fnnn |
| 6 |
19,501 - 26,000 |
MDT, F650, C6500, FL60 |
Freightliner = FLnn |
| 7 |
26,001 - 33,000 |
HDT, Local tractor, F750, C7500, FL70 |
Kenworth = T2000 |
| 8 |
33,000 + |
HDT, Over-the road tractor C8500, VNL, T2000 |
Volvo = VNL |
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By general definition MDT means those trucks between pickups
local/over-the-road tractors. However, the real MDT
characteristics that make them better tow vehicles start in the
Class 6 category.
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A HDT Conversion is taking a used semi-tractor and modifying it
for fifth-wheel towing. The normal conversion for a HDT tractor
is to remove one of the rear axles. This lowers the weight
class from Class 8 to Class 7. Often, the rear axle is moved
forward and the frame is shortened. This makes the converted
truck more maneuverable. |
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A Volvo VNL 610 (left) and a Converted 610 (right). |
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Here is a simple comparison of what a Class 6 generally offers compared to a HDT Conversion:
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Class 6 MDT |
HDT Conversion |
| Horsepower |
250 - 350 HP |
400 - 500 HP |
| Torque |
800 - 860 lb-ft |
1,400 - 1,600 lb-ft |
| Transmission |
Allison Automatic |
Eaton Auto-Shift, stick
Automatic starting 2004 |
| Engine Brake |
Exhaust |
Jacobs Brake |
| Sleeper |
No |
Yes |
| Rear Seats |
Probably |
Rarely |
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Depending on the state, with the addition of a few items, a
converted HDT with a sleeper usually can be registered as a
motorhome. The reason for a motorhome registration is to have a
private vehicle registration. Some states allow an HDT
to be registered as private without needing the motorhome items.
These are the usual additions: |
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| Usual Requirement |
Usual Solution |
| Cooking |
A microwave oven |
| Refrigeration or ice box |
A small refrigerator |
| Self-contained toilet |
A Porta-Potty |
| Heating, air-conditioning, or both |
Usually standard with the sleeper |
| A potable water supply system including a faucet and sink |
Many variations |
| Separate 110-125 volt electrical power supply |
Add AC Extension cord outlet and add an inverter |
| An LP gas supply |
Not usually added |
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Usually the addition of four of the seven items qualifies the
converted HDT as a motorhome. These additions can change the
driver license requirements and insurance.
We wanted a used truck because the Federal Government collects a
12% tax on new tow vehicles called the Federal Exercise Tax
(FET). This FET is bad in regards to RV towers because it is a
disincentive to have the correct vehicle for pulling large
trailers. The tax originally was to collect funds from
over-the-road truckers to help finance the highway system. But
as things go, any tax tends to get stretched to include sources
that were not planned for. And once the government starts
collecting tax money, it is very hard to stop. Be careful of
this FET because many truck sellers are really not clear of when
the tax applies and they are often wrong. The buyer is
responsible for the FET even if the seller does not collect it.
Some sellers are posing that to avoid the FET, you buy the truck
stripped and then add the hitch afterwards. The FET applies to
the truck and anything added to it in the first 6 months. This
includes a truck that did not qualify for FET at time of
purchase but has the qualifying equipment added in the first 6
months. That truck becomes subject to the FET.
Used MDT haulers run in the $60,000 to $65,000 range. They have
the advantage of having automatic transmissions. The
disadvantage to us is the choice of crew or extended cab bodies
only. For some, the issue of the automatic transmission can be
the deciding factor.
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Club Cab
For families where more than two will travel, the club
cab (two doors, two seat rows). |
click on image to enlarge |
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Crew Cab
The crew cab (four doors, two seat rows) available with
MDTs can be the deciding factor. |
We did find one Crew Chief Aero-Condo during our search. We
really liked the layout of the Crew Chief but the one we found
was a stick shift and that was not acceptable for us.
The Eaton Auto-Shift is a standard 10-speed transmission with a
computer and servos added to disengage the clutch and change
gears at the proper time. Since there is no torque converter as
in an automatic transmission to provide slip, the clutch does
need to be fully disengaged at stop. This means the Auto-Shift
Transmission has to have the clutch pushed in at stop.
The clutch is released to start. From that point, the
transmission computer with synchronize the clutch as engine
speed to shift through the gears like an automatic transmission.
The clutch does not need to be used as much as it does in a car
at low speeds because the low gears of the Eaton 10-speed are
very low. You can creep at 1 or 2 mph in second gear.
The Auto-Shift is not as easy as an automatic transmission but
for many, it is far easier to drive that a straight manual shift
transmission.
Starting in 2004, full automatic transmissions were available.
These are sometimes referred to as two-pedal transmissions (fuel
and brake). These automatic transmissions have brand names of
Freedomline and Ultrashift.
A HDT Conversion engine will almost always have a "Jake Brake".
The proper name is Jacobs Brake System. The Jake Brake is an
extensive modification of the engine valve train that when
turned on, alters the valve timing to change the engine into an
air compressor which gives a braking action to the engine.
Typically, the Jake Brake is adjustable to use 2, 4, or 6
cylinders giving the driver an adjustable braking effort. In a
downhill situation, the driver turns on the Jake Brake (actually
it is usually set up to automatically come on). The speed of
descent can then be controller by switching from 3 to 6
cylinders to develop the amount of braking action needed.
This is a superior braking system compared to a simple on-off
exhaust brake.
Many are getting Volvo 610/630 Conversions (more on this later) in
the $30,000 to $40,000 range. Add a deluxe bed and you would be
around $50,000. So you can have more truck for less dollars
than a used MDT. If you are handy, many owners have made their
own beds.
For around the same money as a used MDT, we could get a HDT
conversion with a sleeper that would give us a small motorhome
capability. It would have more miles on it but it still would
be well within the useful life of the truck. We would have more
torque, horsepower. We would have to deal with an Auto-Shift
transmission instead of an automatic.
Drivers licensing for MDT and HDTs varies highly between
states. But let's start with a primary rule; what is legal for
your state in regards to vehicle and drivers licensing is valid
for all the other 49 states under the "full faith" clause of the
Constitution of the United States of America, Article IV.,
Section 1. No state law enforcement officer can override the
Constitution though some may try to overstate their authority.
Not that we recommend arguing with law enforcement officials but
they can be wrong and they can't ruin you day for being wrong.
If you MDT or HDT is licensed as a motorhome, then there is a
good chance that a Class C (auto) license is all you need. Some
states have started to add a license requirement for RVs and RV
combos over 26,001 lb. This includes motorhomes and motorhomes
with trailers.
For more discussion on licensing, see
Drivers Licenses
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In South Dakota, a non-commercial truck pulling an RV
only needs a Class C license. This is one of the
reasons we choose South Dakota as our residence state.
The current SD Code covering this subject is 32-12a-9.
see Drivers Licenses
and State Listing of RV Drivers Licenses. |
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HDT Financing
See
HDT Financing
Therefore we decided that we wanted a HDT Conversion over a
MDT. Now the question was, which HDT to convert. Our research
found the Volvo VNL series to be
the truck of choice.
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