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 At
the recent RV Show there was a Chrysler Intrepid on display,
hooked to a 7,000 pound Airstream trailer. There was a video
of it towing the Airstream to California, so I guess it
actually works. Did this car have aftermarket equipment
to increase its tow capacity? And could the same extension
of tow ratings be applied to pickup trucks that pull fifth
wheels? We have a truck, but do not yet have a trailer.
The pickup is a 2000 GMC 1500 series, 6,400 pound GVWR,
4.8 L V8 with automatic transmission and a 3.42 ratio rear-end.
After a vacation in BC and Alberta we noticed many RVs on
the road. It seemed that with a pickup truck already in
hand, a good fit for us would be a fifth wheel trailer.
However if one visits some of the internet forums, the general
consensus is that you need a huge truck to pull almost anything.
I wonder if this is just the some people's preponderance
to have the biggest truck. Surely if a passenger car can
pull 7,000 lbs. then a pick up truck can pull more! Our
truck is a very civilized vehicle to drive when lightly
loaded. It is much like a passenger car and probably gets
fuel consumption rates not too far below the Intrepid. It
would be interesting to hear your thoughts. Who wants to
run around in a rough riding, noisy, smelly, diesel super
truck when disconnected?
Regards,
John Garnish
Hi John,
Actually some people really like driving their super trucks.
They are becoming much quieter and rarely are they smelly.
However, they are still not as quiet, smooth or economical
as your GMC 1500. The 4.8 L V gives excellent economy, likely
the best in a pickup, and it performs very well. It can
be used to tow a variety of trailers.
The Intrepid you saw at the RV Show was mine. It is completely
stock, except for a transmission cooler and lower profile
tires. Your assumption that because the truck is bigger
than the Intrepid, it has to be a way better tow vehicle,
is an easy one to make and it is often done. Instead of
looking at the size of the vehicles, lets examine some of
the factors that will affect how they perform as tow platforms.
First, lets calculate which vehicle will move weight most
effectively when accelerating in low gear or climbing a
steep hill. The Intrepid's engine produces 241 foot pounds
of torque, which if we multiply by the gear ratios and divide
by the tire radius, will tell us how much torque it can
apply to the road. So, 241 foot pounds of torque, multiplied
by the 2.84:1 first gear, multiplied by the 3.67:1 final
drive, multiplied by the tire radius (measured in feet)
of 1.07, gives us a total of 2,347 foot pounds which the
Intrepid can apply to the road.
The way we were traveling this summer with people, bikes,
water, etc. the combination had a total weight of 12,200
pounds. Or, 5.2 pounds for each foot pound of torque available.
If we do the same math for your truck, it works like this:
285 foot pounds of torque multiplied by 3.06:1 first gear
multiplied by 3.42:1 final drive, divided by the tire diameter
in feet of 1.25, giving 2,386 pounds of foot torque applied
to the road. Since the pickup weighs 900 pounds more than
the Intrepid, the total combination would weigh 13,100 lbs,
or 5.5 pounds for each foot pound of torque available. So
for simply moving weight up a steep hill or accelerating
into traffic the truck really cannot do any more than the
Intrepid. However, the good news is that it can handle 7,000
lbs. (Page Top)
The next thing to consider from a power perspective is,
how much power will the truck have available to overcome
aerodynamic drag and rolling resistance at highway speed?
For this you need horsepower, and you need it at the RPM
you cruise at in high gear. If you do not have enough horsepower
at the right RPM, then the truck will be constantly shifting
back to a lower gear to maintain speed, which is not good
for the truck or for fuel economy.
These numbers are approximate because each combination
is different and I don't have a wind tunnel. At 95 kph or
60 mph, the Intrepid is turning 2,880 RPM and it produces
150 horsepower at that RPM. The truck is turning 2,300 RPM
at the same speed and it is producing about the same 150
horsepower. The Intrepid and Airstream combination requires
38 horsepower to move it through the air at 60 mph. At 60
most truck and fifth wheel combinations will require 70-90
horse power. The Intrepid's front drive has a bit less power
requirement than the rear drive on the truck and the tires
have less rolling resistance. It will need about 25 horsepower
for rolling resistance and drive-train losses, where the
truck will need about 35. Still, if you add it up the total
for the truck and fifth wheel is 105 horsepower, and we
have 150 available. This is great - until we meet a head
wind.
Aerodynamic drag increases exponentially at the square
of the speed. So a 20 mph head wind would increase the drag
on a truck and fifth wheel from 70 to 130 horsepower. This
is where it runs out of power.
We are often asked if a certain vehicle and trailer combination
will make it in the mountains. We usually tell people not
to worry about the mountains, but worry instead about three
days across the open prairies into a 20 mph head wind.
One answer to this problem would be to change the axle
ratio to a 4.10:1 which will increase the engine RPM to
2,760, further up on the power band of the engine, where
it is likely producing a more attractive 190 horsepower.
You also need to be aware of the load capacity of the rear
axle on your truck. Whereas 3/4 ton trucks have a 6,084-lb
rear axle capacity, your truck has only 3,700 pound capacity.
It will have about 2,000 pounds on the rear axle now - by
the time you add a fifth wheel hitch and fill the fuel tank,
you will be able to carry about 1,400 lbs of pin weight.
If you go with a fifth wheel, I would suggest installing
the fifth wheel hitch - one that will slide with the trailer
connected for easier maneuvering - right from the start.
Then you can connect and tow several different fifth wheels
before you finalize your purchase. If you tow into a head
wind you will see how the different models perform aerodynamically.
Further, you can tow it to a scale and weigh the rear axle
of the truck. Remember to allow for loading of the trailer.
Many fifths today have a large rear living area and bay
windows, so there is very little storage behind the axles.
Quite likely, 20 to 30 percent of the weight you add will
be on the rear axle of the truck. I would be a bit wary
of brochure weights, as sometimes the units get changed
but the brochures do not.
The other option to consider is a travel trailer. Your
truck has a short overhang compared to its wheelbase, a
relatively low center of gravity, and good independent front
suspension. A half-ton is generally much better suited to
tow a trailer than a fifth wheel. The aerodynamic drag on
a trailer is generally quite a bit less than that on a fifth
wheel, likely because there's no goose neck area to catch
the wind. You can tow a trailer and fifth wheel of the same
height, weight, and roughly the same shape, and the trailer
will have considerably less drag. The hitch weight on a
travel trailer is less than a fifth wheel's pin weight,
but more importantly, with an equalizing hitch it is distributed
to all four wheels of the truck. A trailer with 800 pounds
of hitch weight will put about 160 pounds on each truck
tire. You may have heard people say that a fifth wheel tows
much better than a trailer. But properly set up, a trailer
can tow every bit as well as a fifth and in some instances,
such as an emergency lane change, even out-perform one.
Stopping distances are generally shorter with a travel trailer,
as the some of the hitch weight is transferred to the front
wheels of the tow vehicle, and because there is a higher
percentage of the weight on the trailer wheels to begin
with. Since the trailer has a much lower center of gravity,
less weight is transferred from the trailer to the truck
when stopping. An added bonus is that you leave the box
free for carrying whatever you like to take with you.
When you equip your truck, I would suggest a direct-acting
brake control, a transmission cooler, and you could change
your P235/75R x 16" (passenger tires) to 225/75R x 16" LT
(light truck) Load Range C tires. These will give slightly
more power, economy and much better handling.
Good hunting, I'm sure you will find the perfect RV for
you and your truck. (Page
Top)
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