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TOW
VEHICLE CONSIDERATIONS
by Greg Murphy
courtesy www.advancedairhitch.com - Newsletter Nov. 2004
VEHICLE
WHEELBASE
Longer is better,
pure and simple - for reasons related to both safety and comfort.
That continual up and down bouncing you feel as you tow is (how clever
a name!) is known as "trailer hitching" When you hit a bump
with the tow vehicle, you will shortly get feedback for the towed vehicle
as the tongue load increases or decreases momentarily. This is considerably better with a longer wheelbase
tow vehicle. My experience (currently) is with a Ford F-250 with 138"
wheelbase. In the past 12 years, I have had 3
"Supercab" Ford trucks with 155"
wheelbase (all 4 had 8' boxes). I can attest that my current vehicle does
not offer as comfortable a ride as the former ones.
From a safety standpoint, a longer wheelbase gives
more resistance to "jackknifing". This is the tendency (semi
trucks are notorious) for the tow vehicle to brake faster in a panic stop
than the towed vehicle -boat in this instance. The tow vehicle starts
to slow down at a faster rate, the boat keeps
right on coming and tries to make the "jackknife" blade swing
shut by trying to force the tow vehicle to "swap ends".
As others have mentioned, this might be the most important
consideration in towing. If you tow with a short wheelbase vehicle and
have no trailer brakes (or they don't work properly)
you could be looking for trouble.
ENGINES
Bigger is
not necessarily better; too small is a problem in both durability and
performance.
As soon as you hook a 22-25' trailerable sailboat behind
your vehicle, you are nearly doubling the demands of everything on the
entire vehicle - certainly the engine. Assuming your tow vehicle (or boat/trailer)
weighs 3,500 to 7,000 pounds you should understand that you are asking
the engine to move about twice the mass as soon as you drop the hitch
onto the ball.
Except for the shortest trips, an engine that is underpowered
for the task is subjected to excessive stress. You also lose the safety
of reserve power for passing.
What if you're not seriously underpowered? You can expect
that it will run hotter than normal and suck a lot of gas! Big engines
never get real great gas mileage; your mpg doesn't change as much, though,
when you are towing. Medium sized engines may get only 60% of the fuel
economy they do when not towing. In fact, they might even be worse on
gas than larger engines when towing.
Why the significant difference? Medium sized engines
are working really hard and are pulling excessive manifold pressure. You
just have to put your foot into it to maintain speed - particularly up
grades.
Is a V-6 or straight 6 enough power? It just depends.
There are plenty of folks who routinely tow C-22's with Cherokees, Astros,
Aerostars, Explorers, etc. with 4L V6's and say that they feel comfortable.
There are even others who tow the heavier C-25 with these vehicles. As
with everything involving sailing, common sense and individual judgment
plays an important role.
I think the biggest factor in engine choice is how the
vehicle is used. If towing the boat is only a very small part of it's
yearly duties, a V-6 may be an adequate choice. If the user takes a lot
of long-distance trailer trips and this accounts for the majority of the
vehicle use, a V-8 is definitely suggested.
What About Diesels?
There is a reason that commercial tractor trailers are
nearly all powered by diesel engines. Diesels have two advantages and,
depending on who you talk to, multiple disadvantages.
The main advantage is twofold: Economy and pulling power.
Diesels (by design theory) squeeze about 12-15% more power out of a given
amount of fuel. A diesel engine of the same approximate horsepower rating
as a gasoline engine, will get 12-15% more mpg - currently diesel fuel
is about 15 cents a gallon cheaper than unleaded regular. Add another
dime or so to go to a premium grade.
Doing some math for a 2,000 mile trailer-sail vacation,
we find the following:
* Diesel: 13.8 mpg, 145 gallons of fuel, Fuel cost of $173.91
* V-8 Gasoline 5-5.4L: 12.0 mpg, 167 gallons of fuel, Fuel cost of $225.00
Of course, the initial purchase price of a diesel can
be a shocker. The 97 F-250 with 7.3L PowerStroke Turbo diesel adds a whopping
4 grand to the price over the 5.4L gas! You have to further math over
the life of the vehicle to see if the increased power and economy really
represent a payback.
On the other downside of diesel economy is maintenance.
Most modern diesels don't require much periodic maintenance beyond oil
changes, filters, etc. And, most modern, Fuel-injected gas engines don't
either. But, if you need anything done on a diesel, it will likely cost
more; some components (like a worn out injector pump) are horribly expensive.
But, many folks who tow big boats or 5th wheel trailers
routinely rack up 250-300,000 miles with no major repairs. You just can't
get this kind of durability from a gasoline engine if you are subjecting
it to the severe service of towing.
The other big advantage of the diesel engine is its performance.
A diesel typically develops it maximum horsepower and torque at a much
lower engine RPM than a gas engine. Not only does this contribute to longevity
of internal components - it makes a significant difference in its towing
abilities. Since the power band is wide and at a lower RPM, diesels have
notable ability to pull hard without working too hard.
TRANSMISSIONS
Most owners manuals have detailed tables for determining
towing requirements. If you look at yours, you'll see that (usually) there
are lower GVW and GCW for manual transmission vehicles. (Diesels are often
as exception, see ENGINES) GVW is Gross Vehicle Weight (of the towed vehicle)
GCW is Gross Combination Weight of both towing and towed.
There are two reasons for this. One is an advantage of
automatic transmissions; the other is a detriment of manual transmissions.
In an automatic, the torque converter provides a fluid
coupling between the engine and transmission. From a dead stop to about
25 mph the torque converter allows something called "torque multiplication"
Simply put, it effectively gives you lower (higher numerically) total
gearing - like a low, low gear. This gives the automatic vehicle a distinct
edge in startability from a dead stop - and pulling a loaded trailer out
of the water.
With a manual transmission, we don't have torque multiplication
and often "feather" the clutch a bit as we pull out of a ramp
or start up uphill from a stoplight. This shaves life from the clutch.
REAR AXLE RATIOS
Lots of folks with pickups, van, Suburbans, Explorers,
Cherokees and the like base their perceived towing abilities on the "capable
of towing XXXXX Lbs. when properly equipped. And, unfortunately, don't
check into what "properly equipped" means.
When you are preparing to tow 3,500 to 7,000 lbs. you
really need to see if your axle ratio allows you the proper range. For
example, consulting the owner's manual for my current truck, I see that
the 7.3L diesel is only offered with 3.55, 4.10 or 5.13 axle ratios. The
maximum GCW goes from a low of 11,000 for the 3.55 to a whopping 17,000
for the 5.13. Even more dramatic figures result when we look at lower
powered engines: For the 4.9L (300 CID straight six) you are (technically)
limited to a trailer of only about 1,000 lbs. with the 2.73 axle Vs 6,000
lbs. with the 4.10.
If you are buying a new vehicle and plan on lots of long
distance towing, consult these charts when you write your specs. Keep
in mind that axle ratios that give you adequate "margin" for
towing seriously compromise you fuel economy. At any given speed, the
engine is turning faster with a lower (higher numerically) axle ratio.
Any rough guidelines? I'd say that vehicles with axle
ratios below about 3.3 should consider limited towing. Chances are, if
the vehicle was equipped with a trailer towing package when new, the axle
ratio was probably in the range of 3.3 to 4.0.
OVERDRIVE
Most manual and nearly all automatic transmissions currently
made have an overdrive. It used to be that "high gear" was about
1:1 - That means that the engine was turning the same speed as the driveshaft
to the wheels. In lower gears, the engine is turning faster, then slower,
then (in high gear) the same speed.
With overdrive your ratio drops down to about .8:1. This
means that the engine is only turning about 80% of the speed of the driveshaft.
This is great for fuel economy (keeping a vehicle at 55-70 mph doesn't
really create much effort for the engine) but not so great when you are
towing.
You'll find that most gear selector levers have a D position
and an OD. If you have it in D you are limited to low, second and high.
IT WON'T EVER GO INTO OVERDRIVE. If you have it in OD, you effectively
have a 4 speed transmission. Some even (I know current Ford trucks do)
have a push switch on the end of the selector lever which prevents OD
engagement.
If you check your owner's manual, you will probably find
that they recommend either avoiding or minimizing the use of overdrive
when towing. THIS IS GENERALLY A VERY GOOD IDEA.
Here's why. As you know, your transmission will automatically
downshift to a lower gear when you either start to pull an increased load
on the engine or you force it (through mechanical linkage) by going to
wide open throttle (calling it a passing gear dates the writer!)
To avoid "hunting" between OD and high as you
go up shallow grades or slow down slightly in traffic, the engineers have
programmed the 4-3 downshift to be delayed slightly. If you are towing
in OD and get slowed down by traffic or a slight grade, you might be pulling
quite an engine load unless you either move the lever or put your foot
into it to force a downshift.
Does this mean you can't use OD when towing? No, it means
you should "feel the vehicle" and use it if you are at a steady
speed (probably 60+) in low traffic/low grade area.
TOWING PACKAGES & EXTRA VEHICLE EQUIPMENT
If you spec a new vehicle with a trailer towing package,
it usually adds the following equipment:
* Heavy duty turn signal flasher, wiring harness and relay.
* Increased cooling capacity
* Limited slip rear axle
* Transmission oil cooler
* Engine oil cooler.
If your vehicle measures up to the towing job you're
going to give it, but doesn't have factory "trailer towing";
Should you consider adding equipment?
Probably. At least an automatic transmission oil cooler!
These can usually be added by a transmission shop for $150. to $200 and
represent cheap insurance against a $1,500 transmission overhaul. When
you tow, you are creating more stress on the entire vehicle - including
making the transmission work harder. When it works harder, it works hotter
- and they are very intolerant of hot fluid!
Update On GM Vehicles With Tow/Haul
My current tow vehicle is a 2000 Chevrolet Silverado
4x4 Extended Cab with 5.3L, 3.73 gearing and factory tow package.
GM has added a very useful feature to certain vehicles with factory towing
packages: The "Tow/Haul" function of the transmission.
In simplest of terms, this is a button on the end of
the transmission selector that, when engaged, uses a shift profile in
the vehicle computer that is much more suitable for having a towed load
than the standard shift profile. Through use in towing up to 7,500
lbs., I've found that it is a very worthwhile feature and should be used
whenever you are towing. As explained earlier, having the transmission
delay upshifts will not only give you more control and durability of engine
and transmission components, but may actually improve your fuel economy
by avoiding the engine overloads caused by pulling torque at lower engine
RPM's.
Additional Trailer Towing Information:
Towing
Your Horse Trailer
- Towing
Tutorial
How To Tow A Trailer
Your
first trip with your Trailer
Tow Vehicle Considerations
Horse
Trailer, Vehicle Safety Check
Trailer Towing Q&A
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