Index

SJ23 Tow Vehicle Considerations - (not towing safety).

INDEX- Wheelbase, Engine, Transmission, Axle Ratio, Overdrive, Towing Package, Maintenance, Brakes.

INTRODUCTION - "Having spent some time as a service engineer for Ford, I'm aware what the manufacturers say on the topic of towing...and why.  Having said that, please understand these are my personal opinions about a tow vehicle.  Hopefully they are informed ones."   Greg Murphy.

• Towing safety is extremely important and maybe this article will help my sailing friends.
• Hopefully your preference in a tow vehicle is based on a happy compromise between the operator manual and your common sense! 

From a safety standpoint, a longer wheelbase gives more resistance to "jackknifing".  This is the tendency (semi-trucks are notorious for this) for the tow vehicle to brake faster in a panic stop than the trailer.  In this instance the tow vehicle starts to slow down at a faster rate, the boat trailer keeps right on coming and tries to make the "jackknife" blade swing shut by forcing the tow vehicle to "swap ends" with the trailer.  Its a frightful feeling for the driver because you know it isn't going to end up good.  In fact it is quite likely that a sailboat would roll off the trailer.

ENGINE - Bigger is not necessarily better and too small is a problem in both durability and performance.

As soon as you hook a 22-25' trailerable sailboat behind your vehicle, you nearly double 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 the engine has to move about twice the mass when you drop the hitch on the ball.

Except for the shortest trips, an engine that is under powered for the task is subjected to excessive stress.  You also loose the safety of reserve power for passing.

What if you're not seriously under powered? - You can expect that it will run hotter than normal and suck a lot of gas!  Big engines never have real good gas mileage; therefore your mpg doesn't change as much, even when you are towing.  A medium sized engine may loose 40% of the fuel economy when towing.  In fact, it might be worse on gas when towing than a larger engine.

Why the significant difference?  A medium size engine work really hard and pulls excessive manifold vacuum.  You just have to put your "foot into it" to maintain speed, particularly up a grade.

Is a V-6 or a straight 6 enough power? - It just depends.  There are plenty of folks who routinely tow C-22's with a Cherokee, Astro, Aerostar, Explorer, etc. with a 4L V6 and claim they feel comfortable.  There are even others who tow the heavier C-25 with one of these vehicles.  As with everything in sailing, common sense and individual judgment play important roles.

I think the biggest factor in engine choice is how the vehicle is used.  If towing the boat is a very small part of it's yearly duties, a V6 may be an adequate choice.  If you tow on flat roads, then a smaller engine might be OK.  If the user takes a lot of long-distance trailer trips and this accounts for the majority of the vehicle use, a V8 is definitely suggested.

What About a Diesel? - There is a reason that nearly all commercial tractor trailers are powered by a diesel engine.  A Diesel has two advantages, and depending on who you talk to, multiple disadvantages.

The main advantages are pulling power and fuel economy.  A diesel engine theoretically squeezes 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.  As of 2000 diesel fuel is about 15 cents a gallon cheaper than unleaded regular gas.  Add another dime or so for the premium grade gas.

Doing some math (US gallon) for a 2,000 mile trailer sail vacation, we find the following:

• Diesel @13.8 m/g - 145 gallons of fuel, cost is $173.91
• Gas @12.0 m/g (5.4L V8) - 167 gallons of fuel, cost is $225.00

Of course, the initial purchase price of a diesel can be a shocker.  The 1997 F-250 with 7.3L PowerStroke Turbo diesel adds a whopping $4000 to the price over the 5.4L gas engine!  You have to do some further math to see if the increased power and economy really represent a payback over the life of the vehicle.

The downside of diesel fuel 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 any repairs to a diesel, it will likely cost more.  Some components (like a worn out injector pump) are horribly expensive.

But, many folks who tow a big boat or a 5^th wheel trailer routinely rack up 250K to 300K miles with no major repairs.  You just can't get this kind of durability from a gasoline engine.  Especially if you are subjecting it to the severe service of towing.

TRANSMISSION - Most vehicle operating manuals have detailed tables for determining the towing requirements.  If you look at yours, you'll see that (usually) there are lower GVW and GCW for vehicles with a manual transmission.  (A diesel is often an exception, see ENGINES).

There are two reasons for this.  One is the advantage of an automatic transmission; the other is the detriment of a manual transmission.

The torque converter of an automatic transmission provides fluid coupling between the engine and transmission.  From a dead stop to about 25 mph (40 km/h) 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 equipped vehicle a distinct edge in starting from a dead stop, like pulling a loaded trailer out of the water.

AXLE GEAR RATIO - Some people with a pickup, van, suburban, Explorer, Cherokee, or the like, base the perceived towing ability on this label, "capable of towing xxxxx lbs. when properly equipped".  Unfortunately, they don't check into what "properly equipped" means.

When you tow 3,500 to 7,000 lbs. you really need to see if your axle gear ratio allows the proper range.  For example, consulting the owner's manual of my current truck, the 7.3L diesel is offered with 3.55, 4.10 or 5.13 rear axle ratio.  The maximum GCW goes from a low of 11,000 lbs. for the 3.55 to a whopping 17,000 lbs. for the 5.13.  Even more dramatic figures result when you look at a lower powered engine.  For the 4.9L (300 CID straight six) you are (technically) limited to a trailer weight of only about 1,000 lbs. with the 2.73 axle versus 6,000 lbs. with the 4.10 axle.

If you are buying a new vehicle and plan on lots of long distance towing, consult these charts to determine your requirements.  Keep in mind that the axle ratio that gives you adequate "margin" for towing also compromises the fuel economy when traveling empty.  At any given speed, the engine is turning faster with a higher (numerically) axle ratio.

OVERDRIVE - Most current manual transmissions and nearly all automatics have an overdrive gear.  It used to be that "high gear" was about 1:1.  That means that the engine turns the same speed as the drive shaft.  For the same road speed the engine turns faster in a low gear than in a high gear.

With overdrive the ratio drops down to about 0.8:1.  This means the engine is turning about 80% of the speed of the drive shaft.  This is great for fuel economy but not great for towing.  Driving a vehicle at 55-70 mph (90-110 km/h) doesn't require much effort from the engine.

You'll find that most automatic gear selector levers have a D position and an OD.  If you select D you are limited to low, second and high.  IT CAN'T SWITCH TO OVERDRIVE.  If you select OD, you effectively have a 4 speed transmission.  Some vehicles (current Ford trucks do) have a push switch on the end of the selector lever that prevents OD engagement.  Its handy to use for extra power when driving up a hill.

If you check your owner's manual, you will probably find they recommend 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 gear as you go up a shallow grade or slow down slightly in traffic, the engineers have programmed the 4 to 3 downshift to be delayed slightly.  If you are towing in OD and get slowed down by traffic or by a slight grade, the engine will be pulling quite a load until you either manually down shift or put your foot into it to force a downshift.

Q - Does this mean you can't use OD when towing? 
A - No, it means you should "feel the vehicle and listen for engine strain."  Use it if you drive at a steady speed in light traffic (probably 60+mph) or on a low grade road.  TOP
 

TOWING PACKAGE - A new vehicle with a trailer towing package, is usually equipped with the following extra equipment:

• Heavy duty turn signal flasher, wiring harness and relay
• Increased cooling capacity
• Limited slip rear axle
• Transmission oil cooler
• Engine oil cooler

Q - 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?
A - 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.  Towing creates more stress on the entire vehicle, including making the transmission work harder.  When it works harder, it works hotter and an automatic is very intolerant of hot fluid!  TOP 
 

MAINTENANCE CONSIDERATIONS - These are mostly common sense, but shouldn't be forgotten.

1. Brakes: Even with good trailer brakes, your tow vehicle does most of the work.  Make sure they are in tip-top shape.
2. Tires: Tires carry a higher load when towing.  Make sure they are all at the maximum recommended pressure when cold.
3. Cooling: The engine is going to run hotter when pulling a load.  Make sure all the hoses and belts are in good shape.  You should probably flush and refill the entire cooling system every other Fall as a cheap precaution.  Definitely check the coolant level before you leave and every time you stop for fuel!  Besides the obvious pain of breaking down with an overheated engine, you risk serious engine damage as well.
4. Transmission Fluid: Absolutely, positively, check the fluid before each and every trailer trip.  You should check your owner's manual for the procedure.  Here's a general procedure:
   • With the engine running and the brake on, select R, N, D, etc. then put it in Park.
   • Pull the dipstick and wipe it off.  Pay attention to the markings on the end of the stick.  Push it back in and pull it out again and check. 
   • Make sure you don't overfill. That can cause foaming and overheating!  Don't operate it below the bottom reading on the dipstick. 
   • If you have to add fluid, make sure it is the correct type.  Most current vehicles use Dextron II or III; some older vehicles use Type "F".  These two are not compatible so don't mix them.  You will likely have transmission damage if you add the wrong type!
5. Rear Axle Housing Vent: Install a tube to extend the vent of the rear axle high in the vehicle so water can't penetrate it.  The absolute worst situation for a hot differential coming off the highway is to immerse it in cold water to launch a boat or to drive through axle deep water.  As the automatic vent on top of the differential closes (some close automatically but don't count on it), water is drawn in through the axle end seals (by the wheel bearings) due to the contracting air in the housing.  This is a recipe for disaster.  A differential housing with a high open vent hose prevents this trouble.  TOP
    

I'd like to know if you found this information helpful or if you have other questions or comments, please email me.

Greg Murphy.
C-25 #5128 Second Wind
Climax, MI, USA
 

The braking capacity of the tow vehicle AND the trailer is the single most important issue when choosing a tow vehicle.  If you can't stop the load, you have NO business moving it in the first place.  There are very few vehicles with sufficient brake capacity to adequately stop a 4000 to 5000 pound trailer in an emergency, let alone normal driving.  Given sufficient gears and time almost any engine can pull up a hill.  It's the down slope that is far more dangerous!  Therefore the trailer MUST be equipped with brakes.  If the trailer has electric brakes, they must be controlled from inside the tow vehicle, within the driver's reach.  An inertia sensing proportional controller is ideal for smooth operation.  If it mounted conveniently close to the driver then manual override for testing or an emergency stop are bonus. 

If you tow on flat terrain, the manufacturer towing capacity is a good starting guide.  If you tow on hilly or mountain terrain, you should either increase the rear end gear ratio and cooling capacity and/or upgrade to a stronger engine.  Keep in mind the traffic and the hills you have to negotiate.  Some examples of tow vehicles in use today (2004) are: Jeep Cherokee equipped with 4L 6cyl, Jeep Grand Cherokee equipped with 318 V8 4WD, 1988 Ford half ton van, Dodge Durango equipped with 4.7L V8, Ford Aerostar, Toyota 4Runner equipped with 6 cyl/auto 4WD, Toyota Tacoma equipped with V6/auto 4WD (my current tow vehicle, 2014).  A friend of mine thinks the ideal choice might be the 2004 Dodge Durango for its combination of towing capability, 4WD on the ramp and great mileage.  The list of equivalent vehicles can go on.  In summary I prefer long wheel base for directional stability on the road, the high gear ratio of a 4WD in 4 low for pulling power on the launch ramp and a tandem axle trailer equipped with electric brakes.  Also See Tech Tip A04, Conversion to Tandem Axle.   Bob Schimmel.  TOP
 

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