April 15, 1966

Dealer Letter DL-140

SUBJECT: HITCHES, TRAILERS AND TOW CARS

Not too long ago, individuals desirous of towing a trailer with a passenger car were given little, if any, assistance by the automotive industry in regard to advice on how to equip a passenger car for trailer towing, or how to operate it safely and successfully. Now, the automotive industry is beginning to cater to the trailer towing public with "special trailer towing packages" that enable the passenger car to double as a trailer tow car. These "special trailer towing packages" are available from most manufacturers, if requested, when the order for a new car is placed. A typical "trailer towing package" consists of such items as: heavy-duty cooling system, automatic transmission, heavy-duty higher rear axle ratio, heavy duty brakes, oversize tires on wider rims, heavy-duty springs with anti-sway bar, heavy-duty turn signal flasher and heavy-duty stop light switch. A noteworthy item in these "packages" is the inclusion of an automatic transmission. The feeling among automobile manufacturers on this point is that manually shifted transmissions should not be used for towing trailers. They feel that the technique required to repeatedly make successful clutch engagements, especially on grades, seems beyond the capabilities of the average driver.

The size, and particularly the weight trailer that is going to be towed should, obviously, be the first major factor considered in the selection of a suitable tow car. For example, a 5800 pound trailer could not be towed with any degree of success by one of the smaller compacts. The trailer weight itself, however, does not seem to be as important as the ratio of gross trailer weight to tow car weight. A rule-of-thumb that seems to work out quite well, when possible to maintain, is a gross trailer to tow car weight ratio of not more than 1:1.

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At this point, it would be appropriate to mention a few words about proper hitch installation, since it is essential to the well-being of the tow car and consequently, the towability of the trailer. The stresses imposed on a loaded trailer are significant, and since these loads can be transmitted only through the hitch, the type of hitch and the manner in which it is attached to the tow car are quite important.

At this point, it would be appropriate to mention a few words about proper hitch installation, since it is essential to the well-being of the tow car and consequently, the towability of the trailer. The stresses imposed on a loaded trailer are significant, and since these loads can be transmitted only through the hitch, the type of hitch and the manner in which it is attached to the tow car are quite important.

If a load leveling hitch is not used, two angular braces running from near the rear of the tow bar to as far forward on the frame as is possible should be installed. These angular braces are necessary to eliminate the back and forth surge that occurs with the back and forth movement of heavier trailers.

Regardless of the hitch utilized, it is generally accepted and recommended practice that not less than 10 percent of the gross trailer weight be on the hitch ball. The enclosed table, established by the Society of Automotive Engineers, shows permissible vertical trailer tongue load weights correlated with recommended types of hitches.

The construction, or more directly, the rigidity of a trailer has important effects on its towing characteristics. A relatively rigid trailer will tow better than a flimsy one, all other things being equal, but it must be remembered that all vehicles whether they be boat trailers or aircraft, are not absolutely rigid. They depend, to a certain degree, on flexing for their strength.

Since it is only practical to achieve a certain degree of rigidity in a trailer, other factors must be taken into consideration so that shock loads imposed on the trailer are distributed or absorbed in a non-destructive manner. This is important if the trailer is going to survive the stresses induced by bumps and a certain amount of off-the-road use. One of these factors, and quite possibly the most important, is the trailer's suspension system.

Presently there are at least five trailer suspension systems that are in vogue, but due to such practical considerations as cost, the amount of vehicle space required, maintenance, etc., certain of these are disadvantageous for boat trailer use.

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Undoubtedly, the most commonly used suspension system is that of semi-elliptical leaf springs. The primary objection to this system, however, when it is used for a boat trailer, is the axle clearance required under the trailer chassis. This can be disadvantageous in boat trailers because of the depth water required to launch a boat when it is sitting "high and dry" above a semi-elliptically sprung axle and the rear wheels of the tow car are already in the water. Add to this the fact that semi-elliptical leaf spring suspension is not a truly independent suspension system and the fact that trailers using it have relatively high unsprung weight and you can see that it is far from the most desirable system to be used for a boat trailer.

Coil spring suspension is equally undesirable because of the reasons stated above and because of the difficulty of rigidly supporting it.

Torsion bar suspension, on the other hand, which is widely used on vehicles in Europe, is one of the best systems in use today. The two big advantages of torsion bar suspension are the ability to obtain a truly independent suspension system and the surprisingly low amount of unsprung weight. The one big, and eliminating, disadvantage about the use of torsion bar suspension, however, is its cost.

Dura-torque, the rubber suspension axle, has all the advantages of the torsion bar system and more. Besides being truly independent and possessing low unsprung weight, the Dura-torque suspension system provides a variable spring rate and, what is more important, it is relatively inexpensive to use in mass production.

The only remaining suspension system that has enjoyed any degree of use is that of the air bag suspension system. The complexity and maintenance difficulties encountered with air bag systems, however, preclude their use to any large degree, even though they do offer good riding qualities.

An important part of any suspension system is the axle or axles. Tandem axles give a somewhat smoother ride than does a single axle, especially when used in conjunction with a fully independent suspension system. However, if the gross weight distribution of the trailer is correct, a single axle trailer is easier to tow. Tandem axles are also more expensive, require a large wheel well, and tend to scuff tires whenever the trailer direction changes.

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This last disadvantage reduces tire life substantially.

In the last analysis, it must be remembered that no matter how well a trailer is designed, its ability to be towed is no better than the tow vehicle and the hitch will allow. Then, too, there is always the tow vehicle driver who must be held responsible for the safe operation of his rig. No amount of tow car tailoring or trailer design can eliminate this individual's responsibility for the well-being of others on the highway, the well-being of his own rig and his own personal safety.

NPH:rs

PERMISSABLE VERTICAL TONGUE LOAD WEIGHTS CORRELATED WITH RECOMMENDED TYPES OF HITCHES