Adventioneering

Sooner or later you are going to want to tow something; possibly even off-road. This is a complex topic made worse for a lot of bar-room math that gets circulated around.

They say a picture is worth a thousand words:

The English translation might be, “be sure to match your tow vehicle to your trailer”. When matching vehicles and trailers you must consider weight, length, hitch, brakes, and lights.

There are several weights for a trailer; it’s curb weight and gross weight just like a vehicle, but also it’s hitch weight. This is the amount of weight pressing down on the hitch (the back of the vehicle). With a standard hitch this weight is entirely born on the rear axle (and reduces the cargo capacity of the towing vehicle). With a weight-equalizing hitch, the weight subtracts from both front and back axles.

It is important to consider the level of the trailer as it rides when connected to the tow vehicle. If the hitch is too high or too low, the trailer will tilt up or down which will cause weight to be transferred either fore or aft of the trailer axle(s). The nose weighting for the trailer (if any was provided) assumes the trailer is perfectly level and has some sort of ideal weight distribution. If in doubt, weigh the nose of the trailer.

The vehicle manufacturer will provide tow specifications; maximum trailer weight and maximum hitch weight. Do not exceed these ratings.

The other major factor is length. The longer the trailer, the more leverage it has to influence the track of the tow vehicle. This may because of bumps, or wind (especially by passing vehicles), and is known as sway. The longer the tow vehicle, the more it can resist. Therefore vehicles with longer wheelbases are better suited to towing longer trailers.

A general rule of thumb is that each 13cm of wheelbase buys you 30cm of trailer. A Disco is 254cm (100″). (254/13)*30 = 586.15cm or 19.2 feet. In my experience towing an Airstream at these length the tow experience is “safe but marginal”. The trailer sways a bit when 18-wheelers pass but is easily controllable – in good weather.

The next consideration is a hitch. In the US with civilian vehicles the most common hitch is a ball hitch. This works well for on-road towing, but off-road a military style pintle-and-ring hitch offers more articulation between the tow vehicle and the trailer. For on-road towing, there are also specialty anti-sway hitches designed to dampen the trailer’s sway effect on the tow vehicle, usually through the addition of a shock absorber.

Hitch balls come in different sizes, which you are going to have to match to the trailer. For most bigger class III applications, this is going to be around a 2″ tow ball.

Trailers will have safety chains that attach somewhere on the hitch of the vehicle. You almost always see these rigged wrong, normally because they are too short. A properly rigged trailer should have the chains cross underneath the tip of the trailer, so that the chain attaches to the OPPOSITE side of the tow vehicle than the trailer. Often people have cut down the chains on the trailer and so they aren’t long enough to do this. The reason is so that when the ball hitch fails and the trailer comes loose, the crossed chains will act as a support preventing the nose from dropping to the highway.

Trailers either don’t have brakes, have electric brakes, or have hydraulic or air brakes. Besides a sense of self-preservation motivating you to want trailer brakes, some jurisdictions require them, usually for trailers over a given weight. Electric brakes typically work using EM clamping force, whereas hydraulic (and similarly air) brakes work the same as brakes in a regular vehicle.

You will usually only run into hydraulic or air brakes on very large trailers. In both cases they are intended to be hooked up to the tow vehicles hydraulic or air brake circuit, so that no additional hardware is required. If the tow vehicle is not equipped to deal with this, it is possible to buy a converter that will accept an electrical brake signal to actuate hydraulic trailer brakes.

Electronic brakes are facilitated by means of an electronic brake controller. These controllers typically sense the deceleration of the vehicle (using a gyro in modern applications), and convert this to a map of brake force. This controls a voltage on a high-amp output circuit where 0v is no brakes and (typically) 12v is max brakes. These units are adjustable because the brakes are always a little different on each trailer.

Generally you want to adjust the controller so that the trailer gets the max possible traction but does not lock up when you panic stop the vehicle. Too much clamping force on the trailer can lead to the trailer locking up and skidding, potentially causing a bad accident.

If your rig can activate the trailer brakes separately from the tow vehicle brakes; as is the case in electric, sometimes hydraulic, and seldom air brakes, this is a major advantage. Should the trailer begin to sway badly and begin to slide out, you must never brake the tow vehicle (which will just guarantee a rollover), but rather your trained instinct must be to grab a handful of the trailer brakes. This will snap the trailer back into line. Mainly trailers have been lost because in the moment of crisis the driver applied the tow vehicle brakes instead of the trailer brakes, which is the natural reaction.

To develop a good habit, I often play with the trailer brakes, particularly when slowing for off ramps and the like. This engenders a comfort in controlling the brakes and teaches me the muscle memory of how and where to reach for the brake control.

Installation of electric brake controls is certainly beyond the scope of this article; but I will give a tip. Don’t fuse the brake circuit; use a self-resetting circuit breaker instead. Otherwise should the fuse blow, you will have lost trailer brakes.

My best advice for learning to tow a trailer is – get some practice! On a good nice day, when you aren’t stressed on in a hurry or on a trip. Be aware of what you are doing and try to develop good habits… but do not become so comfortable you forget you have the trailer and drive into a McDonalds!

This article is part of the Vehicle Dependent Expedition section.