D. AXLE PLACEMENT

To some this is the most important question of all. And certainly if not the most important, it is one of the most common questions. “Where to place the axle?” You may have heard friends talk about building trailers and their biggest conclusion is. “as long as the axle’s in the right place, anybody can build a trailer.” Yes indeed, axle placement is important. and the more poorly designed the trailer, the more important the axle placement. But it is far from the most important thing that determines success of a trailer.

Too many things have to be co-ordinated for the trailer to be successful. Yet, you can’t know how many people have called me on the phone and demanded I tell them simply and plainly where to place the axle on their recently built-at-home trailer. Maybe it was this question with the most complex and elusive of answers that inspired me to write this book.

Axle placement is very integrally a part of many other variables. It is a mistake to assume there is some simple rule to follow and some magic placement that will solve all problems. In the realm of physical possibilities, the axle can be placed just about anywhere along the bed of the trailer. The forward position is usually limited by the tongue or cross- brace at the back of the tongue. The rear hanger position is limited by the end of the trailer. With the newer style crank arm suspensions, an even greater latitude is possible.

We all have probably noticed or can easily imagine, that the axle’s final placement determines tongue weight. Move the axle back and tongue weight increases. Move it forward and tongue weight decreases. If the axle is positioned exactly in the middle under the CG, it can be positioned such that no tongue weight at all is present. Please do NOT do this, as a friend did. Positioning the axle so the trailer is “perfectly balanced” with no tongue weight usually means you’ll find the trailer tows “all over the road.” This indeed is not the answer!

Some people, even some trailer manufacturers, have the mistaken notion that a trailer with a light tongue weight tows better, especially for smaller cars. A light tongue weight may be easier on the tow vehicle but to lighten the tongue on certain trailers could be disastrous. I do believe that a trailer should be designed so it can function well with a lightER tongue load, but this is very different than saying “light tongue weights tow better”. TRAILERS-How to Buy and Evaluate discusses the many considerations necessary when planning axle placement.

Light tongue weights are certainly easier to lift and move around your yard. Just have a buddy jump on the back of your trailer to reduce tongue weight and try it. WAIT! Actually, it’s better if he step up slowly and cautiously so the tongue doesn’t fly upward or the trailer slide out from under him or you. Be sure he has something to hang onto so he won’t slip off, either. Better yet, block the wheels, have everyone stand back and add inanimate weights SLOWLY to the rear until the tongue is lifted. Then find out how easy it is to move the trailer around. This experiment should be done with someone who has experience with this kind of activity. As with anything, I recommend the utmost of caution when attempting to move any trailer.

With tandem trailers, tongue weights can be made light enough so that the coupler sits entirely off the ground. This is convenient because a tongue jack and caster wheel is then not needed to move the trailer around. Building a trailer with this configuration invites trouble, though, because loading becomes critical. The missing tongue weight must be added with load positioning. An unloaded tandem such as this is possibly light enough to be towed by most cars without mishap, however, failure to compensate for this trailer’s lack of tongue weight with loading could easily produce an unstable condition and unpleasant experience.

For small cars, light tongue weights can be very important. But if the weight is too light, the resulting instability of the trailer can have a devastating effect. A light tongue weight in and of itself is more apt to produce instability than a heavy tongue weight. But it is all very, very relative and a trailer can be cleverly designed to tow well with a lighter tongue weight.

The real question is, “How light?” My experience with IRD Trailers is that many of them would tow well with as little tongue weight as 3% of the gross, well below the government tested and recommended minimum of 8%. The IRD trailers though had many features that contributed to towability. The majority of trailers built today do not duplicate the IRD blueprint and often attempt to lighten tongue weight by moving the axle forward and leaving all other parts the same. This is NOT the answer!! Bear in mind that no matter what the tongue weight there is always some critical speed where instability occurs. The real trick is to move that critical speed to a point well above towing speeds. Please understand what you’re doing before you move the axle forward.

In many cases heavier tongue weights do improve towability . . . of the trailer. However, the tow vehicle definitely has to be sufficient to handle it, not only from the stability standpoint but from its other towing capabilities including braking, gearing, drive train and engine power. An interesting solution to the tongue weight question is shown in Figure 2.38. The tow vehicle for this trailer is virtually relieved of the tongue weight as that weight is placed on the 5th wheel style dolly. Almost half the weight of the trailer and its cargo is held by the dolly while the other half is held by the other set of wheels which have been positioned entirely to the rear. And so you rightfully ask, “If this is such a good approach, why are more of these trailers not made?” One reason is the general misunderstanding of benefits to be derived. I can only speculate that another is consumer resistance to additional expense, since more hardware and labor is required to build such a trailer. Laws also prevent individuals towing such a device without a special class license. This also appears to be a relatively costly design project, which may deter new ventures into uncharted territories.

In general, though, and for any given trailer, there is a resulting tongue weight for each trailer/tow vehicle combination which is optimum. Too much tongue weight tends to make the tow vehicle unstable, but too little makes the trailer unstable. The result is the same in either case-an unstable, possibly uncontrollable system. Volume 3 presents a system of analysis which looks at these extremes and TRAILERS-How to Buy & Evaluate summarizes these factors in easily understandable form. Axle position, along with the resulting weight distribution, is one of the factors included here. The axle position and resulting tongue weight is part of the challenge of designing your own trailer. Finding the precise combination for a trailer AND its tow vehicle is only one step in the entire process.