CHAPTER 3. SUSPENSION SYSTEMS

Suspension components along with basic ones fit together with all the frame pieces to create the whole – a trailer with character and integrity. Suspension includes hubs, axle, brakes, springs, wheels and tires. Other components include coupler, fenders, lights and a myriad of accessories. Certain components have a major part in determining capacity, while others are mere add-ons. As your design progresses, you will be answering every question imaginable about each and every component. The more you know about each of these parts, the more certain you will be about combining them into a safe, satisfying and useful product.

Start by selecting the right parts to build your trailer-not just any old axle, hubs, springs and coupler – you know, the one Uncle Harry had stashed behind the shed. Selecting the most appropriate components will produce the most desirable results. Remember you’re building something that will travel down the freeway at 55-65 mph. Please, for all our sakes, take the time to know what you’re building.

Secondly know the technicalities of each component. The more you know about products available on the market, their limitations and their correct use, the easier it will be for you to make the most appropriate selection for the job. Knowing capacities and dimensions of a part is important. Understand also that a newly selected or revised component may modify dimensions or choices you have previously made. Rework your design until all the elements are coordinated. As you study the next few chapters the mystery of many components will gradually evaporate.

Still, your end result may not be perfectly coordinated as to capacity. One part will always be weaker or stronger in relation to some other part. Listing the different components and their separate capacities will provide some insight regarding this puzzle. At first, the trailer’s true capacity appears difficult to assess. Finding the weakest link can be confusing since different components support different things. The true capacity may well be the weakest link. But just what is the weakest link?

Let’s see how this works. Figure 3.1 lists several components on a typical trailer. The maximum gross weight of this hypothetical trailer is 6720 pounds, the maximum net load is 4720 pounds. What is the difference? And why is the gross not 7000 pounds as the hubs and axle would indicate? The net load is what you carry on top of the trailer, the gross includes the trailer weight as well as the cargo. Let’s look at this more closely. The axle carries the framework, the springs and the load on top. The springs carry just the frame and load. While the hub carries the axle, springs and frame and rests on the tire. The weight of the trailer- suspension and framework-must be deducted from the gross to know the capacity remaining for cargo. As you can see from the figure, each component in the chain carries something different and can be rated accordingly.

 Examining each component in this manner provides a fairly accurate physical limitation of your trailer and its parts.

As with many things, though, there are a few traps. All too common is the mistake of rating the trailer at whatever the axle will carry: 7000-lbs in this case. Too many people (some manufacturers included) know the axle/hub rating–and that’s all they know. They then assume this is the rating of the trailer. For example, with 2 axles rated at 3500-lbs, they state the trailer will carry 7000-lbs. Only after being challenged will they concede that the 2000-lb framework and suspension will bring the actual carrying capacity to 5000-lbs. In fact, in many cases, the tires do not have sufficient rating for even the 7000-lb gross. And the down rating is even greater, as in this case to 6720-lbs (4720-lbs net).

Another trap is finding selection of a few items easy and then resting on your laurels and assume everything else is easy. Lack of attention to the more difficult areas can easily produce an unroadworthy trailer. The component selection process involves a complete understanding of the whole system, many reworks of the overall design, and a thorough integration of all the parts into the whole.