Since all trailer manufacturers do not use common terms and reference points in arriving at their ratings, an understanding of those factors is very important to proper trailer selection. A customer with a trailer that is not quite up to the job to be done will find it to be a continual source of irritation, and therefore is unlikely to be a repeat customer. ZIEMAN trailers are rated by Gross Vehicle Weight Rating at an un-restricted speed of 55 MPH. The gross vehicle weight rating, or G.V.W.R., is the maximum total allowable weight of the trailer and its load at the stated operating conditions (i.e.: 55 MPH and above for ZIEMAN trailers). This brings up three critical points.

First, in the case of semi-trailers (See Figure 1 above), the entire G.V.W.R is NOT intended to be carried by the tires and axles. Rather, a portion of the G.V.W.R. is assigned to the hitch. This is in fact the definition of a "semi-trailer": 'any trailer so constructed that some part of its weight and that of its load rests upon, or is carried by, another vehicle'. So any trailer of this general layout is a "semi-trailer", no matter how small it may be. The above leads us to two conclusions.

a.) THE TRUCK MUST HAVE THE CAPACITY TO CARRY THE WEIGHT TO BE IMPOSED AT ITS COUPLER POINT. For example, the rig shown in figure 1 with its 4,000 lb. hitch weight would be inappropriate for a 1/2 ton pick-up truck!

b.) THE DECK LENGTH AND AXLE PLACEMENT MUST BE SUCH THAT THE INTENDED LOAD CAN BE LOCATED TO DISTRIBUTE THE WEIGHT WITHIN ACCEPTABLE LIMITS.

As a rule of thumb, the hitch weight should be at least 10% of the G.V.W.R. for stable towing. Also, the load must be far enough forward so that the tires and axles do not carry a greater share of G.V.W.R. than they are rated for. For example, if a builder rated his trailer 25,000 lb. G.V.W.R., and each of the 3 axles were rated for 6,000 lb., that would leave 7,000 lb. to be carried by the hitch. 7,000 lbs. is an unrealistic hitch weight for anything smaller than a 10 wheel dump truck.

The second critical point is the distinction between PAY-LOAD, the actual load capacity of the trailer, and the G.V.W.R. The actual maximum LOAD that may be carried is the G.V.W.R., less the weight of the trailer itself. When selecting a trailer, be sure to add the weight of any machine options when totaling up the weight of the LOAD. It is good practice to add several percent to the spec. sheet values to allow for fuel, accumulated mud, the addition of minor options at a later date, etc. The weight of all standard ZIEMAN trailers is shown in the current catalog. Also, remember to add the weight of any trailer options. If total weight is near the maximum, that is , near the rated G.V.W.R.., then check with ZIEMAN factory on the weight of any options.

The third critical factor is the speed rating referred to at the beginning of the article. This can be most confusing, since some manufacturers use reduced speed ratings in their literature. The basis of reduced speed ratings comes from tire characteristics. Tire industry standards permit an overload of 9% when speed is limited to 50 MPH, 16% at 40 MPH, 24% at 30 MPH, and so on. However, these criteria CAN NOT be blindly applied to an axle rating. Are the axle, brake, wheel, spring, and other components capable of carrying an increased load? And at what cost in durability?

If you observe a competitive trailer's load rating to be greater than the equivalent ZIEMAN model with identical tires, CAUTION. This can only mean one of two things. Either the tires are rated at some reduced maximum speed, or an excessive amount of the total weight has been assigned to the hitch. Who can be sure that customer or his driver will observe the 50 MPH restriction, or other restriction, as the case may be?

And what about the legality of operation using reduced speed ratings? Many states now enforce tire load laws. For example, California regulations limit tire loading to the unrestricted tire rating unless the vehicle displays a sign on the rear stating 'MAXIMUM VEHICLE SPEED IS XX MPH'. The sign must in 4 inch block letters, black on yellow background. In most cases, a customer with a trailer loaded to the 50 MPH rating would have to follow this procedure and speed limit to avoid an overweight or overspeed citation.

For these reasons, all ZIEMAN models are rated at the unrestricted 55 MPH G.V.W.R. If you select a trailer based on these ratings, following the guidelines above, you can be assured of successful operations under all conditions and in any State. For special conditions and circumstances only, reduced speed ratings are available through the factory. This allows the case by case examinations that reduced speed ratings really deserve.