Where to Install Shackle Straps and How to Determine Axle Position When Building an ATV Trailer


I am building a trailer to haul 2 atvs, 550lb each approx. Going with a single 3500# axle, my ? is are my shackle straps on the front or rear of springs.Is the top bolt on the shackle strap suppose to be loose for pivoting ? Also is there an equation as to where to locate my axle on the trailer. thanks for any help great site !

Expert Reply:

For a 3,500 Dexter Straight axle the springs should be placed between 16 and 18 inches on center from the hub face (flat area the wheel mounts against). Placement of the spring centers in any other position will reduce the axle capacity. If you are using a different brand of axle you will need to contact the axle manufacturer for the full capacity spring center dimensions.

If you are using double eye springs then the shackle straps will be located at the rear of the springs. The top bolt and the spring eye bolt will be snugged tight, but not over tightened so that the weight of the trailer and its cargo will cause the components to move when needed. Locking nuts are used on the axle components to keep the nuts from backing off of the bolts.

The general rule when building a trailer is that there should be a 60/40 percent split in weight when centering the axle. 60 percent of the trailer weight should be placed forward of the axle for proper tongue weight. There is a more detailed method of determining the proper tongue weight for the trailer so that the axles are in the correct location for safest towing and stability. I will outline this process for you.

To determine the correct position for the axle determine the trailer weight under the wheels and the tongue weight. This will require a mock up (pre-build) of the trailer. Once these weights are determined you will need to add the two numbers together to get the total weight. Once the total weight is determined you need to measure the distance to the center point of the axle to the center of the trailer coupler.

Now figure the percentage of tongue weight, total weight divided by tongue weight, if it is greater than 10 percent you will need to subtract 10 percent of the total weight from the tongue weight and divide by the total weight. Next multiply the answer by the distance between axle center and center of the coupler. The result will give you the distance that the center line of the axles should be moved forward.

If you have less than 10 percent tongue weight you will need to move the axles rearward using the following equation. Distance to move rearward will be 10 percent of total weight minus tongue weight divided by total weight then multiplied by the distance between axle center and center of the coupler.

We have all of the parts available online that you should need except the axles (see link).

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