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How to Determine the Capacity of Trailer Leaf Springs and How to Raise the Coupler Height

Question:

2 questions: 1 I have a pair of 3-leaf, 25-1/4 x 1-3/4 double eye springs. Is there a place where their rating is marked or do I have to figure it out from leaf thickness/width and number of leaves? 2 I want to lift a single axle utility trailer about an inch to make it consistent height with my other trailer. Are there any serious drawback to simply swaping the 2-5/8 shackles for 3-1/2 shackles? rather than cutting off the old hangers and welding on new Thanks, once I know the above answers, Ill be ready to order brakes to update the trailer.

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Expert Reply:

Leaf springs typically have the part # stamped on the strap that holds the leafs together at one end of the shorter leafs, or it is painted on the concave side of the springs. The part number can then be used to determine the capacity of the springs. Generally springs with any age on them will be very hard to get a part number off of. You would then need to go by the axle capacity to determine the spring capacity, which would be 1/2 of the axle capacity. If you have a 3,500 lb capacity axle the springs should be rated for 1,750 lbs each. The axle capacity can be determined in one of 4 ways. First, most trailers have a tag that has the VIN # and axle capacities listed on it. Second, there is sometimes a tag on the axle that indicates the axle capacity. Third, the diameter of the axle can help determine its capacity in many cases, see below. Finally, the inner and outer bearing part numbers on the axle will indicate the axle capacity.

The leaf thickness, width and number of leaves has nothing to do with the capacity of the leaf spring, because the quality of steel and how it has been prepared for use as a leaf spring will affect its capacity. Some single leaf springs can carry more weight than a 5 leaf spring that seems to have 5 leafs all the same thickness as the single leaf spring.


Typical Axle Diameters Based on Axle Capacity:

1,000-lb - 2,000-lb Axles: 1-1/2inch - 1-3/4inch diameter
3,500-lb Axles: 2-3/8inch diameter - Can have a 3-1/2inch diameter, but rarely
6,000-lb - 7,200-lb Axles: 3inch diameter
8,000-lb Axles: 3-1/2inch diameter
9,000-lb Axles: 4inch diameter
10,000-lb Or more axles: 5inch diameter

Swapping the shackles, would not be a good idea because the hanger location would remain the same and cause the shackle links to rest at a harder angle more straight up and down which would cause a stiffer ride and would only raise the trailer on one side of the axle, cutting your probable lift in half.

The best way to raise the trailer an inch would be to replace the spring hangers with hangers that are approximately an inch deeper from the center of the bolt hole to the top of the hanger. This would require cutting off the old hangers and welding on the new ones, like you mentioned. You would then use the same length shackle links as originally used.

Another option that does not require welding is an over under spring conversion kit. This would raise the trailer the diameter of the axle plus the spring seat, so you might not be able to use this option since you only need an inch in height.

If you think this might work for you we have an Over-Under Conversion Kit for 3 inch Axles, # K71-385-00, and one for 2-3/8 inch axles, # K71-384-00.

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