Choosing the Right Leaf Spring Capacity for 3,750 lb Trailer Axles to Prevent Sag
Updated 04/21/2026 | Published 04/16/2026
Question:
I recently ordered leaf springs for 3500 axles. When I got them home I did a physical comparison with the existing leaf springs that were on my tandem axle boat trailer and the laid right on top of each other identically. Once I confirmed that they were geometrically correct I installed the new leaf springs on to the trailer. At that point, I let the trailer down off the jack stands and found that the trailer at the rear axle was sagging pretty badly putting the top of the tire within 1.5 inches of the fender. The Axles on this trailer rated for 3750 lbs each so I assumed that the original leaf springs for the pair were rated for 3500 or 4000 lbs. I am considering ordering leaf springs for 5k axle with the same geometry 25 1/4 Eye to Eye. This is for a boat trailer with a Mercruiser I/O. I dont think 4000 lb springs would be stiff enough because it would only provide another 250 lbs of capacity on each side of the axle. I weigh approximately that and so when I would climb up into the boat I would probably see the same sagging problem with my added weight. This also doesnt account for the added gear and gas weight as it currently only has 25 gallons of gas in a 60 gallon tank. I can post a picture if helpful. Thanks for the help in advance.
asked by: Michael N
Expert Reply:
Hi Michael, you’re on the right track and your install and measurements sound solid, but the sag you’re seeing is because the springs you installed are undersized for your axle rating. Even though they matched up perfectly in length, the capacity is what really matters here, and a 3,500 lb spring setup only gives you about 1,750 lbs per spring, which is less than what your 3,750 lb axles are designed to carry. That’s why everything looked right during install but dropped once the weight went back on.
The better move here is stepping up slightly in capacity, etrailer 3-leaf spring part # e57SR rated at 2,000 lbs per spring is a much better fit for your setup. That gives you a 4,000 lb capacity per axle, which matches your needs more closely and adds a little margin for fuel, gear, and normal use without going overly stiff. Jumping all the way to a 5,000 lb spring would likely make the ride harsher and reduce how well the suspension absorbs road shock, so it’s more than you need for this application.
Based on what you described, the e57SR with the same 25-1/4 inch length should bring your ride height back where it belongs and eliminate that fender clearance issue without overcorrecting. If you want to be extra certain, a quick picture of your current setup can help confirm everything is sitting correctly once installed.
It’s also a good idea to take a quick look at your shackle straps and bolts while everything is apart. If they’re worn or elongated, replacing them at the same time can help everything sit and wear properly.
What kind of boat are you hauling with the trailer?
Product Page this Question was Asked From
3-Leaf Double-Eye Spring for 4,000-lb Trailer Axles - 25-1/4" Long
- Trailer Leaf Springs
- Double Eye Springs
- 9/16 Inch Eye
- Nylon Bushings
- 4000 lbs
- 1-3/4 Inch Wide
- 3 Leaf
- 25-1/4 Inch Long
- Standard Grade
- etrailer
more information >
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