Is The 3-1/2" Axle To Frame Clearance On Hanger Kit APTT3 Loaded Or Unloaded?

Question

Question:

On an APTT3 kit 1 1/2inch / 2 1/2inch hangers the dimensions chart states that the distance from the axle c/l to the frame is 3 1/2inch - with a 2 3/8inch axle, then 2 5/16inch between the axle and the frame, without indicating the condition in which this is measured. Elsewhere on your site, you indicate that the hangers provide 3 1/2inch clearance between the axle and the frame when loaded, with a presumed 1 1/2inch deflection at full load. Is the chart in the APTT3 listing measured at full load? And which is correct - 3 1/2inch of available travel or 2 5/16inch of available travel at full load - presuming that the measurements are given at full load. I have a boat trailer, built for the boat with two 3.5K axles that is maybe stretching a little on loading, going to triple for the redundancy, additional axle capacity, and to better disperse the load on the frame rails. Its under a 40 foot boat, so like to stay as low as possible - every couple of inches up is another foot further to back into the water. Thanks for the clarification, Mike

asked by: Mike W

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Expert Reply:

Hey Mike.

I can see where the confusion is coming from.

For the # APTT3, the 3-1/2" measurement in the chart is from the center of the 2-3/8" axle up to the trailer frame (unloaded). That means the actual space between the top of the axle tube and the frame is about 2-5/16" (unloaded).

So, the 3-1/2" dimension is not 3-1/2" of usable suspension travel above the axle.

I also would not consider the chart measurement to be full-load. As the trailer is loaded, the leaf springs will flatten and the axle will move closer to the frame.

Since you are trying to keep the trailer as low as possible, I would use the chart as a starting point for the hanger layout, then make sure you still have enough clearance between the axle and frame through the suspension's travel before welding everything in place.

Hope that helps, Mike. Also, sounds like you will have a nice trusty setup when done! What kind of boat is it?

expert reply by: Andrew N

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Mike W.

10/5/2026

Andrew, thanks for the reply, it does clear up one question. So, per your reply, 3 1/2" minus one half of the axle dia. leaves 2 5/16" between the axle and the spring unloaded. With your stated 1 1/2" spring deflection at full load, the axle clearance to the frame with this kit will then be 13/16" . Given the U bolt diameter of 1/2", clearance between the U bolts and the frame would then be 5/16" = the total suspension travel available. I understand that leaf spring deflection is not linear with load, but given a parking lot speed bump or similar of 3 1/2" height, I fail to see how the suspension would not bottom out. I could relieve the frame buffer piece (2 1/2 x 3 1/2 x 3/8 angle, frame is 5" 9lb channel) to clear the u bolts, leaving 13/16" travel, but is this enough? Researching frame clearance, I came across an article penned by a mechanical engineer/trailer designer on his site, MechanicalElements,com, linking to this particular product, stating categorically that there was insufficient clearance between the frame and the equalize shackles to permit full articulation. Is perhaps the intent that the axle using the frame as a bump stop will limit the travel of the equalizer? Please understand that am wishing to purchase the lowest possible kit - but do want a working suspension at the completion of the project. Thanks, Mike

Andrew N.

10/5/2026

@MikeW Thanks Mike. I looked into this further, and I think your concern about the available suspension travel is valid. My first reply was mainly intended to clarify that the 3-1/2" dimension for the # APTT3 is measured from the axle centerline to the frame. With a 2-3/8" axle, that does leave only about 2-5/16" between the top of the axle tube and the frame, and I would not want to rely on the axle or U-bolts contacting the frame to limit suspension travel. For your build, I think the better option is the triple-axle # APTT5. It uses taller 3-1/4" hangers and, with a 2-3/8" axle, places the axle centerline about 5" below the frame. That gives you roughly 3-13/16" between the axle tube and frame unloaded, providing considerably more room for the springs and equalizers to articulate while still keeping the trailer relatively low. So if the additional roughly 1-1/2" of trailer height is acceptable, I would lean toward the # APTT5 for your application.

Is The 3-1/2" Axle To Frame Clearance On Hanger Kit APTT3 Loaded Or Unloaded?

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