Trailer Axle Leaf Spring Seats or Spring Perches are essential for the Trailers Suspension system. They allow the Leaf Springs to be U Bolted to the Axle and they keep the Axle from Rotating during trailering. On a Round Axle Leaf Spring Seat like this one, the only way to properly install it is to have it welded in place.This Spring Seat is designed to fit around a 2 3/8 inch Diameter Round Axle. Axles in the 3,500lb Capacity are typically 2 3/8 inches in Diameter, but its always a good idea to measure your current Trailer Axle first before placing an order for new Spring Seats.Most Round Trailer Axles are not perfectly straight, most of the time they will have a Camber, or slight bend, to them. The Axle needs to be oriented so that this bend is pointing up, toward the trailer. The bend then levels out when the trailer is under load, and the axle becomes straight. This is important to note when installing new Leaf Spring Mounts, because depending on whether you want your Trailer Leaf Springs on top of or below the Trailer Axle, the direction of the Camber will make a difference.Specifications:-Steel Construction-Weld On Installation-Fits 2 3/8 inch Round Axles (3,500lb)