Torque Rating Requirements for Installing Leaf Springs on a 7000 lb Tandem Axle Trailer

Updated 03/14/2013 | Published 03/11/2013

Question:

What is the torque requirements for all bolts related to the leaf spring on a 7,000 # tandem trailer. U-bolts, shackle bolts, attachment bolts to the frame? Thanks

asked by: Terry B

Expert Reply:

The torque requirements for leaf spring u-bolts depend on the diameter of the u-bolts that are being used. If the gross weight of your tandem axle trailer is 7,000 lbs, then you likely have two 3,500 lb axles. Typically 1/2 inch diameter u-bolts are used with leaf springs for 3,500 lb axles and they have a torque rating of 45 ft/lbs to 70 ft/lbs.

The shackle bolts do not require a specific torque rating because tightening the shackle bolts too much will squeeze the shackle straps against the leaf springs and the whole suspension system will bind and cause problems. The best thing to do when tightening down shackle bolts is to use lock nuts and tighten them until they are snug and then backed off 1/4 turn. Lock nuts should be used because they resist loosening with vibration or torque.

Finally, I strongly recommend welding your leaf spring hangers to your trailer frame. Mounting the hangers with a bolt only will not provide a secure connection between the trailer frame and suspension, even if the bolt is torqued down properly. There are some hangers that come with bolt holes on top, but they should be welded as well as bolted. If you want to use a bolt as well as welding your leaf spring hangers, then you can follow the same torque specs as the leaf spring u-bolts.

A 1/2 inch diameter bolts should be torqued between 45 ft/lbs and 70 ft/lbs. A 9/16 inch diameter bolt should be torqued between 65 ft/lbs and 95 ft/lbs and a 5/8 inch diameter bolt should be torqued between 100 ft/lbs and 120 ft/lbs. I have attached two FAQ articles on trailer suspension for you to check out.

expert reply by: John H