Before we get into the importance of properly setting trailer suspension pivot bolt torque, let’s take a moment to understand what is involved.
To refresh our memories, torque is the amount of energy it takes to run the nut up along the threads of a bolt. The basic way to calculate this is by taking the amount of force and multiplying it by the distance. For the sake of simplicity, if we place a 200-pound person on the end of a two-foot bar, there should be 400 foot-lbs of torque.
The rotation of a bolt (torque) at some point causes it to stretch which is called “tension”. To elaborate, tension is the stretch or elongation in a bolt that provides the clamping force in a joint. When a bolt is tightened the body is stretched so it acts like a very stiff spring which pulls in mating surfaces together. When torque is applied to the pivot bolts, this stretch will clamp the hanger to the bush in the trailing arm.
Ideally, a torque wrench should be used to ensure the pivot bolts are properly tightened and no issues develop down the road. Always refer to the Original Equipment Manufacturer’s bolt torque specifications or refer to a bolt torque chart for recommendations. To maintain accuracy, torque wrenches must be calibrated regularly. According to the ISO 6789 this should be every 5,000 cycles or every 12 months, whichever occurs first. However, they may be calibrated more often if deemed necessary.
Using a rattle gun is not the correct way to set torque. The most common negative outcomes are, over-torque and under- torque. Over-torque will probably yield either the bolt, clamped hanger, or nut. This will permanently deform them, making them unlikely to mate correctly and provide as much clamping force as they originally could. Under torque will prevent the required clamping force to be achieved around the hanger. The lack of clamping force will allow the bolt will shift in the hanger, wearing it out.
If for whatever reason the pivot bolts are removed, some manufacturers recommend replacing them. Reason being, the tightening procedure will permanently stretch the bolt involving high risk of breakage if the bolt is reused. Realistically, it doesn’t make any sense to reuse bolts as the risk far outweighs the replacement cost of used bolts.
As a side note, inspect the thread form of a bolt before installation. Damaged threads can cause galling, or materials adhering to one another, as the bolt is tightened. If the nut seizes to the bolt before the specified torque, then a loose bolt situation is created. In worst case scenarios the bolt will shear of the bolt if removal is attempted.
Once torque is set, pivot bolts are a relatively maintenance free item, however checking the torque during period service intervals may be required. This is done to ensure the bolts do not come loose, cause premature bush failure or create hanger wear at the bolt hole. For more accurate information always refer to the trailer suspension manufacture’s technical publications.