Consider the lowly leaf spring. It’s often the forgotten component, but the leaf spring is critical to vehicle safety and performance, and should definitely be included in any preventive maintenance program.
As any suspension specialist will tell you, U-bolt tightness is the most important factor in leaf-spring well being. The center of a leaf spring, at or near its bolt hole, is the spring’s weakest point. If allowed to flex – the result of a loose U-bolt – accelerated spring failure is practically guaranteed.
Triangle Spring Co., DuBois, Pa., offers the following tips on U-bolt maintenance:
- Never reuse U-bolts. A properly installed U-bolt will stretch under torque. The effects on metal and threads lower a used U-bolt’s ability to sustain required clamping torque.
- Be sure U-bolts are the proper grade.
- Be sure U-bolts are properly aligned. Check the condition of the top plate and saddle alignment holes.
- Torque U-bolts, per vehicle specifications, upon delivery of a new vehicle, or after repairing the suspension on an in-service vehicle. Retorque after the first day’s operation, at least within 500 miles, to adjust for any settling that may have occurred.
- After torquing, rap one leg of the U-bolt with a brass hammer. The U-bolt should ring. If it produces a dull thud, it is not clamping the system. Next, rap the top of the U-bolt at the bend area to settle out any play. Recheck the torque.
- Drive the vehicle for a few miles to settle the suspension and close up any resulting play. Then retorque the U-bolts while they are under load.
Dealing with fatigue
While proper U-bolt maintenance will extend useful spring life, no spring will last indefinitely. Eventually, they will suffer fatigue – a cracking process that’s accelerated by overloading or being used in a severe-service application.
Fatigue cracks occur between a U-bolt and one end of the spring, usually at a surface imperfection on the tension side of the spring. It could be a nick from a loose tire chain, corrosion, weld spatter or fretting (abrasion caused by leaves rubbing against each other). These areas where stress becomes concentrated are called stress raisers.
Once started, a crack progresses until the cross-section at the crack site is no longer able to sustain the load. But the gradual nature of fatigue-crack development gives the astute technician an opportunity to replace the spring before it fails.
Fatigue-damaged leaves should, ideally, not be replaced individually. Depending on age and mileage, the entire assembly should be replaced on both sides of the vehicle.
Leaves broken by loose U-bolts can be replaced individually because this type of failure is not the result of a tired assembly. But this should be done as soon as the condition is discovered, to prevent damage to other leaves from additional loading and from concentration of stress over the edge of the broken leaf.
If a main leaf is broken, regardless of the cause, the vehicle should not be driven for two reasons. First, depending on the type of suspension, the location of the axle may have been altered. Second, unless the second leaf has end wrappers, the only remaining, supporting members are the lesser leaves, tied to the main leaf by small shackles. A broken leaf-binding shackle could, under these circumstances, cause total suspension collapse at that corner of the vehicle.
An often-overlooked factor in leaf-spring longevity is brake balance. One or more brakes out of adjustment will cause the remaining, properly adjusted brakes to do more than their share of stopping work.
The springs at those axle ends get wound-up more than they would if all brakes were pulling equally. The situation is compounded when an axle with a wound-up spring hits a bump. There is little or no reserve capacity available to accommodate the additional loading. Eventually, something has to give.
Finally, when purchasing replacement springs, look for: a smooth surface finish; leaf alignment, both lateral and vertical; proper leaf spacing and stepping; corrosion protection; and manufacturer’s identification.
