Leaf spring/axle question
Leaf Spring/Axle Questions
I had to make several passes to adjust my leaf springs. I lost count and even suspected that my torque wrench was malfunctioning, although it was a quality, nearly new tool. I believe the torque value was only 90 foot-pounds. I switched it to 115 for a test on the front hanger bolt and achieved the torque right away. Returning to 90, I found myself making pass after pass with the U-bolts. This all occurred after the total assembly was completed, with the truck back on the ground, and I began to torque everything.
I think this issue arises partly because the U-bolts stretch slightly during installation as intended. I recommend using new U-bolts; however, hardened washers and deep nuts may likely be reused. Also, don't forget to retorque everything after approximately 100 miles of driving.
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Frequently Asked Questions About Leaf Springs
Can I weld on my springs?
A: This question arises from time to time for various reasons. It is essential never to weld on any spring. Welding or even getting weld splatter on a spring creates a stress riser, which can cause the spring to break. Suspensions are safety items, and their failure can lead to catastrophic damage.
If welding is necessary near a spring, remove the spring before welding, or ensure the spring does not touch the weld heat or splatter.
Should I grease my leaf springs?
A: Prior to the mid-1950s, grease was often placed between the spring leaves. Many older cars even had springs wrapped in metal liners to retain the grease. Some featured hollow center bolts with grease fittings to allow on-car lubrication.
This lubrication aimed to minimize inter-leaf friction. However, in the early 1950s, the steel used in spring manufacturing changed to SAE steel, leading to a cessation of lubrication practice. Grease negatively impacts SAE steel due to chemical reactions that cause steel degradation.
When examining front springs on larger trucks, you may find power steering fluid leaking onto the lower springs.
Are mono-leaf springs safe?
A: To evaluate the safety of mono-leaf springs, we must compare them with multi-leaf springs. A multi-leaf spring is an engineered system designed for support, stability, and vehicle safety. Each leaf is proportionately built to carry load and stress while providing support to the leaves above and below.
In the event of failure—eventual for all springs—this construction helps to support the vehicle. In contrast, mono-leaf springs consist of a single plate of spring steel, which tapers at the ends. The design's taper is critical for durability and safety.
Keep in mind that all springs have a finite lifespan. When designing a leaf spring, possible failure must be accounted for, especially regarding what happens when a vehicle is traveling. A well-engineered multi-leaf spring ensures that each leaf shares the load, is equally stressed, and provides support in case of failure.
What happens if the main leaf breaks?
A: If the main leaf breaks, serious issues can arise. When the main plate of a multi-leaf spring fails in front of the axle, the axle may shift backward. If it breaks behind the axle, multi-leaf springs typically provide enough support to prevent the vehicle's body from dropping onto the tires. However, if a mono-leaf spring breaks behind the axle, the body will drop, causing a sudden stop.
What about the axle shifting rearward?
A: This issue can affect vehicle control, ranging from minor to a complete loss of control. In multi-leaf springs, significant axle movement is rare, but mono-leaf springs lack support to prevent backward shifting.
What is the best way to finish leaf springs: powder coating, painting, or chroming?
A: Powder coating involves spraying dry paint particles onto the spring's surface, followed by heating to bond the paint. If the coating process raises the temperature too high, it can anneal the spring steel, rendering it incapable of supporting weight—correcting this error can cost between $225 and $250 per spring.
Powder coating must be done below 350 degrees to preserve spring integrity. For painting, high-quality, high-temperature engine enamel should be used. Hang the spring by an eye and apply multiple coats, allowing thorough drying between them. Avoid coating the top and bottom of the leaves to prevent added thickness from affecting the spring pack.
When springs flex, wear occurs between the leaves, and coated inner surfaces can lead to additional movement in the center bolt area, resulting in either a sheared center bolt or a broken spring. Note that there’s typically no warranty for springs broken between U-bolts.
How do I correctly measure my leaf springs?
A: The proper way to measure a leaf spring's length is to treat it as if it were flat, rather than measuring eye-to-eye. While the eye-to-eye method might seem easier, it is not accurate because spring distance varies with flex. For instance, a 48” spring with a 6” arch measures 46 ¾” eye-to-eye.
Measuring from the center bolt to the spring eyes must be done by following the curve of the spring itself. Further, the appropriate arch measurements—free arch when unloaded versus loaded arch when under weight—should also be made accurately.
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