Load rate is the amount of weight a spring is designed to carry when it is deflected to a certain height. A better term would be rated load or, better yet, the term we use, design load. In many cases the design load has nothing to do with how much weight the spring will really be carrying. Design load is a number chosen by the spring engineer and is the amount of weight that is really on the spring. An example of all of this would be a leaf spring that has a free arch of 6 inches, a rate of 200 pounds and is designed to carry 600 pounds when it is deflected to 3 inches, but is only carrying 400 pounds. The correct way to describe this leaf spring would be: Rate is 200 pounds–Load Rate or Design Load is 600 pounds–Load of 400 pounds.

All of the above terms apply to all springs, leaf, coil, steel, rubber and air.

The Three Types of Leaf Springs

Multi-Leaf springs consist of heat-treated flat steel bars of diminishing lengths formed to a predetermined arch held together by a bolt through its center.

Mono-Leaf springs consist of only one heat-treated plate of steel whose thickness is uniformly tapered from the center to each end. A tapered mono-leaf spring can equal or exceed the strength of a multi-leaf spring.

Parabolic springs are multi-leaf versions of Mono-Leaf springs. That is, they consist of 2 or more full tapered leaves.

Spring Eyes

Leaf spring eyes hold the bushings through which bolts or pins pass through to attach the spring to the vehicle. Spring eyes can be standard, Berlin or reverse and each has advantages.

Standard eyes are the most popular and easiest to make. Main plates (commonly called the main leaf) with standard eyes can receive additional support by extending the second leaf or a wrap plate.

Berlin eyes places the load through the centerline of the main pate, which reduces lateral deflection.

Reverse eyes will lower a vehicle while providing maximum spring travel.

The disadvantage of Berlin and Reverse eyes is that they cannot be provided additional support from the second leaf. However, this may not be required in most cases.

Measuring Springs

As a spring flexes up and down the eye to eye length changes. A 48-inch spring that has a 6-inch arch will measure 46¾ inches eye to eye. With a 3-inch arch the eye-to-eye measurement is 47¾ inches.

The correct way to measure a spring is to measure as though the spring was flat. Following the curve of the spring, measure from the center of the front eye back to the center bolt, then measure from the center bolt back to the center of the other eye again following the curve of the spring. This method will provide for the correct length no matter what the arch of the spring is.

Additionally, because the axle location is determined by the location of the spring center bolt, measuring this way provides for accurate axle positioning.

Measuring Free Arch

Free arch is how much arch is in a leaf spring when there is no load on the spring. To check the free arch draw a line through the center of the spring eyes. Then measure from that line to the top of the main plate (the leaf with the eyes) next to the center bolt. (C in the above diagram)