Measuring leaf springs correctly requires following the curve of each spring rather than stretching a tape measure straight from end to end. The most common mistake people make is measuring the direct eye-to-eye distance, which changes every time the spring flexes under different loads and tells you nothing useful for ordering a replacement. Here’s how to get accurate measurements that any spring shop can work with.
Why Eye-to-Eye Measurement Is Wrong
It seems intuitive to stretch a tape measure from the center of one eye to the center of the other. But leaf springs flex up and down depending on the weight sitting on them, and that flex changes the straight-line distance between the eyes. A spring under load will measure differently than the same spring sitting on a workbench. The eye-to-eye number doesn’t tell you the actual size of the spring, and it also fails to tell you where the center bolt sits relative to each end. That center bolt position matters because it determines how the spring mounts to the axle, and many springs aren’t symmetrical.
Measuring Divisional Length
The correct way to measure a leaf spring’s length is called the divisional length. You’ll need a flexible tape measure and the spring itself, either still on the vehicle or removed.
- Front half: Follow the curve of the spring from the center of the front eye back to the center bolt. Record this number.
- Rear half: Follow the curve from the center of the other eye back to the center bolt. Record this number separately.
You express the result as two numbers, like “24 inches x 24 inches” for a symmetrical spring or “21 inches x 27 inches” for one where the center bolt is offset. This format does double duty: it gives the total length and pinpoints the center bolt location in a single measurement. When you call a spring shop or order online, these two numbers are what they need.
Following the curve is key. Don’t hold the tape in a straight line from eye to bolt. Lay it along the top surface of the main leaf so it bends with the spring’s shape. This gives you the true arc length regardless of how much load is compressing the spring at the time.
Measuring Free Arch (Camber)
Free arch, sometimes called unloaded arch or camber, describes how much curve the spring has when it’s not carrying any weight. To measure it, imagine a straight line drawn from the center of one eye to the center of the other. Then measure the vertical distance from that imaginary line up to the top of the main leaf at the center bolt. That vertical gap is your free arch.
The easiest way to do this in practice is to set the spring on a flat surface, run a straightedge or string from eye center to eye center, and measure down (or up, depending on orientation) to the main leaf at the center pin. Free arch tells a spring manufacturer how much built-in curve to put into your replacement. A spring with too little arch will sag under load, while too much will sit your vehicle higher than intended.
Width, Thickness, and Leaf Count
Beyond length and arch, you need three more numbers: the width of the leaves, the thickness of each leaf, and how many leaves the spring pack contains.
Width is straightforward. Measure across the flat face of any leaf. Most springs in a pack share the same width, typically ranging from about 1.75 inches on lighter trailer springs up to 3 inches or more on heavy-duty truck applications. A tape measure works fine here, though calipers give you a more precise reading.
Thickness matters more than most people realize. Each individual leaf has a specific thickness, and even small differences change the spring rate significantly. Use calipers for this measurement. Leaf springs commonly range from around 1/4 inch to 1/2 inch thick per leaf, but the exact number is what a spring maker needs. If different leaves in the pack have different thicknesses, measure each one and note them separately.
Count every leaf in the pack, including any shorter leaves that don’t extend the full length. The number of leaves, combined with their width and thickness, determines the spring’s load capacity. Adding full-length leaves below the main leaf increases load-carrying ability without changing the peak stress on any single leaf, which is why heavy-duty springs tend to have more leaves rather than just thicker ones.
Measuring the Eye and Bushing Diameters
The eyes are the rolled loops at each end of the main leaf where the spring bolts to the vehicle. You need to measure the inside diameter of each eye, since they accept bushings and bolts that must match your mounting hardware. Use calipers to measure the inner diameter of the spring eye itself, then separately measure the inner diameter of the bushing pressed inside it.
Don’t assume both ends are the same. Some springs use different eye sizes front and rear, and the bushings may differ too. Measure each end individually and label them clearly. If your bushings are worn or cracked, having the eye diameter lets you order the correct replacement bushings even if the old ones are too damaged to measure accurately.
Double Eye vs. Slipper Springs
The measurement process above applies to double eye springs, which have a rolled eye at both ends and use shackles to connect to the frame. These are the most common type on utility, cargo, and lighter equipment trailers.
Slipper springs look different at one end. Instead of a rolled eye, the rear end is left flat or slightly curved so it “slips” into a hanger bracket. These are designed for heavier-duty trailers. You still measure divisional length the same way, following the curve from each end to the center bolt. But for the slipper end, measure the width and thickness of the flat tip rather than an eye diameter, since there’s no bushing or bolt hole to worry about on that side.
One important note: the spring’s length does not equal the distance between your frame-mounted hangers. Springs extend beyond the hanger points and connect through shackles or slipper fittings that change the geometry. If you’re replacing hangers or building a new mounting setup, you need the hanger spacing dimensions specific to your suspension kit, not just the spring measurements. Mixing spring types or using incorrect hanger spacing leads to axle misalignment, uneven tire wear, and potential suspension failure.
Recording Your Measurements
When you sit down to order or call a shop, here’s the complete list of what you should have written down:
- Divisional length: front half and rear half, measured along the curve (e.g., 24″ x 24″)
- Free arch: vertical distance from the eye-to-eye line to the top of the main leaf at the center bolt
- Width: measured across any leaf’s flat face
- Thickness: each leaf individually, using calipers
- Number of leaves: total count including partial-length leaves
- Eye inside diameters: both ends, measured separately
- Bushing inside diameters: both ends, measured separately
- Spring type: double eye or slipper
Taking photos of the spring from the side and from each end is also helpful if you’re sending measurements to a manufacturer. A clear side-view photo lets them verify the arch and leaf count visually, and close-ups of the eyes help confirm bushing style and condition.