What Does “Constant Load” Really Mean on a Leaf Spring?

If you’ve been researching suspension upgrades for your 4WD, you’ve probably seen leaf springs advertised with terms like “50mm lift” and “500kg constant load.”

On the surface, that sounds straightforward — but in reality, constant load ratings are one of the most misunderstood parts of suspension selection, and getting it wrong is one of the biggest reasons people end up unhappy with their ride, handling, or load-carrying ability.

Let’s break down what constant load actually means, how it applies to leaf springs, and why choosing the wrong rating can ruin both comfort and performance.

What Is a Constant Load Rating?

A constant load rating refers to the amount of permanent, always-present weight that a leaf spring is designed to carry all the time.

For example:

A 50mm lift leaf spring with a 500kg constant load rating is engineered to sit at its intended ride height when approximately 500kg is permanently added to the rear axle.

This does not mean:

• Occasional payload

• A once-a-year camping trip

• Temporary towing or short-term weight

It means weight that lives on the vehicle every day.

Typical examples of constant load include:

• Steel canopies or service bodies

• Drawer systems and storage fit-outs

• Long-range fuel tanks

• Rear bars and spare wheel carriers

• Tools or work equipment

• Regular towball weight (if towing frequently)

Why Constant Load Matters on Leaf Springs

Leaf springs — whether parabolic or traditional multi-leaf packs — are designed to operate within a specific load range.

The spring rate is calculated so that:

• Ride height sits correctly

• Suspension travel is maintained

• The vehicle handles predictably

• Comfort and load control are balanced

If the spring rate does not match the vehicle’s actual constant load, problems start immediately.

What Happens If the Constant Load Rating Is Too High?

This is one of the most common mistakes we see.

If you fit a 500kg constant-load leaf spring to a vehicle that only carries:

• A tub liner

• A fridge on weekends

• Occasional camping gear

The result is often:

• Harsh, stiff ride quality

• Excessive rear lift

• Poor small-bump compliance

• Reduced traction on corrugations

• An unsettled or “skipping” rear end

In simple terms, the spring is waiting for weight that never arrives.

What Happens If the Constant Load Rating Is Too Low?

Choosing a spring with too little constant load capacity causes the opposite problem.

If the vehicle consistently carries more weight than the spring is designed for, you may experience:

• Rear-end sag

• Frequent bottoming out

• Reduced suspension travel

• Poor braking and handling

• Premature spring and shock wear

This is especially common on touring vehicles that slowly gain weight over time as accessories are added.

Constant Load vs Variable Load

This is where a lot of confusion comes from.

• Constant load = weight that is always on the vehicle

• Variable load = weight that comes and goes

Leaf springs must be selected based on constant load first.

Occasional extra weight should be managed through:

• Correct spring selection

• Proper shock absorber valving

• Sensible load distribution

Not by overspringing the vehicle “just in case.”

The 80% Rule for Choosing the Right Leaf Spring

A simple rule that works in the real world:

Choose suspension for how the vehicle is used 80% of the time — not the biggest trip of the year.

Ask yourself:

• How much weight is permanently fitted?

• What accessories are on the vehicle full-time?

• How often is it towing, and what is the ball weight?

• Is daily comfort or maximum load capacity the priority?

Suspension should support the vehicle’s normal life, not punish it for rare use cases.

Why Leaf Springs Get a Bad Reputation

In many cases, the spring itself isn’t the problem.

Poor outcomes are usually caused by:

• Incorrect constant load selection

• No rear axle weight assessment

• Guessing instead of measuring

• Choosing springs based on marketing

• Mismatched shock absorbers

When leaf springs are chosen correctly — parabolic or standard — they provide:

• Predictable ride height

• Good comfort

• Stable load handling

• Long service life

When chosen incorrectly, no spring design will perform well.

Final Thoughts

A 50mm lift leaf spring with a 500kg constant load rating is not a universal solution — it’s a specific spring for a specific vehicle setup.

Leaf springs are not magic. They don’t adapt on their own. And they don’t forgive poor selection.

If you want a suspension setup that rides well, carries load properly, and lasts — the spring must match the vehicle, not the brochure.

You don’t need hype. You need the right spring.