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What is Trailer Spring ‘Growth’?
Trailer springs do not remain the same size forever. Over years of service, repeated flexing under load causes the spring steel to relax slightly. This gradual change is often referred to as spring ‘growth’ and presents as a small increase in the spring’s free length.
Even a few millimetres of extra length can be enough to alter fitment and suspension geometry. If you measure only a worn spring and order a like‑for‑like replacement without accounting for growth, you risk poor fit and unwanted handling changes.
How Spring Growth Affects Trailer Performance
Ride height changes
As free length increases, chassis height and axle angle can shift, affecting coupling attitude and load distribution.
Axle alignment and tracking
Small geometry changes can compound into noticeable tracking issues, making the trailer wander or feel nervous at speed.
Tyre wear and component stress
Misalignment accelerates tyre wear and increases stress on shackles, hangers and related suspension hardware.
How to Measure a Trailer Spring Accurately
Always measure the spring in its unloaded state. Remove the spring from the trailer where practical so you are not fighting bushing compression or chassis weight.
For eye‑to‑eye leaf springs, measure from the centre of one eye to the centre of the other eye. Record eye diameter and bush type to ensure correct hardware selection.
For slipper springs, measure from the centre of the eye to the extreme end of the slipper tip. Note the style of the slipper end, as profiles can differ between manufacturers.
When you are measuring a worn spring, compare your figure to the manufacturer’s standard length for that model. If your measurement is longer than the catalogue dimension, adjust for growth and order to the correct specification rather than the worn length.
Common Mistakes When Replacing Springs
Ordering from worn length alone
Aged springs can read long. Use catalogue or OE dimensions as a cross‑check before purchasing.
Ignoring bush and eye wear
Ovalised eyes and tired bushes distort measurements. Replace bushes and record the true eye centres for accuracy.
Choosing by model name only
Trailer models change over time and may ship with multiple suspension options. Direct measurement is more reliable than assumptions.
Over‑specifying spring rate
Heavier is not always better. Springs that are too stiff for the actual load reduce compliance, harm ride quality and can increase stopping distances on poor surfaces.
Choosing the Correct Replacement Spring
Select a spring that matches the measured length, eye style and bush type, then confirm the rating against your trailer’s GVM. Consider the axle configuration as weight is shared differently across single axle, tandem and tri‑axle setups.
If you are changing trailer ride height, confirm guard and tyre clearance and check brake lines and wiring for chafe risk after installation.
Where to Buy Correctly Sized Trailer Springs
Browse our full range of trailer spring sets engineered for New Zealand conditions. Choose from individual leaf springs, complete single axle kits, tandem axle kits and tri‑axle kits, plus matching suspension hardware and accessories to finish the job properly.