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Do you agree caravan overlay axles can be inverted?

Bill asked me a great question on the Couplemate website yesterday. He asked “I want to invert my overlay axle, so my caravan gains extra height. Currently, it hits the top of the mudguard.”

So Bill here is our answer.

overlay axlesOverlay axles have changed over the years, so it is not possible for a straight up and down answer.

Overlay axles were traditionally built with a 5mm camber so inverting these axles would have disastrous results. The image on the right is an overlay axle in its normal position.

The function of an overlay axle is to lower the car trailer or horse-float a little so that cars and horse have easier access. The centre of gravity is also lowered.

The question of inverting an axle has advantages and risks. Here are our recommendations.


How much extra clearance do I gain by inverting my 45mm Square Axle?

The total gain by inverting a 45mm Square Axle is 90mm

Overlay axles can be inverted under the following conditions.

  • The stub axle has been welded into position with zero camber.
  • The overlay is installed on top of the springs.
  • Axle spacer plates must be repositioned and welded to the axle.
  • Park brake to be relocated on top of the axle for 10” electrics.
  • Park brake to be relocated on the bottom of the axle for 9” mechanical brakes.
  • Stub axles to have a minimum of 50mm welded section.
  • Standard U-bolt are changed for heavy duty U-bolts.

inverted overlayTaboo issues are

  • Axles located under spring is a safety issue. A damaged U-bolt can easily detach and cause your axle, also to become separated.
  • Couplemate does not recommend under slinging axles except in exceptional circumstances. Example, where the keel on a boat trailer prevents an axle from being installed. Axles can be gull-winged or inverted by the manufacturer.
  • Not welding in axle spacers in between spring and axle.
  • Stub axles welded on top of the hollow box section.
  • Welded section less than 50mm
  • 1/2″ u-bolt (higher centre of gravity required more clamping force to counteract the higher moving load forces)
  • 5/8″ u-bolts may be required for inverted axles.

Furthermore, higher centre of gravity you have installed by inverting your axle will require a change in driving habits.

As a result, the top-heavy rig will corner differently and behave differently on variation in road camber.

Long, wide overlays can create vibration fatigue.

There is a good article here about how to remove vibration and how to install shock absorbers to remove the effects of vibration.

Is it possible to invert straight beam axles?

Finally, you may need an engineering certificate when considering under-slinging your axle to raise the van, here are some considerations.