40mm Square Axle

40mm Square Axle


Engineered for Quality - Couplemate

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40mm Square Axles to suit Standard Bearings

40mm square axle suits wider small box trailers and boat trailers up to 1000kg.

Axles are measured from the tip of the axle to the tip of the axle. When measuring from the face of the axle, add 100mm to this measurement to get the tip of the thread to the tip of the thread length.

Standard Bearing Sizes

Couplemate manufactures 40mm square trailer axles in Wynnum, Queensland. BlueScope or One Steel provides the best axle money can buy. If you can find a better product, please tell me.

For more information about why we use quality steel, check out my article, Axle Steel for Caravan and Trailer Axles.

Your 40mm square axle may be perfect for your trailer, caravan or camper if you are towing your rig straight and true, perfect. If not, then below are a few tips.

Common problems:

  • Too much weight will bend this 39mm Round Axle causing axle flex.
  • An overloaded axle causes axle flex. You can read about axle flex and vibration here.
  • Axle flex occurs when heavily loaded trailer springs rebound on the highway, causing the axle to flex up and down in the middle of the axle.
  • Springs to fatigue - fail prematurely under axle flex loading conditions.
  • If your load is over 1000kg, you will start to experience axle flex.
  • If your axle is 40mm Square and bending, buy a 45mm square axle and update to 1.5t.
  • It is not expensive in comparison to replacing springs that have suffered fatigue.
  • Rough seal journals can cause a loss of grease through the rear seal.
  • Are your tandem tyres wearing unevenly? How to Align Caravan Wheels
  • Some suppliers supply non-virgin steel or scrap steel and forge this steel into a solid square or round shape for axle production.
  • Chinese bearings are the second biggest cause of axle failure. If you plan to drive on the highway, use Japanese bearings. Combining Japanese Bearings and a 39mm round Axle will help you significantly with lower maintenance for your rig.

Finally, after reading the above information on axles, I invite you to comment in the review box below. We never get too old to learn.

Calculating The Correct Axle Measurements

As a result of viewing the video above, calculate the face to face measurement.
  1. Measure the offset wheel. The wheel offset is the distance between the inside of the hub and the outside of the rubber.
  2. Add 50 mm to the offset. Alternatively, choose your own distance between the rubber and the chassis.
  3. Multiply by 2 for both sides. Add your chassis width to the above number.

Metric to Imperial Converter

Convert Imperial (inches) to metric (mm)

  • =

Still unsure of what measurements to take? Check out this handy guide below:

[P] - Check material specifications. What size and profile is the axle (for example, 45mm Square)
[T] - Overall axle length (tip to tip)
[F] - Hub face to hub face measurement
[A] - Chassis width
[C] - Tyre clearance
[B] - Back offset
[H] - Tyre to chassis
[G] - Axle pad centres

How To Measure An Axle

Identifying Bearing Combinations

You will also need to identify your bearing combination, which can be done by pulling off a hub and bearings and locating the number laser engraved onto the bearing. You will require both the inner and the outer bearing number.

Where to identify bearing number. Look for the laser engraved number on the bearing.

Caravan axles and their maximum load capacity