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Loose Axle U-bolts Cause Highway Havoc

by | Jul 19, 2019 | U Bolts | 3 comments

Loose Axle U-bolts cause highway havoc for unsuspecting caravaners. The reality of not checking your rig before heading on your family holiday can quickly become a nightmare.

The axle in the photo is a heavy-duty 50mm square, Australian made axle with 12″ electric backing plates and Alko drums.  All of the equipment is heavy duty and of good quality so the problem did not arise from this. The trailer was also nearing its total capacity of 4.5t, so this is quite a severe accident.

The cause of the accident was found to be loose U-bolts. Notice the u-bolt axle clamp still intact on the right-hand side of the trailer? The leaf spring has broken; however, if the U-bolts were adequately tightened, some portion of the spring should be retained by U-bolt clamp.

The left side of this axle is severely damaged. The u-bolts have become so loose that the axle has moved back, possibly under shock loading and become detached from the u-bolt clamping system.

The axle had then rattled around inside the spring camber area until it had broken the spring and bent the axle when it exited the trailer undercarriage.

How to avoid this disastrous event? A very simple solution is to ensure you tighten the nuts on your U-bolts.

U-bolts and Australian Standards for Caravans

Currently, an Australian Standard for U-bolts used in the caravan or trailer industry does not exist.

The standard U-bolt used most commonly is SS400 grade steel coupled with a 1/2″ BSW thread. Although higher tensile steels are available, the industry is unmoved by demanding a better standard of clamping axle and spring to the caravan chassis.

Couplemate is currently investigating and testing higher grade steels for use within the caravan industry that does not require re-tightening at the 1st service. As an essential chassis component that is critical for safe travel, Couplemate is dedicated to investing in research and development for this product.

Manufactures of note, including Al-ko and Couplemate, specify torque settings for caravan and trailer u-bolts.

A problem is created when dealers and customers ask for torque settings. It also creates issues removing the hydrogen embrittlement that often causes u-bolts to bend or break easily.

Typical sizes of u-bolts and their ratings are:

  • 1/2″ BSW – up to 1500kg per axle
  • 5/8″ BSW – up to 2250kg per axle
  • 5/8″ UNF High Tensile – over 2250kg

Cutting U-Bolts

It is often the case that buyers of U-bolts do not measure or take a sample of a U-bolt to their trailer shop when purchasing new u-bolts.

This procedure sounds straightforward; however, the nylon inside the nylock will melt when given enough heat. The melted plastic inside the nylock will cause the nut to unwind on the highway.

The fix is to measure the U-bolt and cut it before installation. An ounce of care is worth a tonne of cure.   

Couplemate U-Bolt TestingRetightening U-bolts gave me piece of mind for the entire trip.

We all agree this is a pretty scary event that has a simple solution.

Couplemate U-bolts have been tested to rupture at 23t, evidenced by the photo from our testing laboratory.

The test was undertaken with a 200t press utilising a 50mm square K1045 axle section.

We estimated the forces need to bend this axle when exiting the undercarriage were more than 23t.

This is especially true when carrying heavy loads. Yes indeed, loose axle U-bolts cause highway havoc.

The image below was taken when I was checking my boat trailer for the first time in many years.

u-bolts caravan accidents Loose U-bolts cause highway

It surprised me the nuts on my boat trailer were re-tightened by 1/2 turn. Retightened U-bolts gave me peace of mind for the entire trip. I encourage you to check the tightness of your u-bolts with a socket today before you forget.

Loose u-bolts are one of the most underrated issues in the Caravan and Camper industry.

  • Check yours and see if I am right?
  • Did your mechanic check them on your last service?
  • Did you ask him to check the u-bolts?

Why U-bolts may be the Major Cause of Accidents

Finally, if you found this article helpful, I invite you to leave a comment at the bottom of the page.

Loose Axle U-bolts cause Highway Havoc


  1. Doug Dukes

    Tight and NEW NYLOCK nuts

  2. Ian lawson

    Suggestion; fit retainer nuts as secondary once torque value is reached or recess a situation into the retainer plate that will effectively capture and retain the nut (s) at least to a degree thereby reducing the requirement to check but not eliminating it. If this situation is such a problem then why on earth didn’t manufacturing companies allow for this, afterall a lot of older retired operators are out there suggesting that they are not exactly overly flexible enough to go crawling under rigs checking stuff that shouldn’t in all reality, be set up to fail. .

    • Steve Wotherspoon

      Hello Ian, We found there is no problem with nut slippage however there is ample evidence nylock nuts should not be re-used.

      Personally, I would prefer that steel did not stretch and higher tensile steel was used. To date, manufacturers or OEM are unconvinced about stretching u-bolts and this is partly because there is no Australian Regulation for the use of u-bolts on caravans and trailers.

      Some manufacturers insist on 1st service that u-bolts are tightened however, the process is not mandatory.

      Sadly, retired old operators must continue to check their rigs.

      Thank you for your reply Ian, it has triggered a review of this article. ie, the cutting of long u-bolts can cause the plastic inside the nylock to melt.

      Steve Wotherpoon

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