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