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U-bolts may be the Major Cause of Accidents

U-bolts are not usually assessed as the culprit in Caravan accidents or rollovers, possibly due to the enormous accident carnage. Our industry has no Australian Standards for caravan or trailer u-bolts, they have not been seen as a problem.

Steel has an exact amount of carbon for each grade of steel. Carbon steel has a variety of mechanical properties that produce medium or high strength steel.

When steel is heated, hydrogen gas attacks steel and applies stress to the steel. Hydrogen embrittlement occurs at this time producing stress corrosion cracking only visible under x-ray.

Bad bearings, broken springs, loss wheel nuts are blamed as the cause of most caravan accidents.

Hydrogen Embrittlement

However, the culprit in many caravan and trailer accidents is the hydrogen embrittlement embodied during manufacture.

Hydrogen embrittlement is the most underestimated and underutilised processes in the manufacturing industry.

Fact: A Hydrogen content or embrittlement is embedded in all steel products that have undergone some form of a heating process.

Example, plating tow balls install hydrogen embrittlement into the tow ball. Tow balls must then be de-embrittled via an elevated temperature and then slowly cooled to room temperature, to remove the embrittlement.

Read more about hydrogen embrittlement here.

How do I know if I have the right equipment on my caravan?

U-bolts are often overlooked and just not on the maintenance checking schedule before your next caravan trip.

It is difficult for manufacturers to produce a standard u-bolt as there is no tensile strength or Australian Standard.

broken u-boltsCouplemate have decided to undertake tests similar to rated Dee Shackles. The test for dee shackles is breaking load limited (BLL) / 6 = working load limit (WLL). (not the ATM). To calculate the ATM divide the WLL by 1.5.

Example, if heavy duty u-bolts break at 23t, the rated load limit is 5.75t.

ATM is calculated at 5.75t / 1.5 = 3.83t max load.

Therefore, a 5/8″ Couplemate u-bolt kit has been tested to a maximum ATM capacity of 3.83t

The image on the right is from our 200t test rig. A force was gradually applied until breakage occurred at 23t. We called this the BLL or break load limit.

By applying the Dee Shackle or Bow shackle testing regime, we can apply a maximum safety rating of 3.83t per axle for this type of u-bolt.

Heavy Duty U-bolt Kit specifications are

  • 2 x 5/8″ BSW Galvanised mild steel u-bolts
  • 4 x 5/8″ BSW nylocks and washers.
  • 1 x 10mm Galvanised mild steel plate.

Spring and Axle Connection

It is a common practice by some caravan manufacturers to put spring under the axles to give extra height. Contrary to popular opinion, Couplemate not only thinks this is a bad idea, but it is unsafe and contributes to road accidents.

bent u bolt by handAxles are usually located above the spring and enclosed by the spring and the chassis rails.

What buyers do not know is there is no Australian Standard to test u-bolts used by caravan and trailer manufacturers.

A process called hydrogen embrittlement causes steel to become brittle and fracture on the bend during a tensile stress test. The steel rod on the left was bend by bare hands and suffered a loss of ductility

This is caused by absorbed hydrogen or absorbed hydrogen atoms and is the principal cause of hydrogen embrittlement.

The effect of hydrogen embrittlement results in U-bolts becoming. There zero chance of effectively clamping your axle to your caravan spring when residual stress remains in your u bolts..

Fractured U-Bolts

A fractured u-bolt when the axle is under the spring means the axles can easily break free once the u-bolt fracture and discharge itself out the rear of the caravan. The potential for road carnage to innocent travellers behind you is horrific.

Couplemate tests its u-bolts to destruction under its standards, due to the lack of Australian Standards, so we are well aware of this problem.

Our remedy to combat hydrogen embrittlement inserts an additional process into the manufacture of u-bolts. This process removes hydrogen embrittlement.

When making your spring choices be sure to purchase Couplemate u-bolts.

Snapping U-bolts

Hydrogen embrittlement can be readily determined by breakage at the corner of the u-bolt when it has been bent.

All bends should have a least a 5mm radius otherwise they will not match the 5mm radius curve on your axle. This radius is a critical pressure point. It is easily eliminated.

U bolts are the only thing holding your entire caravan onto the road. They bind your axle to your springs and then your chassis.

U bolts are not a disposable item like grease or bearings or tyres. They are in place for the life of your caravan,

Please excuse me if I am OCD about u bolts after witnessing images from accidents.

How important is a u-bolt check?

I installed new u-bolts in Brisbane and headed down to the Hawkesbury River for Christmas with the boat in tow. (top image)

Lovely, uneventful trip but now its time to check a few things.

Top of the list this time is u-bolts. Told my brother-in-law, all about mild steel stretching and how these nuts will probably tighten 1/2 turn.

As suspected, all the nuts needed 1/2 a turn to make them tight again.

When was the last time you checked your nuts?

Especially on new caravans. Do not wait for the first service for what looks like a faulty spring is actually lose u-bolts.

Grab your wrench and socket. Test them and tell me what you find!

U bolts are a major factor in correcting caravan wheel alignment.


Check your u-bolts for tightness every time you pull your wheel off to check your bearings. If you have nylocks, they will need replacing if removed.

Further, u-bolts are made from mild steel, stretch and even break under shock loading. I always check mine, and I ask you to do likewise for your safety,

Finally, I would love to hear your comments below and your opinion on broken U-bolts and how close you were to disaster.

The video below is about a boat owner who had a shock loading event that snapped his u-bolts. What he did not know, was his old u bolts were likely to have been suffering hydrogen embrittlement.

Couplemate U-Bolts

Round U-Bolts


Square U-Bolts





Part Number
1 ½” (39mm)½” (13mm)4″ (100mm)U39R4GH
1 ½” (39mm)½” (13mm)4½” (112mm)U39R45GH
1 ½” (39mm)½” (13mm)5″ (125mm)U39R5GH
1 ½” (39mm)½” (13mm)5½” (137mm)U39R55GH
1 ¾” (45mm)½” (13mm)4″ (100mm)U45R4GH
1 ¾” (45mm)½” (13mm)4½” (112mm)U45R45GH
1 ¾” (45mm)½” (13mm)5″ (125mm)U45R5GH
1 ¾” (45mm)½” (13mm)5½” (137mm)U45R55GH
1 ¾” (45mm)½” (13mm)6″ (150mm)U45R6GH
1 ¾” (45mm)½” (13mm)6½”(162mm)U45R65GH
2″ (50mm)½” (13mm)4″ (100mm)U50R4GH
2″ (50mm)½” (13mm)4½” (112mm)U50R45GH
2″ (50mm)½” (13mm)5″ (125mm)U50R5GH
2″ (50mm)½” (13mm)5½” (137mm)U50R55GH
2″ (50mm)½” (13mm)6″ (150mm)U50R6GH
2″ (50mm)½” (13mm)6½”(162mm)U50R65GH
2″ (50mm)½” (13mm)7″ (175mm)U50R7GH
1 ½” (40mm)½” (13mm)4″ (100mm)U40S4GH
1 ½” (40mm)½” (13mm)4½” (112mm)U40S45GH
1 ½” (40mm)½” (13mm)5″ (125mm)U40S5GH
1 ½” (40mm)½” (13mm)5½” (137mm)U40S55GH
1 ½” (40mm)½” (13mm)6″ (150mm)U40S6GH
1 ¾” (45mm)½” (13mm)4″ (100mm)U45S4GH
1 ¾” (45mm)½” (13mm)4½” (112mm)U45S45GH
1 ¾” (45mm)½” (13mm)5″ (125mm)U45S5GH
1 ¾” (45mm)½” (13mm)5½” (137mm)U45S55GH
1 ¾” (45mm)½” (13mm)6″ (150mm)U45S6GH
1 ¾” (45mm)½” (13mm)6½”(165mm)U45S65GH
1 ¾” (45mm)½” (13mm)7″ (175mm)U45S7GH
2″ (50mm)½” (13mm)3″ (75mm)U50S3GH
2″ (50mm)½” (13mm)4″ (100mm)U50S4GH
2″ (50mm)½” (13mm)4½” (112mm)U50S45GH
2″ (50mm)½” (13mm)5″ (125mm)U50S5GH
2″ (50mm)½” (13mm)5½” (137mm)U50S55GH
2″ (50mm)½” (13mm)6″ (150mm)U50S6GH
2″ (50mm)½” (13mm)6½”(165mm)U50S65GH
2″ (50mm)½” (13mm)7″ (175mm)U50S7GH
2″ (50mm)½” (13mm)8″ (200mm)U50S8GH
2″ (50mm)½” (13mm)10″ (250mm)U50S10GH
2 ¼” (56mm)½” (13mm)4½” (112mm)U56S45GH
2 ¼” (56mm)½” (13mm)5″ (125mm)U56S5GH
2 ¼” (56mm)½” (13mm)5½” (137mm)U56S55GH
2 ¼” (56mm)½” (13mm)6½”(165mm)U56S65GH
3″ (75mm)½” (13mm)3″ (75mm)U75S75GH
3″ (75mm)½” (13mm)4″ (100mm)U75S100GH
3″ (75mm)½” (13mm)5″ (125mm)U75S125GH
3″ (75mm)½” (13mm)6″ (150mm)U75S150GH
3″ (75mm)½” (13mm)7″ (175mm)U75S175GH
3 ¼” (82mm)½” (13mm)4 ¾” (120mm)U80S120GH
4″ (100mm)½” (13mm)3″ (75mm)U100S75GH
4″ (100mm)½” (13mm)4″ (100mm)U100S100GH
4″ (100mm)½” (13mm)5″ (125mm)U100S125GH
4″ (100mm)½” (13mm)8″ (200mm)U100S200GH
1 ¾” (45mm)⅝” (16mm)6″ (150mm)CM600GH
2″ (50mm)⅝” (16mm)6″ (150mm)CM602GH
2 ⅓” (60mm)⅝” (16mm)7″ (175mm)CM604GH
1 ¾” (45mm)⅝” (16mm)6″ (150mm)CM601GH
2″ (50mm)⅝” (16mm)7″ (175mm)CM603-7GH

© Steve Wotherspoon 8th August 2017