Updated June 2017

Wheels falling off trailers, wheel studs breaking and wheel nuts loosening still happens. Here is why it happens, and how to prevent it. Wrecked wheel bearings and fractured stub axles are also common. That fastenings such as wheel nuts may be caused, not just permitted, to loosen is rarely covered in engineering training. The causes and prevention are however well known. This referenced article by Collyn Rivers explains how and why in rigour and detail.

Wheels falling off trailers – and fastenings work loose

The thread of a screw fastening is necessarily spiral. Some side clearance must exist to enable the nut (or stud) to be turned. Tightening however causes the stud to stretch slightly. This marginally decreasing its diameter, thus increasing inter-thread clearance. Inter-thread friction normally prevents or limits sideways movement between the threads but, if any repetitive loads exert sideways force, that force may momentarily overcome that friction. This in turn allows the nut or stud to be thrust slightly from side to side. As the thread is spiral and in tension, momentarily relaxing the frictional contact may cause the fastening to ‘ratchet’ itself undone. Wheels falling off trailers are enabled by this. 

To see how this can happen, hold (by its head) a large diameter clean dry coarse-threaded bolt, with a loose nut, pointing downwards. Whilst stationary, the nut (restrained by inter thread friction) stays where it is. If shaken from side to side, however, gravity will cause the nut to unwind. With a similar bolt and nut under tension, repetitive sideways movement above a certain force will inevitably ‘ratchet’ that nut loose.

That is what can happens with wheel nuts or studs. As the wheels encounter bumps and pot holes, side shock loads impact the wheel studs. Unless correctly tightened or somehow restrained, the fastenings may work loose. Again – its a cause of wheels falling off trailers.Broken Ttailer in Cape York

The stub axle sheered off this tour group’s trailer on the corrugated track road to the tip of Cape York. It had no shock absorbers, nor provision for fitting them. The OKA in the distance belonged to the author. Pic: Author.

Preventing the effect – including wheels falling of trailers 

By adding yet further elasticity, spring washers make matters worse. Nyloc nuts add friction and limit sideways movement but in more extreme cases they too may work loose. Castellated nuts and split pins prevent nut movement but accurate re-torquing is not feasible unless new holes are drilled each time. Double nuts however work well because the upper one is not subject to sideways loosening.

Left-hand thread nuts (on left hand wheels) were used at one time on the assumption that anything they strike whilst the wheel is turning tightens rather than loosen them. This does not however restrain from loosening as above. Once the main causes of fastenings working loose became understood, most vehicle makers used conventional right hand threads both sides. Left hand threads are still used on mining and heavy quarry vehicles where direct impact is likely.

The approaches described below are very effective.

Wheels falling off trailers – reducing loosening forces

Fastenings mainly loosen where there are repeated side shocks. It also occurs where bolted assemblies bend, Furthermore, with cooling and heating cylinder head studs etc. It happens particularly with wheel studs and nuts and is usually why wheels fall off.

Far more wheels fall off caravans and campertrailers than ever from cars. Nevertheless, some trailer makers deny any failing causes this! It does not affect all trailers. It is mostly those lacking shock absorbers.

Wrecked trailers with wheels torn off, or stub axles broken, litter corrugated tracks. Almost all not just lack shock absorbers. They lack provision for fitting them.

Wheels falling off trailers – what actually happens

A wheel encountering corrugation (etc) is thrust upward. This compresses the spring. Inter-leaf friction absorbs a small part of the impact energy on the upward movement. Most, however, remains (as elastic energy) in that spring. Once over the bump, the now unrestrained spring jackhammers the wheel and axle downward. The force is proportional to its mass and the square of its velocity. The wheel strikes the road with huge force. That force is now transmitted to the trailer – via the unfortunate wheel studs.

There are typically 1100 plus corrugations per kilometre. Thus 1000 km of corrugation imparts over one million hammer-like blows. All via those studs. Once it works even slightly loose, further shock loads cause it to fail. Typically, others too work slightly loose. Impact forces then shear the studs in half. Such repeated shock loads also wreck wheel bearings. Furthermore, they may eventually cause axles to break.

This rarely happens now with cars. They are close to undrivable if a shock absorber fails. Let alone not even there.

How shock absorbers work

Much as firing an arrow into water, shock absorbers absorb and dissipate energy. They convert much of the released elastic energy – into heat. Once that’s understood, the need for shock absorbers seems obvious.

Some trailers and caravans have springs so stiff there’s little movement to dampen. The springs do not break. The trailer contents, however, take a beating. Furthermore, wheel studs and stub axles are even more likely to shear.

That’s why wheels fall mainly off trailers that lack shock absorbers.

Shock absorbers are essential

Friction shock absorbers were fitted to cars as early as 1905. They had them despite travelling at low speed and thus incurring far lower forces. Some trailer makers still argue that shock absorbers are unnecessary. As a result, some caravans’ ‘suspension’ is identical to that of trailers carting garbage to the local tip.

If a trailer maker says shock absorbers are not needed – ask this. Why are your products immune to Newton’s laws of motion? If they still argue, go somewhere else.

fwcs-3

The omission of shock absorbers on trailers (and consequent problems) is confined mostly to Australia. The above is an ultra-cheap but well-engineered Finnish garden trailer. Large shock absorbers are standard. Pic: caravanandmotorhomebooks.com.

In over 500,000 km and fifty years of mostly off-bitumen driving I have yet to have a wheel nut even loosen. Let alone fall off. And that includes twice across Africa, and over twelve return trips across Australia on dirt tracks via Alice Springs. (i.e. 24 crossings).

All I do is to tighten, via a torque wrench, to the amount the vehicle maker advises. And using truly high quality shock absorbers.

(Whilst an applied research engineer I devised a test rig simulating bump action on damped and undamped axles. Undamped axle assemblies loosened and/or broke wheel fastenings. Adequately damped ones did not.

Other springs

Coil springs have next to zero self damping. They are never used without shock absorbers. AL-KO rubber sprung assemblies are available with and without shock absorbers. In Europe, however, most AL-KO suspended caravans have shock absorbers. Air bags must be so damped.

Correctly tightening studs and nuts

Rattle guns are prone to over tighten, thereby stretching the stud. This reduces its diameter, thus increasing inter-thread spacing. This alone causes studs to crack and/or sheer off. The fastener and automobile industries emphasise that wheel nuts and studs must never be finally tightened by rattle guns (impact wrenches). They insist that such tightening may only be done via a high quality torque wrench. This wrench must have known accuracy. It must tighten to vehicle manufacturer’s specified amounts.

Recheck (and tighten if necessary) after 50-100 km. And again after 1000 km. But many tyre fitters sadly ‘know better’. They use only rattle guns.

If you use a lubricant (most authorities recommend against it) you must reduce tightening torque by about 20%. Do not use anti-seize materials, for any but totally static applications. Their intended role is easing undoing.

Follow this sequence:

1. Clean threads thoroughly. Ensure nuts are free to spin along the stud’s full threaded length. Discard any that do not. Never use a nut or stud that is or has been corroded. Studs and nuts need be totally clean and dry.

2. Locate the wheel on the studs. Finger-tighten using a diagonal sequence. Give the wheel a few wriggles to allow correct location.

3. Tighten diagonally and progressively.

4. Use a torque wrench for final tightening and only to vehicle maker’s specifications. Never exceed specified tightness. That ‘more is better’ has no foundation. It is counter-productive.

5. Recheck after 50 km, and a further 100 km. If further re-tightening is needed, whatever is being clamped is under-engineered and bending. (This occurs with U-bolt axle clamping plates on early OKAs). Any such needs addressing by an engineer. Urgently.

If really concerned, apply Loctite 290 after the final bedding down. See below re how it works.

If employing a tyre fitter, insist beforehand that a torque wrench be used to finally tighten. Never allow one to use a rattle gun for this. If used, the risk of serious risk over-tightening is very high. Ideally, have a high quality torque wrench and do the final tightening yourself. I always do.

Insist on the above. Many mechanics and tyre fitters believe they can ‘feel’ correct tension. Extensive research shows that few can do so. Tests show variations of plus/minus 30%, and sometimes far more.

Loctite – and similar products

If there is no inter-thread gap, no side movement is possible. There is hence little likelihood of such threaded fastenings undoing. Rather than ‘glueing’ threads together, Loctite (and similar products) thus expand. They fill the gap between threads. This specifically precludes sideways movement – that enables, or causes, undoing.

The specialised Loctite 290 product (only) is designed for fastenings that subsequent re-tightening (e.g. wheel and U-bolt nuts). It is applied after the initial re-tightening period. It is a self-wicking fluid that works its way between even horizontal threads. As the product effectively precludes nut unwinding, further torque checks are (claimed to be) unnecessary. It must, however, be reapplied after wheel changes.

This product is also used to prevent catastrophic failures. It is thus used in aircraft, and also roller coasters. It even prevents jackhammers falling apart.

Wheels falling off trailers – further information

Our published books are written much as this article. They are technically competent but in plain English. They include the Caravan & Motorhome Book, the Caravan & Motorhome Electrics, the Camper Trailer Book. Solar is covered in Solar That Really Works – for cabins and RVs. Solar Success is for homes and properties. For information about the author please Click on Bio.

References

Designing with Threaded Fasteners, Havil G.S, Mech. Eng., Vol 105, No 10, Oct 1983.

Medium/Heavy Truck Wheel Separations, National Transportation Safety Board, Report No. PB92-917004, NTSB/SIR-92/04, Sept 1992.

Myths That Must Be Shattered, Automotive Industries, (April, 1982) p.43.

Design Handbook, Loctite Corporation, (1968) 160 pages.

A Logical Approach to Secure Bolting, Havil, G.S. Soc. of Manufacturing Engineers Ref: AD80-329.

Fastening and Joining, Machine Design, Issue 1967, Vol 14, Penton Publishing.

The Use and Misuse of Six Billion Bolts a Year, Kerely J. J., NASA Goddard Space Centre. Delivered, 35th meeting of the Mechanical Failures Prevention Group, NBS.