WHY WHEELS FALL OFF!

Reports of wheels falling off trailers through stud failure and/or retaining nuts loosening have existed for decades. The problem exists, but in determining how commonly, reality becomes sequentially enhanced myth. The RVMAA (Recreational Vehicle Manufacturers Association of Australia), for example, recently asked that incidents be reported to them (and had negligible response). A large internet website also sought specifics – with zero response. There is however ample evidence that the problem exists.

My own extensive dirt road experience (I live in the Kimberley – where almost all roads are dirt) is that box trailers, and the cheaper camper trailers and caravans are prone to losing wheels. Yet whatever tows them are not. Caravanning publications have reported likewise since the early 1920s.

Reliable data shows that three quarters of known failures are due to wheel nuts being over or under tightened. Also blamed is lack of ongoing checking but, essential parts needing routine checks to ensure they do not fall off, reflects serious failings in design.

I have for some years suggested a further cause that combines a known defect of screwed fastenings, and a likely cause of that effect occurring.

Why Things Work Loose

The mechanism that enables/causes screwed fastenings to loosen is known, but rarely covered in engineering training. Research papers describe it in considerable rigour and detail (eg. Girard S. Havil), ‘Designing with Threaded Fasteners’, Mech. Eng., Vol 105, No 10, Oct 1983.

The threaded section of screwed fastenings is by necessity spiral. One side of the tightened nut pushes up on one side of the corresponding stud’s threads; the other side pushes down on the other side of that thread. The tightening force normally maintains these faces in tight contact and prevents sideways movement between the nut and the stud (a slight clearance must exist to enable the nut to turn).

As the nut is tightened, the stud or bolt stretches very slightly – it is in tension. Even when correctly tightened however, repetitive side loads can cause the nut to move slightly from side to side. As the thread is spiral and marginally stretched, this sideways movement momentarily reduces the frictional contact between the threads, thus minutely relieving the tension in the stretched stud. This acts as a ratchet mechanism that causes the nut to undo.

The above is not conjecture – try it yourself!

Hold a large clean threaded stud and nut vertically downwards – and shake the assembly vigorously from side to side. The nut will be seen to unwind. A similar effect happens with a nut and stud under tension – excepting that the briefly but repeated loss of tension more or less replicates what gravity does in that experiment. If repeated sideways movement exists, nuts ‘ratchet themselves’ loose.

Spring washers do not help: by adding elasticity they may make matters worse. Nyloc nuts add friction and marginally limit sideways movement. Castellated nuts and split pins prevent nut movement but accurate re-torquing may not be possible. Double lock nuts help because the upper nut is freer of loosening forces. Left-hand thread nuts (on left hand wheels) are claimed to reduce the amount wheel nuts undo, but do not restrain initial loosening.

Of the above, only the Nyloc nut (partially) addresses the initiating cause. There are better solutions, either of which is effective. Used together (although my experience indicates doubling up is not essential) it is probable that wheel falling off can be eliminated.

Filling the Gap

The first approach is to eliminate the gap between the threads. If there’s no gap, there is little likelihood of nuts undoing - and that’s how Loctite (and similar products) work. They do not ‘glue’ things together – but by filling the gaps between threads they eliminate the movement that causes the problem.

Most such products are not however suitable for wheel nuts. Wheels ‘bed down’ and need re-torquing at about 50 km, and ideally after a further 100 km and any subsequent torquing destroys the effect of the product.

The specialised Loctite 290 product (only) is designed for things known to need bedding down and retightening (such as wheel and U-bolt nuts). It is a self-wicking fluid that works its way even between horizontal threads. It is applied only after final re-torquing.

As the product effectively precludes nut unwinding, further torque checks are (claimed to be) unnecessary. It must be reapplied after wheel changes. (It is also used on roller coasters to prevent fastenings loosening with high risk of consequent catastrophic failure).

Reducing Loosening Forces

A further approach is to reduce the enabling forces. These include side shock loads, repeated bending of the bolted assembly, and repeated heating and cooling (eg. cylinder head studs). But vibration alone may not cause a nut to loosen unless it generates sideways movement.

More wheels demonstrably fall off trailers than ever do off cars. Yet for close to a century, trailer makers have denied there’s anything about their products that might cause this to happen! It does not affect all trailers: wheels that fall, or stub axles that break were mostly from the cheaper products.

I’ve travelled the 4000 or so kilometres each way of dirt tracks from Broome to the east coast and back eleven times, and have been sixteen times across the 1100 km Tanami. I have observed and photographed very many trailers and wrecked caravans that had wheels torn away (or stub axles broken). All had something in common. They were leaf sprung and none had provision for shock absorbers.

Bumps are not just bumps

When a wheel encounters a bump, it is driven sharply upward (with a force proportional to the square of the driving speed), compressing the spring in so doing. Friction between the spring leaves absorbs a small part of the imparted energy, but only on the less important upward travel: a lot of pent-up energy remains and the now totally unrestrained leaf spring hurls the road wheel downward like a demented jackhammer.

As the wheel strikes the ground, and is only partially damped by the tyre, a reactive shock load is transmitted up through the road wheel and to springs and trailer structure. It does so via the wheel studs.

The conical nuts and locations limit side movement, but the forces are so large (and often repetitive) that even slight sideways movement causes (not just allows) them to loosen. One pass of the Tanami track (at roughly 900 corrugations per kilometre) imparts three-quarters of a million hammer-like blows via those wheel studs. And another three quarters of a million on the way home. Once a nut works loose, even a few hundred shock loads may cause it to ratchet itself off. Others may work slightly loose and then bind. The studs are then abraded until they shear.

The Action of Shock Absorbers

Despite outback Australia being littered by products demonstrating the contrary, some trailer makers still argue that shock absorbers are not necessary on leaf sprung trailers.

At GM, I built a test rig that simulated bump action on damped and undamped axles. Many undamped axle assemblies loosened and/or broke wheel studs. Adequately damped ones didn’t. I later data-logged the shock loadings on a leaf-sprung QL Bedford whilst I drove it twice across Africa. The difference in shock loading on rebound (between a damped and undamped rear spring) was on average thirty five times greater.

Other Springs

Coil springs have close to zero self damping and are never used without shock absorbers. Alko rubber sprung assemblies are available with and without shock absorbers but, in Europe, where roads are generally better, most Alko-suspended caravans have shock absorbers as standard.

The argument that leaf springs are self damping overlooks that it applies only marginally, and where it matters least - on the upward thrust. A lot of pent-up energy remains. On the downward thrust the spring leaves are not constrained (and thus damped by friction). Their acceleration is limited only by the mass of the wheel assembly: it is virtually unchecked.

The purpose of shock absorbers is also to control both bound and rebound movements and to dissipate the imparted energy, in the form of heat.

Once this action and effect is realised, the need for shock absorbers is obvious. The only exception is where the springs are so stiff there’s little spring movement to dampen. Here, wheel studs and stub axles are likely to snap instead.

I believe it is disgraceful that, in 2008, trailer makers omit shock absorbers despite their common usage on cars as early as 1905 and that at a fifth of the speed. If a trailer builder tells you that shock absorbers are not needed ask why his/her products are uniquely immune to the effects of Newton’s more fundamental laws of motion. If they still argue, buy from somewhere else - they display no understanding of the basics of suspension.

In over 450,000 km and forty years of mostly off-bitumen driving I have yet to have a wheel nut loosen – let alone fall off. And that’s without Loctite 290.

Brief bio

Collyn Rivers W8054 is an ex GM research engineer. He changed careers in 1970 to found and edit the eventually global Electronics Today International, and many other technical publications. He was Technical Editor of The Bulletin from 1982-1990. He has subsequently written four books on various aspects of RVs. His website is www.caravanandmotorhomebooks.com.

Collyn’s books are sold directly by the CMCA.

Note: Several other organisations have already asked permission to reproduce this article. Such permission will not unreasonably be withheld subject to full acknowledgment and express written permission from Collyn Rivers (who holds copyright of the text); and from the CMCA (which has copyright of this particular presentation).

Tightening Wheels

1. Clean the threads thoroughly and ensure the nuts spin freely on the stud for their full threaded length (discard any that do not, and never use a nut or stud that is or has been corroded). The studs and nuts need to be totally clean, dry, and unlubricated.

2. Locate the wheel on the studs and finger-tighten using a diagonal sequence. Give the wheel a few wriggles to allow it to locate correctly.

3. Tighten the wheel nuts progressively and working diagonally.

4. Use a torque wrench for the final tightening and torque to the vehicle maker’s specifications. Do not exceed specified tightness.

5. Recheck after 50 km, and after a further 100 km. If really concerned, apply Loctite 290 (ONLY). It will travel through the horizontal thread.

Insist on that torque wrench being used because many mechanics wrongly believe they can ‘feel’ correct tension. They cannot. Personally observed research showed that over half of 300 (tested) experienced mechanics failed to gauge wheel nut tightness by plus or minus 30%. Some were out by over 40%.
Stress beforehand that that you need the final tightening to be to the vehicle maker’s specifications. Never allow a ‘rattle gun’ to be used for tightening as there is a real risk of over serious overtightening. This increases the stretch, and hence the ‘undoing tendency’ and also marginally narrows the thread diameter, thus increases the thread gap.

Ideally, have your own high quality torque wrench and insist on doing the final tightening yourself (I always do this).

Fifth Wheelers

There are increasing reports of wheel nuts loosening and/or wheels falling off fifth wheelers - all ultra-low priced imports. Checking showed they had suspension identical in every respect except scale, to my cheap twin axle box trailer that I use to take garbage to the local tip. None had shock absorbers. In my opinion (as an ex research engineer with specific experience in this area) such suspension is grossly inadequate for Australian use.