4WD MODIFICATIONS - TYRES & WHEELS
The Caravan Council of Australia is accustomed to receiving reports of broken wheel studs and loose wheel nuts: sometimes with the nuts unwinding completely. However, since 2018 there have been a spate of such reports.
There are several reasons why wheel studs
break and nuts work loose, said the CCA.
Firstly, don’t blame the studs or the nuts themselves, without good evidence of low-grade-steel manufacture. The reason is likely to be some other factor or factors.
Many stud breakage and loose-nut issues result from the fitment of after-market wheels that aren’t compatible with the original equipment (OE) hub and stud assemblies.
There’s some amazing misinformation about hub-centric versus stud-centric wheels and I’m talking here about 4WD-sized wheels, not truck wheels, where twin-nut, hub-centric types are quite different.
A popular website thread is that, in the case of a hub-centric wheel, the weight of the vehicle rests on the hub projection and, in the case of stud-centric wheels, the weight of the vehicle rests on the wheel studs. Both these claims are myths.
In all cases of a wheel being attached to a hub the clamping effect of tightened wheel nuts and studs makes the wheel and hub a solid piece. Correctly torqued wheel studs are never in shear (a force acting at right angles to the stud), but are in tension (a force acting along the length of the stud).
The critical factor in all wheel to hub attachments is the clamping force of the stud-nut combination. The friction generated between the inside face of the wheel nave and the outer face of the hub creates a load path and no weight is transferred to the hub projection, or to the studs.
The correct title of a ‘hub-centric’ wheel is a ‘hub-piloted wheel’. The key word there is ‘piloted’. The close fit between the hole in the centre of the wheel nave and the projection on the hub is there solely for the purpose of easily aligning (piloting) the wheel holes concentrically with the studs, while the nuts are centred accurately and are tightened.
Once the nuts are correctly torqued the projection on the hub is redundant. (It’s like the dowels in a cylinder block making sure the head is aligned, but once the head bolts are tensioned the dowels don’t have a function.)
In the case of non-standard wheels that have a larger centre bore than the diameter of the projection on the hub it’s necessary to take great care in ensuring that the wheel nut holes are centred over the studs.
Push-in hub-centric rings are available, to convert the wheel bore diameter to match the hub projection. They’re usually aluminium or even hard plastic, because, as we’ve seen, once the wheel is centralised and the nuts are tightened, the rings bear no weight.
Australian Vehicle Standards Bulletin VSB 14 insists that hub-centric rings are necessary for all such fitments, to aid proper pre-torquing alignment.
Because the wheel-hub interface is critical in achieving a good clamping force it’s absolutely vital that the inner face of the wheel nave and the outer face of the hub are scrupulously clean. Also, the nut taper must exactly match the wheel hole taper angle and the holes in the wheel face need to be free of paint and debris.
If 4WD vehicle, trailer, caravan and camper-trailer owners and dealers fit after-market wheels and nuts, they must check thoroughly to ensure the replacement wheels and nuts are completely suitable for the vehicle and axles.
The OE wheel specifications and instructions spell out the required fitment details and these must be adhered to. Just one example of these very important instructions is the quite different recommended torque settings for dry studs and lubricated studs.
It may seem bleedingly obvious, but ensuring that the pitch circle diameter (PCD) of the OE stud circle exactly matches that of the wheel is paramount. However, there are instances where a metric-measurement (mm) wheel PCD has been mated to an imperial measurement (inches) hub PCD.
In this case the studs are not exactly centred in the holes in the wheel nave and, when the nuts are tightened, all the studs are bent slightly. Stud failure and/or nut loosening is only a matter of time.
Another should-be-obvious problem is when the hub hole in the centre of the wheel nave is smaller than the hub, so that the wheel doesn’t seat properly on the hub.
Less obvious is the case where the taper on the clamping face of the wheel nuts doesn’t match the taper on the wheel nave holes, so that the friction face is much smaller in area than normal. Nut loosening will occur.
Rust or paint on the tapered friction face of the wheel nave holes must be scraped off before the nuts are tightened, leaving a bright surface on which the tapered nut can seat. Residual rust or paint between the nut and the wheel will break down gradually, reducing the nut clamping force and allowing the nut to loosen.
Aluminium wheels have insert nuts and some are deeper than others. Long nuts in a thin-nave wheel will bottom out before the wheel face is fully clamped to the hub.
Another wheel-nave-related issue concerns highly-dished, thin-nave pressings that flex slightly as the nuts are tightened, becoming virtual spring washers that gradually lose tension over time, letting the nuts loosen.
Stub-specific issues are the use of cheap, low-grade-steel studs or serrated studs not fully driven ‘home’ when pressed into the hubs. These studs can gradually creep into their proper, seated position under tension in the nut-stud assembly, thus causing the nuts to become loose.
The use of rattle-guns that are set at high-torque levels (aren’t they all!) to tighten wheel nuts, rather than only to undo them, causes the studs to stretch, and thus become weakened.
Wheel nuts must be tightened repeatedly in a criss-cross pattern, with progressive torque increases, not given full torque on each nut in a circular progression around the PCD.
All wheel nuts must be tightened to the correct torque and in the correct pattern, in strict accordance with the instructions provided by the wheel or vehicle OE manufacturer.
It is strongly recommended that pencil lines are made on one face of each nut , with a mating line on the wheel, so that a quick visual inspection can detect any loosening of a nut.Clip-on plastic ‘indicators’ fitted to all nuts, with their adjacent ‘arrow-heads’ aligned, show up any loose-nut issue. They’re compulsory on mining vehicles.
Continual vibrations and occasional heavy impacts from road surfaces have an adverse effect on the wheel assemblies and this is severely aggravated if the tyre pressures or the spring rates are too high for the actual wheel-loading.
Stresses on the wheel assemblies are further increased if shock absorbers are not fitted.
Leaf springs do provide some damping of vibrations, but unfortunately it is mainly on the ‘bump’ (upwards) movement of the wheel, rather than on the ‘rebound’ (downwards) movement of the wheel, unless rebound leaves are fitted to the top of the spring packs.