4WD MODIFICATIONS - TYRES & WHEELS
There’s something magical in the reference to ‘racing’. You find it in all sorts of 4WD equipment and, like references to ‘marine’, almost certainly brings with it a price hike.
There’s often an element of truth in claims made for products that have proved themselves in racing conditions, where the stresses and strains are much greater than those in normal on and off road motoring. However, mostly these claims need to be taken with a grain of salt, because what works in the heat of competition isn’t necessarily what you need for the long haul.
An example is the small number of on-road performance cars fitted with racing-style centre-lock wheels – one big nut instead of five or six – and we’ve been asked about the viability of race-style centre-lock wheels for on and off road 4WD vehicles.
Surely, the attraction of one-nut wheel attachment is obvious for vehicles that are more likely to suffer punctures and need quick, easy wheel changes. Also, we’ve been asked: “It must be quicker and easier to undo and fasten just one nut than to have to do five or six times that much work?”
As with all things that look simple; it’s not quite that simple. Firstly, some wheel attachment history.
The good ol’ days
In the early days of the automobile, most vehicles were fitted with wooden-spoke or metal-spoke wheels that had a single attachment nut. The wheel was fitted at the factory and the nut served to fasten it in place, as well as allowing for adjustment of the wheel bearings, to compensate for wear. Unless damaged or the wheel bearings needed replacement, wheels were not regularly removed.
Early ‘tyres’ were made of iron, fitted by being heat-shrunk onto the wheel rim. The next step was solid rubber tyres, clamped to the rim. These ‘tyres’ weren’t replaced unless they wore out or broke.
When the pneumatic tyre arrived, there was a need to replace the tyre periodically, because of punctures and wear. That posed a problem, so the early tyres were fitted to an outer rim that was attached and detached from the wheel spokes, using circumferential bolts.
As loads and engine torque increased, these skinny tyres on their flexible rims weren’t up to the task, so the one-piece wheel – consisting of a central nave and a rim – became the norm. The nave was drilled with holes that allowed attachment to a separate hub. That’s what most vehicles use today.
Interestingly, most heavy trucks and trailers in the USA and Australia retained fixed, ’spider’ hubs and detachable rims until relatively recently. The heavier nature of truck rims allowed them to have stand-alone strength and, for truck drivers who experienced a flat tyre, it was easier to change just a rim with tyre, rather than a full steel disc wheel with tyre. The detachable rims were held to the spokes of the spiders by bolt-on wedges.
Forged aluminium disc truck wheels have largely supplanted the spider hub and separate rim, because they run much ‘truer’ and are no heavier than the old style separate rim and tyre.
So, what’s all this have to do with centre-lock car wheels? Parallel with the move by most car makers to a steel one-piece wheel with multiple-nut attachment to a fixed hub, was the need for rapid wheel changes for racing cars. Also, racing car wheels needed to be as light as possible, to reduce unsprung weight.
All racing cars and many sports cars used wire-spoke wheels that combined one-nut attachment and light weight. Also, a ‘wire’ wheel allowed cooling air to flow though the wheel to help cool the brakes.
Various types of one-nut attachment were developed, but the most popular were variations on the 1920s’ Rudge-Whitworth patented system. It worked so well that it was the normal fitment to sports and racing cars until the 1970s.
The system is deceptively simple. The wheel centre is tubular and conical, and designed to be a snug fit on the wheel hub. When installed, the wheel centre does not come into contact with the brake face, but the taper on the inner face of the wheel contacts the rear taper of the hub.
The cylindrical outer face of the wheel centre is designed to sit inside a V-shape on the inner face of the hub locking nut. This V-groove serves two purposes: it ensures that wheel is clamped in place as the nut tightens and it also imparts a self-tightening, friction action as the wheel rotates.
That self-tightening action works through the rotation of the vehicle wheels, because the nut threads are right-handed on the left side of the car – in the direction of travel – and left-handed on the right side.
The most common attachment piece was the ‘winged or ‘eared’ knock-off nut. When replacing a wheel, you undid the nut by loosening it with blows from a lead or copper-faced hammer and tightened the new wheel in place by reversing the procedure. It was quick and you knew the nut was secure when extra blows didn’t move it.
To reduce the likelihood of braking and acceleration torque loosening the clamping force of the nut, fore and aft load-bearing splines were machined into the hub’s outer circumference, with matching grooves on the inner tube of the wheel centre.
We drove on-road and raced wire-wheeled sports cars and never had a problem with wheels coming off.
‘Knock-off’ wheels were common on road-going sports cars until the 1970s, at which time there was a growing awareness of how vehicle design was affecting the level of injuries caused in accidents – particularly to pedestrians.
Legislators pounced on sharp-edged vehicle fronts, bonnet mascots and…knock-off wheel nuts – especially the triple-eared one fastening the beautiful Boranni triple-laced example above.
Because knock-offs were installed by tightening them with hammer blows, the winged nuts were always outboard of the wheel rim – positively offset – so that when you swung the hammer you didn’t smack the rim or the bodywork.
There was also the ‘Isadora Duncan factor’. In 1927 the famous American dancer was heading off in an open-top car, in Nice, for a liaison with a racing driver she fancied. As the Amilcar (sometimes wrongly branded a ‘Bugatti’) roared off from the kerb, her flowing scarf became entangled around the rear wheel nut, jerking her backwards out of the passenger seat and breaking her neck.
To safety specialists the ‘winged’ or ‘eared’ knock-off nuts looked like Boadicea’s ‘scythe’ chariot hubs or Duncan-stranglers and they were banned in Germany and the USA in the late-1960s.
That effectively ended their reign, but the loss of knock-off wire wheels was compensated by the release of ‘fake’ knock-off wire wheels, in which the nut was purely decorative and the wheel was attached by several conventional nuts that hid behind the spokes.
A below example is the Buick Skylark wheel of 1959.
Later still came cast aluminium wheels with thousands of different patterns that soon became fashion items.
A modern racing car wheel with centre-lock nut usually has drillings on the inside of the wheel’s centre boss and these align with tapered pegs on the hub face that replace traditional wheel studs. The pegs align the wheel, during fitment and aid the clamping force of the nut to absorb fore and aft forces during acceleration and braking.
Some car manufacturers have sold production vehicles with centre-lock wheels as factory options. Porsche has been using centre-lock wheels on their flagship vehicles for many years – most prominently the 911 GT3, Carrera GT and the 918. Ferrari also incorporated centre-lock wheels on its F40, F50, Enzo and LaFerrari. All Koenigsegg production vehicles have centre-lock wheels.
Of course, modern centre-locks have low-profile nuts that aren’t hazardous, but there are other challenges.
Torque, torque and more torque
It’s important to note that a typical racing car imposes a ground pressure of between 200kg and 400kg on each of its tyres. In contrast, a loaded 4WD wagon or ute imposes between 700kg and 1100kg on each of its tyres, meaning that its spindles, hubs and bearings need to be proportionate.
Don’t forget, also, that racing car wheel ends are made from expensivium or unobtainium, and can be very small and light, while providing ample strength.
So, apart from the sheer cost of centre-lock hubs and wheels that are designed for cash-rich car racers, there’s the fact that there are no such fitments for 4WDs that have larger spindles and hubs.
Even if someone developed such fitments there’s another issue: the necessary wheel nut clamping force. Because one large nut has to do the clamping work of five or six smaller ones, it needs to be torqued up appropriately. A typical five- or six-stud wheel requires around 120Nm of dry-thread torque on each nut, but a centre-lock wheel nut may need 600Nm or more!
Check out the pneumatic wheel-changing tools used in race-car pits and you’ll see what’s necessary for rapid wheel changes.
The minimum manual alternative would be a 3/4-drive torque wrench that’s rated for 600Nm ± 50-percent and that’s a $500+ purchase. (Yes, we know there are $200, 600Nm battery-operated ‘rattle’ guns available, but they have a very small-diameter chuck and, also, how can you calibrate one?).
A cute solution is a socket with a ‘handle’ attached, so when the vehicle is driven slowly forward or backward, with the handle locked between the wheel spokes, the nut undoes itself!
The final argument against centre-lock wheels on 4WDs is the fact that Dakar race vehicles don’t use them. These very expensive one-off racers have cutting-edge everything and they frequently lose time on special stages through having to change punctured tyres. If a one-nut wheel fixing was viable, these cashed-up vehicle developers would certainly be using it.
So, in summary, the centre-lock wheel has its applications and most of them are on bitumen race tracks, or on rich boys’ toys. Maybe, in the future…