Popular Mechanics (South Africa)
The performance wheel has been reinvented
LIGHTER WHEELS ARE FASTER WHEELS. And after decades of incremental advancements in wheel-casting and forging technology, additive manufacturing (3D printing) is primed to transform how little performance wheels can weigh and what the wheels look like. An early example of what we can expect was produced in 2019 by wheel-building boutique HRE in collaboration with GE Additive, GE’s 3D printing subsidiary. Their HRE3D+ wheels mated printed titanium components with a carbon-fibre barrel. The 20” × 9” front and 21” × 12.5” rear wheels weighed just 7.2 and 8.6 kg respectively. For high-end wheels of the same size, this saves roughly 0.45 kg in the front and 0.9 kg in the rear.
Traditional wheels are manufactured using cast or forged aluminium alloys. These existing processes are costeffective but suffer from material inefficiencies where extra meat is left on the wheels to give the manufacturer margin for error in meeting strength standards.
Casting’s standard process, gravity casting, is the simplest and cheapest: molten aluminium is poured into a mould and left to cure. Because the aluminium casting process suffers from porosity – which creates weak points – the wheels require more material and designs are limited to large, simple elements.
Forged wheels are lighter and stiffer (and more expensive) than their cast counterparts. The most common method is machine forging, which involves tempering a block of billet aluminium to add strength, then CNC-machining it into a wheel.
HRE’s concept wheels used GE’s electron beam melting (EBM) technology to print five interlocking individual components that formed the face of the wheel. The EBM process uses a 3 kW electron beam to solidify layers of metal powder into potentially complex 3D shapes at temperatures in excess of 982°C. The remaining powder can then be vacuumed away and recycled.
The final part requires minimal finishing work thanks to a beam diameter of 140 microns – one micron is equal to 0.001 mm. Unlike traditional gravity casting’s porosity, these machines can achieve nearly 100 per cent material density.
While additive manufacturing offers an excellent opportunity to disrupt the performance wheel industry, it necessitates vastly different design requirements than forged and cast wheels.
‘Step one in learning to design for additive is forgetting everything you know about everything else,’ says Josh Mook, chief engineer and innovation leader at GE Additive. Mook stresses that EBM technology pushes the boundaries of what’s required from a material science standpoint. Despite HRE using a robust Ti-6Al-4V titanium alloy, the complicated geometries of the HRE wheels required additional support structures to suppress thermal distortion during the printing process. These scaffolds, which get removed during the finishing process, prevent the hot 3D-printed metals from sagging during manufacturing.
But how will lighter wheels make your car faster? This all has to do with the unsprung mass of an automobile, the components that aren’t supported by the suspension system – wheels, hubs, brake discs, callipers, brake lines, and tyres. While these components only make up around 10 per cent of the overall weight, they have an outsized effect on how your car drives.
As unsprung mass decreases, so does the workload on the suspension system. In bumpy situations, this allows the tyre to remain in contact with the road for longer, leading to more traction. Lower unsprung mass also means lower rotational inertia, which allows for better acceleration and shorter braking distances.
While additive technology has yet to be seen in most production vehicles, GE Additive is currently developing a more scalable and affordable additive manufacturing solution to fill this gap.
HRE has no plans to sell these wheels at the moment, but doesn’t doubt that these types of wheels will see production at some point. The technologies behind casting and forging wheels have essentially plateaued, but additive manufacturing is just beginning to unlock a higher level of performance in the automotive industry.