Development of F1 mag wheels

As with many of the parts that contribute to the success of some of the fastest machines on the planet, Formula One wheels have been perfected over the course of more than 60 years of professional racing.

The wheels you see adorning cars driven by Lewis Hamilton and Nico Rosberg have morphed substantially from the rims-with-spokes design that supported F1 cars in the immediate post-war years.

The light but relatively brittle design used back then was, in fact, not technically a wheel, although the greater load that came with faster speeds meant that change had to occur during the fifties.

The early versions of the first true F1 wheel did not resemble the modern wheel in a technical sense. The axle was connected to the flange by a metal plane, and the wheels, which were made of steel, were usually composed of two parts--the wheel itself and the rim. These were fastened together by welding or rivets.

Subsequently, the flat wheel was shaped to improve its ability to cope with lateral loads, although wheels of this design still continued to be relatively heavy.

The next major development came with the introduction of six-spoke cast wheels made from light aluminum and magnesium alloys. These wheels appeared in Europe by the early sixties, and continued as a feature in professional racing cars until the early nineties. Initially, racing teams cast the wheels themselves before sourcing them from specialists.

Given that racing car wheels are subjected to extreme speeds of more than 300 kph, research and development is continually trying to discover new ways of creating an extremely lightweight yet strong disk, and this is where magnesium comes in.

A magnesium wheel is approximately one-third lighter than its aluminum cousin. The density of magnesium is 1.78 g/cm 3, lower than that of aluminum by a factor of 1.5 and lower than the density of steel by a factor of 4.4.

Yet magnesium’s strength is on a par with aluminum, and only slightly less than steel. The vibration damping properties of magnesium alloys--which absorb shocks and vibrations-- exceed those of aluminum alloys by a factor of 100, and those of steel by a factor of 23.

Cast metal structure

The result of all this is a significant reduction in vibrations transmitted to a vehicle’s suspension and cabin, thereby extending its service life.

Following the choice of material in a wheel’s manufacture, the second vital factor is the method used to produce the blank. Since the eighties, light alloy wheels have been produced using gravity and low pressure die casting, the simplest and cheapest method of manufacturing wheels.

While complex parts can be produced with minimal waste, this process has its shortcomings. Even the most technologically advanced casting can lead to a formation of concealed holes and cavities in the wheel.

The free, non-directional, crystal structure of the metal can also reduce its mechanical properties and this produces a lightweight but brittle wheel which would struggle to cope with the forces exerted by an F1 car.

Such a wheel is susceptible to splitting rather than deforming, the latter more often seen in steel wheels; this phenomenon is difficult to counteract since strengthening them would also mean increasing the weight.

Cracked cast wheels

Cracked cast wheels

The most common alternative process is called flow forming. A work piece comprised of hub with spokes and a forerunner of the future rim is cast. The rim is then heated and fixed in a lathe using rollers instead of tool bits.

The heated rim is then formed to the required shape by the rotary drawing method. The properties of such a rim are similar to those achieved by hot closed die forging although the grain direction is longitudinal. The hub remains cast and heavy as a result of this process.

A variation of flow forming involves using a cast work piece that is upset, such as being subjected to preliminary forging by a press. While the hub of such a wheel is stronger than a cast version, the grain direction and metal properties change at the point where the hub joins the rim, leading to reduction in the strength of the piece.

Critically, however, this is the point of maximal stress and manufacturers therefore need to increase the thickness of the walls to compensate for these stresses. This results in the wheel becoming heavier than a properly forged one.

Flow formed rim wheels are affected by rim flange break-off due to the longitudinal grain. Such damage can occur when the wheel hits a high curb stone, for example.

Structure of wheel rim following flow forming

Break-off of flow formed alloy wheel

Another common alternative method is liquid forging or high pressure die casting. This technique requires less powerful presses than hot closed die forging, typically between 3,000-5,000 tons.

Here, molten metal is poured under high pressure into the mold where metal crystallization takes place, resulting in a finer but divergent structure. These liquid forged wheels are superior to cast wheels, but their ductility and elasticity are still less than those of properly forged wheels. Rim flow forming is occasionally performed following liquid forging.

Diagram of high pressure die casting process

Metal structure following high pressure mould casting

Component wheels are those in which the rim is fastened to the hub by bolts or welding. In this case the characteristics and performance of the finished wheel depend on the type and integrity of the techniques used to produce each part.

These can be cast, flow formed or forged although the advantages of such wheels are dubious, as component structures inevitably reduce durability and increase the weight. Bolt joints require maintenance, including occasional tightening.

These alternative techniques, however, cannot compete with true forged wheels in terms of quality and strength. Higher manufacturing cost is no longer an issue as a result of both equipment costs and low yield (after forging, the blank is machined, resulting in more than half the wheel’s weight as chips, filings, shavings and other waste).

True, high quality, strong and light forged wheels continue to be popular among racers and, increasingly, motorists. As manufacturing techniques become more efficient and production costs fall, it is inevitable that the use of forged wheels will widen. They now come with a five-year manufacturer’s warranty, made possible with implementation of innovative anti-corrosion anodizing and paint. And they are guaranteed to noticeably reduce fuel consumption, tire and brake wear, carbon dioxide emissions, and braking/stopping distance-- which means higher safety on the road. The case for forged magnesium wheels is quite compelling indeed.