Particularities of the World's Fastest Wheels

F1 teams have been reinventing the wheel for decades, and motorists around the world are now beginning to benefit from modern manufacturing techniques pioneered by the famous racing series.

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Development of F1 mag wheels

F1 teams have been reinventing the wheel for decades, but motorists around the world are now beginning to benefit from modern manufacturing techniques pioneered by the famous racing series.

Reinventing the wheel

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.

Forged magnesium alloy wheels

Mg F1 wheel front

SMW forged magnesium alloy wheels on a Sahara Force India racing car

The world of racing wheels was revolutionized in the nineties, however, when Italian based OZ Group, founded in 1971 to produce car and motorcycle wheels, proposed the use of forged magnesium alloy wheels to the Jordan Formula One team.

These wheels were manufactured using closed die forging, as opposed to sheet metal stamping. In fact, the wheel forging process involved the innovative technology and manufacturing equipment of VILS (All-Russia Institute of Light Alloys).

A forged magnesium wheel is 25 percent lighter than any other wheel, and all Formula One teams would soon benefit from this technological development. Today, these are used by each team within motor racing’s most famous competition, including many teams from other series.

The leading forged wheels manufacturers are SMW Engineering (Russia), Washi Beam and TWS (Japan). All the most renowned manufacturers, including BBS and OZ, purchase blanks or finished wheels from these manufacturers.

The wheels used on F1 racing cars have recently attracted a renewed focus, with specific attention on unsprung weight. This is the combined weight of the complete wheel set, including the brake system elements.

The ratio of sprung to unsprung weight is significant since the force exerted by the unsprung components on the car from the bottom upwards must be offset by the sprung weight; otherwise it loses grip on the track making it harder to steer.

Unsprung weight can also help to improve the vehicle dynamics, as the heavier the wheels are, the more energy and time required to alter their rotation speed.

For these reasons reducing the overall weight of a racing car, including the wheels, has been a key goal of design and manufacturing teams for many years.

Mg F1 wheel back

Magnesium wheels

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

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

Cracked cast wheels

Cracked cast wheels

Forging process

The next stage in wheel manufacturing techniques is hot closed die forging. Forged wheels are stronger and far more ductile; this process can reduce the weight of the wheel by 25 percent compared to a cast wheel.

A forged wheel does not crack. Instead, it bends without the formation of cracks, and it can be straightened if required. Defects such as voids and cracks, common in cast wheels, are not present in a forged wheel.

Grain direction in forged wheel

Grain direction in forged wheel

The closed die forging method eliminates the free, non-directional, crystal structure of the metal and reduces the thickness of the wheel walls by about 20 percent. The structure is fibrous and fine grained, with the grain radiating from the center of the hub, along the spokes and passing to the rim. The grain is arranged transversely at the rim.

In the closed die forging process, pressed bars are used as the feed stock. The bar is cut to size and heated before being placed in the die and pressed. The hot metal flows into the die tooling cavities and assumes the shape of the wheel.

Depending on the complexity of the wheel configuration, the forging process may be performed in one or several stages. The initial stage is the preliminary forming. The work piece first assumes a pot-like shape before being forged in the final die to assume the shape of the wheel. The more precisely the die configuration follows the contour of the finished piece, the higher the quality of the completed wheel.

Structure of forged wheel

The structure of a forged wheel

Hot closed die forging

Diagram of hot closed die forging in final die

F1 wheel following forging

Blank Formula One wheel following forging

The grain is arranged along the load directions allowing for the maximum possible wheel strength. The measure of material strength depends on whether the load direction is along or across the grain. The strength along the grain direction can be almost twice that of across the grain.

Forged magnesium alloy wheel

Distribution of stresses within a magnesium alloy at various loads

All wheels are subjected to special heat treatment to further increase their strength. The combination of these processes creates a light, strong wheel that can withstand extreme loads.

Formula one crash

A Formula One car crashes, but the wheels remain intact

Producing wheels using hot closed die forging is a costly process and few manufacturers can afford to invest millions in such a powerful press. Yet drivers demand the advantage of the forged wheel’s superior characteristics.

For this reason, many companies purchase blanks from large manufacturers. The supplier performs all the upfront operations that require expensive, specialized equipment while the final machining, polishing and painting are completed by the buyer. Although this arrangement is popular in Europe, there are very few forged blank manufacturers worldwide.

As a result, a number of alternative processes have emerged. These produce wheels that are typically better quality than cast wheels and come at a lower cost. Manufacturers often call the end product ‘forged wheels’ but their quality is still inferior compared to a true forged wheel.

Flow forming

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.

Wheel flow forming diagram

Wheel flow forming process

Structure of wheel following flow forming

Structure of wheel rim following flow forming

Flow formed wheel break

Break-off of flow formed alloy wheel

Liquid forming

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.

High pressure die casting process

Diagram of high pressure die casting process

Metal structure following high pressure mould casting

Metal structure following high pressure mould casting

Component wheels

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.

No substitute for the true forged wheel

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.

Magnesium wheel

SMW forged magnesium alloy wheel

Example of forged magnesium wheels in use

Comparative testing was conducted between Porsche 918 Spyder standard forged aluminum wheels and the Weissach package containing forged magnesium wheels, which over 60% of buyers have opted for. The vehicle having factory standard configuration accelerated from 0 to 100 km/h in 2.8 seconds, from 0 to 200 km/h in 7.7 seconds, and from 0 to 300 km/h in 22 seconds.

The same vehicle with the Weissach package, which includes forged magnesium wheels along with the sports-version belts and several carbon fiber components, altogether delivering weight savings of 35 kg in total, had demonstrated improved acceleration results amounting to 2.6, 7.2 and 19.9 seconds respectively. The quickening by two (2) seconds is attributed largely to the reduction of rotating mass delivered by the forged magnesium wheels. Braking/stopping distance reduction was not measured as part of the study. More detailed information about aftermarket magnesium wheels can be found at magnesiumwheels.com", target="_blank") magnesiumwheels.com]

Porsche SMW magnesium wheel

Porsche forged magnesium wheel

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