Forged wheels combine lightness and sturdiness and are fruit of a refined technology. A product commonly used in competitions, but also recommended as an upgrade for the best super sport production motorbikes.
For years, street and racing bikes were always equipped with wheels made with spokes obtained by fusion. The only exceptions, as is also true today, were models that sported wheels with spokes designed for aesthetic reasons (custom, old-style, some naked) or destined at least partly for use off road.
These wheels have been developed, mainly with a view to making them lighter, and this has created some exceptional models, with hollow spokes and hubs. Wheels are not suspended parts and therefore reducing their weight is always beneficial, enhancing the performance of the bike and improving the conditions in which the suspension is required to work.
But that's not all, because wheels turn and this has important consequences.
To begin with, accelerating rotating parts is not the same as accelerating parts that don't rotate.
The variation in the linear speed (let's say of the motorbike, and therefore of the axles on which the wheels are mounted) is indeed then also accompanied by the variation in the rotation speed.
The indicator that lets us know how much an element can resist the changes in the speed with which it is turning is its moment of inertia, linked not only to the mass, but also the range of rotation (squared).
When this reduces, the speed with which the wheel is able to "take the turns" increases, in line with the power provided by the engine.
And this is where how the material is arranged also comes in.
The most effective arrangement, is that which, with the same wheel diameter, is furthest away from the axle , and that is, in line with the rim.
When comparing two wheels of the same weight and dimensions, the one with the lower moment of inertia is advantaged.
The giroscopic effects, not only tied to the moment of inertia but also to the speed of the wheels, must also then be considered.
In this case, forces that fail to cause variations in how the motorbike leans are particularly important.
And in this case too, a lower moment of inertia is an advantage; indeed, this makes the bike more agile and quicker to respond to changes in direction.
Less weight where it is not required
The wheels must therefore combine sturdy consistency and reduced weight, with the smallest possible quantity of material in the most outer part, with a view to achieving a lower inertia moment. And here, the structure and the density of the material make their entrance.
For many years now, racing wheels have been made of magnesium alloy (density 1.8 kg/dm3) and the wheels of production bikes are almost always made of aluminium alloy (2.7 kg/dm3).
The design of the wheel and the sizing of its components (hub, spokes and rim) are strongly linked to the mechanical properties of the material used: indeed, the higher these are, the lower the number of sections there can be and so the weight is also lower.
Therefore, the type of alloy used and the heat treatment to which it can be subjected are two elements that take on a significant importance.
But another, even more important element is the switch to a production procedure that enables the material to be enhanced with superior characteristics than those possible when the parts are created by fusion.
This is the forging process, during which the metal is brought to a high temperature with a view to making it sufficiently flexible, and then broken down into a mould, with the help of a press.
In this way, a rough piece is obtained, made of material which is free from porosity and features a fine, crystal clear, smooth grain. Additionally, the fibres can also be pointed in an advantageous direction.
The technology of forging
For over fifteen years, Marchesini wheels ( Marchesini is the brand of the Brembo Group, leader in the design, construction and sale of lightweight alloy wheels for racing and street motorcycles) have been exclusively produced, both racing and for high performance street bikes, using the forging process.
Over the last 5 years, the motorcycles equipped with Marchesini forged wheels have won both the Moto GP World Championships and the Superbike World Championships 4 times.
Available in a wide range of measurements and with different designs, they are made in both aluminium alloy and magnesium alloy, with the latter version dominating the scene on Gran Premio motorbikes for a long time now (in 2015, no less than 14 drivers of the MotoGP used Marchesini forged magnesium wheels) and on superbikes.
The design of the wheels is fruit of the latest computerised simulation and calculation techniques, using the data provided by tyre and motorcycle manufacturers and those identified using sophisticated systems that rely on the use of extensimetric techniques and transmitted torque detection, with wheels equipped with specific instruments for this purpose.
Much of the information obtained by way of these technologies is then provided to the technicians of the motorcycle manufacturers and the teams, with whom there is a close collaboration.
One noteworthy innovation is the recent creation of racing wheels fitted inside the hub with a sensor that counts the turns these make; in this way, the technicians at Marchesini are able to know precisely when the end of their useful life is drawing near.
The manufacturing process begins when the raw wheels are created, using a particularly state-of-the-art procedure named multiforging.
This consists in forging in many directions, and using several presses, which act simultaneously on different parts of one mould.
After the heat treatment, the wheels are processed using ultra-modern control number machine tools in the factory located in Curno.
From the rough forged products, we go to the end of the turning and milling operations, with which a significant quantity of material is removed, to the finished wheels.
In-line checks are carried out during the various processing phases.
At the end, all the wheels produced are subject to tests using penetrating liquids (to detect the presence of any small cracks).
The magnesium alloy wheels then undergo a treatment that forms a thin layer designed to convert the surface, capable of ensuring an excellent level of protection from corrosion.
After painting, the rim is fitted with an antislip application, which prevents the tyre from moving about and slipping on the rim in more critical conditions such as maximum acceleration and sharp, borderline braking.
The wheels are then completed by installing pads (which, in those to be used in the Moto2s and Moto3s, from this year on, feature spheres in ceramic material) and spacer plates. Finally, almost all the rear wheels are fitted with disc couplings before being sent to the storage warehouse.
