Physics metallurgy and manufacture.

First a reminder of Newtonian Physics.

Law 1: Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

Law 2: The relationship between an objects mass m its acceleration a and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.

Law 3: For every action there is an equal and opposite reaction.

Therefore it follows that lighter wheels are better than heavier ones. Unsprung weight is that portion of a vehicle that is not supported by the suspension (i.e. wheels tires and brakes) and therefore most susceptible to road shock and cornering forces. By reducing unsprung weight light alloy wheels provide more precise steering input and improved turning in characteristics. By reducing the weight of the vehicles rotational mass alloy wheels provide more responsive acceleration and braking.

Metallurgy

Wheels have to stand up to a lot of abuse. They have to be strong to avoid denting falling off your hubs and holding your car on the road.

Obviously you want to start off with a very light yet strong alloy. Making this alloy stronger you need to consider the configuration of its intermolecular bonds. Ideally you want to achieve a crystalline structure. This is why a diamond is stronger than a lump coal although both are made of carbon. Consequently metal can be made strong by employing heat and pressure just like how diamonds are formed.

By using stronger metal you dont need much material to make the wheel... making it lighter still.

Manufacture

• GRAVITY CASTING

Gravity casting is the most basic process of pouring molten aluminum into a mold utilizing the earth’s gravity to fill the mold. Gravity casting offers a very reasonable production cost and is a good method for casting designs that are more visually oriented or when reducing weight is not a primary concern. Since the process relies on gravity to fill the mold the aluminum is not as densely packed in the mold as some other casting processes and the molecules are aligned randomly as they cool. Often gravity cast wheels will have a higher weight to achieve the required strength.

• LOW PRESSURE CASTING

Low pressure casting uses positive pressure to move the molten aluminum into the mold quicker and achieve a finished product that has improved mechanical properties (more dense) over a gravity cast wheel. Low-pressure casting has a slightly higher production cost over gravity casting. Low pressure is the most common process approved for aluminum wheels sold to the O.E.M. market. Low-pressure cast wheels offer a good value for the aftermarket as well. Some companies offer wheels that are produced under a higher pressure in special casting equipment to create a wheel that is lighter and stronger than a wheel produced in low pressure. Once again in the quest for lighter weight there is a higher cost associated with the process.

• SPUN-RIM OR RIM ROLLING TECHNOLOGY

This specialized process begins with a low pressure type of casting and uses a special machine that spins the initial casting heats the outer portion of the casting and then uses steel rollers pressed against the rim area to pull the rim to its final width and shape. The combination of the heat pressure and spinning create a rim area with the strength similar to a forged wheel without the high cost of the forging.

• FORGED OR SEMI-SOLID FORGED

The ultimate in one-piece wheels. Forging is the process of forcing a solid billet of aluminum between the forging dies under an extreme amount of pressure to align the metal molecules into a crystalline structure. This creates a finished product that is very dense very strong and therefore can be very light. The costs of tooling development equipment etc. make this type of wheel very exclusive and usually demand a high price in the aftermarket.

• Chrome Plating

Chrome is a heavy metal. Much heavier than the alloys that many wheels are made of. Its also relatively brittle and can easily crack after being subjected to trauma. For this reason all chrome wheels are made through a plating process.

Alloy wheels are plated by submerging the wheel in a heated solution containing the chrome plate and running an electrical charge between the solution and the wheel. The charge encourages deposition of chrome on the wheel and combined with the heat encourages bonding of the chrome to the alloy. The additional chrome adds weight to alloy wheel.

And the wheel is weakened at the alloy/chrome interface. The bonding process disrupts the crystalline configuration of the alloy metal. Close contact between different metals creates an electrical potential (ie. a battery) that is inherently unstable.

Conclusion

So chrome wheels are heavier and weaker than alloy wheels. IE. 17 Niche chrome wheel - 35lbs... 17 Centerline Forged RPM Wheel - 13lbs.

By definition if a modification is done for cosmetic reasons and reduces the performance potential of the car... it is rice.