The wheels are among the most critical components as far as handling and stability of a car is concerned. Searching for suitable alloy wheels made me realize that I need to look much further than the "R" rim size. First I came across "PCD". Searching for PCD first got me to Pussy Cat Dolls Searching again, this time for 98PCD got me to the right places and from there I came across "J" and "ET". Is that all or is there anything else? How do I ensure that I get everything right when I get my wheels. And so my research started on choosing the right wheels for my car. Having a Fiat with a not so common 98mm PCD ensured that my research went on for months and it was an very valuable learning experience. So here I am putting all that I learned in a single thread so that others can benefit out of it. Why Alloys? Why do you need alloy wheels. Other than looks does it have any benefits? The most important benefit of alloy wheels is from its reduced weight. Lighter wheels can improve handling by reducing unsprung mass, allowing suspension to follow the terrain more closely and thus improve grip. To understand the theory behind it you need to learn a whole lot of Physics about Sprung Mass and Unsprung Mass. I shall briefly explain the same below. Sprung Mass and Unsprung Mass Basically the wheels, tires, braking system and part of the suspension form the unsprung mass of a vehicle. The sprung mass is the body, the engine, the passengers and everything else that is supported by the suspension. For better handling and ride comfort, you should reduce unsprung mass. Why? Remember your high school physics class? F=Ma, Force = Mass x Acceleration. The higher the unsprung mass, the more force exerted on the sprung mass and this is what affects handling and ride comfort. The higher the force exerted on the spring of the suspension, the higher the force required to keep the wheel planted to the ground and this is what affects handling. That same force is also reacted by the sprung mass (body of the car) and so the passenger in the vehicle feels that force and its effect on ride comfort. The higher the force, the worse the ride comfort. Basically you want wheels to lightly move over bumps in the road or track and the lighter the wheel, the lower the forces involved. This is a pretty simplified explanation but I hope it gets the point across. Rotational Inertia The other major factor affecting performance is the rotational inertia of the wheel/tire/brake package. Rotational inertia is essentially the rotational mass of an object. It is defined by the distribution of the mass to the square of the distance from the center axis of rotation. That is the more mass you have farther from the center of rotation, the more energy that is going to be required to speed it up and slow it down and the effect is squared!! The simple example is the ice skater moving their arms in or out while spinning. The closer they bring their arms in, the faster they spin. Their mass didn’t change, but their rotational inertia definitely changed and by a lot. Basically the heavier and larger in diameter the wheel, the more rotational inertia it is going to have and the more energy required to spin that mass up under acceleration and to spin that mass down under braking. With light weight and properly engineered alloy wheels you can maximize acceleration and minimize your stopping distance. This isn’t only a performance issue, it can be a safety issue as well. Lower rotational inertia also means you will be using less fuel. Heat dissipation Another benefit from using alloys is better heat dissipation. This helps to dissipate heat from the brakes, which improves braking performance in more demanding driving conditions and reduces the chance of brake failure due to overheating. Reference: http://en.wikipedia.org/wiki/Unsprung_mass http://en.wikipedia.org/wiki/Alloy_wheel http://cysunnychang.tripod.com/personal/useful_information/mass.htm http://www.hrewheels.com/blog/unsprung-mass-why-is-this-important/ http://www.hrewheels.com/blog/rotational-inertias-effect-on-performance/ Getting the right size and the size right The size of a wheel is determined by 4 different measurements, these are the Rim Diameter, Rim Width, PCD and the offset. In addition to these you need to consider the centre bore of the wheel also. Rim Diameter is the diameter of the inner surface of the wheel, and is where the tyre is seated. Rim Width, is the distance between then inner surfaces of the wheel, and is where the tyre is seated against the edge of the wheel. These two measurements are usually specified in inches. So a wheel with 15 inch diameter and 6 inch width is specified as 15x6 or 15J6. PCD (pitch circle diameter) is the diameter of a circle drawn through the centre of your wheels bolt holes. PCD is measured in millimetres and also indicates the number of studs or bolts the wheel will have. So a 4 hole 98mm configuration is specified as 4x98 PCD or sometimes PCD 98x4. Offset (also marked as ET) is the distance between the inside facing plate of the wheel and the centre of the wheel. Most of the passenger cars has a positive offset whereas a negative offset is seen in high performance vehicles. Too little or a negative offset would result in the wheels projecting outside the bodyline of the car. ET for Grande Punto (Indian Edition) is 41mm - This is stamped on the steel rim. You can safely go to about 5-7mm off the manufacturer's figures. And one more thing, there is a difference between the Indian edition Punto and European version Grande Punto when it comes to PCD, ET, etc. Reference: http://www.gomog.com/allmorgan/wheeloffset.html http://www.nice-wheels.co.uk/pcdandoffset.html http://www.fiatforum.com/punto-faqs/ Centre bore, Hub diameter and Spigot rings The center bore of a wheel is the size of the hole in the back of the wheel that centers it over the mounting hub of the car. Factory wheels have a center bore that matches exactly with the hub to reduce vibration by keeping the wheel centered. Wheels with the correct center bore to the car they will be mounted on are known as hubcentric. Hubcentric wheels take the stress off the lug nuts, reducing the job of the lug nuts to center the wheel to the car. Most wheel manufacturers design their wheels with a center bore large enough to fit on most cars. Therefore, since wheel manufactures make their center bore large enough to fit most cars, most wheel fitments have a gap between the hub and the center bore. O’Rings / Spigot Rings or Alloy Wheel Hub Centric Rings are designed to fill in the gap between the hub of the car and the center bore of the wheel. The Inner Diameter of the Spigot Ring is the size of the outer diameter of the hub on the vehicle. The Outer Diameter of the Spigot Ring is the diameter of the central bore of the wheel. Spigot rings are normally made using high quality Poly Carbon (Plastic). It is a shame that very few shops in India stock these components. I am told that these are available at A1 Car Care Centre, #1/1, Ashoka Pillar Road, Jayanagar. # 08026577438. Some one from Bangalore please verify and confirm. Reference: http://www.justforwheels.com/index.jsp?cat=hubcentric&sub=how http://planetsoarer.com/offset/studs.htm http://planetsoarer.com/offset/offset.htm Corrosion and Other factors The hubs are made of steel and the alloy wheels are made of, well, alloys of Aluminium or Magnesuim. So when these two metals come together in an electrolytic medium (water) corrosion will happen. This is known as galvanic corrosion. Now it is impossible to avoid water getting on to the wheels right?. So it is definitely a good idea to paint the hub prior to fitting your alloy wheels. Reference: http://en.wikipedia.org/wiki/Galvanic_corrosion http://www.corrosion-doctors.org/Corrosion-History/Eight.htm Well that's all about it. I have avoided pictures and diagrams here as there are enough and more in the reference links provided. I am going to get the hubs painted tomorrow and after that will put on the wheels. So have I achieved perfection? Well that's something only time can tell and they say perfection is not a destination, but a never ending journey. Hope you all get a head start in your quest for perfect alloy wheel fitment from this.