Here is an explaination (it saves me having to type it, lol):
http://www.n54tech.com/forums/showthread.php?t=1890
"In the process of shopping for wheels for the 5er...I was doing a bit of research on the virtues of proper wheel/tire sizing and weight. I stumbled onto a pretty cool article about the subject...
Unsprung Weight and Inertia
Unsprung weight and polor moment of inertia. Sounds like physics mumbo-jumbo. However, this is key to understanding the pros and cons of plus sizing your wheels and tires.
Unsprung Weight
Unsprung weight is the term used to describe weight that is not damped on the vehicle. The wheels, tires, brakes, and some suspension components are unsprung, whereas everything else is sprung. This buzz word is often thrown around with the implication that reducing a car's unsprung weight will make the car faster.
This is untrue. (Rotational inertia is what impacts speed.)
Decreasing a car's unsprung weight will increase its sprung-to-unsprung weight ratio, and that directly leads to improved ride quality. When your tire hits a bump in the road it sends a shock upwards into the chassis that must be absorbed. If it is a 3G shock, then the the chassis must absorb three times its unsprung weight. This jolt will cause transfer into the chassis, which causes the unpleasant feeling caused by hitting a speed bump. Cars with stiffer springs and harder bushings transfer this jolt more directly, which is why they seem harsher.
If the car has 80lbs of unsprung weight per tire, than a 4G shock could send 320lbs of force upwards per tire. That's 640lbs per axle hitting you from below to be damped in just one or a few inches of suspension travel, which can be a lot. This is why some sports and most racing cars use forged suspension parts- it reduces unsprung weight, so the suspension does not need to counteract as large of a force. Dropping the unsprung weight by 25% (admittedly a difficult thing to do) would decrease the upward force the springs need to counteract by 160lbs per axle, which in turn can allow the use of slightly firmer springs (reducing body roll) without a degregation of the stock ride quality. This is also the most significant drawback of solid rear axles suspensions- they add the entire weight of the differential and driveshafts as unsprung weight- that's usually 80lbs or more!
The easiest way to decrease unsprung weight for the tuner is to use lighter wheels and tires. While a steel 16" wheel can weigh 22lbs or more, a cast or forged 16" wheel could be found 6lbs or 9lbs lighter, respectively. Lightweight tires, such as those from Continental or Toyo (or Hoosier for racing slicks) can further reduce unsprung weight. Saving just 8lbs of unsprung weight is an improvement of 10% or more on most cars, which can make a marked improvement in ride and responsiveness.
Rotational Inertia
While often considered to be synonamous with unspring weight, rotational inertia is a different term altogether. It is possible to have a heavy wheel that has little inertia, or a lightweight wheel with lots of inertia. A wheel and tire with a lot of inertia takes a greater armount of torque to slow or accelerate, making the car sluggish.
Have a pencil in front of you? Try this: hold the pencil upright by its eraser. Now spin the pencil in your fingers as if you were trying to make a dot on a sheet of paper. It takes almost zero effort to spin the pencil this way, right? Now hold it the pencil's center, between your finger and thumb. Rotate the pencil back and forth, as if you were shaking it to hear loose parts. Feel how it takes more effort to rotate it this way? That is because the mass you are rotating is further away from its center of rotation. Ever notice how an ice skater or karate man tucks their leg in to rotate faster? It's the same concept. If you don't feel the difference, try the same experiment with a larger object such as a broom handle or a baseball bat. Spinning a broom handle like a propeller will take more effort than turning it like a giant drill.
Likewise, the further a wheel and tire's mass is from the axle, the more torque will be required to accelerate it. If a car has 16" wheels and those wheels are replaced with 17" wheels of identical weight, the amount of inertia that wheel carries will probably have risen between 7 to 8 percent. Going up another inch would add another 7 to 8 percent, and so forth. It adds up quickly.
That's assuming the larger wheels weigh the same, which is often impossible. With a larger diameter wheel comes exponentially more surface area needed to create the outer edge of the rim, which is the worst possible place to add weight. Increasing wheel diameter AND increasing weight (even if only a modest amount) will produce rather noticable drawbacks- you can loose 1-2 car lengths (or more) in a 1/4 mile race. Going with wider wheels raises the amount of metal required too, although it does so only linearly.
However, the largest contributer on the entire car to rotational inertia is the tire. Tires are even further out from the axle than the wheel, and usually weigh more too. Some street tires weigh as much as 4 pounds less than their competitors through the use of lightweight materials. Hoosier racing tires, until recently, were made of fiberglass belts instead of steel for the purpose of weight savings (until new regulations prohibited this). That mere four pounds per tire extra will require about the same amount of force to spin as it would take to carry a date riding shotgun! A small difference in tire weight can make a large difference in rotational inertia.
This is a compelling reason to run smaller diameter tires, as larger diameter tires have more inertia per pound and are heavier due to the increased amount of rubber. The weight and inertia savings of going to a skinnier tires is comparitively smaller than decreasing diameter.
Of course, going to a skinnier, softer wheel and tire combination can sacrifice handling (detailed in another article), so it's up to the tuner to find the proper balance for their car.
Do the Math
Andy Welter has created a spreadsheet for calculating the gains or losses of various wheel and tire combinations, found here:
http://www.mazda6tech.com/files/rotational.xls
Kevin K has revised the original spreadsheet to more correct specifications. Remember that the weight calculation is PER wheel/tire.
http://www.mazda6tech.com/files/wheel_inertia.xls
Article is from: http://www.mazda6tech.com/index.php?...d=16&Itemid=50"
