You probably hear the term unsprung weight bantered around in suspension circles, what is it and why should I care
What is unsprung weight- well it's weight that is not supported by the springs- that's easy! Sprung weight is your chassis/transmision/engine and other assorted sundries- obvioulsy there is more sprung weight than unsprung weight.
What components comprise the unsprung weight- well we have tires/wheels/lug nuts/rotors/calipers/knuckle/hub assemblies- it also includes a portion (roughly half) the weight of the axles/control arms and struts- these latter components are partially sprung and unsprung.
Is less unsprung weight better than more- yup. Actually any loss of weight is good when it coms to accelerating/braking/cornering- loss of unsprung weight has the added advantage of improving the ability of your suspension keeping the tire to the ground (it's primary job). This is what ultimately makes a car handle well- grip!
The WRX in stock trim has a ratio of ~ 7:1 sprung/unsprung (if anyone is interested in the calculations -basically just determing the unsprung weight/corner by weighing a bunch of parts- I can post them)- this is pretty good, some solid axle cars are in the 3 to 4:1 range- not so good.
What's the easiest way to reduce unsprung weight- well it's not replacing the axles
Your wheels are probasbly the easiest place to shed unsprung weight, often tires that are equally grippy sometimes have some differences in weight, the use off aluminum in suspension arms can reduce unsprung weight, aluminum calipers can reduce unsprung weight as can two piece rotors (aluminum hats) and many aftermarket strut assemblies are lighter as well.
You can trim 10+/corner of unsprung weight w/ careful choices of parts, but as you can see you can just as easily add to unsprung weight as well.
You'll notice that many of the unsprung components are also rotational- tires/wheels rotors- having these lighter also enhances acceleration and braking- to a much higher degree than simple loss of non-rotational weight (again all weight loss is good, just some is better)
clif note version- buy them damn lightweight rims you've been drooling over