PCA SDR

Okay, so I'm an engineer. And, somewhere I still have my old sliderule. But, I can never keep straight the issue of "sprung" versus "unsprung" weight and which is the greater concern and why. Any help out there?

Steve,

Unsprug weight is what we want to reduce. The rule of thumb is-removal of 1 pound of unsprung weight is like removing 3 pounds of sprung weight. That is why a set of Kinesis wheels can make a car (RSA for example) feel as if 100 pounds were removed from the car.

Definitions:

Unsprung weight is all the weight that is directly connected to the wheel and the ground. (wheel, hub, rotor, caliper, suspension carrier) This must be as low as possible so the suspension can control the wheel and maintain contact with the ground as much as possible. Try doing curls as fast as you can with a bowling ball and then with a wiffle ball. The wiffle ball is the Kinesis wheel. The stock wheels are bowling balls.

The sprung weight is everything that moves when you step on the bumper.

Heavier wheels are also harder to accellerate and decellerate due to the extra inertia they have.

I hope this answeres your question

Steve's last line about braking is particularly germane for those of you contemplating the purchase of dubs for your pimped-out Escalade. A recent test (in R&T I believe) examining the effect of adding insanely heavy 22" wheels (with spinners) to a Chevy Tahoe the result was doubling (yep times 2) the stopping distance. Scary.

Yeah..................
it's a matter of mass moving up and down.
The lighter your wheel, tire, rotor, caliper,........the quicker it can return to the ground after the compression stroke of your shock absorber. Quicker back to the ground means more rubber on the road sooner, hence, more traction for a longer amount of time.
The suspension "arms" are always going to be a ratio of sprung weight, i.e. held up by the springs-versus unsprung weight, or what the springs are pushing down. So the control arms on one car may be 70 % sprung weight and on another car, only 30% sprung weight. Depends on the design angles. But the wheel, tire, rotor, and caliper will almost always be unsprung weight. (Not counting those Alfa Romeo inboard brake discs.........sneaky Italians!)
But smart......