Well I thought about that, but thought to keep it simple...

However: Less unsprung mass that is rotating too will lead to best results;)

To further demonstrate the effectiveness of unsprung weight. In Excellence magazine, Manthey Racing had released an upgrade package for GT3's and they tested it. Unlike their competitors, who concentrated on upping the HP and putting lighter weight body panels on the car, Manthey Racing concentrated on building lighter suspension components and worked with BBS to develop a be-spoke wheel package. Olaf Manthey said they concentrated on reducing unsprung weight because in testing he believes that reducing 1lb of unsprung weight is the same as reducing 7lb of sprung weight.

The 50% thing is just an engineering shortcut factor. To get exact answers you need special programs- ADAMS I believe- which do multi-body dynamics. It is very hard to model all of those individual pieces interacting with each other as they move without a computer. The systems are coupled. It is much easier when you "fix" or "ground" a link like the wheel- constrain it to remain on the ground. Then you can just use energy methods to come up with an equivalent inertia for a 1 degree of freedom quarter-car model. This would take into account an equivalent quarter car sprung mass weight as well as the inertia of the suspension links. If you go 2DOF on the quarter car model then you model the tire as a spring and things get more complicated with a coupled 2DOF system. Add the 3 other corners and I believe you end up with a 8 DOF model. A car with wheels grounded has 4 DOF- heave, roll, pitch, warp. The excitation can be several things- one wheel bump or roll being common.

I have the basic skills to solve a problem like that, but I would need to know the inertia properties of the sprung mass most importantly and also of the suspension and wheels.

To paraphrase Colin Chapman (I think) : To go faster, add lightness ;)

Yeah, the bottom line is less is more when it comes to unsprung weight. The hard part is knowing when adding some sprung mass is worth the reduction in unsprung mass. This would happen when using inboard brakes on the front wheels of a rear wheel drive car, as was seen in F1 years ago.

The RATIO of sprung to unsprung weight is the critical factor. If you strip a car down by removing the interior trim and fitting lightweight fenders, for instance, and then fit heavy wide wheels/tires with big (heavy) brakes you have effected this ratio negatively.

Thank you for the education. I love this place.

This is a really good point. We reduce unsprung weight so we can get control of the tire movement for improved grip. Getting control of tire movement also adds to ride quality. Having the unsprung weight be a smaller percentage of the mass of a car improves ride quality.

When you look at a land barge you will see that they have a small percentage of unsprung weight. They just left out the part about "adding lightness" that is so important in a sporty car.

Yeah, but...

Unsprung weight is important but not overwhelmingly so. Camber change matters, too, as it maintains maximum tire contact with the road surface. Some sports cars (old Alfas, for example) have straight rear axles that weigh a lot but keep the wheels straight upright in corners, minimizing camber change, so the tires stay in better contact with the road. Larger wheels and tires can be heavier (more unsprung weight) but provide better traction, too.

Like many other aspects of automotive engineering, suspension design involves compromise.

Frank

As long as the road surface is relatively smooth-and most race tracks, and some public roads are. But, introduce roughness into that road surface and low unsprung weight comes to the fore. Which may have contributed to the Porsches beating the Alfas in the TransAm...
Ferinstance...:)

It's why I get frustrated that there isn't a large selection of good 15" tires. The reduction in unsprung weight by swapping out my 16's for equivalent sized 15's is significant. I'd keep the same width and aspect ratio too, so it wouldn't be a compromise.

Wheels have been mentioned, but I would point out tires matter too... There is a HUGE variation in tire weights out there, is similar sizes... Some folks spend $$$ getting lighter wheels or lightweight calipers, only to unsuspectingly slap on a set of tires tire that is 20 lbs heavier than it could be. Tire rack has weights listed under "specs", it's shocking at times...

For instance the michelin supersport in Cayman S rear size (265) is 4 lbs lighter per tire than a bridgestone Re11.... you WILL feel that...

The best contrast between low and high unsprung weight would be a rally car 911 with Fuch wheels and street tires on it. Handling, good.

Take them off, put a 12" lift kit on the 911 and mount some 33" tall mud tires and 20" rims on it.

Would you notice a difference in ride quality and performance? Yes.

Brake rotors too. Everyone wants the biggest they can fit within a rim but the weight increase is a penalty.