Christer

I am sorry as well!

Wikkid, I really didn't understand what you were talking about. I was thinking 'is my unsprung weight more than yours?' - what the hell does that mean? I wasn't trying to keep it all serious - I am less serious than most, or so I thought!

Try to use the big boy's voice in the main chamber, OK Scott?

tonytaylor

Its worth noting unsprung weight`s relevance is relative ( - to the sprung weight of the car)
Increasing unsprung weight by eg. larger brakes, lower profile tyres/wheel combination may improve overall performance even though unsprung weight is generally antagonistic to suspension performance.

Jim Michaels

To increase unsprung weight switch to 23X10" Enkei Kriminal wheels at 60# each. That's the heaviest wheel listed by wheelweights.net.

Wikkid911

Jim,
You may be in for a roasting if Adrian or Christer see you recommending people fit big heavy wheels.
It does seem a complicated subject this unsprung weight though. You may improve a cars overall performance even though you have incresed the unsprung weight by fitting more effective components (from what I can gather). I would doubt that many tuning companies selling aftermarket products go into things in as much detail as is required.
Cheers,
Dave
'90 C2

Adrian

Guys,
Have you heard of the words inertia and momentum. The unsprung weight of a car is anything that is not supported by the suspension and rotates. This includes everything connected to the wheel hub and the axles. The axles are not supported by the suspension in the terms of unsprung weight. The higher the unsprung weight the higher the inertia. This means it will require more energy to move it from the stopped position. When moving it has more momentum which means it takes more energy to stop it. The aim is to always reduce unsprung weight which means using lighter wheels, lighter brake rotors etc. In laymans terms. Increasing unsprung weight reduces acceleration and increase braking distance by reducing brake efficiency. You must remember that the unsprung weight when moving carries with it the entire weight of the auto, sprung or not. The brakes have to stop the entire weight and you are not helping them by making them overcome more momentum.
There is no way that you can improve the performance of any auto by increasing unsprung weight. It is not a complicated subject at all it is simply physics. What weighs a lot is hard to get going and harder to stop when it is going. Dave you are quite correct, tuning companies do not consider such things when they are selling you an expensive set of wheels. One of my complaints against the tuning world is that they do not inform their customers of any potential negative impacts of the products they are selling. Not surprising, never investigated but in my opinion potentially dangerous.
Ciao,
Adrian
964C4

tonytaylor

I`m not convinced that fitting larger wheels and tyres and larger brake discs is not going to improve the performance of the car even though unsprung weight will increase.

johnfm

Adrian

pretty sure that unsprung weight is not the term used to refer to the angular inertia (I) of the rotating components. The 'unsprung' bit refers to the linear springing of the coil springs.

If you do a simple energy analysis of the system (ie the car) as it goes over a bump, you will discover why sprung mass and unsprung mass is important. If the car was totally unsprung (thing wooden wheels on carriages for example), if the system went over a bump, the entire mass would need to be raised over hte bump.Raising a mass requires energy. In a sprung system, you only need to raise the unsprung mass over hte bump. The system becomes more efficient (oh, and more comfortable too!)

964porschedude

Wikkid911,

I believe the real reason to reduce the un-sprung weight is to allow the suspension to keep the tire on the ground. When a vehicle encounters a bump, the wheel and suspension react either up or down.

The sum of the un-sprung weight needs to react well and efficiently. When the wheel moves upward during a bump, the wheel and components accelerate upward, then decelerate to stop or peak, then accelerate and travel downward to a normal ride height. The less the mass or un-sprung weight, the easier it is to move the suspension to the terrain. This keeps control of the vehicle by keeping the tire on the ground.

The un-sprung weight has little to no effect on the overall braking. It is the total vehicle weight that is the real concern when it comes to braking. The total mass of the vehicle is being stopped by the brakes and not just the un-sprung weight. The effect of having very large wheels on a vehicle is more of a flywheel effect that an un-sprung weight effect. The rotating mass of the wheels resists the braking effort like that of you engine flywheel. The flywheel stores energy and allows you to use it during peak times, say starting from a dead stop. You will notice a change when a vehicle is converted to a lightweight flywheel. From a standing start the vehicle is a little harder to control the staling when you let the clutch out.

What brakes do!

The brake system basically takes a moving or dynamic mass at a certain velocity, which contains a certain amount of energy, and converts the forward velocity or energy to another form of energy. The converted energy is in the form of heat.

Keith

Adrian

I will try and find the article on this subject written by Porsche Engineering on the subject. It was about wheels brakes and the impact of unsprung weight on braking. I am pretty sure it was about the GT-3 brakes. Let me have a look and at least I can provide you all a reference to go dig up. Porsche engineering certainly disagree with the comment that unsprung weight has little effect on braking.
Ciao,
Adrian
964C4

tonytaylor

Unsprung weight per se has the same effect on braking kg per kg as sprung weight but the effect of rotating masses is a diffrent thing altogether - a relatively small mass rotating at high speed will take a disproportionatly higher braking force to stop compared to the effect of its weight alone.

Wikkid911

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"It is not a complicated subject at all it is simply physics"
Adrian
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Adrian, got to say this made me laugh. It really was very difficult when I was at school. Maybe if they had said this subject may have some bearing on the ownership of that Porsche you are dreaming about it may have made more sense. I printed the responses off for this and have re-read them a few times so thank you for your views and everbodys input. It seems that unsprung weight means different things to different people.

Cheers
Dave
'90 C2

J richard

It is a generalization that an increase in unsprung weight will negatively affect the performance of a vehicle. Not true if you consider the net result. For instance, larger/wider wheels and brakes will undoubtedly improve the overall grip and braking performance (to a point), as long as the shocks/springs have the ability to dampen the momentum of the unsprung weight and keep the tread in correct contact with the pavement. It has to be looked at as a system.

Rotating mass of the wheels and tires are a different issue than unsprung weight, but the impact can be considerable on stopping and acceleration (flywheel effect).

Overall the best strategy is to increase surface area of tire contact patch and braking surface and reduce weight, ie; wider tires on lightweight racing wheels, carbon fiber rotors with aluminum calipers, and then increase the dampening capacity of springs and shocks to compensate.

After all isn't that exactly is done to increase track performance of every race car?