Hello all, it appears that the expensive technology(sp) wheels weight 6 lbs less than their similarly visual counterparts.

In theory, they should afford better handling and quicker acceleration.

Is the 6 pounds of weight completely removed from the unsprung weight? Or is it a fraction of that ( being 1/2 the weight is above the axle center line?)
My guess is that being it is attached under the spring to a moving part, the calculation is more complex.
Please help me understand this.

Also, has anyone ever tested these wheels and noticed any difference at all?

From my research, the 8" fronts weigh 19.5 lbs and the 10" rears weigh 23.3 lbs.

I am a big fan of weight reduction, as it is an often overlooked ingredient in performance.

Does anyone have any experience with the lighter wheels and the benefits they offer?

Thank you

Rich

 

For a street driven 993 with 282 HP this is really a non issue. Buy the best set of wheels in the best condition for a fair price, and move on. Hollow spoke or solid spoke, it really won't matter.

 

it is all unsprung weight.

above or below the axle center line makes no difference, things that are attached to the chassis at one end and wheel assembly at the other end are partially sprung and partially unsprung.

lighter certainly makes a difference but it's difficult to quantify, for street use I wouldn't worry overly much about it other to use the lightest wheels you like.

same can be said for tires, there is a significant difference to be found there too.

 

I believe unsprung weight is multiplied by 4.

 

if it were only that simple

the rotational energy cost a a wheel depends on where the mass is located and the distribution of that mass, every wheel/being different in this regard.

A solid disk energy cost is not the same as a hoops

an annular cylinder is not the same as a hoop

etc

you have to take into account the energy cost of change rotation speed and direction in 3 dimensions as well as the energy cost of linear motion changes in 3 dimensions.

It all gets very complicated

the best method would be to go to a drag strip and make multiple passes w/ multiple wheel sets, both accelerating and braking then average the results to see the time differentials

or put the wheels in question on a wheel dyno



As a rule of thumb w/ all sorts of simplifying assumptions I've seen any where from just under 2 to ~4

I gave up trying to come up w/ even a ballpark figure, closer to 2 seems to be better than closer to 4, but I really dunno, so if you want to believe a different #, I wouldn't argue

 

With Bill on the math here.

Regarding subjective feeling - I honestly believe I can feel the 1.5# difference btw. some of the wheel/tire combos on my race car.

But no data I have will tell you that any set is faster than the other. One could think the lighter set would show a bit of an advantage, but after two years and many, many sessions, nothing different.

 

One real benefit you feel is the difference in weight when removing them and putting them back on!

 

Ain't that the truth!!

An additional facor in feel is that often the tire rolling radius is different, relatively small changes here can be felt fairly easily

 

The guys over on the Jetta TDI pages often run 15" rims to maximize their mileage by getting the lightest wheel/tire combinations they can find. I couldn't translate this into Hp loss but they claim losses up to 5-7 mpg by going from 15-17" wheels. Part of the losses are from rotational mass and part is from increased rolling resistance/friction.

The changes in rotational inertia are effect more by moving the center of mass outwards than by purely adding weight. Adding 1 lb at the center of a shaft for instance would cause a very small change in rotational inertia, where-as adding 1 lb 9" out from the center of the radius will have a much larger effect.

This is a good webpage which explains rotational inertia and has some good formulas that you can play with. http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html

 

Yes, the additional mass and weight would have a slight bearing on their speed but I would be willing to guess that the Jetta TDI folks haven't matched their tire sizes for rolling circumference. The smaller the rolling circumference to original, the faster the speedometer reads but I seriously doubt that they're going that much faster.

 

Here is a test and look at what it does to MPG & performance

http://www.caranddriver.com/features...d-tires-tested

 

David,
Thats a great article, I read it a few years back.

There are a few guys in the TDI world that run larger OD tires to keep their revs down on the highway, but most stick to tire/wheel combinations that keep the speedo reasonably accurate. You notice changes in the acceleration more when you only have 100hp to start with, but again they are shooting for mpg's not traction or acceleration.

 

is that the new RWB?

 

You guys are all terrific. Knowledge and comedy all at once. Priceless!!!

So, what I take from here is there should be some theoretical benefit. If I buy light wheels, and match light tires, I may notice a slight difference.

At the very least they are easier to remove and replace as I service it!!!

As always, I thank you all

Rich

 

Good summary of form over function ...