sportscarclassics

Chris,
I wanted to do the test myself and I probably watched the same show. I have both stock and turbo twist II with 295. In a couple weeks I will look around for a dyno.
Carl

Tony

Rule #1 do not tell the wife!
Ya try to be the good guy...up front ..honest...she still gives me " the look" and i know im in trouble.

I just called Steve 10minutes ago stating that i cant take them. These are now still for sale. He sold the set with the tires...below are the pics of the whels he sent me. I figure i would plug them for him since i had to back out. Slight paint flake on one area...little rash on antoher. Both minor and can easily be put right.

Wife gave me speech #4 in her line up.......
the "need or want speech"....
Plus she needs a new matteress set for the new bedroom set.. I suppose if i ever want to break in that matteress set i probably need to pass on the wheels this time around.
Live to fight another day.


The wheels are a gret deal and IMHO the best looking wheel you can put on a 928

BC

Just for conversation, I believe the wheel set for the GT2 are solid spoke. Possible point, if true, in that the "track car" gets solid spokes.

Randy V

Here ya go, Tony - we ordered you a subscription to this instead - read it when you're sleeping on the couch :

Richard S

Tony, so I guess I don't get that beer.....can I break in your new matress set?

Rich

Ketchmi

The GT2 wheels are in fact solid. Sorry about the hollow's Tony, there are more of them out there. I'd let my set with P-Zero Corsa's go for $1900ish if someone really needed some sticky's. Tires are almost new.

Jim bailey - 928 International

One of the ricer magazines did comparative dyno (dynojet) tests with different size weight width rims and tires and the smallest combination recorded the highest "calculated" power. Part of that is the increased rolling resistance of larger wider tires. A Tirerack comparison of 16 inch rims to plus one 17 inch rims they recorded fuel milage figures dropping by over 5% as I recall . The energy required to acccelerate the mass of a wheel and tire is not as great as most people think. It does have some impact on turn in due to the gyroscope affect and greater unsprung weight would need heavier shock absorbers to control wheel movement on irregular surfaces as well as they did with lighter wheels. FYI a small motorcycle Dynojet uses a 18 inch diameter roller 20 inches wide and the roller weighs 875 lbs while I can not easily find the roller weight of the automotive Dynojet the total package weight is something like 8,000 lbs . Point is a few lbs plus or minus in the wheel and tire is pretty insignificant compared to the mass of the roller.

Ketchmi

But considering that the 4000# rollers in a dynojet can measure horsepower to the 1/10th, it is significant.

Jim bailey - 928 International

Ok so if the two rear wheels/ tires happen to weigh 20 lbs more, then worst case 20lbs divided by 4,000 lbs roller very simplistic it looks like an ERROR factor of .005 or max 2 HP on a 400 hp run ?

AO

Tony-
It is important to have your own bank account just for these reasons. Both my wife and I have our own "special" accounts. I put a couple hundred $$ in each paycheck as does she. She doesn't bitch at me, I don't bitch at her. Everything else comes out of the joint account. Makes for marital bliss.

Now, padding your account with special one-time deposits is a different story.

Ketchmi

Jim, just give up. Lighter wheels do matter and if you ask any true racer you will get confirmation. You can come up with excuse after excuse but it's still a fact.

I know you are just trying to sell your wheels but geez man, don't try to if, and or but the laws of physics. If the extra weight isn't noticeable to your customer, it doesn't matter then does it? You will have plenty of folks buying those wheels for appearance and ease of fittament without trying to falsely prove that the weight difference doesn't matter. To those of us that are performance minded, it does matter.

You owe me another beer...

slate blue

FYI I was at a BMW dealer when the Williams team was in town. The F1 tyres and wheels are superlight. One of the Williams mechanics picked up a wheel and through it to one of the BMW staff. I thought, man you knock him out, as he was only half ready for it. The catcher couldn't believe how light these things were. He could just toss it up in the air one handed.

Isn't there a saying that 1kg from the wheels is worth 4 kgs from the car? I mean the lighter the wheels and tyres are there is less flywheel effect. I suppose it depends on what these heavier wheels weighs to see if there is much real world difference. My 5 spokes weigh 8 and 10 kgs respectively. They are also manufactoring the tyres lighter too these days and given these go on the most external part of the wheel they will also have quite an inpact.

As to forging versus cast, I would say this a reasonable take on the issue. The factory wheel we are talking about are forged. Well I believe them to be and I know my 5 spokes are. BBS the maker of the rims goes to a lot of trouble to make them light and strong. You can read about it on their website. They are the developer of new products. The Italian made wheels are copies and cast. If a like material is used, the forging will always be stronger, from the work hardening and the graining. However this strength gives BBS the ability to pare weight. This paring may reduce the strength down to that of the heavy wheel. Given I don't know the thicknesses, I can't comment on this.

However the copies are more price sensitive, so I don't believe they would have gone to the trouble of R&D and developed some super strong alloy that was comparable to the BBS forging, they would just be using an adequite alloy. One that casts well. I think you pay for what you get one this issue.

Cheers Greg

CMW

Tony... you and I have the same issue... but since I have been talking trash about the wife's spending lately my honor and my holier than thou fiscal resposibility will come crashing down to the ground.

Ketchmi

We have probably 20 sets of champ car wheels in house, 1 piece forged magnesium. Super light for the size, we leave one on the floor just to mess with people. It's 16" X 14" and about 1/2" thick on the wheel and 3/4 to 1" on the center. It weighs about 15#. Most people look at the mass of it and guess 30ish pounds at least. Now if I could find a way to fit them on my 928 racecar...

sportscarclassics

Weight on a wheel does count because you have two components to over come in an acceleration, centrifugal force (straight out) and rotational force (90 degree in direction spin. The larger the distance of the mass from center the harder to spin. If I keep it real simple and do just one component...centrifugal.

Assume car does 14 sec 1/4 mile. 1/4 mile = 1320 ft
Acceleration = 1320/14 = 94.3 ft/sec
Mass 1 = 30 lbs
Mass 2 = 20 lbs

F=MA Force=Mass X Acceleration
F=30 X 94.3
F= 2829 lbf per sec required

F = 20 X 94.3
F= 1886 lbf per sec required

This is over simplified but it gives us an idea of the difference 2829-1886=943 lb force per second is just one of the component. However if you are not accelerating then the mass increase will not a difference. The tire width increase will make a difference because of the rolling resistance.
Carl