IcemanG17

I am looking at some larger 18x10x65mm wheels for my race 928....the goal is to put 285/30-18 tires all the way around....since the tires weigh the same on any wheels....here is my ???

How much difference does "sprung" weight matter....my options are to buy some nice OZ racing 19.5lb wheels at $400 each or el-cheapo italian knockoffs of stock Porsche wheels for around $175 per wheel, that I'm guessing are maybe 27lbs each.... So how much does 32lbs of sprung weight hurt?

My current setup of tire-wheel weight is very close to the weight of the OZ setup I am looking into above....with only about 1lb per corner more.....of course the el-cheapo wheels would make the gain per corner at 8 lbs...

To make NASA GTS2 class I am going to have add a bit of weight to the car anyway, but I don't know if....about 100lbs total with the lighter wheels...

Jim Michaels

That would be 32# more of ugly unsprung rotating mass; probably the worst place you could add weight (with the possible exception of a tall and wide 32# block of wood on the roof). And I worried about switching to a set of tires that weighed a total of 8# more than my previous set.

"They" say that adding unsprung rotating mass reduces performance to about the same degree as adding about four (or was it five?) times the static sprung weight. So how much would adding 130 to 160# in the passenger footwell reduce performance?

IcemanG17

this is what I thought....so its $$$ well spent to get the larger tires & lighter wheels....I can live with +1 lbs per wheel-tire.....since the other wheels tires I raced on where +4lb per wheel-tire and those performed "okay".... But I did drop 7 seconds (now 8.5) per lap when I took the 225/50-16 street tires off and put on the 245/45-16 BFG R1's too....

I would be happy with a 2 second improvement going from the 245/45-16 tires to 285/30-18 all around....watching video of my old race 928 that had HUGE tires (295 front 335 rear) turn a 2:09.03 and my current 928 turn a 2:09.8 at the same track, the larger tired 928 seems positively effortless in its lap, vs the ragged edge in the smaller tired 928....yes the larger tired 928 had more power and more gears....but the car just flat isn't trying in the corners compared to its smaller tired cousin...

I will have a comparo video next week

race911

I've got all manner of data from racing RSA #1 way back when, and I used it all from stock wheels to Cup 1 knockoffs, to Fikse/Kinesis/HRE, and even some spiffy BBS that I was able to borrow. I'd relegate the Kumho V700s to the Cup 1 knockoffs, and I'd be maybe 2 seconds off what I could do with fresh Hoosiers on the lightweight wheels.

va122

It's 4X and its significant enough that you can feel it in the corners.

JackOlsen

First off, start with good data. Tires with shorter sidewalls weigh less than tires with taller sidewalls -- so tires for 18" wheels will weigh less than tires for 15" wheels in the same width and with the same overall diameter. With lightweight wheels, you often get a net weight reduction going to larger wheel sizes, so long as you don't increase overall diameter.

Overall diameter varies because tire sizing is kind of strange. I know that when I was going from 16s to 17's, there were some wheel tire combos that would decrease total weight and some that would increase total weight.

You've got to look at the actual wheels and tires you're planning on running. Beyond that, it's difficult to know if you're going to get heavier or lighter or whether your overall diameter is going to increase or decrease.

IcemanG17

Jack
Great points....I am considering both total weight and actual diameter (gearing) in my decisions.....since the 285/30-18 is very close to the diameter of the 245/45-16 I am running now.... The tire weight is 2lbs per tire heavier but the wheel weight is 1lb lighter.....so I am looking at a net gain of 1lb per corner to gain almost 2" of actual tire width at every tire.....I would say thats a good trade......

Of course knowing how my life works.....I'll drop the $3k on the wheels tires and not go any faster.........

bobt993

I went from 245/40 front and 285/30s rear to 275s front and 315 rear. Added 3 lbs per corner, but net affect was faster lap times. The tough part was learning to enter at a faster pace and hold on midcorner more. Less sliding. Bigger tires should give more grip, but your setup and driving will need to change to take advantage of the mechanical grip.

Mahler9th

I am a bit of an old school guy... I think you should factor in robustness. You seem to post quite often and I'd guess you drive on the track quite often. I suggest that whatever you choose be a forged product. Back in the day I decided between Forgeline, Kinesis and Fikse as they were the best available forged choices close to my budget. I chose Fikse... still have a set I use when I drive my car on the street. I only ran cast wheels in the first few events.

I have always recommended forged wheels to my students.

IcemanG17

Forged wheels are always a good idea.....the 16x8 wheels I run now are forged... The OZ's I'm looking at are cast

http://www.tirerack.com/wheels/Wheel...All&sort=Brand

Hmm....I wonder how much weaker they are? How much would this effect their performance?

Ron Cohn

I have a set of 997S Lobster Froks for sale with mounted MSPC's that should fit your 928. They are lifht, strong and cheap.

ronaldcohn@gmail.com
914-474-4151

333pg333

http://www.vividracing.com/catalog/e...0-p-24876.html

I would seriously consider these Enkeis. They are light (21-lbs), cheap (but not cheaply built), attractive (IMHO) and available.
The process of casting vs forging may not be always 100% in favour of forging all the time. There are different levels of casting.
There is cheap and nasty Chinese vs high level Japanese casting such as Enkei utilises. The et 60 will work fine on your car.
I have Big Reds on my 951 and they clear these and fit inside the rolled guards just fine.

cobalt

In general forged should be stronger but not necessarily. Some of the forged designs use a very small cross section and are susceptible to failure just as much as cast given there is less material supporting the same load. So think about the design as much as the process.

F1 Wheels are cast using a rare earths ZE41AT5 magnesium alloy. It is not overly strong (no where near the tensile of forged aluminum) but has a decent yield strength which is more important. These wheels handle the most grueling loads although the cars weigh considerably less.

I know of many people who have been using the OZ's and have nothing but good experiences, Most BBS wheels are cast and are very strong. Their magnesium centered wheels have cast centers. They also have a process for rolling the wheel through pinch rollers after it is cast to give it it's strength and shape.

The disadvantage of most cast wheels is weight over their forged counterparts. Although most of the quality wheels that are light are 2 or 3 pieces with cast barrels and spun lips which can have inherent issues by themselves.

I use Fikse's myself and have noticed a considerable performance difference between these and the CupI's. The car accelerates and decelerates quicker and the tires seem to be more planted over rough surfaces. If weight is a factor I agree Forged is the more likely choice however there are some quality cast wheels that should hold up and are not quite so heavy as the wheels Porsche supplies from the factory.

Ed Hughes

I know you're talking about wheels that are a bit bigger than mine, but ~20lbs still seems heavy. My 7.5/9 X 17's are 15.2lb each in front and 15.9 in the rear.

va122

They're great, very reliable. I have them. They use some new casting method that's supposed to be stronger.