bfd365

Dear Forum,

Looking for input on the Pro's and Con's of changing to a larger wheel diameter from the stock 18"s. Besides the added weight, are there performance drawbacks to mounting 19" wheels on a 993?

Also, is there a rule of thumb regarding what size/width tires you can fit on an given sized rim? For example, with the factory turbo twists the fronts are 8X18 and Porsche recommends 225/40/18 sized tires. How much wider or narrower a tire could you safely mount on this particular sized wheel?

Lastly, would mounting an aftermarket 18.5" width front wheel on a 993 have any adverse effects? Are the correct wheel offsets for a WB 993 as follows?

Offset

front - 46
rear - 40

Thanks,

Rob (waiting for my RL membership to be activated)

ca993twin

Rob,

1). There are a few 19" wheels that are not much heavier than the standard 18" wheels. Given the correct sizing, few if any performance drawbacks.

2). An 8" wheel can usually accomodate a 245 section tire. I wouldn't go any narrower than 225.

3). I assume this is a typo, and you meant "8.5" front width wheel. No problem. You can go as wide as 9" if the offsets are just right.

4). I think you have the correct numbers for the WB offsets.

My car uses the very heavy Ruf 19" aluminum wheels 19x9 and 19x11 (I think). I am using 255x30x19 front and 305x25x19 rear (Pirelli assymentricos). I LOVE the way my car handles, but the Ruf suspension may have something to do with that.

bfd365

Steve,

Because the Ruf wheels you are running are 1" wider than what comes from the factory as well as your tires being wider, does that mean the offsets have to be change so that the wheels dont look like they are sticking out of the side of your TT?

Thanks in Advance,

Rob

ca993twin

Rob,

Your question is good, but beyond my abilities. It probably depends on whether the extra inch is tacked on to the inside, outside, or split between the two. I don't have the answer. FWIW, Ruf does not make a special wheel for the wide-bodied cars... the rears are designed to fit a narrow-body, and they use spacers to create the needed offset. Why I thought that was worthy of mention is not clear to me anymore.

bfd365

Appreciate the info. Steve.

Nikoncamera

19 inch wheels on our model 911 is simply about looks, not performance. Function following form.
Our chassis were simply not designed for 19 inch wheels, and they do ( in all but the most extreme cases) increase unsprung weight, and lets face it, well and truly "over tyre" these cars: an possible increase in roadholding, with perhaps a negative effect on handling.
Cheers

c993k

18's give you more a selection and cost less. They are what engineering tested and recommended for our cars.
The best way to answer your question would be to test drive your car with both and let us know what you think. Chris

AVoyvoda

19 inch wheels have many disadvantages, some serious:

a) Increase unsprung weight. On average, 19 inch wheels are 1kg (2.4 lbs) heavier than 18 inchers.

b) Increase rotational mass of wheel. Can be felt through greatly increased steering effort in high speed corners. For example, cars w/o powersteering become impossible to drive.

c) Change suspension geometry. Your suspension is optimised for smaller wheels.

d) Wrong spedo readings. The wheel/tire outside diameter increases but system assumes you are still with your old wheels. Therefore, you speedo will always show the wrong road speed.

e) Unsafe. Your ABS is calibrated to the standard wheel diameter. 19 inch wheels are outside the envelope your ABS is programed for.

f) More stress on suspension components, stress they weren't designed for. Our cars are are not young any more.

g) Not sure of my grounds here (perhaps a lawyer could offer some insight) but it's possible your insurers would reject a claim, as your car is outside manufacturers' recommendations. This could be especially serious in terms of third party claims.

Statements by some that "I got them and they are OK" don't cut it, in my book. They'd need to be tested over many thousands of miles, over different road surfaces, under varying loads and speeds and as importantly, in different emergency situations. Just like the manufacturers do. The odd test drive just isn't enough.

Moral of the story: Lot of risk for a little bling.

Rickard 993 Turbo

and if the roll circumstance is the same, do you hate 19" then also..?

AVoyvoda

How can the "roll circumstance" (I think you mean circumference) be the same? There's an extra inch of wheel diameter to take into account. Super-skinny tires may help a little but at the cost of serious driver discomfort as you crash over every bump. And you'd increase the risk of wheel damage with even less rubber to absorp the impact.

993-CT

That made me wonder about what size Porsche engg actually recommended for our cars. I thought 993 came with 16", 17", 18" options. The 95 was clearly not intended for 18" (steering rack brace?), but could the suspension changes for 96+ have been partly geared to 18". Anybody know what the engg/designers "sweet spot" was 17 or 18?

ca993twin

AVvoyda,

My experience... and those of many other 993 owners with 19" wheels runs contrary to your arguments. Have you any real evidence to support your claims? Perhaps our fairly decent roads in California present us with fewer problems then you describe. In a word... hogwash.

AVoyvoda

Surely RUF did a splendid job on your car. But that's a different ballgame to "just" buying and fitting 19 inch wheels becasue they look "nice". A proper conversion would at the very minimum need:

a) The correct suspension components (like yours), and
b) Different ABS rings on all four corners. This also takes care of speedo issues, and
c) Chassis strengthening, and
d) Lowering the car some more (since bigger wheels = more height from the ground) or you'd end-up at super-SUV height, which then gets into bumpsteer issues i.e. probably new uprights and tie-rods.

Evidence? Plenty. But most of it is kinda obvious.

mborkow

it was my understanding that the move to 18s was dictated by the size of the big red calipers. i think the car was originally designed and tested with 16s.

rdurff

Rob,

I'm not an expert on this myself, but want to pass along some interesting information you may want to consider from the 911 Buyer's Guide by Randy Leffingwell (no affiliation). In the notes on '95 MY 993s, he notes that many owners had problems with the the front and rear window glass when fitting oversized wheels to their cars. The seals for the windows were rigid -- and in a number of cases the windows cracked or shattered (!) due to vibrations. (Unclear from the text whether it was resonance or just greater transmission of harsh bumps.) In any case, something to think of if you have a '95. In later years Porsche sealed the windows differently, with a less rigid substance.

Cheers,
-Robert