augustbailbonds

I was checking Tire Racks catalog and saw some great wheel/tire combos. I never was one for the larger than stock wheels until I saw what was out on the market and visualizing it on my 928.

How does one figure out what works? Are there speedometer issues with larger diameter wheels? I hear the term "offset" on the list. How does that figure into the preliminaries before choosing a custom wheel.

I saw some 19" wheels on a current generation Mustang that looked killer. However, I would rather enjoy driving the car at speed than make a mistake purchasing for aesthetics than function.

August Spillers
81 928s with Hurricane Isabel love tap

Garth S

You can use any wheel diameter provided the tire diameter stays at ~25". This preserves the rolling radius and speedo calibrations. Many are using 17" and 18" - perhaps some 19". Risk of wheel destruction due to the all pervasive pothole attack increases almost exponentially with increasing diameter - avoid the 20" 'bling' thing unless you drive on 747 runways or like to park a lot.
928s use an uncommon 130mm 5-bolt circle, and wear front offsets ( called 'ET') from negative 65mm (oem) to ~ negative 50mm max. The rears can operate in the 40 to 65 mm range as a rule of thumb - this can be played with using spacers, etc.
Other than custom $$$ 3-piece wheels, there is a somewhat limited selection of ones that fit.

Brent 89-GT

One comment that needs to be made is that no wheels larger than 17" are "approved" for use on the 928 by Porsche. Many here are running 18's with no problems but it would be a good idea to keep a closer eye on wheel bearings. Larger wheels are also heavier generally. My car had 18's on when I bought it. I sprung for a new set of custom wheels to be able to fit larger than stock front tires. I went down to 17's and lost some of the look. I ran the car on the track some and didn't need to be taking chances with "un-approved" wheels cornering at 130 mph. Big wheels are the thing though, I ordered my Cayenne S with 20's and it looks great. Switching to 18's for winter takes some getting used to.

928drvr86.5

Why is it that 18" wheels add more strain to the wheel bearings? I think my new 18" Fikses are close to the same if not lighter than the 16" flat dish wheels they replaced when you consider total weight with the tire mounted. In addition i have pretty close to the same rollout circumference by using a 35 series tire. Offsets are pretty well matched as well. Is there something i am missing about these wheels adding more stress? I do notice the ride is a little more harsh, but i think that is a result of the new Bilstein/Eibach setup on the car. I did run the 18" wheels for about a week before changing the shocks and springs and the difference between the 18" and 16" wheels was not very dramatic as far as ride comfort.

Brent 89-GT

I think the reason is that the wheels are obviously stiffer than the tire sidewall. With a shorter sidewall and larger wheel there is more leverage on the bearings when cornering HARD. With the smaller wheel and taller sidewall the tire gives under hard cornering before the taller wheel shorter sidewall.

Somebody with a better explaination will probably chime in. I think that is what I read though.

928drvr86.5

That makes sense to me. That is also why it handles better as well... less sidewall flex.

mark kibort

Its all BS. trust the guys that have been running 17s and 18s for years and racing them at speeds over 150mph. the differences are so subtle , its not even worth discussing.
as far as weight, 18s vs 17s, hardly a difference, and even if there was, the tire weighs more, so its actually better to keep less of the weight off the outside of the diameter. meaning, if a wheel weighs 20lbs and a tire weighs 25lbs. its better to grow the wheel and shrink the tire!. the only valid point here is sidewall flex. Racing requires careful monitoring of pressure and wear patterns. generally, 1-2psi less on an 18" tire vs a 17" tire is enough to keep the grip equal.
as far as wear to the wheel bearings?? right! grip is grip. if you can pull 1 g, its all going through the tire, so it doesnt matter if it is an 15" rim or a 20" rim. it all gets transfered to the axle the same.

so to repeat the point, generally, 16s vs 18" wheels and tire packages are generally the same overall diameter based on the tires commonly mounted to them. (ie 225x50x 16 is the same diameter as a 315x35x18) Or close enough. right around the 25" range. a racing wheel and tire can be the same weight as the stock 16 " rim and 225 stock street rubber too!

MK

mark kibort

porsche also approved Mobil one oil, but for one, ive never run it and seem to have more luck with other oils than MOST. same will go for all sorts of other things on a porsche.

Just because porsche recommends or certifies a part, doesnt mean its best. it means the part manufacturer paid money to have porsche say it, and porsche has determined it works. doest mean, by any stretch that other things wont work as well or better.

MK

928drvr86.5

Good points Mark. Thanks!

cobalt

Sounds like Mark is right on.

The first car approved by Poirsche to use the 18" wheels is the '94 964 turbo. The TSB was released not long after. Yet the 91-92 964 turbos are identical in body and suspension as the 94's and they do not approve 18's for use on the 91-92 turbos.

Sounds like Porsche was taking the easy road and just decided to say no so they wouldn't have to deal with any issues that may arrise.

Jim bailey - 928 International

Garth said it best..."Risk of wheel destruction due to the all pervasive pothole attack increases almost exponentially with increasing diameter "....... It almost becomes a matter of WHEN will you bend them not IF you will bend them if you drive the car at all. But it does provide an ample supply for wall mounted wheel face clocks Even the three original Porsche GTS 17 rims on the old very brown 1980 that I use on the street are ALL bent a bit.........which is why I got them ! Try to buy a wheel style that is popular and should be available for a while in case you do need a replacement in the future for what ever reason. And keep your old wheels for spares !

jetrainor

Having just mounted my 17" Hartman twists purchased from 928 Int, i'll chime in here. Fronts are 235/45, rears are 275/40 Yoko AVS Sports....looks awseome and as everyone running aftermarket wheels knows, it transforms the look of your car.... no fit issues at the front...rears are real close to rubbing the inner fender lip...this has me thining about rolling the rear fenders but am worried about cracking the paint and generally screwing up the fenders...i've read some of the threads on this and have a general idea what to do but what i'm wondering is if anyone else is running this wheel/tire size combination without rolling the rear fenders.

1987 Euro auto -Silver/black-

mark kibort

rolling the lip is done best with a wood baseball bat. minor rolling is done with really no chance of cracking the paint or ruining the fender. when you have to not only roll it, but get the fender bowed out a bit, that takes some "flare" if you know what i mean.

mk

Garth S

A question on fender rolling technique: what approximate ratio of tire to fender gap verses diameter of the hardwood bat is workable? ie. if the rubber/fender gap is 1/2", then presumably the handle of a bat at ~ 1" could be rolled through.
I know the deal about a warm day, warm paint, etc - but I have a bad image of rolling the business end of a 35 oz. Louisville Slugger through a finger sized gap. Does one fit old tires, narrow wheels to get a gap of x inches before starting ... .
I could use a mere 10 mm (~3/8") for a new set I'm playing with, but have never seen it done. TIA for any advice ...

jetrainor

Thanks for the response Mark...i would like to 'bow' the fender out a bit and to carry on from your question Garth, i've wondered the same thing....i can barely get my finger between the rubber and the lip so i assume i'd have to swap my old rims/tires on the back to roll the rims...heat them with a hair dryer first?

87 Euro auto - Silver-Black-