Speedtoys

I suck on wheel sizing..7x17 ET55 front and 9x17 ET55 rear Cup II wheels on an 85 or an S4.

Good, bad, passable?

Comparing to stock 85 wheels for sure.

The Forgotten On

Passable. It will give you a 0 scrub radius though.

Speedtoys

Wassat mean?

Speedtoys

Trying to make the RIGHT sense of this.

S4 wants +65 front, +52.3 rear, from:
http://www.928registry.org/928-resou...ic/wheel-guide

A broader set of wheel data..
http://members.rennlist.com/flyingdo...%20Fitment.htm

If I add a 10mm spacer up front, does that make the 993 front wheel now +65mm?
And leave the rear alone?

hacker-pschorr

Just the opposite, that will make them an et45.

Bigger number = wheel's move inward. Adding a spacer moves them out, the opposite of what you need.

I have et52 8x17's on the 87 right now. No issues so a 7" wide et55 will fit fine up front.

Speedtoys

Ok...I was afraid I was backwards, thank you.

Mostly this is for the kid's car, her 85. And I don't want to introduce a performance issue for her.

The Forgotten On

As far as for our cars the higher the offset the better. The more positive scrub radius we have on the front the less of a chance our cars will tramline and become twitchy.

That is why we got +10 scrub from the factory, at least for the pre 86.5 cars. Not sure about S4's though.

I run Panamera rims on both of mine and they have an offset of 59 in the front (the lowest the 928 ever got from the factory was 60) so they have no real tramlining issues.

A lot of people run Carrera rims which have an offset of 50 in the front and some have reported tramlining issues because they now have a negative scrub radius.

jcorenman

ET65 Cup-IIs are harder to find, but the right answer for the 928. They are also 7.5" wide, a better fit for 225/45 x 17 tires.

Same. Stock 16" wheels were ET65, club-sports were ET60-- which I believe was the smallest offset that Porsche ever fit to a 928. We ran ET52 Mille Miglia 18" on the front of our GT for a few years and about 60K miles, tramlining was a bit worse but could be compensated by increasing toe-in a bit... Not ideal, and they stuck out a bit more than I liked.

zekgb

I believe this is backwards, stock suspension and ET65 wheels provide -10 scrub radius and moving to ET55 gets you to zero.

Speedtoys

She can deal with the tramlining, my C3 wheels do it some at a 57mm offset, and she drives that fine.

Rears, seem close enough, and she ends up with way better tire choices.

FredR

When it comes to negative scrub radius we are seemingly talking about "fine margins"- the difference between "good" and "better" as it were.

For a while I ran with a set of 18 inch off takes with ET52, looked good, filled the wheel well nicely and no real issues- until I did a bit of auto-cross type driving wherein the thing really seemed to struggle compared to an earlier 928S a friend was driving. Decided to sell the things there and then to a friend who had a 911 - a win win scenario.

Cannot compare the wheels to that I replaced them with as they are a different kettle of fish altogether but with plenty of front ET at 68mm and plenty of rubber capability.

The research I did suggested that the worst thing you can do is run with zero NSR in that a positive scrub radius is better than "zero scrub radius"- I cannot verify this - just commenting on my research.

I concluded that Porsche knew exactly what they were doing when specifying the wheels/suspension design and decided to stick with the design principals they mandated. I tweak the suspension/alignment for [hopefully] optimal cornering capability and I am satisfied it works enhancing the original design. No tramlining issues, no other issues other than perhaps knocking out the front suspension a bit earlier than might have otherwise been - performance that made factory instructors take notice up at Dubai Autodrome.

Does NSR have any safety connotations for day to day driving?- a difficult one to answer with any certainty but probably "yes" when driving in performance mode. The 928 always seems to have a "bit more up its sleeve" when you need it most which is usually in the middle of a brain fade moment or a "self inflicted" crisis.

IMHO not a good idea to mess around negatively with Porsche's very advanced suspension design intent- improve it or stick with the stock design but do not go backwards. My personal point of view.

Rgds

Fred

mark kibort

Fred, there is no bettter feeling than big meats on wide rims that go to the end of the fender widths. (the only limit )
those rims that speedtoys is considering will fit fine.
dont worry, the 225 fits nicey on the 7s and you can then use a 245 or 255 on the rear.

Jim Chambers

Here are a couple of sites that could be helpful.

http://www.rimsntires.com/specspro.jsp

http://www.1010tires.com/Tools/Wheel-Offset-Calculator

anonymousagain

Speedtoys, FWIW my 86 (with factory 10mm spacer) fits perfect to fender w/out rubbing using 7x17 ET55 and 9x17 ET50, 235/45/17 and 275/40/17 respectively (for similar diameter match).

Speedtoys

Thanks.