Red Flash

Mark, please bear with me, I don't 100% understand this:
1.) It sounds like the key point for you is the location of the edge of the rim on the outside, right? A 9.5" wheel with an 85mm offset puts the edge of the rim about 1.4" further out from the wheel mounting surface.
2.) When you say backspacing what do you mean? The edge of the rim to the back of the wheel well?

Thanks for your insight here.

FredR

Backspacing and offset are two different methods to determine the same characteristic- i.e. where the wheel rim sits relative to the mounting flange.

Our "Colonial Cousins" tend to like to use the term backspacing which is the distance from the inner rim face to the mounting flange. In Europe I think it is fair to say that we tend to refer the term "offset" which is the distance from the hub face to the rim centreline expressed in mm. Thus Porsche wheels ar eonly stamped ET65 or whatever [cannot remember what the ET stands for- a German expression to reflect offset I seem to remember].

The 928 steering front wheel steering geometry is somewhat different to most other Porsche sports cars [specifically 911's and Boxsters] in that 928's are designed for a higher offset to achieve the neutral scrub radius [NSR] the design demands to make it work correctly. Thus for themost part other Porsche wheels do not allow the suspension to work correctly when fitted with smaller offsets [typically around 50mm for the 911 crowd].

To fit bigger wheels in the 928 front wheel well the constraints are fender clearance, sway bar clearance, caliper clearance and amount of lock before rub occurs.

Thus the 928 geometry needs bigger front wheel offsets and with greater overturning moments due to the overhang the wheel design has to be that much stiffer to resist excessive flexing during hard cornering. With a one piece cast or forged wheel construction that results in a "heavy wheel" and more unsprung mass- enter the three piece forged wheel concept wherein custom offsets can be realised by utilising different width half sections. Thus my font wheels have a 1.5 inch outer and 8 inch inner sections to give a 9.5 inch wheel and the hub design is such that the end result has an offset of 68mm. For street 928 that is about as perfect as can be.

For a track racer like Mark, he can afford to run an even higher offset to accomodate a wider rim/tyre section [10 inch/275 with 85mm offset] but I suspect that would have severe lock limitations for a street car- not a problem on the track though unless you have a lot of very tight hairpins. Compared to my setup [9.5 inch/265/68mm offset] Mark is going to lose 23mm of inboard clearance compared to my setup [17mm of offset plus 5mm section width] and for street car that would be very limiting in car parks and the like.

Thus I doubt Mark would use his "track wheels" on the street. It is all "relative".

Regards

Fred

mark kibort

The backspacing is the mounting surface of the wheel to the inner wheel edge. you can find this by using a level on the rim inner surface, to the mouting surface (where the lugnuts go in)

for a 9.5" rim with a 8" backspacing, means the outer portion of the rim will be 1.5" from the mounting surface. (where the rim aligns with the fender edge) . an 11" rim with a 8" backspacing puts the edge 3" toward the fender edge.

offset is the total wheel width, divided by 2, and then the distance to the mounting surface from that centerline.

Wisconsin Joe

Damn. Some of you guys are smart.


"Offset is usually stamped or engraved into the wheel and is measured in millimetres of ‘ET’ [ET is the short form of the German word 'Einpresstiefe' which literally translates as 'insertion depth']An example would be “ET45″ for a 45mm offset."

http://www.wheelsrus.com.au/blog/tec...ogy-explained/

Vilhuer

ET means positive offset. There's another german word for negative offset. Those wheels are stamped with something other than ET followed by number. I have memory it's AT but since those wheels are used only in old Opel's and such there's very little need to remember it.

Red Flash

Fred, Mark K., thanks for your comments! That really helps me and gives me something to think about. I hope it helped the OP as well.

Erkka, I did not really understand what your comment related to, but the I think I'm good to go with the other comments now.

Jhoffmann

Jim Doer of 928 Classics has a couple of charts (Phone Dial Project) posted on his sight that might help make it a little easier to understand...
http://www.928classics.com/project-phone-dial.html

Josh

mark kibort

there needs to be another number that is standardized, but there isn't. I know the backspacing, because I can do the math in my head for my widths of rims, but it takes time.... I know what works, what is a push, and what can use a 1/8" spacer and still work.... plus, what tire manufacturers and sizes I can use that will work and not rub.... if you see the clearances, you can see its the absolute limit...... plus, I have even moved out the fender up front for even larger width tires, or the .5" less backspacing.

anyway that number we need is the number from the outer rim edge to the mounting surface, SINCE that is all we really care about. backspacing will then be the number that tells you if the tire is going to hit the chassis. 8.5" is about the limit, as I use 10s up front with 8.5" backspacing, and the 1.5" to outer edge remains constant...... U turns become a little dicy at full lock.

so, if you can get a rim with less than 1.5" distance from mounting surface to outer edge, you can use any width rim you want.... up to about 10 up front. rear, use 3".......

devek a long time ago found the optimal back spaced rims for 11s and 9.5s on the 928.... both where 8" backspacing. the rear rubs a little on the inside and requires a 1/8" spacer for a 305 tire, and slight fender lip rolling which is not hard to do and looks great. otherwise, 285s fit with ease in the rear, and 225 to 275s up front fit well too.

hacker-pschorr

It must be something with Porsche, it's not like cars back then didn't have wide front tires. An 87 Vette came with 255's, and by 1990 they were putting 275's on the front of the ZR-1.

Even the Camaro had 245's up front.

Vilhuer

There was huge development going on in tire tech over few years in late seventies and early eighties. Whole car design had to change to make room for wider rubber. 928 was first car to use Pirelli P7 which was what started it all. Porsche didn't see far enough into future to be able to benefit and accommodate this development when basic 928 body was set in stone. It was too expensive to change it in entire 17 year production run. C4 Vette was designed maybe 8 years later and GM engineers had that much more knowledge on tech changes what were happening.

Mark R.

Okay, so great comments and discussion...
But still no truly definitive answer to my original question.

Let's disregard the wheel width for now, and focus on tire width.
So that leads to a slightly broader question:

-> Who has ET-60 front wheels with 245, or wider, tires..?

I agree with all the comments about larger offsets (ET-70, etc.).
But I am specifically interested in a certain set of ET-60 wheels.

Thanks guys !

Vilhuer

I have 245/45-17 on 8" ET70 Cup II with 6mm spacer making it ET64. I don't see any problem for this particular tire (Falken FK452) to work with ET60 wheels. It's basically same as 225 on ET50 wheel. Some other tire brand can be so much wider in real life than it would cause problems. There can easily be 10-20mm difference in actual width between different brands and models. In general more expensive tire is wider it is compared to what number it has in sidewall. Some Michelin or Bridgestone might cause problems while Falken works well. When geting close to the limits there are too many variable to say for sure what works and what causes problems. Old or S4 style suspension, suspension height, specific tire measurements etc all make a difference.

Mark R.

Lots of great comments. I really appreciate all of the feedback and guidance.
When I first started this thread, I was actually more concerned about the inner wheel/tire surface growing inward by 0.5".
(at least when compared to my current 8" front wheels with ET-60).
I was thinking there would be just as much likelihood of contact problems on the inside of the wheel or tire.
But since folks are able to run the same width (or wider) tires with ET-70, that should completely eliminate that concern.
A higher offset (like ET-70) moves everything even further inward, which should confirm that it will be fine with ET-60.
Correct? I think so... That would eliminate half of the issue (the outer edge of the tire being the other half).

Vilhuer

As long as tire diameter remains around 25" first place where problems occur on inside is when wheel is at full lock. That can be handled by installing thicker spacer rings to steering rack ends. It's very difficult to find wheel with so large ET that there would be problems with spring or suspension parts when steering is pointing straight. In almost every case fender is causing problems much earlier and spring can become problem only if wheel is very wide and fender is modified to make it fit.

FredR

Mark,

It is a question of what you plan to stick on your 9 inch 60mm offfset wheel. Assuming we are talking about a 245 section and a similar tire geomery to my Continental tires first point of contact concern would be the outer shoulder surface relative to the wing [fender]. I run two degrees of front camber and my wings are rolled - I have no problem there whatsoever albeit my wheels fill the wheel well very nicely.So in your case I think you will be OK but it might be close with a stock [0.5 degree] camber and stock fender. Try putting a 255 section on and I suspect you may well have issues rubbing the fender.

In my case with a 265 on a 9.5 inch ET68 I had to fit an additional spacer each side of the rack to stop contact on the Devek roll bar. I believe the Devek bar is a bit more restrictive than the stock front links. You will not have this problem or anything close to it.

Rgds

Fred