Dean_Fuller

Dr. Bob....my ET47 is on the REAR with a 10" wheel....not the front. I would never put a ET47 on the front. I may of worded my statement oddly....The REAR is only 3/16" off stock. I agree the fronts are off 9/16" from stock...my 3/16" was not correct for the front...just the rear. Sorry for the confusion.
Please correct me if I'm wrong. The way I am understanding it now is the critical number is centerline ( pivot point ) of what ever wheel regardless of width to the ball joints ???
I think as you said the stock wheel is ET65 and my fronts are ET50 so they are out board 15MM which is 9/16". I know thats not ideal at all.
BTW...somewhere I read the stock front offset was ET-65...I HOPE that was a mis print.

dr bob

Stock offsets are 65-67mm, depending on year/wheel. I think the GT/CS wheels had 65. If my memory of the measurements is correct, about 57mm has the wheel centered exactly over the ball joints. So the original wheel has 10mm more tire inside that point than outside. 55mm is really close to neutral, and 50mm is about 1/4" from centered, towards the outside. 50mm is about three to four times the displacement of a 55mm offset wheel. I can tell you first-hand that the difference between 55mm and 50mm offset is significant. My 'testing' involved no geometry changes other than the spacer in/out, so no fudge factors or wildcard variables like different tire sizes or brands.


Again, what works or doesn't work for me is hardly a Rule for you. Go with what works for you. The extra front width may, for you, more than justify the difference in offset. I just said that --I-- noticed a difference and wouldn't got less than 55. MHO, that's all.

Dean_Fuller

Dr. Bob...Its clearer to me now than before....centerline of the wheel has really nothing to do with ball joints and pivot point its a combination or overlap of the two. I always have to really think about that when it comes up in conversation. This has been the first time some mentioned ball joints and pivot point....at my age I have gotten to the " can't see the forest for the trees" stage more often than not.

I will say that with the ET50 on the front the car tends to ride pavement changes more than one would think. Uneven roads side to side effect it as well....Maybe that IS trampolining. So far I have just manually adjusted for it.

Thanks for making it clearer for me....I assume the rear offset doesn't matter as long as everything clears as far as driving is concerned.

stealth

I had a chromed version of the wheels (996 factory wheels) you have on your avatar and I thought the front offset was 52mm (the 17x7 et 52 I cited) and rear 55? Maybe I remember it backward.

Whatever it was was an improvement over 47mm.

I can also say that, before I got the 944 CS wheels on the S3, I had the original flatdish et65's and noticed that the ruts trucks tend to make in the right-center lane of an 8-lane expressway jerk a 928 around regardless of offset or tires...the track width lands in the sunken spots and it's hard to handle even with correct offset.

dr bob

Shane--

The fronts I have are 17x7 with 55 offset. Not the ideal width, would rather have 7.5 or 8, but for me no worse that the 16x7 fronts that came on the car from the factory. I didn't really care for the look initially, but a few compliments from valued 928 friends convinced me that I could survive for a while with that look. I've only seen a couple other 928's with them, only one in person. I didn't jump quickly enough to grab his set so I could have 9" all the way around, but such is life.

I'm with you on the ruts and the steering tracking. There's a section of I5 south of me where the trucks have impressed the pavement a few inches. Those tracks grab at the wheels and tell them which way to go. It takes a firm effort to get out of them no matter what. As you say, offset makes not so much difference. I try to stay out of that lane if I can.

I suspect that there will be more deals on used and takeoff 997 wheels in the future. If a set happens for me I'll probably change to get the correct front geometry again. Right now, for my grandpa gray driving style, these will be OK.

sendarius

dr bob:

I have the chance to pick up some 18" Porsche rims, and the vendor is offering "spacers so they will fit a 928".

The front wheel offset is quoted at ET50 and my understanding is that adding a spacer will REDUCE this number, thus making the discrepancy WORSE.

Am I right, or can these wheels be made to fit without violating the "never less than ET55" rule?

AO

Yes. Spacers would make the front worse. and a ET50 would not be that bad (depending on the width of the wheel.)

Oh... and there is no such thing as a "negative" spacer. If the offset does not work, it is highly inadvisable to have the wheels machined to make them fit.

sendarius

Thanks Andrew. That's what I thought.

The wheels are 18" 996 hollow spokes: fronts 996.362.136.04 rears 996.362.142.11

I think that makes them 8x18 and 11x18.

What makes them attractive is the range of tyres that are available for 18" rims. I had a tough time sourcing new shoes for the original 16" wheels.

AO

Be careful with the 11" rear width. You may need to roll your fenders to get them to fit. 10" will not be a problem with up to a 275 tire.

sendarius

Yeah, I thought of that.

Thing is I can get a set of 3-piece Simmons custom-made to fit, with exact offsets for not much more than the going price on these.
Of course tyres are extra if I go the Simmons route, and a set of part used Falkens is included with the hollow spokes. It's never easy, is it?

AO

Nope...

rawky

Right, so would an offset of et50 and 225/40/18 tyres be okay for not tramlining with an 8J front wheel? Or would the difference from the stock 7-slots be unbearable?

Cosmo Kramer

I test fitted my 996 rears 11 x 18 et 45 and they have about 1/4" clearance to the suspension on the inside. You can't use spacers with them. I haven't had a chance to drive on them yet as I am still doing winter maintenance.

Here is a thread of a member that has them fitted. He managed to wedge 295's on the rear with no fender rolling and has 8 x 18 et 50 fronts.

https://rennlist.com/forums/928-foru...-my-shark.html

76FJ55

Another thing to take into consideration is tire diameter. with an increase in tire diameter you will need to decrease offset to maintain scrub radius. I don't know off hand what the factory kingpin inclination is, but assuming 15 degrees a change of .3 inchas on the rauis (going from 235/45-17 to 245/40-17) of the tire will require 2mm of offset change to maintain the factory scrub radius.

dr bob

Good catch! Reality is that most tire fitments maintain the 25.5 rolling diameter. That's really the critical diameter, not necessarily the rim diameter.