Justin O

Hi all, I've been searching the forum for good threads to guide me on rim sizes, offsets, etc. for a 928 project - so far I've found a few tidbits and LOTS of debate, but nothing too definitive. So I'm going to try a new thread (apologies in advance for any redundancy):

I'd like to fit the following rims to my 1982 928: 17x8J up front; 17x11J in back. The car has a 3" blister in the back - front wheelwell is stock.

For the front rims, I have read that an 8" rim should work fine, as long as you keep it as close to the factory 65mm offset as possible (ie no late-model 911 rims with the 55mm).

For the back rims, I've seen lots of threads for guys who want to put wide rims on the back without modifying the fender (or, at most, with a mild roll) - but have not yet seen any offset recommendations for a blistered fender. My working assumption is I would simply need to add the extra 3 inches to the existing 65mm offset, for a total offset of 141.2mm. That way, I would keep the width between the inside edges of the two back rims exactly the same as factory, and avoid hitting suspensino components or rubbing the inner wheel well. I guess i could also use spacers, but 3 inches is a lot of gap to make up with those. Plus i'd rather do it the "right" way.

Of course with that kind of offset, I assume I'm looking at custom rims. Time to raid the the rainy day fund...

So, do i have the numbers right? If not, any guidance greatly appreciated.

Thanks,
Justin

PS - I also considered 18" rims, but have read in several technical forums that they are a no-no for 928s. Plus I like the look of a bit more sidewall. Please let me know if you feel otherwise.

davek9

Many here run 18" rims, so your last "PS ..are a no no.." is not correct.

Justin O

Fair enough - thanks. Could be the article I was reading was a bit... conservative (PCA forums). Any guidance on the offsets?

dr bob

Justin, without seeing the amount of extra room you have (3" all the way across or 3" at the center, tapering back to meet the original metal shape withing the span of the tire) it's virtually impossible to make a definitive recommendation.

But you can pretty easily figure out what will fit if you know what you have now. Assuming from your original post that the original wheels are 7" wide, with 65mm offset.

Figure out how much clearance you have now between tire and fender. Now, every inch of width that you add at the rim reduces the clearance to fender by half an inch, since half goes on the inside and the other half goes on the outside of the wheel. Assuming the same offset then, the 11" wide wheel will reduce the tire-to-fender clearance by 2". How much room is left? You can add that remaining space back in with any combination of offset and spacer. Keep in mind that your measuring should consider the full suspension travel, and an allowance for bushing displacement during cornering.

Rob Edwards

What are the offsets of the wheels you want to use? And the sections widths of the tires you want to use? Any discussion of offsets is always fraught with a little hesitation to comment when one starts to try to get close to the limits, as no one wants to say, yeah, your $X000 set of wheels and tires will fit, when in reality it won't.....

[old man] I assume 'blister' means 'flare'? What's the contour of the blistering? [/old man]

Adding 3 inches (76.2 mm) to a 65 mm positive offset wheel doesn't get you 141.2 mm of offset, it gets you -11.2 mm. Is the plan to add 3" spacers?

FredR

18 inch wheels were never certified by Porsche for use on the 928- that being said many of us run them and I have never heard of them causing any mechanical problems- some feel the ride quality is too harsh but then some of us like a firm ride.
For the front wheels you really should seek to maintain the 65mm offset and more if you go for wider front rims [my fronts are 9.5 inches and offset 68mm and I consider that perfect.

For the rear it seems you seem to have the wrong concept of what offset does. More offset pushes the rim centre inboard. If you have flared arches and they sit 3 inches outboard of the stock wheel arches then the last thing you need is more offset- quite the opposite.

If you fit an 11 inch rear rim then I presume you are looking to fit a section something like 325 wide. You are then looking to fit spacers like the GTS [38mm] and maybe greater to make sense of the look. The thing you have to be careful about is the over hang to ensure you do not knock the wheel bearings out.

If the wheel arch is flared 75mm that is a big gap to fill and I suspect you will need a GTS type spacer [with appropriate length wheel studs] and an ofset in the region of 40mm [probably less] to avoid the installation looking silly. Thus important you define what you mean by "75mm".

As I remember, the GTS chassis flares an additional 28mm compared to the S4, with my ET60 rear wheels with 10 inch rims and 60mm offset, I found that a 295 tire sits nicely with a 10 inch rim section and a 24mm spacer. This data can be used to determine the offset for a set uo of the type you mention.

Regards

Fred

Justin O

Thanks very much. Merging a couple of questions - i think if a "blister" as making the entire fender bulge outward (similar to the strosek kits for 928), while a "flare" is a more or less straight (or maybe slightly angled) flange coming directly out from the wheelwell. But it seems there are several points of view on the terminology here.

Anyway, this is a blister based on the above definition - but not a kit - done custom. Cut out the wheelwell itself, move it out 3" and fab the sheetmetal to close the resulting gap. So wheelwell geometry stays exactly the same, just 3" outward on both sides (6" total width gain).

Thanks,
Justin

jetson8859

Justin, can't comment on the offsets, plenty of much smarter people can provide that information. Here is a picture of my 87 S4 running 18 inch Turbo Twists from a 2000 996, front offset is 52 and rear is 65. No rubbing or tram-lining issues what so ever. Tires are 235/40ZR18 front and 285/35ZR18 rear.

dr bob

Nicely done on the metal work!

-----

Offset is correctly described in the negative and positive, so correctly stated the offset on my factory fronts might be -65mm. Add offset, and it goes from that -65mm through zero and on into the positive offset numbers. Can be confusing if you don't have the rule book.

It's a lot like camber, where we look at a camber number without concern for the plus-or-minus, since "everybody" knows that our stated-as-positive value really means negative.

Ben

If you have a 3" 'blister' you could go much wider than 11" and go with the 335/35R17 sized tire that is still available from various suppliers.

I am running 11" ET50 on the rear of my 85 Euro with only a minor tickle of the fender lip.

Something like a 13" wide rim with an offset of 25mm should go nicely and give plenty of clearance to the suspension and fender lip.

My own ultimate plan is to do similar for my car.