WyattsRide

I can't wait to see those on Brunhilde.

Eplebnista

Magnus - Thanks for the heads up, but the front rims are a 55 offset. The article you refer to warns against an offset different than the original wheels but also says that many have used a 55 offset with no problems but going to a lower offset will certainly cause issues. I chose not to go below 55 for that reason.

Magnus Rostadmo

Eplebnista & Docmirror !
I can see now that my reply was more adressed to Docmirror than to Eplebnista.
I believe an Et55 rim on an OB will work out fairly well as a marginally scrub-radius is retained.
But i still do believe that your car will not be as stable as Porsche intended as they are like on rails with the right Et rims and tires matched. I have never driven through standing water on just one side without the need for counter-steer with any car before. But with my -87 S4 i can do that with my hands off the steering wheel without any weer at all. Impressive for sure ! :-)

FredR

Otto,

The whole point of these forums is to get the collective knowledge of the group as a whole. As you say many have used 55mm offset with no problems [that they know of] but then many have never [and will never] drive these cars at the speed and limits they were designed for- some of us have and a very few beyond those limits!

Our 928's have what is by today's standards, a very advanced suspension system that was designed in the early 70's and developed some more with the S4 in the mid 80's [your model has the same front suspension as the S4 I believe].

Some folks quite innocently manage to concoct somewhat strange modifications that quite frankly beggar belief [to those appropriately enlightened] and that is their choice but what no one wants to see is someone else doing the same because they were advised it was "OK to do so".

I had a very nice set of reverse spoke 911 wheels on my late S4 with front ET of 52mm. They felt OK for the most part and then we had an autocross type event in which I struggled against a beat up old 928S- hmm! I sold the wheels to a friend with a 911 and purchased a custom built set of 3 piece forged wheels. Next time out I went 6 seconds quicker on a 60 second lap and that despite not having the front camber where I wanted it- and duly wiped the floor with a bunch of 911 turbos and the like. I have a tunable anti roll bar, I have welded reinforcing brackets on the mounts, I have a 928 Motorsports lower front brace, I have uprated suspension units, I have a custom alignment such that my front end turns in like a demon and despite two degrees of front camber I have no issues with uneven tire wear. I suspect most folks on this list probably have little to no idea where their alignment is other than it lies somewhere in the "acceptable range" and probably have little to no idea how to tune such.

Porsche designed a NSR of something like 10mm into the 928's excellent suspension which was the forerunner of all todays Porsche suspension system designs. To achieve this NSR the ET is all important and this requires a wheel with an offset of ET65- at ET55 the NSR is zero and at ET47 it is minus 8mm. So who do you think knows best? My front wheels have an ET68, are 9.5 inches wide and carry a 265x35x18 section, I have additional spacers each side of the steering rack to prevent rub and I have no problems whatsoever other than having to make an occasional 3 point turn where others cars do not.

Beyond that, a given tire width has a range of "acceptable" rim sizes it can be mounted on and whereas a 255x40x17 is a fantastic size for 928 service, the optimum rim width is 9 inches with a minimum of 8.5 inches and a max width of 10 inches. Note how the max width has greater range from optimum than the narrow end of the allowable range. There is a reason for this and it is to do with stability. Now, if you want to run a section on a rim narrower than that proscribed as a minimum, give your insurance company a call and ask them if the cover is valid if you run that way -try it in Germany and you may end up in prison- Ya volle!

Of course if you drive well within the envelope then obviously you can get away with more. I have had my S4 on the Dubai Autodrome chasing a factory instructor in a twin turbo and giving him a very hard time in the bends [overtaking was not allowed]- he was utterly astonished at what my S4 was able to do on the track. My knowledge came from some of the best 928 brains in the world and I have the evidence to prove it.

On the other hand fitting tires to rims that are too narrow is [to my mind] most unwise [and that is putting it diplomatically]. One might get away with a 235x45 but if you do check with the specific tire manufacturer as to whether they agree with that fitment but do understand that you have a less than optimal setup NSR wise which to my mind defeats the object of driving a Porsche 928 in the first place.

Personally, I like my 928 to do what Porsche intended it to do [and a bit more] and to do it safely.

"Chance favours the prepared mind". Up to you to decide who knows what they are talking about.

Regards

Fred

Magnus Rostadmo

FredR !

Great info for the community and i could not agree more. You also put it so well sayin`
-"defeats the object of driving a Porsche 928 in the first place"-. I`ll add -"want a ricer-get a ricer"

I may be biased but i prefer these cars looks (mostly) to be "the way Porsche intended them to look"
I am no "originality police" and tweaking the looks somewhat can also be very nice. So i am using 10-spoke Panamera wheels on my S4 and some will say they ruin the classic look.
But each to its own talking about some hideous (in my mind) styling and such things that make me wonder how can someone have such bad taste and almost "rape" their cars the way they do.

But i don`t really care as long as safety is maintained so that themselves or no other gets hurt, or in worst case killed by technical modifications done by people that has little or no knowledge about what they are doing to their car. And even adviced "Ok to do so" by people similar or with even less knowledge.
Thanks for sharing :-)

Gary Knox

Fred,

Very good write-up regarding wheels/tires. Should be a 'sticky' all by itself for that purpose!! Well done.

bureau13

So is there a range of offsets that can be said to retain factory handling in all reasonable driving scenarios (range might be different for the street vs track). Or is anything other than dead-nuts on going to be considered a noticeable detriment to handling? I just don't see a huge number of wheels...especially in the more affordable categories...that use the factory 928 offsets. I personally have no plans to track the car, and I don't go around driving at 9/10 of the car's capability (which would probably exceed my capability as a driver by quite a bit) but I still don't want to degrade it significantly.

SMTCapeCod

I can certainly see posting to ask for peoples' experience with a particular setup.
And posting good tech info about the physics/basis for specific fitment recommendations. But
another broad wheel rehash seems all

It gets into expected use, and the usual recommendation to not dip below ET55 based on NSR, and then the subjective views and opinions about the range above and below that, right? The last part gets back to oil territory...