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928 Wheel offsets

 
Old 12-28-2018, 08:11 AM
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Jaybrandnew
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Default 928 Wheel offsets

Hey guys.
First time poster, long time avid reader. I am the proud owner of a 1990 s4. I am based in Edinburgh, Scotland. I have begun what I hope to be a very comprehensive restoration of the car, both mechanically and cosmetically. I want to say at this point I have little to no mechanical knowledge.

My question is this. I have been offered a lovely set of 18" turbo twists at a great price I would very much like to use on my car. The problem is the offsets. As I understand it. the s4 offset is 65mm all round? The wheels I have been offered are ET65 at the rear, but only ET52 at the front. I have been getting a lot of conflicting views on this. With some saying they have been using a similar set up for years with no problems, to others saying it is outright dangerous, and many others somewhere in between.

What are you guys take on this? Is it do-able? What are potential problems? Some have mentioned machining a small amount off inside of wheel to increase offset? Is anyone using a similar set up? Will increasing tyre width in any way negate wrong offset potential issues. Any input would be much appreciated.

Thanks very much
James
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Old 12-28-2018, 09:42 AM
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Adamant1971
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ET 52 will work, but it moves the wheel out by 13mm. Any time you stray from the factory geometry it will have an effect on handling.

I used to run 18 x 8 @ Et 57. When I went back to 17 x 8 @ Et 65 I noticed the car handled much better. But Iím an aggressive driver and like to hammer it in the twistes.

So allot depends on how you drive the car and what you expect from it.

My .02 cents
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Old 12-28-2018, 10:40 AM
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James,

The wheels you have been offered are typical offset values for Porsche sports cars like the Boxster and 911 families. Plenty has been written on the list regarding this topic.

The reason for the ET65 up front is to create a negative scrub radius of 10mm. With ET52 you would have a positive scrub radius of 3mm. The significance of this is that on the track a positive scrub radius can be favourable to performance. The worst case scenario is probably an ET55 giving zero scrub radius which is likely to create a tendency to squirm [tramline]. The main benefit of the NSR is that the steering geometry causes a safe condition if you hit standing water on one side [i.e. the near side]- in such a scenario the steering naturally pulls straight ahead. Without the NSR the car in such a scenario will pull to one side as a yaw effect is created and the car will tend to veer to instability. Hitting such a scenario is rare so not at all surprising most have never experienced it and going sideways at 100 mph or whatever is no time to find out Porsche knew what they were doing. On the track such a scenario is not likely to happen but on British roads very likely I suspect.
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Old 12-28-2018, 10:45 AM
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Originally Posted by Adamant1971 View Post
I used to run 18 x 8 @ Et 57. When I went back to 17 x 8 @ Et 65 I noticed the car handled much better. But Iím an aggressive driver and like to hammer it in the twistes.
Not trying to be nit-picky but the wheel size changed and so did the tire, so the effect on handling was not all in the wheel offset. The only way to do a proper apples to apples comparison is two wheels of the exact same size (but different offset) with the same tires. Even then, if the wheels are not the same weight......
This is why this discussion is so controversial, many variable to consider.

If anyone wants to learn about the specific effects this has on the suspension, do a google search on scrub radius and wheel offset.

This past summer for no specific reason, I was playing with different wheels on my 87 and 79. Two of these wheels have very new tires on them, I'll stick to those in this thread
One set phone dials with 245 / 255 tires, ET 65 with BFG G-Force Sport Comp II's
The other set C-III's with 235 / 285 tires, et 57 and 58 with Sumitomo HTR Z III

Bottom line, the C-III's with the Sumitomo handled better. The steering feel was much more direct and the car just overall felt better planted.
Was this due to the difference in offset? Doubt it.
If I had the exact same wheel with an et65 would it handle even better? Probably.
In my opinion the significant change in sidewall and a possible more aggressive tire on the 18's is what I attributed it to. The BFG is a specific tire marketed to the "Retro Rod" tuner crowd, may not be as sticky of an overall tire as the HTR Z III's

I also have a set of 17" et52 and et55 wheels I've run on my cars in the past. Never experienced anything negative with regards to handling with those either, even on the track.

The most noticeable aspect to changing the offset I noticed is the desire for the car to "self center" when coming out of a corner or going straight down the highway. The "correct" offset this will feel normal, as you increase the offset it felt like I had to put in a bit more steering effort to bring things to center. Caster also effects this, but I'm pretty sure over the years my cars had the correct alignment specs this was not a factor.


Side Note:
Just remember Georgen Suenned has 18" et57 wheels out front while running in the open road races at over 210mph's without issue.
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Old 12-28-2018, 10:53 AM
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Fantastic info guys thank you very much, I had no idea about the issue hitting water on one side. I thought the issues were contained to wheel bearing stress and change in handling.

I certainly dont hammer my car, but I absolutely need it to be safe. I guess Im going to have to give up the 18" Turbo Twist idea. It is a pity as the rears are readily available in ET65. I am guessing rears would be too wide to fit on front were I to buy them individually to make up a set?

Thanks again I appreciate your time
James

Last edited by Jaybrandnew; 12-28-2018 at 11:19 AM.
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Old 12-28-2018, 11:27 AM
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Thank you Hacker. And this is exactly why Im confused. It seems to split opinion right down the middle.
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Old 12-28-2018, 11:37 AM
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Originally Posted by Jaybrandnew View Post
Fantastic info guys thank you very much, I had no idea about the issue hitting water on one side. I thought the issues were contained to wheel bearing stress and change in handling.

I certainly dont hammer my car, but I absolutely need it to be safe. I guess Im going to have to give up the 18" Turbo Twist idea. It is a pity as the rears are readily available in ET65. I am guessing rears would be too wide to fit on front were I to buy them individually to make up a set?

Thanks again I appreciate your time
James
I have been running those exact wheels on my 87 for the past 4 years. I also have a set of Cup I wheels in correct offset but they are the 17 inch wheel. I ran those for about 3 weeks while the Turbos were being refinished. I can honestly say that in normal daily driving I didnt feel any significant difference other than ride quality, as the 18s provide for a bit stiffer ride. I'm sure in more aggressive driving it would be much more noticeable but the daily commute and occasional day trip to the mountains they are fine.

if your looking for a more modern wheel in 18 inch then I suggest you check out the Panamera wheels, fronts are ET63 much closer to original. I have both the 5 spoke and 10 spoke on 2 of my 4 928s and they give you a much bigger tire selection.
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Old 12-28-2018, 11:56 AM
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Originally Posted by Hacker-Pschorr View Post
Not trying to be nit-picky but the wheel size changed and so did the tire, so the effect on handling was not all in the wheel offset. The only way to do a proper apples to apples comparison is two wheels of the exact same size (but different offset) with the same tires. Even then, if the wheels are not the same weight......
This is why this discussion is so controversial, many variable to consider.

If anyone wants to learn about the specific effects this has on the suspension, do a google search on scrub radius and wheel offset.

This past summer for no specific reason, I was playing with different wheels on my 87 and 79. Two of these wheels have very new tires on them, I'll stick to those in this thread
One set phone dials with 245 / 255 tires, ET 65 with BFG G-Force Sport Comp II's
The other set C-III's with 235 / 285 tires, et 57 and 58 with Sumitomo HTR Z III

Bottom line, the C-III's with the Sumitomo handled better. The steering feel was much more direct and the car just overall felt better planted.
Was this due to the difference in offset? Doubt it.
If I had the exact same wheel with an et65 would it handle even better? Probably.
In my opinion the significant change in sidewall and a possible more aggressive tire on the 18's is what I attributed it to. The BFG is a specific tire marketed to the "Retro Rod" tuner crowd, may not be as sticky of an overall tire as the HTR Z III's

I also have a set of 17" et52 and et55 wheels I've run on my cars in the past. Never experienced anything negative with regards to handling with those either, even on the track.

The most noticeable aspect to changing the offset I noticed is the desire for the car to "self center" when coming out of a corner or going straight down the highway. The "correct" offset this will feel normal, as you increase the offset it felt like I had to put in a bit more steering effort to bring things to center. Caster also effects this, but I'm pretty sure over the years my cars had the correct alignment specs this was not a factor.


Side Note:
Just remember Georgen Suenned has 18" et57 wheels out front while running in the open road races at over 210mph's without issue.
Yes, there are many variables. Rim width also changes geometry signifactly. The other issue not often written about is that different brands of tires have different tread widths in the same sizes.

For example a portenza is a very square tire with a wider tread width as compared to the same size conti.

When I get home I will post a vector drawing I made overlaying various tire sizes and rim sizes. In the end if you can mathematically keep the centre line of the tire as close to the centre of the stock wheel you will maintain the geometry.

In the end Iím in the camp of stay with how Porsche designed the car for the offsets. Many are happy to go outside the factory specs and everything is subjective based on driver skill, roads, tires, and all the other parts of the suspension that may be perfect or may be worn.

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Old 12-28-2018, 12:01 PM
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Im very grateful guys, thank you. Do any of you know what would be the maximum width of rim I could safely get away with on the front? Just to see what my options are. The Panamera alloys is a great idea, thank you.
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Old 12-28-2018, 02:59 PM
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IIRC the Panamera 18s are ET 59 not 63 up front, but regardless they work great on my S4. Without spending big bucks they're the best match.
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Old 12-28-2018, 03:31 PM
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Originally Posted by chart928s4 View Post
IIRC the Panamera 18s are ET 59 not 63 up front, but regardless they work great on my S4. Without spending big bucks they're the best match.
Correct they are ET59. I have a set that will be for sale in the spring once I dig them out of winter storage.
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Old 12-28-2018, 06:48 PM
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Originally Posted by Jaybrandnew View Post
Im very grateful guys, thank you. Do any of you know what would be the maximum width of rim I could safely get away with on the front?
You can get a 10" with a 65mm offset up front, and then run a 265 tire. It is the way to go.
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Old 12-29-2018, 09:38 AM
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On my 79, I'm running an 18x8 - offset 57 with Nitto Invo 235/40-18.

Going any further out (lower et number or wider tire), you run the risk of having the tire contact the fender if in a higher speed turn with a mid corner bump, unless at the high stock ride height - ask me how I know. ..

Had Carrera LWs on the car when I bought it with the low 50s 911 front offsets and caught the fender lip with the tire. No damage to the fender, just the lip. Moved to these wheels with a slightly higher offset number and rolled the fender lips and no issues since.







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Old 12-29-2018, 10:46 AM
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Originally Posted by Petza914 View Post
The ride-height and stance on this car looks very good!
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Old 12-29-2018, 11:51 AM
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I just impulsively purchased four D90 16 x 9, 5 x130 ET60 (rear) wheels that are chrome and need a bit of love.
The part number appears to be: 928 362 119 30?

To me they look like Clubsports, but I thought I'd ask the experts here if they are the real thing and worth restoring?




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