I have been reading inside and outside of the forum for weeks before typing this.
I rounded it down to 3 setups and would like to know if any makes any sense.

EXECUTIVE SUMMARY
I want more grip front -> less understeer so I am considering custom made wheels done by Augmented Wheels.
They would be 17" or 18" wheels, factory ET , front/rear width depends on the setup
- 245 on all corners - fully avoid matching total diameter issues
- 245 front 255 rear - 24.7" and 25" not ideal but is it that bad?
- 255 on all corners - probably 9" front 10" rear would allow me to mount larger tires in the back if/when the car will add power

Would love to get the opinion of the knowledgeable guys that tried it in real life.


LONG VERSION

CAR: 1989 S4 Manual Silver/Burgundy originally from Switzerland, fully stock for now, bought it mid July I drove it 2000 miles between July and October.
The car just crossed 100k total, I drive her only in the week ends/holidays for family trips or personal joyrides on highway and hills/mountain roads, I avoid driving town traffic or short distances.

PLACE: Oslo and southern Norway. 8 months without snow or ice, hot but can rain multiple times a day -> tires with a rain A rating are preferable.

CONTEXT: The Continental SportContact2 now on the car have barely 2600 mile but will be 5 years old in spring.
Summer tires in 245/45/16 only 6 options, of which A rated only 2 options
ZERO options for having the same product in 225/50 front and 245/45 rear.
I would have to mix brands or designs, like Pirelli Rosso Asimmetrico in the rear 600$ a piece, and some other Pirelli up front.

CAR CONDITIONS and IDEAL PROJECT DEVELOPMENT
- Just Done: bilstein all around, timing belt, fuel pump, A/C
- Almost done: CE panel refresh waiting for stabilant... Led lights, refoamed speakers because I love the 80s, DAB radio adapter and antenna because family trips, black anodised aluminium pedals because downshifting

Further development depends on the end of lockdown
- 2021: Intake refresh, MAF/Brains checkup, Hans short shifter, engine mounts, wheels, paint correction and sealing
- 2022: Headers, original or new cats, xpipe, Porken chip
- 3/5 years: Mild Stroker for street only purpose and all the fine tuning that comes with it

This for completeness but lets stay grounded - in this thread I would like to tackle the rubber issue

I favor the function over looks approach, but for those into design, my ideal wheels would be a custom augmented version of in order:
Club Sport - not S4 model correct but they are just really nice and should look in any size
Manhole - period correct and make the car look solid like a tank but might look huge in 18"
D90 - not ideal but Augmented has already made them which simplifies the whole process

Thank you for your patience

 

Most race 928's run square tire setups....I did on mine.... 18x10x65mm offset is a common Porsche wheel that I ran, just took a touch of fender rolling to clear 275 tires all around. If you want to run a smaller tire like 265-255mm you could find a narrower wheel too.

The single biggest perk of the big front tire is way more front end grip and braking. I did have a set of 18x8 front wheels that I used on occasion with yokohama slicks in 235 up front....it was fine with the 275 rear if you didn't push the pace...it would understeer at the limit braking was weaker...

I always setup my racers to be right on the edge of loose.....since I HATE understeer

 

This is the other reason for my question, more grip and better braking = more safety and I guess feel more confident in oversteering.

I am by no means a pilot, just by my house 4 blocks away, there is a 3 lane wide roundabout with an upgoing exit and straight on a bridge, followed by another fully flat roundabout. Completely empty road at night so every time i can, on warm tires I test the grip, do a full circle at limit around the roundabout or push full throttle right before having to turn right for exiting on the long straight that follows.

The feeling is that at limit I lose the front first every time, for sure on wet conditions. I tried to suddenly close the gas but the back stays planted, if I keep the gas opened the steering wheel gets very light and the car goes straight.

One thing i noticed after having all suspensions changed, alignment and ride height checked, is that the car at times fells like it tramlines

 

[Edit: Nevermind! It was a typo.]

Re: tramlining - what wheels do you have on there now? It sounds like you might have wheels with too little offset up front (65 is ideal, 60 might be OK. Also depends on who you ask ).

 

This sounds like a weight transfer issue and not a tire issue. Even so I'd strongly suggest Michelin Pilots. 4S or Cup2's depending on what's appropriate for your usage case.

Consider cutting 1 coil off your front springs and 2 coils off the rear springs. That should allow the whole car to feel more planted and drastically shorten weight transfer time which is what I think your are feeling when you say the "wheel gets light" I believe the rear springs on the 928 are way too long and soft.
You also want to check your bushings. Take up or flex in the rack bushing and upper control arms can cause vagueness on transitions.

Anyone know what the rebound is like on the Bilstein's compared to compression? I stiffened the rebound on my Koni's and modified the springs... the transitions were drastically improved.

I also hate the lower control arm bushing arrangement on a 928... The power flex move freely and let the suspension do what it should be doing... reacting quickly and that translates to more feel in transitions also.

 

It was a typo it is supposed to be 24.7", thanks for pointing it out so i corrected it.

Car has D90s 7.5" front 225/50/16 and 9" rear 245/45/16 running Continental Sportcontact2 with only 2600 miles but 5 years old

 

Ah, thank goodness!

Nice stock setup, so you are good there. Continental states to replace tires at 10 years even if no visible problems, so unless you are in some super harsh climate (or have been enjoying the roundabout daily), you are probably fine there too.

I'm not racing, but I like having the same wheel size all around to simplify tire size selection for both correctness and looks, and allows for interchange (the rears wear out faster for me... ). As you probably know, early 928's up to '86 had the same size wheels and tires all around.

 

Your 928 is incorrectly aligned. Either:

Your 928 was aligned before the suspension was settled(*).
Alignment was simply incorrect.

A 928, correctly aligned, on OE wheels, OE-sized tires, mostly-correct tire pressures does not tramline.

(*) A 928 cannot be lifted before or during alignment.

 

What tyre pressures are you running front/rear?

 

What you describe above is classic understeer- no surprise given you are running stock wheels/rubber. If you are not a "pilot" then the last thing you need is oversteer. Porsche designed the car with natural understeer for a reason- to stop the average punter going through hedge rows *** first! Early 911's were infamous for this. Try fitting new front tyres with old knackered rear ones and see what happens in a bend if you come off the throttle- chronic lift off oversteer- BTDT! The 928 handling characteristics give the driver a lot of feedback compared to say a 911 or so I found- in part that is what makes it such a safe car to drive. As you get close to the limit you can feel things getting lighter

In your case you could simply try stickier rubber, especially on the front wheels- given your low mileage the wear rate would probably not be relevant. Such tyres in my experience age much faster though. If you do go for wider wheels/rubber then you need to ensure the car retains a small amount of understeer or is neutral [as I aim for]. Having an oversteering car on public roads is not the best of ideas for the average driver/enthusiast.

When you fit wider rubber up front it is a game changer. The impact appears to be much more profound on the front wheel grip level.

 

Good tires always help but something is wrong with the OP's setup.
No such thing as 'classic' understeer. There are generally 3 types of understeer and the solutions or remedies are different.

1. Corner Entry Understeer (aka transitional understeer)
2. Mid Corner understeer
3. Corner Exit understeer (has a transitional component)

If you have #1 you have a weight transfer issue which is clearly what is going on with the OP becasue he has stiff shocks and soft springs, perhaps OE bushings... AKA bad setup. The remedy is the right combo of springs, shocks and sway bars, bushings and tires last. The tell that the issue described is transitional is the OP indicated the use of throttle affects and indicated the transitional or dynamic nature of this problem by saying "right before having to turn right"

2. Is generally Tires or grip when the setup is settled (corner weight balance). Still can be tuned to balance out the load front to rear. Grip is a function of load forces placed on the tire. Camber plays a major roll here, Too much load and you slide, too little load...you slide that axle. The 928 gains camber as the corner loads and that is a great aspect. If the suspension is very stiff you wont get as much camber gain so more static camber is needed to solve mid corner understeer on a 928 in this case.

3. Is generally considered a drift and sometimes useful for fast laptimes and usually managed with throttle to transfer weight off the front and on to the back which loads the rear (grip) and unloads the front (loss of grip, understeer) This is the most tricky dynamic problem to mange for a driver, To much throttle (rwd) you rear grip goes to Zero and a spin occurs but get it right and the car moves well through the end of the turn. This is where throttle response is super important, the kind of throttle response you can only get with a manual transmission with the engine around its torque peak. Electric cars really shine here, Think Tycan

The time between these three phases is impacted by speed and the ability for the suspension component to react quickly. Suspension compliance in the form of bushing materials and type, tire sidewall construction, sway bars, and spring / damper rates play the major roll here, For the 928 the front lower control arm busing design is the slowest reacting bushing and a change there offers a very noticeable improvement in dynamic performance.

Over all the 928 front suspension bushing design needs upgrades and is mismatched against todays tire technology, but the aftermarket has resolved these issues, .Lower control arm is stiff and slow to react. Upper control arm is fast and too lose . Rack bushings are just mush. Aftermarket upgrades in these three areas transform the 928 and when used with a quality UHP tire / wheel of the right size and offset, ride quality is good.

 

Another Norwegian 928 owner here. My 80 S has about the same setup as yours, except ET of 52 on the front. I fitted a kit called a rear swaybar stereoid kit. It moves the attack point of the rear sway bar further in. This made my car understeer much less.

My car was set up for the same tire width from factory, it may be that the S4 has a stiffer rear sway bar from the factory.

 

This is exactly what i noticed in my roundabout testing.
At first I was afraid of loosing the back by pushing the throttle more, but it was soon clear the back has plenty of grip and would just not let go.
The big issue would be the front suddenly regaining grip before you have the time to straighten the steering and to turn right to exit the roundabout, because that would suddenly transfer all the weight to the opposite side in a difficult to control way.

It feels like once you decide to open the gas and transfer more weight back, you have to keep it opened while steering the car in the exit direction. To me that feels a bit more scary than losing the rear and having to counter steer, especially since the engine is in the mid-front unlike the 911.

 

Which part of Norway if I may ask? I am actually Italian but to Oslo in 2014

 

edit wrong quote