Suspension tuning, tips, and suggestions

Howdy everyone -- This forum has been very helpful over the years and I am back with another "Request for Advice" - I have a 2003 C4S with the Bilstein B16 pSS10 kit from FSI on it. Absolutely love it and the stance that my local shop was able to get it to when they set it up. There are a few things I would like to address with the help of the rennlist mind. 1-- when I go over bumps, box culverts, etc (at speed) the car tends to bottom out with the tires impacting the wheel wells. The car has HRE 19" wheels + PS4s on it so they sit outboard more and are catching the lip. I would think that I should move the spacers "up" more, or compress the spring more. I think the car needs "more" suspension, rather than less. Any insight on that is greatly appreciated. 2 -- At speed the car feels kind of floaty in the front. Over 100mph it feels like the front wants to catch too much air. Seems like if I stiffen the rear suspension it might solve both of these problems. Any other suspension bits and pieces I should be considering? I dont want a racecar but being a car guy, cant help myself but enjoy tinkering. Are the front strut braces between the shock towers useful at all? Will it keep me from putting my groceries in the frunk? Will it lower body roll in the car so that my groceries dont tip over when I am racing home from the store? TIA!

Your rear wheels probably are not the correct offset. Your 4S has the Turbo aero and it definitely should not be floaty even at 150 mph. Front floaty could be a hundred different things... Too high tire pressure Too low tire pressure Front toe out Not enough rear toe in (my guess) Ride height too high in front or too low in rear Not enough rebound damping in front Very excessive front camber Wrong tire sizes messing with the AWD Post up the alignment sheet if you have it.

What tyre size on those wheels? Your overall (rear) tyre diameter is most probably too tall, trying to fix that by increasing rear ride height is the wrong measure. What wheel dimensions and offsets? What spacer thickness, if fitted. Was a wheel alignment done front and rear when the coil-overs were installed? Strut bar is typically e only worthwhile if it is in the way for any practical use of the frunk. Strut bar will not lower body roll (for that you'd have to get stiffer sway bars...). Nothing but cornering slower will prevent your groceries from tipping over unless you hang m off the strut bar. Then they'll swing... That shop is useless if they sent you on your merry way with such a crap handling sports car, unbelievable.

Another thing to think about are the settings on the PSS10s. I agree with you, they are excellent coilovers. I run PSS9s. Too stiff of a front and too soft rear can cause a floatiness feeling and understeer. Same with sway bars.

Quotes source posts: 20120540, 20121614

Thanks for all of the input, all. I will do some investigating with the alignment. The rubbing on the tires makes sense given they are probably more "outboard" than the OEM products. I have the HRE Flow Form FF01's -- https://www.hrewheels.com/wheels/hre-flowform/ff01 in 19" wheels, 8.5 on the front (High Offset) and 11 on the rear (Medium Offset). No spacers. I have 305/30 ZR19 on the back. 245/35 zr 19 on front. Logical steps for figuring out how to dial in the handling a bit more: Step one seems like monkeying with the air pressure and seeing how it feels. Cost effective and easy. What PSI would make the most sense? I generally have this set at 35PSI Step two would likely be to get the car on a rack. Getting the alignment sheet may provide a lot of clues. I had instructed the shop to get the stance lower, so any ill prep of the car was at my direction. They did have it aligned after coil-overs installed. I do have a lot of faith in the shop as one of the regions pre-eminent Porsche shops. like the look with as little of wheel well gap as possible, but will certainly give that up in exchange for getting the handling dialed perfectly.

Lowering the car can change rear toe a lot more than the stock eccentric adjusters can handle. If that be the problem the answer is adjustable rear toe links.

Your tyre outside diameters Fr and Re are much bigger than the biggest factory option tyre sizes. That opens a whole can of worms regarding roll center heights front and rear, greater ride height (as in: more distance between road surface and underbody) that you can not dial out without sacrificing other aspects of good handling. You gotta choose: go for the looks and live with compromised handling or go for convincing handling and live with a look that is 'nothing special'.

Those are pretty big.

19s are not ideal on a 996! Plus the extra width and offset are possibly half the problem here/ A lot of us have 18's and run the GT3 sizes 295/235 All mid/rear have a little lightness to the front, but a little rake is good for that (rear higher than the front). Also perhaps just roll your guards to help with the rubbing. Really low is not always best, mine was dumped super low and it was just painful, very little suspension travel and just not dynamically good.

He's got a C4S which runs Turbo tire sizes. I run 245/40-18 front and 305/35-18 rear. I have 19 inch wheels on my other Turbo though I don't remember what sizes the tires are.

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Offset is key then.

According to tiresize.com your rears are more than an inch taller than 295/30-18. Your fronts are fairly close to stock. So you have about 1/2 inch of rake. That extra height may have something to do with the handling. But the loose front end has me thinking you have something else happening up there. Did you change the chin spoiler from stock to something else.? I think you are pretty close on revs/mile to set off the pasm.

I ran the Ruger�s that were on the car when I bought it for 100K+ miles of its time. Don�t know the offsets of hand. Never had any drivability issues. Always ran 225/40/19-F and 295/30/19-R. Stock height and lowered on B8�s and vogtland springs, all components are adjustable except coffin and caster arms. No rubber bushings. Only ever a minor rub on the front fender liners, and of the three different brands of tires only one rubbed the rear lip when new. Switched back to the stock twists two years ago. Prefer the ride on the 18�s overall. 18� e88�s or LM�s would be perfect

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I disagree, your car looks perfect with those Turbo Twists. That�s beautiful.

Quotes source posts: 20191965, 20192126, 20192240, 20193465, 20193692

I'll add that to my list of things to investigate. That is some great insight. These are the type of things I would never have known if not for the forum. Thank you. I never actually really thought that much about this. Very valid and interesting point though. I am considering the tradeoff. For goofing around my locale, the looks of the larger wheels is more fun. For longer trips I suspect I'll be throwing the factory twists on and going to convincing handling. This is where I am at. I may have dumped too much out of the suspension in pursuit of "low" -- a huge draw to these coilovers was the ability to adjust them and try to add a bit more suspension in. I'd like to land in the happy land of a bit more ride and a small bit of trade off of looks. No change to the chin spoiler or body modifications for that matter. No PASM on this one. I am thinking about putting the stock wheels back on and doing the tuning, then seeing what happens when I go to the larger variety. Always an adventure. Thank you, everyone for the very appreciated and valuable input!