My car's high speed handling over about 160km doesnt seem right particularly on bumpy highways. Despite speed limits here, we can get away with quite high speed driving. Car bucks and weaves and generally feels unstable.

Replaced suspension with PSS10s and HR sway bars, and while we were at it replaced other suspect components (wishbones, control arms, bushing etc).

Net result, very expensive and zero change in the high speed handling. Steering rack was replaced a year ago. Only other suspect is I have 19s and are running 12s at the back with the recommended 315/25. I do wonder whether the tyres are a bit stretched perhaps leading to sidewall flex. Though I know guys running 12 inch rims with only 305s (not 996 Tt) who seem to have no problems

Any ideas on what else to look at before I go experimenting on spending more money on 19x11. Current setting for the PSS 10s and the sway bars (gather they are adjustable as well) are both ' medium' or so I'm told by my guy.

 

Did u have the car properly aligned and corner balanced?

If so, check your tire pressure. I went through that a while back , did everything and like an idiot I forgot to check my tire pressure. Adjusted to recommended pressure and it was that, and I'm running 19s with same tires sizes as you.

 

The 315 is the correct size for 12" rims so that's not the issue and they are definitely not stretched since 315 is the exact recommended width for that 12" rim. What kind of tires are they? Are they a high quality, high speed rated tire? What size front tires are you running? Were you already running the 19" wheels before the suspension changes?

I'd lean towards an alignment issue personally. I just had a very similar issue on my Turbo and it turned out to be the passenger rear camber adjustment which had come loose. The handling of the car was drastically changed and felt incredibly unstable.

On a side note, what's your goal with the sway bars? Typically people put them on to get more oversteer and dial out the understeer the cars have stock. The normal setup most people run is the middle hole in the rear and softest hole in the front. If you really want to go more towards oversteer, then set the rear to the stiffest hole and the front to the softest.

Last comment, if you haven't noticed a difference with the new sways and coilovers then something is definitely very wrong. I've only done my sway bars with Eibachs front and rear and those alone made a huge difference in the handling of the car, not only when cornering, but just in the general flatness and stability, especially at higher speeds.

 

Both, Thanks for feedback.. Am running Michelin PS 2 in recommended sizes 235/35 fronts and 315/25 rears. Front rims are 8.5 x 19. I earlier had a wrong size tyre combo and changed shortly after I bought the car and I 'think' the handling went downhill from there. My mechanic aligned and corner balanced after the recent change of suspension so hopefully he got it right ..will try changing sway bar settings and see where we go from there.

 

actually every tire manufacturer that makes a 315/30-18 tire has the rim width at 11.5 " max. i dont know why porsche put them on a 12" rim but it goes against what pirelli, michelin, falken, toyo, and so on recommend

 

PS2 will sit nice n square on 12 without stretch. Toyo and Nitto run narrow and will have a bit of stretch.

Start with tire pressure, uneven tire wear, slightly bent rims, move thru the suspension

 

after installing pss10's and sway bars, did you get car corner balanced and aligned? Also, when I had the rake checked on mine it was actually higher in the front than in back, so now the car's rake is more even and it is corner balanced. What you may be suffering from is more of a corner balance and a higher in the front than in rear situation which at high speeds destabilizes the car. It did on mine.

 

My 996tt with 19x11 and 315-25-19 (Advan Sport)and KW v3 ,handling is better and better as the speed goes up!!I have the factory Aero Kit,maybe this is the reason!

 

I had the same problem: My car was lowered and had some toe out in the rear. The road undulations which caused bounce made the toe out worse (similar to bump steer but in the rear)along with the tram-lining from the wide tires I wagged like a dog, and it was definitely scarey. I installed a thicker adjustable sway bar and it reduced the bounce and ALMOST eliminated the problem. The bounce before the sway bar install was very minimal, but at high speeds, it was accentuated enough to slow me down.

 

Typically if you have a decent alignment with a little bit of negative camber to it, your main source of loss of control happens when rebound is too weak, often in combination with compression being to hard. The bad behavior increases at speed (dramatically). PSS9 and 10 unfortunately have the worst possible valving bc it makes these cars worse except on 100% smooth racetracks, unless you are ready to run the shock settings at all times in a way that make your teeth fall out.

On 997, Porsche gave a solution to this, sport mode. It doesn't change the swaybar in the middle of driving, it doesn't change the tires, or the alignment. It tightens up the shocks on the rebound side, and in some cases a bit on compression too, giving better control.

Tighten your shocks rebound 3 clicks front and 2 clicks rear then try it (safely of course).

It will reduce your ride comfort, but if you liked the improved control, it's time to toss those shocks for something where you can adjust compression/rebound separately.

Also make sure you don't run higher tire pressures than recommended in the manual, possibly drop it a couple of psi if you know you have a hard sidewall.

Another note, heavier wheels will escalate the behavior (greater unsprung weight, makes it harder on the shocks to keep the car controlled) but at the relatively low speed you are trying to achieve it will have a minor impact unless you run extremely heavy wheels.

 

Recently I was going to try a high speed run on a very narrow 2 lane desert road without a shoulder and scrub growing right up to the pavement. I tried several times to get above 200 KM (125 MPH) but the poor road base and bumpy pavement really made the car rock and roll. It felt like if I went much faster the car may go out of control. I'm on fairly new tires and stock suspension. It felt too firm for the roughness. I have ridden motorcycles in excess of 150 MPH here several years ago without issues but with a shorter wheelbase and only 2 tires it was manageable, even with one hand on the bars.

On bumpy roads at speed, you may need to soften the spring rate and dampening both directions to keep the tires in contact with the pavement. Too stiff suspension will have you skipping over the bumps and make the car unstable. You will need a very sophisticated set of coilovers where spring preload, ride height, compression and rebound dampening are all independently adjustable. Rough roads may require light duty anti-sway bars as well. Ideally, you want the chassis to remain level and each tire to follow the uneven pavement without upsetting the opposite tire or axle.

 

I agree with MrWhite - stiffen the settings. I have KW V3's and after stiffening the compression and rebound as well as adding adjustable front droplinks the difference was amazing.

 

Dumb thought here.... But it could be that you're too low on the ride height settings and bumping on the internal bump stops of those PSS10's, so you've got no travel left on them. I have the H&R's and I had it set too low. I had a GT2 alignment spec on my car. I felt the car was extremely fidgety at high speed. I ended up raising my car about 1/4" all the way around (and got it re-aligned) and the handling improved dramatically. Raising the car put it back within the desired adjustment range specified by Bilstein/H&R.

If you start looking at the ride height and determine it is too low, I would raise it to within the adjustment specs and then be sure to get it aligned and corner balanced. Not corner balancing could definitely cause this problem.

Just something for you to take a look at as you're trying to diagnose the problem.

- Chris.

 

how much are you lowered?

My suspension is a 9ff based pss9 that does have higher spring rates than the avg pss9. Despite that, i think you should ensure youre not lowered too far and hitting bumpstops.

these cars get better the faster the speeds get...

 

All, many thanks for the fantastic advice and opinions. Have gone softer on the front roll bar, and cranked up the pss10s from 5 to 7. Feels better already but haven't done a high speed run yet so will try and feedback when I have. Thks again to all.