Recently, during a somewhat spirited drive on twisty country roads the front end kind of my Carrera S felt like it was skipping across the slightly uneven though otherwise smooth pavement. Is this normal?

 

Lori, were you in PASM sport?

The front end of these cars can get kinda light at times... just my observation - and my C4S has a hundred or so add'l pounds up front, too.

 

Is that primarily where the awd adds weight? Having the front end feel light would be a new one for me - did the car feel it was really losing traction?

 

Were you on the brakes, the gas or just going a bit too fast?

Without knowing much about the circumstances you do state it was a “spirited drive on twisty mountain roads” my guess would be the slightly uneven surface, twisty roads (lots of turning going on) and spirited driving caused the front tires to have more slip angle than the rears. In other words, you experienced understeer.

 

Sounds like it could be classic understeer which, in these cars, is usually experienced while on throttle in the turn, and a gently lift should help get some front traction and rotation started. Also, the PASM sport setting will definitely result in some nervousness over uneven pavement. It is a bit over-damped for rough surfaces. The car suits me better in the normal mode on all but the very smoothest roads.

 

front end can go light when there is an elevation change during a turn/curve.

 

Thanks for all the replys. I was in PASM with sport on and steady on the gas as I entered the turn with more throttle about midway through. Isn't it a no-no to lift the gas pedal while in a turn in a Carrera? How do you compensate for understeer?

 

If you are experiencing understeer, you need to shift the weight of the car to the front, which can be accomplished by either lifting your foot off of the gas, or lightly stepping on the brake pedal. The idea is to provide greater traction to the front wheels by shifting more weight to the front. If you step on the accelerator, the weight shifts to the back of the car, thereby promoting understeer.

 

That trait is widely exagerated in these cars. If you jump off the throttle in mid turn, any car can get loose, and the 911 is certainly more prone to it than a front-heavy car. But, everything is relative, and if the car is understeering, you have to get more traction up front, so lifting is appropriate. Just be moderate with it while you're getting used to it. And try the same situation again with PASM in normal mode. I think it's very possible the over-damping was causing much of what you felt.

 

The "no lift in a turn" rule barely applies to any of the 911's built in the 80's and is irrelevant to any of the cars built since then. Porsche made all sorts of improvements to the 911 that effectively dialed out that problem that affected only the early cars.

And if that's not enough, remember that you have PSM, which actively corrects traction problems. I have tried as hard as I can to induce oversteer with PSM active and I can't. PSM simply won't allow it.

How do you compensate for understeer? Several ways, but the easiest is probably with tire pressure. Increasing front tire pressure and/or reducing rear tire pressure both will result in less understeer.

 

Lori all Porsche's come aligned (or set up) from the factory with understeer programed in. Their reasoning is it will keep you from driving too aggressively and perhaps crashing which is one of Porsche greatest fears. They've had lots of litigations over the years, especially with the early 930's so they've set them with understeer to keep you from driving too fast (if you can't get the car to turm-in ,then you can't possiby get it to oversteer and possibly spin out).

The cure is relitively simple, have the car re-aligned with as mmuch negative camber as possible. The difference in handling is significant....

911 Dave: Please explain how adding pressure to the front tires help cure understeer. I don't understand that theory....

PS: Lifting mid-turn or just after turn-in is a totally acceptable way to rotate a Porsche and simultaneously get the front end to bite. You should try some seat time on a skid pad or an empty parking lot. Stick it in 2nd. gear, turn off the PSM, and just use the accelerator to push the rear end out as you drive in a circle....

 

I don't completely understand the physics myself, but increasing tire pressure increases its load capacity, which affords it better shape retention on hard cornering, as well as crisper turn-in.

If you do a web search on understeer, you'll find countless references to using tire pressure to increase/decrease understeer or oversteer.

Here's an article at Tire Rack

Here's a list of handling corrections

Here's the most concise explanation I found:

"Using tire pressure to eliminate oversteer and understeer by balancing slip angles between front and rear:
Slip angle is the difference between the vector drawn thru where a tire is aimed and the vector drawn thru its actual direction of travel. Slip angle is determined by how much your contact patch distorts in cornering and you can change that by changing tire pressure. Increasing pressure will reduce contact patch distortion and reduce slip angle.
Neutral steer occurs when slip angles are balanced front to rear.
Oversteer occurs when rear slip angles exceed front slip angles.
Understeer occurs when front slip angles exceed rear slip angles.
Balance between front and rear is what matters, so you can reduce oversteer by increasing rear pressure OR decreasing front pressure and you can reduce understeer by increasing front tire pressure OR decreasing rear tire pressure."

And you are quite correct that 911s (and indeed all cars) come from the factory set up for understeer, because it's much safer that way. There is a huge difference in recommended front/rear tire pressures listed in the owners' manual. They did that to cause the car to understeer. I've always found better handling in my 911s is achieved by roughly halving that difference.

 

911Dave,
what pressures f/r are you running for spirited driving? If I were to go from my current 33/39 cold to 35/38 would that make the rear wheels significantly more vulnerable to damage from bad roads? I live in ny/ct, thus my concern.

 

erv, I grew up in rural CT so I know exactly what you're talking about. Worst roads I've ever seen. I don't think 1 lb. of reduced pressure will increase your risk of wheel damage by any considerable amount.

For pressures, my 86 Carrera's OM calls for 29/36 but I've found 33/36 to be about optimal on the street, running 225/50/16 on 7" rims and 245/45/16 on 9" aligned with some negative camber. On the 997, I've been running 35/39 on stock wheels/tires and factory alignment but I only have about 1,000 miles on it so far. 36 might be a little better in the front, but it will take me longer to find out than in the 86. With the 86, I can reach the handling limits a lot easier than I can in the 07! And I don't track the 07.

 

If you like sport on try putting shocks in the softer of the 2 settings (shocks off). Also slight trail braking is necessary to keep the weight forward during turn in. Do not get on gas hard until steering wheel is straightened out.

If MJones was around I think he might suggest PDE.