I've been driving my '07 GT3 on the roads for just a few months now. I have found that when I drive at moderate speeds (say, 40 mph) over certain types of road depressions with abrupt pavement transitions, the worst being sunken manhole covers, that I get a sharp, whacking noise from the front end of the car when the suspension hits bottom. By eye these road depressions don't seem to be more than a couple inches deep, so I find this kind of surprising. The result is pretty much the same from both the left and right front wheels. The car does not seem at all upset by the impact, but the noise does catch you off guard when it happens.

It sure sounds like the suspension is bottoming out. Do you guys consider this normal?

FWIW I made a rough measurement of suspension travel from its neutral position to full downward extension of the front wheels by raising the front end of the car so the wheels lifted off the ground. I measured a hair more than 2 inches of travel, which seems to suggest there isn't much margin against bottoming out. Does that sound right?

 

I have a 996 GT3 that had been fitted with aftermarket 19" wheels. Every highway divider, dip or other minor undulation resulted in the sound that I thought was bottoming out. However, there was no evidence of strikes on the underside. I switched out to stock 18" wheels and the problem is gone. I am still careful about drops or ledges and I would be very careful about a sunken manhole. These cars are low.

 

If switching to 18" wheels eliminates the problem, does this mean that on your 19" wheels, the tires are being compressed down to the rim when you take hits from road irregularities? I know that's how rims get damaged sometimes with very low profile tires, but so far my rims seem fine.

 

I believe the larger tires were hitting some part of the suspension. The 996 stock wheels are 18" in diameter. The 997 may have been designed with 19" wheels. The aftermarket 19" wheels on my car may just have been too big.

I would get the car in the air and see if you can identify any part of the underside that has kissed the ground. Failing that, you may wish to pull the wheels off and check for enounters on the outer or inner fineder and the suspension. Once you have identified the source of contact you should be able to come up with a solution, even if it means being a little more vigilant on city roads that are rough.

 

I don't believe you can tell much from this, since you were measuring from full extended to normal load bearing travel, when your interest is really from that point to fully compressed. You should be able to see the distance on the main strut shaft to the bump stop, which limits the compressed travel. But I don't know if you can see this with the wheel attached. If not, you will need to remove both front wheels, then lower the car so its full weight is carried on the suspension. Obviously, you will need to use a soft wood block or similar under each LCA so as not to damage it.

 

Check to see if your sway-bar is hitting your tie-rods (there will be a mark on the tie-rod if this is the issue). If so, soften the sway bay by one hole or change your ride height.

 

Do you have the PCCB floating rotors ?

I had a similar issue at low speed with my previous 997 GT3 and I was told that this noise comes from the floating rotor.

 

I had the same problem with my 07 GT3 and took it into the dealership for diagnosis.

My front shock was bad and they replaced it under warranty. After the replacement, it was very quiet and smooth.

4,000 miles later, I'm hearing the same sound. Now, both front and rear. Guess I need to make another trip to the dealership.

Seung

 

Do you mean floating calipers? I have the steel rotors.

I've looked at things inside the wheel well and under the car. There's no evidence of anything hitting. But I'll look again specifically for this.

I was wondering myself if this could be the problem. If it is, then I think the shocks are bad on both sides because I get the whacking noises from both the left and right sides.

 

put a tie wrap on your strut shaft and go drive...if its at the top when you are done its bottoming out and you have your answer..... if not, its something else.

 

Excellent idea! I'll try that.

 

Some followup on this...

I was not able to put a tie wrap on the strut shaft to measure suspension movement because the entire damper, including shaft, is enclosed in a flexible rubber housing with nothing exposed.

I ended up taking the car to the dealer (Porsche of Nashua NH) to be checked out. The tech there believes the suspension is hitting the stops (and they are hard stops) simply because of the limited amount of travel that's available. It was quite revealing to see how little wheel drop there was on the GT3 while it was on the lift compared to another 911 that was on a nearby lift. The regular 911 must have had 4-5 inches of wheel drop compared to about 2 inches on the GT3.

The tech also stated that the dampers tend to be noisy in cold weather and it is cold here, with typical driving temperatures recently in the teens and 20's (F). He said my GT3 was the first one he ever saw with winter tires on it. Yes, I am driving it through the winter, but only when the roads are clear of snow and ice. We'll see what happens in the spring...

 

Which winter tires did you go with and what are your impressions so far?

 

I've got Pirelli 240 Sottozero Serie II. So far they have been fine in the cold (temperatures as cold as 10F). I did take them out on packed snow once to see what it was like, and the GT3 had remarkably good traction (at least as good as my Saab 9-5 on its winter tires).