GrantG

I assume part of the steel tub...

Airbag997

981/GT4 has same issue. Couldn't find 991, but assume same. Looks aluminum to me? Steel and or steel composite should not fatigue like the pictures shown...

kyrocks

GrantG

Thanks! I guess I'm just used to seeing that part on my old 911 being a continuous part of the steel unibody...

Petevb

It's an aluminum casting. The newer gen cars have minimal jounce travel- not just before you hit the bump stops but in general. Combine that with a very stiff chassis and they are sensitive to ride height. Lowering the car dramatically increases the loads the parts see over bumps.

Based on the numbers I'd run previously I suspect this isn't fatigue, it's the result of bottoming the suspension out. Lowered ride height may have been a contributing factor.

johnsopa

Stiffer spring rates will reduce the chance of this failure happening.

maspirito

Solution on a C6 Z06
This is a build for the track that will have extreme down force and we were concerned about tub failure with the aluminum frame Z06

Larry Cable

great info Pete, then your recommendation would be not to lower a car significantly, and presumably do not reduce spring rates - but perhaps increase them - I guess my suggestion of a strut brace would be ineffectual in this scenario since it is the "brisance" of the energy transfer thru the fully compressed suspension that is the root cause - although there might be some bracing effect I guess?

Petevb

Yep, that's my read. Reducing the ride height with smaller OD wheels is fine, which is what many guys running 19" hoosiers do out of necessity. Lowering the spring perch on the other hand is where I get nervous, and I know that was a contributing factor in at least some of the GT4 failures. Personally I'm not lowering my car much in part due to this issue, also the change in when the bump stops come into play. The elastomer bump stops are very much part of the spring package on a modern Porsche; they create a highly progressive rate which in turn changes behavior on turn-in and over bumps. If you're tuning purely for speed on a smooth course (ie autocross) you can lower the car to ride on the stops and get more spring rate even in a stock class, however doing so brings significant risk if you do find a bump...

ipse dixit

Ouch.

evilfij

Thinker23

James88

I'm sure others will take your allocation in a heartbeat.

Rac10000

This looks pretty much identical to the issue a few GT4 owners have dealt with. Surprised Porsche didn't add additional reinforcement to the shock towers on the .2s.

Petevb

IMHO no way. If you're aware of the issue and take two basic precautions you should be in very good shape. First- stick within Porsche's recommended ride height range when on the stock wheels. Second- avoid curb surfing at the track and over big potholes on the street. If you can do those two things you'll almost certainly never have an issue, and worst case it's an insurance claim.

That said it's a shame Porsche didn't address the problem with a design change (ie longer jounce travel) and/ or a ride height warning given that they have known about this issue for a few years. Between GT4s and 991 GT3s we've seen half a dozen cars on this site with the issue? On the order of half a percent of the represented perhaps?