When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I'm with you. I'm well aware of what I don't know.
If your 991 somehow suffers this issue you're covered under warranty. There's a known fix to this problem. Heck, Manthey even manufactures a reinforcement kit for their GT4 and Cup cars racing in VLN so you can strengthen your top mounts as a preventative measure.
I've not heard of a single instance of this being covered under a factory warranty.
991 gt chassis is way more stiff than the 997,996,964,993, 911. More energy dissipates into the mount point as a result of less chassis flex. Price you pay for flat cornering.
This is a misunderstanding. A) the 991 is not significantly stiffer than the 997 or many other vehicles that don’t exhibit the same issue (including the 918, etc). B) there is no need to rely on chassis flex if the suspension package is properly designed. Additional travel with the right elastomer is far more effective at preventing a failure than chassis flex.
Flat cornering and a chassis that doesn’t fail catastrophically are not mutually exclusive. Don’t kid yourself: this is not a necessary compromise, it’s a design flaw. If Porsche engineers are worth half their salt the 992 won’t suffer from the issue.
This is a misunderstanding. A) the 991 is not significantly stiffer than the 997 or many other vehicles that don’t exhibit the same issue (including the 918, etc). B) there is no need to rely on chassis flex if the suspension package is properly designed. Additional travel with the right elastomer is far more effective at preventing a failure than chassis flex.
Flat cornering and a chassis that doesn’t fail catastrophically are not mutually exclusive. Don’t kid yourself: this is not a necessary compromise, it’s a design flaw.
As I understand it based on data from Porsche the 991 chassis is 25% more torsionally rigid than the 997.
I'm not sure many 918's are being pushed to their limits to make that model a valid comparison. There are way more 911's on the road than 918's and they all share the same suspension mount design. So far I've only heard of a handful of failures.
I understand what your saying but adding suspension travel and more elastomer in the suspension train is great to dissipate energy but does not make handling any better. In fact the more energy dissipating elastomers you use the more difficult it is to model and predict suspension behavior. The best way to control the suspension damping and overall performance is to have everything rigid except the shock. Then you can adjust spring rates and shock damping and get the exact suspension behavior you want.
Understandably there is a trade off between weight of the car and strength of the suspension mount points. You could make them infinitely strong and never fail, sure, but that would mean increased weight in places that may not make the car perform very well. All road cars carry some compromise in design. I'm not sure this constitutes a design flaw if cars that are subject to significant force show failure at high stress points.
Any part on a car is susceptible to failure given a force application that pushes its material past the plastic portion of the deformation curve. It seems that a few of these cars have had similar failures. If 25% of the mount points failed I would worry that there is a design flaw. I haven't seen any data that suggests mass failures and many cars are taken to the track and many 991's, not only gt cars, are driven every day and I'm sure hit large potholes and don't have these failures routinely.
As I understand it based on data from Porsche the 991 chassis is 25% more torsionally rigid than the 997.
Less, but take the above as a given. 25% increase in chassis stiffness = over half a dozen failures from a small sample size where before there were zero?
Originally Posted by CobaltCr
I'm not sure many 918's are being pushed to their limits to make that model a valid comparison.
Red herring, but there are dozens of cars with higher chassis stiffness that don’t have an issue. Virtually everything with a carbon tub, obviously, but also lesser models- R8 or BMW 5 series, etc.
Originally Posted by CobaltCr
I understand what your saying but adding suspension travel and more elastomer in the suspension train is great to dissipate energy but does not make handling any better. In fact the more energy dissipating elastomers you use the more difficult it is to model and predict suspension behavior. The best way to control the suspension damping and overall performance is to have everything rigid except the shock.
Incorrect. Modern “bump stops” are integral parts of the suspension system, and come into play when the suspension is compressed only slight from static ride height. They are the primary way Porsche achieves a progressive spring rate, and thus they are one of the main ways Porsche has been able to lower their cars and reduce bump travel. Thus they are integral to improving the handling on modern Porsches. Furthermore any extra travel at the stiff end of the bump stop only comes into play when a massive bump is encountered, and thus is transparent in normal handling and has zero negative impact. These are not your father’s bump stops- the evolution of elastomer second springs is one of the biggest trends in Porsche suspension design over the last 1/4 century.
Back to the basics: adding back the extra 10mm of jounce travel that was cut from the 997 and putting a very stiff bump rubber over that range would almost certainly cure the issue with no ill effects.
Originally Posted by CobaltCr
If 25% of the mount points failed I would worry that there is a design flaw.
I’d prefer my car had less than a 1 in 4 chance of catastrophic chassis failure thanks. Must be the “conservative” engineer in me. However if that’s your bar then I understand why you don’t think there’s a problem.
Originally Posted by CobaltCr
many cars are taken to the track and many 991's, not only gt cars, are driven every day and I'm sure hit large potholes and don't have these failures routinely.
While a number of regular 991s have experienced failures (google) the GT cars are proportionally much more prone to the issue. For obvious reasons: lower ride heights, larger wheels and shorter sidewalls all cut into jounce travel. This issue was a sticky on the GT4 board for the better part of a year, we’ve got numerous examples on both generations of the 991 GT3, the cup and clubsport cars are reinforced to try to mitigate the issue. Not sure how we can pretend that’s not a problem?
Flat cornering and a chassis that doesn’t fail catastrophically are not mutually exclusive. Don’t kid yourself: this is not a necessary compromise, it’s a design flaw. If Porsche engineers are worth half their salt the 992 won’t suffer from the issue.
^ This.
CobaltCr,
To put it in another way, do other cars catastrophically fail when similar loads are applied? You are implying that Porsche knowingly made a trade off between “performance” and strut tower durability. Hard to fanthom the wisdom and logic that was applied that netted the results we are seeing. This car is not meant for competition racing. Adding some weight to make it more robust for driving in typical target market environments would be the obvious choice.
To put it in another way, do other cars catastrophically fail when similar loads are applied? You are implying that Porsche knowingly made a trade off between “performance” and strut tower durability. Hard to fanthom the wisdom and logic that was applied that netted the results we are seeing. This car is not meant for competition racing. Adding some weight to make it more robust for driving in typical target market environments would be the obvious choice.
Well I'm not exactly sure what forces have been applied in the failure scenarios. I haven't really delved into the rennlist archives but the few reported cases I've seen so far have either no information provided or second hand information about possibly hitting curbing and there is one case in this thread of hitting an animal carcass at high speed- usually an event that leaves a car totalled but in this case the car was repaired back to normal working order.
Of course the engineers made compromises. All engineering of this sort requires some compromise between strength, cost, and target function. I agree that the tower can be made stronger to resist this type of failure mode, but I'm sure there would be other complaints from this board associated with that level of robustness.
My point isn't that the tower isn't failing, it's just that it doesn't seem to be failing under normal load conditions. It's failing under extreme conditions. I guess if your point is that a shock tower should never fail then I agree it's a design flaw. But if the tower doesn't fail something else is going to. And maybe more unpredictably and maybe more dangerously.
Less, but take the above as a given. 25% increase in chassis stiffness = over half a dozen failures from a small sample size where before there were zero?Red herring, but there are dozens of cars with higher chassis stiffness that don’t have an issue. Virtually everything with a carbon tub, obviously, but also lesser models- R8 or BMW 5 series, etc.Incorrect. Modern “bump stops” are integral parts of the suspension system, and come into play when the suspension is compressed only slight from static ride height. They are the primary way Porsche achieves a progressive spring rate, and thus they are one of the main ways Porsche has been able to lower their cars and reduce bump travel. Thus they are integral to improving the handling on modern Porsches. Furthermore any extra travel at the stiff end of the bump stop only comes into play when a massive bump is encountered, and thus is transparent in normal handling and has zero negative impact. These are not your father’s bump stops- the evolution of elastomer second springs is one of the biggest trends in Porsche suspension design over the last 1/4 century.
Back to the basics: adding back the extra 10mm of jounce travel that was cut from the 997 and putting a very stiff bump rubber over that range would almost certainly cure the issue with no ill effects.I’d prefer my car had less than a 1 in 4 chance of catastrophic chassis failure thanks. Must be the “conservative” engineer in me. However if that’s your bar then I understand why you don’t think there’s a problem.While a number of regular 991s have experienced failures (google) the GT cars are proportionally much more prone to the issue. For obvious reasons: lower ride heights, larger wheels and shorter sidewalls all cut into jounce travel. This issue was a sticky on the GT4 board for the better part of a year, we’ve got numerous examples on both generations of the 991 GT3, the cup and clubsport cars are reinforced to try to mitigate the issue. Not sure how we can pretend that’s not a problem?
Hard to compare carbon tubs to aluminum and say that because a carbon tub doesn't fail that stiffness doesn't play a role in this instance of failure.
Why was 10mm of jounce travel cut from the 997 if that didn't benefit suspension behavior and handling characteristics? I think you're arguing my point that increasing stiffness has led to a small number of failures in extreme circumstances. My point is that these failures don't constitute a large scale design flaw but maybe we've found the tail of the bell curve.
I'm all for better engineering. Why doesn't Porsche add a stiffer bump stop? What are the bump stops like in the current suspension?
Its clear that this issue is a common issue in a sense that suddenly newer porsche GT cars have a number of failures in the same manner while other brands dont have this issue (or previous generation cars). Stop saying its not a common issue, its not as common as engine failures but something like this can be called if the occurance rate is 1/10th of engine failures but still happen occassionally. Strut tower should not fail like this, period.
I'm with you. I'm well aware of what I don't know.
If your 991 somehow suffers this issue you're covered under warranty. There's a known fix to this problem. Heck, Manthey even manufactures a reinforcement kit for their GT4 and Cup cars racing in VLN so you can strengthen your top mounts as a preventative measure.
Do you have any more details about the manthey kit? Could it be applied to street cars?
It would not shock me (haha pun intended) if it was designed to sheer in an accident. Looking at the red part from the accident car, it looks easy enough to replace and apparently Porsche sells the part so I don’t know why people are so concerned.
My point is that these failures don't constitute a large scale design flaw but maybe we've found the tail of the bell curve.
Clearly Porsche thought it wasn't serious enough to address. On the GT4 we seem to be over ~.1% per year failure rate based on the data collected on the GT4 board, maybe approaching a 1 in 100 chance if the car's a keeper? Very tough to fix properly- chassis jig, ~$25k, etc. Enough to suck if you're one of those guys.
Originally Posted by CobaltCr
I'm all for better engineering. Why doesn't Porsche add a stiffer bump stop? What are the bump stops like in the current suspension?
They didn't leave enough room in the GT cars for the larger bump stop, and/ or the engineers cut it too close.
The GT guys are hot-rodding a chassis that was designed at Weissach, not Flacht. The basic 991 only sees failures extremely rarely- when it goes curb surfing or hits something very big. Thus the factor of safety on that design seems acceptable. Now the GT guys get it to turn it into a GT3/ GT4. They lower it significantly and increase the rim size. Both of these decrease the margin before you run out of travel and metal hits metal, so the failure rate goes up. They also give the car adjustable spring perches for corner balancing, which owners then use to lower the car further. So clearance before metal hits metal is reduced 3 times from the original design, and each time the size of the hit that can create a failure goes down. In the end you're left with the situation where a large pothole (as seen on GT4s) can cause a catastrophic failure.
Why are we only seeing it now? Older base generations (ie 997) were designed with softer spring rates and more total travel. Thus there was still sufficient travel left when the GT guys lowered the car. Also when room ran out the chassis would deform plasticly rather than fracture. Apparently 10mm less travel and a more brittle part means we found the tipping point.
Typical Porsche bump stops look like #9 below, labeled "additional spring" in the PET. They can also be internal to the shocks on some models.
I'm with you. I'm well aware of what I don't know.
If your 991 somehow suffers this issue you're covered under warranty. There's a known fix to this problem. Heck, Manthey even manufactures a reinforcement kit for their GT4 and Cup cars racing in VLN so you can strengthen your top mounts as a preventative measure.
A.) Not covered under warranty.
B.) If this is the strut tower bracket that you are referring to, I really dont see how this can be very effective.
This has already been fairly well discussed in the GT4 forum.
Clearly Porsche thought it wasn't serious enough to address. On the GT4 we seem to be over ~.1% per year failure rate based on the data collected on the GT4 board, maybe approaching a 1 in 100 chance if the car's a keeper? Very tough to fix properly- chassis jig, ~$25k, etc. Enough to suck if you're one of those guys.They didn't leave enough room in the GT cars for the larger bump stop, and/ or the engineers cut it too close.
The GT guys are hot-rodding a chassis that was designed at Weissach, not Flacht. The basic 991 only sees failures extremely rarely- when it goes curb surfing or hits something very big. Thus the factor of safety on that design seems acceptable. Now the GT guys get it to turn it into a GT3/ GT4. They lower it significantly and increase the rim size. Both of these decrease the margin before you run out of travel and metal hits metal, so the failure rate goes up. They also give the car adjustable spring perches for corner balancing, which owners then use to lower the car further. So clearance before metal hits metal is reduced 3 times from the original design, and each time the size of the hit that can create a failure goes down. In the end you're left with the situation where a large pothole (as seen on GT4s) can cause a catastrophic failure.
Why are we only seeing it now? Older base generations (ie 997) were designed with softer spring rates and more total travel. Thus there was still sufficient travel left when the GT guys lowered the car. Also when room ran out the chassis would deform plasticly rather than fracture. Apparently 10mm less travel and a more brittle part means we found the tipping point.
Typical Porsche bump stops look like #9 below, labeled "additional spring" in the PET. They can also be internal to the shocks on some models.
Good analysis. Thanks for that. I see your point. I'm guessing the .2 mounts are not materially different in design or execution?
05-25-2018, 02:27 AM
