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.
My dealer has a 981.2 GT3 on the floor . I was curious to see if they had done any redesign or reinforcement on the strut towers . i took pictures of three different cars a 718GTS 991.2GTS 991.2GT3 .I hate to say is they are all the same as my GT4 which i posted earlier in this thread . The one thing that could increase the failure rate on the GT cars is the top mounting plate on the strut is much smaller which would cause the force of impact to be more concentrated . The larger top plates used on the 991s,981s,718s would disburse the impact better maybe not totally eliminating the failure but probably greatly reducing them. Looks like it would be simple the make a larger top plat out of 1/8 steel to replace the factory plate. Hope maybe one of the forum sponsors may want to look into it ?? carl
P.S just ordered a M2 completion today should be here in sept. I have a 17 M2 great car not a track star compared to the 4 but really a fun DD. When the new one comes im going to do the same thing i did with my first GT4. Sell it to a forum member for what the dealer was going to give me on trade. If you want a white 6sp ex pack 10k mi M2 for wholesale let me know.The catch is you have to wait till the new one comes ..
It's my opinion that failures like this are easy to deny.
You have a stress riser(s) which cause micro-factures that are undetectable or visually hidden. These grow until a seemingly innocuous bump or jarring road feature that would ordinarily be absorbed causes macro-failure. Given the massive costs in repairing strut tower failures, it's easy for the bean counters at Porsche to say it was the impact that caused the failure; and imply driver causation. It also doesn't aid seeking warranty coverage that by and large these are still rare failures by absolute percentage. I'm not sure what the threshold percentage failure rate would be to convince Porsche that it's likely to be a manufacturing/design flaw. It seems that the GT3 engine failure issue was far more prevalent.
its interesting the it looks like the same part number for the cast strut tower on the GT3,4,981,991,718 . No reinforcements for the street gt cars only the cup cars. more failures on the GT cars because of less shock travel lower ride height and a smaller mounting surface on the top of the shock to absorb impact??. i did see a reg 991 coupe with a blown out tower at the dealer but the car hit a curb hard. carl
Porsche doesn’t seem to be talking. I do know I saw a white GT4 this year with a strut failure from going off course at a DE here in Florida and Porsche fixed it. I believe the car was still under warranty. The body shop said there was some back and forth but they finally agreed to fix it. As soon as it was fixed the owner traded it in to the dealer. Who sold it in less than a week. I have been following this for a long time I think the prudent thing to do is try and lower the unsprung weight on these cars. The GT4 has all 991 components under the front end. Bigger wheels and tires, big breaks, 991 control arms, tie rod ends ect,ect. The base,S, and GTS. caymans have smaller 981 parts. (Don’t seem to suffer from this issue) The GT4 rides lower on stiffer struts with less travel. But the chassis and strut tower are 981. I have not seen a GT4 with PCCB’s have a strut failure. (Not to say it hasn’t happen) I think adding DSC module and lighter wheel tire and brake combo is the safest insurance against this issue for the time being.
I don't think it's a go/ no go question. The shock tower is the weak point- if you drive over a curb even with stock ride height it will fail. However the size of the impact the car can tolerate will be inversely proportional to ride height.
Good question...
It sounds like the ride height spec is +2/ -0mm. If true there's zero room to lower the car.
I believe X51 is shorter than stock, not sure how it compares to the GT4. However keep in mind that the 20" wheels and tires are larger OD. This raises the spindle height, reducing jounce clearance even at an identical ride height.
I'm not saying this isn't a design issue. I think it's a serious flaw. However the root cause may be that the GT4 doesn't have the camber and ride-height adjustability we all expect of a car with its design mission in mind. I don't see the aluminum shock tower itself as the flaw- that part is clearly sufficient in other applications. One could argue that steel would bend rather than break, but I'm not sure a bent chassis is all that much better...
A friend and race car designer once mentioned a rule of thumb: design front suspension components with a 7x factor of safety- take the max corner weight the wheel is likely to see and multiply by 7, likely closing on 10k lbs for the GT4. However if you run out of travel even that's going to be wildly insufficient.
Lower any car improperly, even your 914, and you're going to get issues. The failures we're seeing look like a car that's been lowered improperly. I'd be shocked if Porsche engineers built that flaw in at stock ride height- this is 101 stuff and they are teaching graduate classes. However I suspect they didn't build in the margin that we (I'd argue correctly) expect, particularly since we did buy the ability to adjust ride-height.
If this theory proves correct then perhaps Porsche needs to issue a warning bulletin re ride height. I'm not sure that they fully understand that many here in the US will crank in more camber as step 1, possibly seriously compromising the design intent...
As it stands I worry that much of the standard setup advice (as seen on the sticky thread just below) may unwittingly be compromising the car and promoting this failure.
Originally Posted by Petevb
I've been looking into the issue as well- so nice to have a little time (on vacation).
First, I'm still convinced that bottoming the suspension is the primary cause. Below you can see the load vs deflection curve of the front suspension of a 997. This is from a K&C machine- it cuts off at the high end for obvious reasons, but you can extrapolate to say that somewhere around 2.6 inches of deflection you'll see forces in the 10k lbs range that could risk failing the strut tower. We're looking at a slightly progressive base spring rate followed by the bump stop coming into play around .4", then it gets serious around 1.5" before going fully compressed around 2.5".
If we use this data to run a simulated impact we need to know things like shock curves, tire rates, etc on top of this to get an accurate answer. However using plug values some trends jump out:
With any significant impact the tire will fully compress, so sidewall height (minus .5" or so) becomes a big factor. In addition, even with high damping rates the chassis doesn't have time to move out of the way significantly at freeway speeds before the forces get excessive- on the above I calculate a maximum of ~5mm of upwards movement on the body when encountering a 3" obstruction at 60 mph.
This leaves the main variables as the amount of jounce travel and the tire sidewall height. Run out of those and it doesn't matter much if your strut top can take 10k lbs of load or 20k- either way it's going to fail at nearly the same point.
I know the 991 has 10mm less travel than the above, and I'd bet the farm on the GT4 having less than that still. We also know the tire 20" sidewalls are very short, so the GT4 likely has as little headroom for obstacle impact as any street Porsche, possibly less.
Once that headroom runs out the big difference is the failure mode: in steel strut-tower cars you probably won't even know that you've tweaked the chassis by .5" until it comes time for an alignment. In the aluminum strut top cars it's going to be immediately and painfully obvious.
My bet is that we're not seeing these failures on standard Boxters or 991s because they generally have more suspension travel and sidewall height. On top of this lowering our cars at all cuts into an already thin margin.
If I wanted to avoid the failure my first thought wouldn't be to reinforce the shock tower. Instead I'd a) keep ride height relatively unchanged, and b) look for a place to add compliance to the system. One option might be running "race weight" 3 piece wheels- these are weaker than stock and would likely bend before you exceed the critical forces on the shock tower. Other options (beyond new shocks) would be to look into designing a crush structure at the shock-top, etc.
I have PCCBs, DSC and Forgeline CF205s, 16 lb F/17.5 lb R. In my opinion there is some combination of a perfect storm that causes the cast tower part to fail. Wheel, tire, pressure, speed, geometry of what was hit, resultant force on the tower and any casting anomaly of the cast strut tower part. There are a lot of variables.
I agree totally. The problem is all the variables. It’s next to impossible to know what to do or not to do. I do know the less the unsprung/rotating mass/weight the easier it is for the strut to control the force of impact. add the DSC module that significantly increases the PASM systems functionality and in theory you should drastically reduce the stresses on the tower. Unfortunately without Porsche talking and no direct engineering analysis I think that is about all anyone can do.
Porsche doesn’t seem to be talking. I do know I saw a white GT4 this year with a strut failure from going off course at a DE here in Florida and Porsche fixed it. I believe the car was still under warranty. The body shop said there was some back and forth but they finally agreed to fix it. As soon as it was fixed the owner traded it in to the dealer. Who sold it in less than a week. I have been following this for a long time I think the prudent thing to do is try and lower the unsprung weight on these cars. The GT4 has all 991 components under the front end. Bigger wheels and tires, big breaks, 991 control arms, tie rod ends ect,ect. The base,S, and GTS. caymans have smaller 981 parts. (Don’t seem to suffer from this issue) The GT4 rides lower on stiffer struts with less travel. But the chassis and strut tower are 981. I have not seen a GT4 with PCCB’s have a strut failure. (Not to say it hasn’t happen) I think adding DSC module and lighter wheel tire and brake combo is the safest insurance against this issue for the time being.
991 GT cars have suffered this same shock tower failure. This thread somewhere shows the exact same parts used for shock tower in 991 and 981 cars. Front tub of the car is same also between 991 and 981 cars. My armchair theory is these failures are caused by a combination of the different springs and end-of-travel snubbers used on GT vs. non-GT 991/981 cars and the smaller contact area of the GT car camber plates to the underside of the shock towers. I have no objective data to support this theory, only armchair analysis of what we do know combined with some experience with mechanical design and cast aluminum parts.
So wouldn't increased spring rate springs help this issue by providing more counter force to prevent contact of the upper strut mount?
Seems unlikely. Yes, stiffer springs are going to absorb more energy, gracefully, but certainly the tower is failing well into the vertical portion of the deflection/load curve. So the small increase in spring rate (even if you went "big" at 50% stiffer) is dwarfed by the exponential load increase in the failure scenario. My guess is you need more bump stop, not more spring rate. (where the bump stop provides exponential spring rate at high deflection)
I'm no engineer though, so maybe that's wrong and worse, maybe it's gobbledygook.
05-25-2018, 11:57 PM
