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Joe, it would be good if you can get a picture of the inside of the top spring seat. If in fact the shock body hit it you should be able to see. If it did hit it that means either there is no bump stop or if there is one internal it's not doing its job. Thanks carl
Update: Plan B to obtain a new shock tower part has failed, moved on to plan C today. Should know if plan C pans out within 10 days, will move to plan D after that. If plan D fails, my options become about zero. If I get a new part in my hands I will provide details here about the efforts to obtain.
EDIT: Plan C has failed, moving to plan D but it'll be early next week before I can engage on it.
Interesting! Wonder if someone is trying to stop you from getting the part. Perhaps it is in your C or D already, but have you thought about using a dismantling service who might have a used one? That could be pulled off an existing car?? One would think somewhere there is an 991/981 etc that have been written off due to rear end damage. I know this is not an earth shattering overall Porsche issue, except for those who have had the failure, but the engineer in me is still very curious. Thanks for trying!
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.
Interesting! Wonder if someone is trying to stop you from getting the part. Perhaps it is in your C or D already, but have you thought about using a dismantling service who might have a used one? That could be pulled off an existing car?? One would think somewhere there is an 991/981 etc that have been written off due to rear end damage. I know this is not an earth shattering overall Porsche issue, except for those who have had the failure, but the engineer in me is still very curious. Thanks for trying!
A salvage part might be an option past my plan D, but due to the complexity of extracting just this part from the adhesive and rivets that attach it to the structure, it might not be something a salvage yard would mess with.
Regarding why I've been unable to source a new one yet from 3 different sources, I'm not on the conspiracy bandwagon, yet. The reasons I've been given so far make sense from one perspective, just not mine . Plan D is a pretty different approach from my first 3 attempts, and I'm fairly optimistic I can make it happen, but we'll see.
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.
At least that's how I see it. $.02
Thanks for the info and post, with excellent data and insight to the issue.
What I'm attempting to add to this thread if I can obtain a new OEM part is an analysis of the stress in the shock tower structure with the GT3/4 camber plate versus the non-GT camber plate on 991/981 cars, when the same force is applied by each camber plate design to the underneath of the shock tower. It may be nothing more than a geek exercise, but it will provide some relative data points for further use, if nothing else. I'm not supposing I can provide a magic solution or smoking gun of some sort, just some data to examine and add to the mix, or maybe just go "....hmmm", and then go drink beer.
Fully understand the complexity of removing a part that is both riveted and glued/sealed in place. But that might mean it is a part that would just be sent to the final metal salvage scape yard after the remainder of the 'good' parts are stripped and sold. So the residual value might be very small. I just went and had a look at our 981 in the garage. If the frunk area was stripped clean and the front suspension removed, I think I could fully cut out around a strut tower with a Saws-All in a few minutes. Now cleanly removing the actual strut tower would be a lot more work for sure, but if I got that far I would give it a try. During my early aviation training days I worked in a rebuild/overhaul shop for damaged Helicopters. Lots of rivets and structural adhesives where dealt with in that shop on a regular basis. I really can't help with the analysis, but on the dirty hands side I could.
Fully understand the complexity of removing a part that is both riveted and glued/sealed in place. But that might mean it is a part that would just be sent to the final metal salvage scape yard after the remainder of the 'good' parts are stripped and sold. So the residual value might be very small. I just went and had a look at our 981 in the garage. If the frunk area was stripped clean and the front suspension removed, I think I could fully cut out around a strut tower with a Saws-All in a few minutes. Now cleanly removing the actual strut tower would be a lot more work for sure, but if I got that far I would give it a try. During my early aviation training days I worked in a rebuild/overhaul shop for damaged Helicopters. Lots of rivets and structural adhesives where dealt with in that shop on a regular basis. I really can't help with the analysis, but on the dirty hands side I could.
If I can find a Porsche salvage shop willing to do the sawzall job you describe for not much more than a new part cost, I'll give that a shot if my plan D fails. I might have to do some de-riveting and adhesive separating myself, but that's doable. I need confidence the part I scan is dimensionally accurate to the OEM new state with no distortion, bends, etc..
I will start a seach up here and see what I can find. We have about 1/10th the cars you do in the US,.......but you never know what you will find until you look.
Leaving the spacers in raises the ride height and would reduce impact to the strut tower. Just the opposite of what you would think. Carl
Probably right...as a comparison I have JRZ Pro on the car now...way stiffer than stock and when I hit a bump or compression, its nowhere near the physical impact (and noise) caused by those spacers. There was no compression with the spacers, therefore there was no travel...which is why it launched me airborne on a few occasions!
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.
Does that mean going to 19" wheels/tires with a taller sidewall could help?
Originally Posted by Petevb
...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.
one thing to also keep in mind is that aluminum weakens with fatigue cycles - each small load change is a fatigue cycle. some of us NSX owners looked into this 20 years ago. that said, given this is happening to relatively new cars - maybe the strut tower was under-engineered for the combination of suspension and tire sidewall described above.
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.
At least that's how I see it. $.02
I like your explanation the best. "Reduced suspension travel."
The Cayman GTS is lowered 10mm and the GT4 is lowered 30mm below the Cayman.
Last edited by Track Junkie; 01-01-2017 at 09:50 AM.
12-29-2016, 10:17 PM
. Plan D is a pretty different approach from my first 3 attempts, and I'm fairly optimistic I can make it happen, but we'll see.
