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I don't know anything about welding aluminum but on steel the BMW OEM solution was to weld the reinforcement to the underside of the strut tower. The plate is designed to match the curvature of the strut tower and better spread the impact force of the shock mounts across the hat area. Since the suspension is already adjustable, it would be easy enough to just drop the front ride height to match the additional stack height of the reinforcement. Pic for reference:

If the Porsche OEM replacement parts are inexpensive it could be worth cutting the tops off to fit and double up.
It's great that there's not been many failures, however it could be catastrophic & also is a super expensive fix.
I've read this entire thread & been following. Agree with Karl & others that placing steel brackets on top of tower would add minimal reinforcement. Underneath the strut will move fore/aft & continue to stress aluminum tower top of bracket, & eventually, after enough metal fatigue will fail.
Thinking about a more robust intervention, I agree with Stout in that a steel bracket should be placed on the underside of the tower. The strut tower top can be substantially fortified, by placing a larger diameter forged steel base shim on the underside of tower & also bolting the P motorsport steel brackets on the top. Simply think of it (strut tower top), being an aluminum sandwich with steel as the bread. High impacts to the shock/strut would cause the mount to directly stress steel, & indirectly aluminum (over larger surface area), further supported by steel frame top. Could run over a tank mine before it busts.
Will discuss this idea with engineer & machinist friends, to get their input.
Bruce
Due to the materials involved (aluminum strut tower, steel reinforcement "cup"), I'd be curious about bonding the "upside down" steel cups into the strut tower. While they won't need anything to hold them in place—generally speaking—that might help transfer the loadings through a larger area. If I was keeping my GT4, it's the first aftermarket suspension part I would *definitely* buy—if only for peace of mind (so long as it was engineered/developed by someone I trust).
Considering how these fail I don't think those sort of cups are a solution. Typically those types of reinforcement plates are used when there is a steel mushrooming effect due to the strut mount radius being too small and thus distributing the impact across a smaller area. With the GT4 failures the crack appear exactly around the bent area of the aluminum so those plates would accomplish nothing since the weight of the car will still be ultimately carries by the flat area. The real solution I think is something that is cast into the exact shape of tower from the inside extending 2-3 inches lower than the flat face of the tower. Placed inwards and welded around the lower edge of the cup thus spreading the weight not only to the top flat surface of the tower but along its circumference few inches below the where it it bent to flatten.
Can anyone following this thread conclusively explain why the OE Cup car strut tower plates (991 cup on right, 981 clubsport on left) will NOT work? Usually these parts have a reason for existence. The arguments so far are solely based on intuition and conjecture. It it takes an FEM simulation to see what the deformation mode(s) actually is (are) to accept/dismiss the functionality of these parts.
The fact that "ears" of these plates are attached differently in the cup cars than in the street cars is (I guesstimate here) inconsequential. If you have held these plates and actually seen them, you will notice that the force transfer to the ears is low - any force will bend it (1.5mm steel plates). So, the effect must come from the fact that there are now two layers of metal on top of the shock tower vs. just one in stock config.
One more: How the shock tower deforms on impact will also depend on where the camber adjustment happens to be. It seems (again guess-timating here) that the support is least / bending is max. for max. neg. setting (most inwards).
Regardless -- pending proof that they conclusively don't work, I am with the factory engineers and their FEM simulation software and racing experience. And - why would the attachment points happen to be in the street cars also? That's not a coincidence. Maybe they considered these a judgement call during development and last minute left them out because all is fine in most use cases.
In practice, for me, this means that I have them in the shop and will install as soon as I have some spare time. Thanksgiving weekend is coming ...
I disagree, I don't think the strut tower base failure is at the perimeter. The first one posted certainly looks that way, however Joe's is quite different. Dependent upon the moments of force, I suspect a strut mount pad corner initiates the failure, then the thing busts apart.
Considering how these fail I don't think those sort of cups are a solution. Typically those types of reinforcement plates are used when there is a steel mushrooming effect due to the strut mount radius being too small and thus distributing the impact across a smaller area. With the GT4 failures the crack appear exactly around the bent area of the aluminum so those plates would accomplish nothing since the weight of the car will still be ultimately carries by the flat area. The real solution I think is something that is cast into the exact shape of tower from the inside extending 2-3 inches lower than the flat face of the tower. Placed inwards and welded around the lower edge of the cup thus spreading the weight not only to the top flat surface of the tower but along its circumference few inches below the where it it bent to flatten.
^
Yep, and this is what I mean by "upside down cup". Needs to spread the load over as much area in/on the tower as possible.
This, spoken as a mechanically inclined person but no engineer—and it really needs to be addressed by an engineer. I have to think there is a solution to prevent the kinds of failures we've seen, rare as they are.
The GT4CS has a strengthened front. The cage extends through the front "firewall", ends at the shock towers and provides the correct attach points for these plates.
No sir, it has the exact same part for the shock tower as the other 981 and 991 cars, as least for GT4, Boxsters, Caymans, 991.1, 991.2, and GT3. But as you will see below, the GT3 and GT4 (maybe others) have a difference in the shock mounting itself (besides the rubber snubber on the non-GT cars) that I believe is a contributor to additional stress on the shock tower.
Quite possibly true, but I believe there are other contributing factors that would cause a random cast part with compromised integrity to be more likely to fail. See below.
First, a request: If anyone is aware of a non-GT 981 or 991 car that has this failure on the front of the car, please post or send me links or info. AFAIK, there have been none, but I haven't been extensively searching.
The first photo is my 981 Boxster S driver's side shock/spring, left side is front of the car. Note the shape and size of the aluminum mounting pad that mates the shock/spring to the shock tower. Note how close it is to the aft end of the shock tower, and it's overall size.
The second photo is my GT4, same side of car and orientation. Note the difference in size and shape of the mounting pad. If you zoom in on the aft (right) end of the pad you can see the flat machined surface of the shock tower casting left uncovered by the GT4 (and GT3) part, and you can get a feel for how far away this edge is from the aft end of the shock tower cavity. The aft end of the mounting pad is a straight line, the curve you see is part of the machined surface of the shock tower's mounting area. This machined area appears to be almost the same shape as the Boxster's larger shock mount.
The shape, size, and location of the GT3/GT4 part is causing more bending moment on the areas of the shock tower where ruptures have occurred. The further away from the vertical walls of the shock tower cavity the spring loads are applied, the more severe the stress is at the failure points we've seen. This is due to the longer lever arm and higher bending moments. Also, looking at the Boxster and GT4 springs and their location relative to the three mounting bolts in the shock tower, I think the GT4 strut is further aft, I suppose to provide more caster? If so, this shifts the center line of spring force to the aft end of the shock tower, relative to the Boxster strut. But the pad location and size relative to the non-GT part is the key focus for causing higher stress on top of the shock tower.
I am not saying this is the cause of any shock tower failures. I am saying there are more stresses on the upper shock tower structures with the GT3/GT4 shock mount.
The next three photos show the areas on the shock tower where the Clubsport brackets are installed, including the Clubsport gusset and white tube that extends forward to the outboard aft portion of the shock tower. After looking at my GT4, I think that white tube is inserted into a steel gusset that has a through bolt going vertically into the hole shown in the center of the 3rd photo, just left of the 90 degree bend in the aluminum air conditioning tube. This hole is part of the shock tower casting. And I'm supposing that steel gusset is also welded to the high strength steel that runs horizontally left and right just above that hole.
The 4th photo is just for orientation reference.
The 5th photo shows another bolt into the forward part of the shock tower casting holding some electrical connector in place. This bolt together with all the other bolts shown in the Clubsport photos in this thread could be used to assist holding down a metal hat machined out of steel or aluminum billet and bolted and bonded to the top of the shock tower for reinforcement. If someone so desired.
Oakie981 nicely detailed the design & placement of the strut mount pad onto the strut tower hat. The GT4 strut is more aft than other 981 cars (of which there's no reported strut tower failures). A stronger base (steel) & larger diameter flange/cup, would distribute moments of force over a greater area and most likely prevent failure. The bolted in motorsport plates on top would reinforce all that much more.
^
Yep, and this is what I mean by "upside down cup". Needs to spread the load over as much area in/on the tower as possible.
This, spoken as a mechanically inclined person but no engineer—and it really needs to be addressed by an engineer. I have to think there is a solution to prevent the kinds of failures we've seen, rare as they are.
^ I agree that the fix should be over engineered. Although as, Okie981 pointed out the "regular" 981's aren't experiencing (or not reporting?) these failures, their strut design is different with the same strut tower hat part #
^ I agree that the fix should be over engineered. Although as, Okie981 pointed out the "regular" 981's aren't experiencing (or not reporting?) these failures, their strut design is different with the same strut tower hat part #
Shock and spring stiffness is key to how much force propagates into the chassis.
Shock and spring stiffness is key to how much force propagates into the chassis.
^ Yes.
Too many variables between tire sidewall construction, strut-top shape, spring rate, dampers, suspension arms, brake weight, etc. It's why I'd be very interested in listening to a Burvill, Levitas, Moreland, or Watkins on this one.
electon mike's original post:
This brings back bad memories as this happend to a 991 GT3 I was buying when the tech did the PDI. He hit a curb on the test drive and did significant damage... I didn't take the car as it was messed up pretty badly, and they couldn't get another GT3 allocation.
in electron mike's photo, the jagged edge of strut tower hat failure shows the strut mount adjacent to the fractured aluminium hat. I think that mount "corner" transmitted excess force initiating the aluminium hat failure. It's possible that force could have been dissipated over a larger area with no failure. As Okie981 noted in a previous post, GT4 strut mount is smaller & more aft, which could cause this pattern of failure.
I'm not going to weld anything into the strut tower of my GT4. What I will do is talk to a local Porsche Meister tech who has a special interest in suspension, to see what they recommend. I hope known experts chime in on this thread soon. That would alleviate some anxiety & give us good idea on how to fortify this weakness.
Last edited by G-forceGT4; 11-20-2017 at 01:19 AM.
Reason: citing original post as electron mike's
electon mike's original post:
This brings back bad memories as this happend to a 991 GT3 I was buying when the tech did the PDI. He hit a curb on the test drive and did significant damage... I didn't take the car as it was messed up pretty badly, and they couldn't get another GT3 allocation.
in electron mike's photo, the jagged edge of strut tower hat failure shows the strut mount adjacent to the fractured aluminium hat. I think that mount "corner" transmitted excess force initiating the aluminium hat failure. It's possible that force could have been dissipated over a larger area with no failure. As Okie981 noted in a previous post, GT4 strut mount is smaller & more aft, which could cause this pattern of failure.
I'm not going to weld anything into the strut tower of my GT4. What I will do is talk to a local Porsche Meister tech who has a special interest in suspension, to see what they recommend. I hope known experts chime in on this thread soon. That would alleviate some anxiety & give us good idea on how to fortify this weakness.
This photo is precisely why I don't think any flat or near-flat solution will be a solution. May even make the situation worse.
^ Yes, I just referred to a previous post with a fix that had been used in BMW problem. The simple "discs" will not suffice & a machined steel piece fitting as a liner would provide the most strength.
11-19-2017, 08:44 PM
Boxster & Cayman towers aren't failing
