ExMB

I thought that had been debunked because the premise in that thread was an extrapolation of a guess without factual data to confirm. Until that has been cleared up, confirmed, IMHO it shouldn't be used as a factual answer.

Petevb

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.

Track Junkie

Your description of the GT4 ride over bad roads attributed to the the strut tower to cast aluminum chassis mounting point is how a bicycles feels when comparing aluminum vs steel frames.
Aluminum bicycle frames are stiff and you feel all the bumps in the road and are a harsh, noisy ride. While steel bicycle frames dampen and absorbs road shock well and are much more comfortable and quiet ride.
The GT4 cast aluminum is definitely at a disadvantage here over steel.

Track Junkie

If the cast aluminum shock tower isn't a flaw, why did Porsche add 1.5 mm steel reinforcement plate to the shock tower for the 991 GT3 Cup and the GT4 Clubsport and it wasn't added to the 996 GT3 Cup or the 997 GT3 Cup with the steel shock tower. If the cast alumimum shock tower isn't a flaw maybe it's just weaker and needs reinforcement.

mqandil

True, there is definitely a different feel for the steel structure over bumps, but please bear in mind 4 of the 6 porsches other than GT4 I own (991 GT3, 2016 Spyder, 2013 911 4S, & 2017 911 Targa 4S) also all have aluminum frames and none of them felt or sounded the same as the GT4 over bumps. So Petevb theory has lots of merits and I do believe it has something to do the bump travel of the GT4.

My wife couple of years ago went over a curb at +40 mph with our 2013 981 Boxster S which has the same exact design & dimensions as the GT4 and ended up severely damaging both wheels and tires and nothing happened to the frame or shock tower. After replacing the wheels and tires the car drove great for more than 2 years until we sold it which proves Pete theory.

If you also followed my earlier posts, I did mention the GT4 shock tower contact area with the upper shock mounting plate is roughly 30% smaller than standard 981 Boxsters or Cayman which results in yet higher stresses on the shock tower. In addition on the GT4 the shock centerline axis where the force is transmitted to the shock tower is offset toweard the back compared to other 981 Caymans and Boxters resulting in uneven loading distribution on the GT4 shock tower. So aside from bump travel concerns, there appears to be other design concerns than can add to this issue. But I think Pete theory makes so much sense, and there could be other contributing factors. Mark

mqandil

I agree Steel is going to perform better under impact than aluminum but with manufacturers quest to reduce weight we are stuck with Aluminum. Would not call it a flaw but rather a reality as most manufacturers are heading this way. As I stated in my earlier post, our 2013 981 Boxster S with Aluminum structure that is identical to GT4, survived a major impact when my wife went over a curb at high speed and the structure survived, which proves the structure by itself is not the issue.

I think the steel reinforcement plates added as these cars have probably shorter bump travel compared to the street cars and they also see much more abuse on track. It will be a nice addition to the street cars. Mark

4carl

As i theorized in a previous post. With The lower the ride height they didnt want to use a shorter bodied shock so to have the travel they eliminated the bump stop. Now the shock bottoms out on the brittle cast alum tower with no dampening of the blow and impact. What could possibly go wrong?? carl

Petevb

I would blame cost control more than weight. The amount of material needed to double the part strength here would be trivial, and it was likely calculated to have plenty of factor of safety as-is across all the street cars... in design conditions.

The race cars use much higher spring rates (and triple the chassis stiffness/ strength with a cage) so they likely require (and can use, without twisting the frame) stronger parts.

It looks fairly straightforward to design a part to reinforce this area, but if the shock is bottoming as I suspect you'll just be moving the problem. Of course if that makes the weak link something that fails gracefully and is easier to repair that might be ok, but I'd rather address the root cause...

This might be a good application for crushable foam aluminum or similar replaceable energy absorbing structures if you could fit them.

mqandil

I agree, about the weight saving, as it is minimal and especially if they allow for the same safety factor as steel. With the added thickness to compensate for the lower strength, the net weight savings is minimal. Not sure though about the cost savings being so significant as we experimented ourselves of making our large centrifugal pumps out of Aluminum vs steel, and yes there was a cost savings but it was not significant as we have to add additional thickness to compensate for the lower strength, but the castings sure look great, but at higher volume I guess it adds up. I guess the corrosion resistance would be an added value for Porsche with the Aluminum. Mark

okie981

I believe a previous post in this thread showed an internal, albeit shorter, bump stop in the inverted monotube shock/strut used on the GT cars. There was a photo included in the post.

neanicu

..........

4carl

I could be wrong but I thought the internal bump stop that was shone wasn't verified to be from a GT4 . It was just an internal bump stop?
Again I'm not sure was it in fact from a GT4 shock. If Joe could diasect his that would be great. Carl

okie981

Lots of good posts on this thread in the last two days.

I'm still waiting on the new shock tower part to arrive at the dealer, was delayed last week "...due to the holidays", per the parts guy. If it doesn't arrive in a week or so, I will go to my plan C to obtain one. I don't have access to the 3D scanner until after the new year anyway....

Having real data on the shock/spring/force/travel as mentioned in Petevb's earlier post would be useful for the FEA analysis I'm working towards.

Petevb

All recent Porsche sports cars use highly progressive spring rates achieved by a combination of an elastomer secondary and a rising rate primary spring. I can't imagine the GT4 would be any different in philosophy, and thus I'd be shocked if there wasn't a snubber hiding inside that shock. The issue is that any bump rubber/ spring/ shock is going to get squashed flat eventually- it's only effective before that happens.

If you encounter an object on the road while traveling at 60 mph and the suspension can't articulate/ comply to clear without lifting the car itself the forces get astronomical. That's true regardless of the bump rubber package. And in this case the chassis stiffness of the modern cars works against it- the older frames would happily flex for some additional compliance.

okie981

Here's the post, you're correct, it was never confirmed to be GT3/GT4 OEM. I can obtain an OEM GT3/4 part from a Rennlist member. Just need time.

https://rennlist.com/forums/gt4/9483...l#post13807855