Ground Control rear shock mount failure
10-29-2008 | 11:32 AM
#32
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Joined: Jun 2007
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From: Palm Beach, Florida
I agree with Jay's recommendations to tighten the bolt to 85 ftlb as that is the industry standard for that size bolt into that material - I have now done mine to that spec 
What I did notice was that one of the bolts went in quite easily and the other was much tougher to get in. From research I found that a lot of bolts suffer from "thread friction" and the larger the bolt, the higher the friction. So when torquing down that bolt, the thread friction acts against the torque wrench causing a 85ftlb reading to only reflect an actual torque applied that could be some 20% lower. The other thing that could affect the reading is the use of any thread sealant, which causes additional friction on the threads.
There's a bit of a write-up on this Here
I think what we have here is that the manufacturing inconsistency of the arms combined with the large bolt thread causing innacurate torque readings. Before I redid mine I ran a tap through the threads on the arm to clean them up real good. I test fitted the bolts and they went in very smoothly and evenly. Prior to cleaning them up one of the arms was very tight. I then used a very small amount of thread locker and torqued them down to 85ftlb making sure they went in all the way and that the face of the bolt was flush with the arm.
Only time will tell.
What I did notice was that one of the bolts went in quite easily and the other was much tougher to get in. From research I found that a lot of bolts suffer from "thread friction" and the larger the bolt, the higher the friction. So when torquing down that bolt, the thread friction acts against the torque wrench causing a 85ftlb reading to only reflect an actual torque applied that could be some 20% lower. The other thing that could affect the reading is the use of any thread sealant, which causes additional friction on the threads.
There's a bit of a write-up on this Here
I think what we have here is that the manufacturing inconsistency of the arms combined with the large bolt thread causing innacurate torque readings. Before I redid mine I ran a tap through the threads on the arm to clean them up real good. I test fitted the bolts and they went in very smoothly and evenly. Prior to cleaning them up one of the arms was very tight. I then used a very small amount of thread locker and torqued them down to 85ftlb making sure they went in all the way and that the face of the bolt was flush with the arm.
Only time will tell.
10-29-2008 | 12:52 PM
#33
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From: Soowanee, GA
I agree with Bill. However, I will also add that you are using the control arm in a way that it was not (to my knowledge) designed to be used, so expect s**t to happen. I'm sure Porsche over-designed the crap out of that part of the arm as a shock mount, but not to hold the weight of the rear of the car on heavy springs. In my opinion, even if the adapter is "seated" against the control arm surface, if there is not sufficient preload and any movement/flexure of the adapter is possible, then it will tend to deform the aluminum control arm surface, which makes more room for flexure and ultimately results in fatique of the adapter bolt.
Somebody should make those adapters out of titanium, and maybe this wouldn't be an issue. Or, send clear instructions on the 150 ft-lb torque requirement.
Somebody should make those adapters out of titanium, and maybe this wouldn't be an issue. Or, send clear instructions on the 150 ft-lb torque requirement.
C.
10-29-2008 | 01:11 PM
#34
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From: Cincinnati
Well, I did say "to my knowledge", so I was correct!
So, did the 968 CS use the exact same control arm? If so, if we can find out what torque spec Porsche used for the shock/coilover mount, then I would say that is a good one to go by for the racer's edge or ground control adapters.
10-29-2008 | 01:34 PM
#35
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From: Soowanee, GA
Well, I did say "to my knowledge", so I was correct!
So, did the 968 CS use the exact same control arm? If so, if we can find out what torque spec Porsche used for the shock/coilover mount, then I would say that is a good one to go by for the racer's edge or ground control adapters.
So, did the 968 CS use the exact same control arm? If so, if we can find out what torque spec Porsche used for the shock/coilover mount, then I would say that is a good one to go by for the racer's edge or ground control adapters.
Yes, same trailing arm.
C.
10-29-2008 | 01:38 PM
#36
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From: Hudson Valley
Ok, thanks for the information guys (and Jay - did you guys get my email? I'm at work by the time you guys are open and can't call during reasonable hours until friday but I can use the computer)
Here are some pics (sorry I'm terrible at taking pictures!) I took this morning before I went to work:
Somehow the stud broke off not flush, but inside the trailling arm. It took out a tiny chunk of the trailing arm threaded hole with it
While under the car I decided to check the other side (and retorque to 85ftlbs) and while I thought it was seated properly on the trailing arm when I installed it... there seems to be a tiny gap (.030 using feeler gauges) on one side of the mount and it is flush on the other side. That seems to indicate it is slighty bent or my trailing arm isn't perfectly flat..
Man.. it seems like if anything can go wrong with anything.. it will happen to me. I think I am the perfect test bed for new products
Here are some pics (sorry I'm terrible at taking pictures!) I took this morning before I went to work:
Somehow the stud broke off not flush, but inside the trailling arm. It took out a tiny chunk of the trailing arm threaded hole with it
While under the car I decided to check the other side (and retorque to 85ftlbs) and while I thought it was seated properly on the trailing arm when I installed it... there seems to be a tiny gap (.030 using feeler gauges) on one side of the mount and it is flush on the other side. That seems to indicate it is slighty bent or my trailing arm isn't perfectly flat..
Man.. it seems like if anything can go wrong with anything.. it will happen to me. I think I am the perfect test bed for new products
10-29-2008 | 02:08 PM
#37
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From: Soowanee, GA
Hey Mike...Don't fret too much. Could have happened to anybody.
There is a seciton in the Racer's Edge piece that mentions filing the mounting surface smooth for the bolt to contact it completely. That may be the only thing you need to do. (Other than getting a new bolt.)
C.
There is a seciton in the Racer's Edge piece that mentions filing the mounting surface smooth for the bolt to contact it completely. That may be the only thing you need to do. (Other than getting a new bolt.)
C.
10-29-2008 | 03:41 PM
#38
Three Wheelin'
Joined: Nov 2004
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From: Northville, MI
I would be a bit suspect of the other adapter as well since its not flush. The first definitely failed in bending, otherwise it would be almost flush with the trailing arm. Its probably just a matter of time until it fatigues to failure too. My advice would be to buy two replacements. One for the failed adapter and one to replace the other thats still on the car. Wouldn't be surprised to see the other one fail as well.
10-29-2008 | 03:57 PM
#39
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From: The Heart of it All
The 924CGTS however, had coil-overs and no t-bars.
10-29-2008 | 04:29 PM
#40
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From: Phoenix, AZ - NJ Runaway
Yeah, the later cars all used the t-bars with coil over helpers. The 924CGTS weighed 2200lbs, not 3100 like the later cars so that's a big difference on loading. Also I don't believe the CGTS use aluminum trailing arms (I could be, and probably am, completely wrong).
I've seen this failure happen way to often when deleting t-bars completely. Maybe for a lightened track car it's fine but I am skeptical when doing this on a full weight street car. I've seen the extended bolts break, the trailing arm snap, and the upper shock mount crack when utilizing this method. I "believe" the casting is too thin to support the added loads of an extended bolt coupled with higher spring rates.
There are however plenty of guys who have been running a full t-bar delete for years without an issue but caution should be used when selecting this method. If only 5 out of 100 suffer failure......It's still to high for me to recommend. It may be easier to swap, but not necessarily better.
Since I occasionally drive my car on the street and have my family in the car, I chose to keep the t-bars with coil over assists and the proper off-set shock mount instead of the extended bolt method.
I've seen this failure happen way to often when deleting t-bars completely. Maybe for a lightened track car it's fine but I am skeptical when doing this on a full weight street car. I've seen the extended bolts break, the trailing arm snap, and the upper shock mount crack when utilizing this method. I "believe" the casting is too thin to support the added loads of an extended bolt coupled with higher spring rates.
There are however plenty of guys who have been running a full t-bar delete for years without an issue but caution should be used when selecting this method. If only 5 out of 100 suffer failure......It's still to high for me to recommend. It may be easier to swap, but not necessarily better.
Since I occasionally drive my car on the street and have my family in the car, I chose to keep the t-bars with coil over assists and the proper off-set shock mount instead of the extended bolt method.
10-29-2008 | 04:38 PM
#41
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From: San Diego, CA
If i remember right, there are picture of the rear trailing arm floating from a 924CGTs (or maybe it was the GTR). As i remember it, it was a steel rear trailing arm with boxed mounting points for reinforcement. I cannot find the picture again naturally, im surprised i didnt save it.
Im wondering, would a machined down torsion bar (say the popular 10mm) have any effect at lessening these weakness point when running a coil over? Or were guys simply doing this to get by with the rules.
Im wondering, would a machined down torsion bar (say the popular 10mm) have any effect at lessening these weakness point when running a coil over? Or were guys simply doing this to get by with the rules.
10-29-2008 | 04:39 PM
#42
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From: New York
wow sorry to hear mike. Where is this GC shop located? I need a local shop that is confident in working on these cars now that im up here.
Can someone pm me the info on GC. Looks like they could be a good place to send the p-car.
Can someone pm me the info on GC. Looks like they could be a good place to send the p-car.
10-29-2008 | 04:50 PM
#43
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From: The Heart of it All
If i remember right, there are picture of the rear trailing arm floating from a 924CGTs (or maybe it was the GTR). As i remember it, it was a steel rear trailing arm with boxed mounting points for reinforcement. I cannot find the picture again naturally, im surprised i didnt save it.
Im wondering, would a machined down torsion bar (say the popular 10mm) have any effect at lessening these weakness point when running a coil over? Or were guys simply doing this to get by with the rules.
Im wondering, would a machined down torsion bar (say the popular 10mm) have any effect at lessening these weakness point when running a coil over? Or were guys simply doing this to get by with the rules.
I took pics of all of the undercarriage, the interior, and engine bay of that car. There are lots of interesting bits on it.
Yeah, the later cars all used the t-bars with coil over helpers. The 924CGTS weighed 2200lbs, not 3100 like the later cars so that's a big difference on loading. Also I don't believe the CGTS use aluminum trailing arms (I could be, and probably am, completely wrong).
I've seen this failure happen way to often when deleting t-bars completely. Maybe for a lightened track car it's fine but I am skeptical when doing this on a full weight street car. I've seen the extended bolts break, the trailing arm snap, and the upper shock mount crack when utilizing this method. I "believe" the casting is too thin to support the added loads of an extended bolt coupled with higher spring rates.
There are however plenty of guys who have been running a full t-bar delete for years without an issue but caution should be used when selecting this method. If only 5 out of 100 suffer failure......It's still to high for me to recommend. It may be easier to swap, but not necessarily better.
Since I occasionally drive my car on the street and have my family in the car, I chose to keep the t-bars with coil over assists and the proper off-set shock mount instead of the extended bolt method.
I've seen this failure happen way to often when deleting t-bars completely. Maybe for a lightened track car it's fine but I am skeptical when doing this on a full weight street car. I've seen the extended bolts break, the trailing arm snap, and the upper shock mount crack when utilizing this method. I "believe" the casting is too thin to support the added loads of an extended bolt coupled with higher spring rates.
There are however plenty of guys who have been running a full t-bar delete for years without an issue but caution should be used when selecting this method. If only 5 out of 100 suffer failure......It's still to high for me to recommend. It may be easier to swap, but not necessarily better.
Since I occasionally drive my car on the street and have my family in the car, I chose to keep the t-bars with coil over assists and the proper off-set shock mount instead of the extended bolt method.
10-29-2008 | 08:36 PM
#44
Thread Starter
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Joined: Aug 2001
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From: Hudson Valley
Ground Control is overnighting me two new lower shock mounts 
I stand by this post.. excellent customer service
https://rennlist.com/forums/showpost...66&postcount=4
luckily it is something fairly easy to take care of, just an inconvenience
I stand by this post.. excellent customer service
https://rennlist.com/forums/showpost...66&postcount=4
luckily it is something fairly easy to take care of, just an inconvenience
