Some of you may remember Valerie from her Introduction to Driver's Education posts from a few weeks back. Today's post comes from Vaelrie's brother Ernesto. Ernesto is a Porsche 993 owner, enthusiasts and PCA DE instructor. Recently, while tracking his car at Waterford Hills Raceway, Ernesto experienced a catastrophic suspenion failure at 90+ MPH. Fortunately, he was able to maintain control of his 993 and he was uninjured. While I would have preferred to wait and post this article with pictures, given the fact it could happen to any one of us, I wanted to post it up immediately. The following is a description of what happened and is important for all 993 owners to read and understand.
To my 993 family friends,
The suspension failure on my car this past Tuesday has an interesting background and is worth understanding if you own a 993. Please note that the 996 and 997 use very similar suspension components and geometry, so my final conclusions/recommendations at the bottom do apply.
I went through the car yesterday and have the following details to offer:
Issue: Rear left suspension collapsed under very high lateral loading and moderate impact from road surface. For reference, I was in a right hand corner, traveling about 90mph, under maximum acceleration, at Waterford Hills Raceway.
Root Cause: Rear left knuckle failure at lower attachment point. The lower attachment point joins the base of the shock absorber to the knuckle and is held geometrically in place by a cast aluminum link that attaches to the rear-most point of the rear crossmember.
Failure Sequence: The lower attachment point on the knuckle has two 'ears' that wrap around the shock. The cast aluminum link has a ball joint stud that goes through the knuckle ears and the shock. This bolts the assembly tight with a large nut on the opposite end. The left 'ear' (forward on car) of the knuckle failed first, allowing slop in the overall joint. This caused extreme bending of the other 'ear' on the knuckle, which eventually failed also. At that point, the rear left suspension had significant slop. (That is what I was feeling going through big bend, which I thought may have been a corded tire.)
Please note that all the events described here happened in a few seconds, but the failure modes of the components show that there were various phases.
The sustained corner loading on the car (during the corner) caused the suspension to push rearward, its travel being limited only by the shock and the cast aluminum link. The cast aluminum link bottomed out on the crossmember and began bending dramatically. Eventually it failed also. The shock rod then broke and also collapsed, ripping the sway bar link off of the shock in the process. At that point, I had the car stopped and was limping back to the pits with the suspension collapsed.
Background on Ernesto's Porsche 993
I bought my car with only 36,000 miles and clean car fax. The car drove perfectly, but during the first few drives I noticed that under moderate cornering acceleration, I could generate fairly violent wheel hop. Under mild acceleration, the car would not do it. Under very aggressive acceleration, the car would not do it. But under moderate acceleration, with only mild wheel spin, the wheel hop was severe.
I learned after talking with various Porsche folks, that the original shock absorbers on the car tend to blow out fairly quickly, allowing the suspension to hop around uncontrolled. I thought that was an insane recommendation given the low mileage on my car, but through forum chats I discovered that people had seen this issue on cars with as little as 20,000 miles. So, I took the shocks out of my car and confirmed that I had a left rear shock completely blown. It's incredible how well the car drove considering the condition of the shock. I replaced all 4 shocks with slightly better quality Bilstein HD's. During reassembly, I noticed that the car had a slight amount of wheel end play when performing the traditional top-to-bottom and side-to-side check for loose bearings.
Even though the play was slight, I decided to replace the culprit link / ball joint. This link was the lower, rear-most cast aluminum link described in the content above. With new links, the suspension was firm, slopless.
Over the next year, during my routine vehicle checks, I noticed that the rear suspension had again developed a very slight slop. Very slight. Most people would ignore it. I monitored it but it didn't seem to get worse.
Looking back now, given the severity of the original wheel hop issue, I suspect that the knuckle integrity had been jeopardized. The lateral slop and violent wheel hop the car experienced may have stressed the knuckle enough to create a hairline crack, or the start of a crack, that I never noticed. It took four years of dedicated track use and sticky race tires for it to let go.
Conclusion / Recommendations
The failure I experienced on my car can be prevented. So,
- Check your rear suspension looseness regularly.
- Be very strict about slop or endplay.
- If you have end play, have a thorough inspection performed and replace any component that contributes to this end play.
I want to thank Ernesto for sharing this post with my readers. Has anyone else experienced anything like this or something similar? Ernesto is going to be repairing the car next week, so hopefully I will have a follow-up post with some good pictures.