damenh85

Hi Guys-

I was getting ready to do a brake bleed before the PCA track event at TWS this weekend, and immediately when I jacked up the left rear, it was clear there was a problem. Upon further inspection, I found that the top of the pin top / shaft mount had sheared off at the bottom of the threads. The other side is fine (for the moment). I mounted these about 18 months ago (after purchasing used) and have done +/- 6 track days since. Clearly something was seeing load in a non-intended direction. Thoughts on the root cause? Has anybody else experience this?

I know that clevis mounts are the standard for coilover installations now, and I can imagine that would have prevented this failure.







-damen

cruzin2

Hard to tell from the pictures but its more than a pin failure but looks like the shock rod has sheared off. Typical side load failure of a shock rod. Usually coilovers have larger diameter rods to withstand some of these loads. Can't tell from the pictures if it was a crack initially and then a fatigue failure or if its a "true" shear failure. Either way, this is not good. It's tough to know what's the event cause though with used track parts. I would consider another supplier for my next set.

froggy47

You need this for top mounts.

http://forums.corvetteforum.com/auto...available.html

Think of the top of the shaft like a straightened paper clip. Grab it with vise grips.

Start flexing the bottom a few times.

sperkins

I have those mounts on my Penske's and they are the bomb.
The problem is they don't sell them anymore.

travisnd

Looks like you have the delrin upper mounts. They have to be loose enough to allow the shock to move some, but tight enough to prevent the nut from backing off. I'll bet whoever installed them tightened them to where it couldn't move. Thus, every time the suspension moves up and down the shock shaft is getting stressed/bent. With the stock rubber mounts it doesn't matter because the rubber is flexing.

I drilled the tip of the shock shaft and used a castle nut with a cotter pin. This allowed me to keep the mount loose enough to allow movement, but not worry about the nut backing off.

Shaka

Correct. That is a fatigue failure in tension. There can only side loads if the wheel hits it.

yakisoba

Clevis

briannutter1

Call up Anze and get their spherical upper mounts for pin top shocks. The rears are very easy to install and allow the shock to articulate much better. You will have a lot less wear on the shaft as well.

I wrote up an install article and posted it up a few months ago..you can probably search it under Anze.

RX-Ben

If you are using delrin spherical type mounts the (self locking) nut should be tightened to just barely snug up the pieces (or slightly looser). Torque value is 0.
That is how tight I had mine for thousands of street and track miles, no issues.

KeaRveK

Hardbar sells them as well,

http://hardbarusa.com/hardbar/produc...roducts_id=141

froggy47

Pfadt Racing

Thanks for the support Scott! But here is something much better:

This is the proper mount for Corvettes: The Pfadt BOA mount. This mount is specifically designed for the Corvette packaging area and address all of the shortcomings of other systems. We have been selling this for well over 2 and a half years now and not a single failure like the one displayed in this thread. This is the new standard for Corvette shock mounting as it does NOT reduce your shock travel like a clevis system, provides full articulation with no binding, and it fully supports all of the loading that the shocks see with and without a spring (shock only or coilover). All the degrees of freedom needed and virtually zero friction introduced with this system.

We have had a design in our back pocket for aftermarket Penske shocks due to customers calling us about those issues. Below are pictures of that exact design. With enough customer interest we could make a batch so they never have to experience those problems again! But this is not something we have stock of at the moment. Just posting this to gauge the community's interest in such a product.

Let us know what you think. Here is a quick video of the BOA in action and how it works!




Pfadt BOA mount for Penske series shocks:

froggy47

So this is only Penske, right?

sperkins

The BOA mount looks really nice too.
Here's a quick video showing the articulation on Pfadt's previous delrin version.

jeremy_sanchezC6

pfadt is the only one with a real solution for this problem and the BOA mount is (as far as I know) used exclusively on their Pfadt coilovers. It sounds like they can make a version of their mount that can be installed on a penske to correct the problem...... if we are interested.

Ls2_robertdrewrose

Doesn't make a whole lot of sense to make a mount for a competitors product....they should let penske figure it out for themselves. Engineers obviously aren't the best business men

jeremy_sanchezC6

I think its good for business. They already know what they offer is the bst coilover design, why not help others who alredy have a different system and help make it better.

autoxer6

This set up would be great on my penskes. I wonder if this would be legal for SCCA SS autocross? The rules require you to maintain mount type, and suspension geometry. I would still consider this a pin top mount, however the pivot point has been moved down slightly. Definitely splitting hairs here.

damenh85

Thanks for the responses everyone-

I have/had these mounted with the spherical delrin mounts from Hardbar as linked earlier. I can't remember how tight I made those nuts during the install, but I didn't use an impact gun or a vise during the install. I have a hard time believing that I applied enough torque with a hand wrench to pull apart the stainless mount- does anyone really think that's likely? I admit my material science knowledge is quite small. Also, they aren't so tight that the shock couldn't rotate in the mount.

I've been told that the angle through which the shock rotates is (deliteriously) large. It seems to me that as the suspension reaches its compression limit, the conical bore of the delrin mount doesn't allow for enough angular movement of the pin top, thereby introducing a bending load. This load would be alleviated by exactly a design such as Pfadt kindly posted, a clevis mount, or a "wider" opening angle in the cone of the delrin mount.

Thoughts?

In related news, my buddy let me drive his E36 M3 track car for the weekend, so all was not lost!

-damen

Shaka

This type of failure is rare. How many hours does the shock have? I build scratch chassis and I always use Heims and never mess with those type of mounts. Thet are for street.
Spherical mounts are for stick axles like in NASCAR and don't belong on a Vette.
There was too much clearance in the upper mount bushings causing impact loads. The nut wasn't tightened enough. Shock mounts do a lot of work and should be checked every event. The top mount brackets upper and lower bushings are designed to move like bearings. They also absorb a lot of energy that is transferred to the chassis. The slightest amount of clearance causes severe impact loads even in rebound depending how the shock is tuned.
This shaft failed due to these shock loads. There is nothing wrong with the shock and shaft is replaceable. The top mount nut must be at the correct torque and checked every meet. Replace them every year.
You should have been able to feel or hear this shock prior to failure. Inspect for cracks on mount hard point also.
You can over tighten the bottom mount bolt also which loads the shaft adversely.