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  #1 (permalink)  
Old 03-25-2006, 06:54 AM
mrmustang's Avatar
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Lightbulb Third party shock failure analysis with pics

Ok, let's start off with the Bilstein shocks as submitted by Marty D. who wrote to me the following:

"the shocks were mounted with body up on the front suspension. Lower control arms are from Fortes. I had some spacers between the spring hat and the heim joint to prevent any interferance from occuring with the lower arm. When I looked at the shocks I couldn't see any signs of contact between the arm and shock. There are about 5k miles on the shocks. Many of those miles were on bumpy back roads driven at speed."

Ok with that out of the way, let's look at cause and effect with the following four photo's:






From the looks of things, there was not only some unusual lateral loads (wear on the outside of the lower spring seat and threading), but my shock guy (a custom rebuilder and master WD for Bilstein, QA1, Carrerra, and Pro Shock) sent me the following email:

"From the shock it looks like the center to center of the mount on that car are at 14.5 " center to center this will allow a 5" stroke on the shock and looking at the shock, also looked like there was only 2" of shock left when the car is at ride height so that means there is only 2" of shock available to extend so constant battering on that shaft at full extension is what caused that shaft to rip off. By no means was that a shock manufacturers fault but a poor choice of mounting a street car should have at least 7" of shaft length to deal with at ride height 3/1-2 " of shaft for bump and rebound the reason that some other shocks might get away with the constant beating full extension is that the QA1 shocks pro's and carrera shocks use a thicker shaft that is much softer than the bilstein but this still does not make it right".


Just thought you would like to know his thoughts on first glance, he is going to take them completely apart in the next day or so to check the valving to see if anything failed there as well. If he finds anything else of interest, I'll let you know.

Sincerely,

Bill S.

PS: I also want to add that the jam nut still had a thread showing, so we know it was not over tightened on the shaft assembly.

PSS: Sorry for poor photo quality, but all I have at the moment that is working is my 10 year old Sony Maciva with a floppy 1.44m floppy disk.
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  #2 (permalink)  
Old 03-25-2006, 07:56 AM
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Analysis-----Let's see- we fasten a wheel on each corner with 5 studs of at least 1/2-20 size? and then suspend all that same weight on the end of a 3/8-24 stud? I think coil over suspentions with heim bearing ends should be limited to smooth race tracks--where benefits of them are balanced out by there short comings

Jerry
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Old 03-31-2006, 03:53 AM
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Default Shocks

Jerry,

I dont think the 3/8 thread is a problem whether its on the road or track, coil over shocks, with Heim joints are good for many many street and race miles. The problem is without me knowing much about it, is more than likely a build problem with the suspension. The shock guy in the previous post mentioned it may be working on the outer limits of droop and the shaft can take a hammering if its right out at the end. Some people use droop straps to limit travel, and its alway good to have proper designed bump stop installed as well!

I had a guy who supplied a set of Konis for my first Cobra, he held the shafts in a vice and damaged the hard chrome, then his business went belly up, the Koni suppliers felt sorry for me and supplied a new set free of charge, so I am about to go through the design installation of shocks on my present car soon, and its doesnt like most Cobras have a lot of travel on the front suspension. probably 4-5" max.
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