Originally posted by transio
..... Re: fluid dynamics, you say that all that matters is the speed with which the shock is compressed. What you're missing, though, is that the axial load placed on the shock is specifically what determines the compression rate....
Haha! Man you're not going to let me slide on outta this discussion are you? Here's the deal - I'm missing a lot of things in life but that little tidbit of info is not one of 'em! I was just using the million pounds to illustrate the point that it doesn't matter how much "force" you apply to one end of the shock body, the other end of the shock will only transmit as much force as the internals will allow (i.e., as much as the damper is resisting the movement of the rod). What YOU'RE missing ;> is that in that example I specified a rate of 1 inch per second - as if the shockwere being compressed by a huge ram or something. It was just for demonstation purposes if you will - another gedanken experiment.

I can see why you might think I was missing something though because I used "pounds" to describe the force - which assumes acceleration due to gravity. I'm not an engineer or physics guy so I talk of force in pounds instead of proper terms like "Pound Force" or "Newton". Sorry. We both know if you put a million pounds on top of an extended VX shock here on earth and let it drop it would (for all practical purposes) fall at 32 ft/sec squared since the resistance generated by the damper would be nil compared to the force of 1 million pounds. But of course while it was falling through the travel of the shock, the bottom of the shock would only be pressing down on the earth the amount the compression damper could resist the shock rod's movement. If you had on steel-toed shoes you could - for an instant - be a hot dog and balance a million ponds on the end of your foot! And about a 30th of a second later - when it reached the end of the travel and the shock bottomed out - well then you'd have a crushed shock and a million pounds on your foot. Ouch!

But the heck with all that outrageous stuff. In the real world our VX shocks only have a thousand pounds or so above them. And practically speaking, that weight doesn't vary much during the typical drive. What determines the shock rod speed is simply the speed of the vehicle and the slope of the obstacle it's rolling over. If you hit a six inch high, four foot wide speed bump and you're doing 30 mph the shock will compress at a speed of about 11 feet/sec (just ignore tire sidewall flex, the car pitching up, etc .) I have no idea how a VX shock plots out on the shock dyno but let me tell you from personal experience - a speed bump like that will give the ol' shims a workout but the VX copes just fine. Haha - I hit that exact speed bump at 30 mph today - was tooling around Duke Univ. Med Center - late for an appt - never been there before - looking for the parking garage - not paying a bit of attention to the road - WHAM I nailed that hump. Fluid dynamics in action...

And to answer the other part of your question - yes, if you have a cheap shock with welded body and the rate of compression is extremely high you can end up in a "hydraulic lock" scenario and the shock body can rupture. This is much more likely with old style damper rod type shocks since they use a fixed orifice. There is a velocity squared relationship between flow and resistance - that is - if rod velocity is doubled you get four times the amount of resistance - quadruple the rod speed and you get 16 times the resistance. It adds up in a hurry and pretty soon you end up in a situation where the fluid won't pass through the damper quickly enough and internal pressure builds and if you've got a weak weld the shock could split. That hardly ever happens though. Most of the time the seal will blow or the shaft will bend before the body will split. With cartridge style dampers like what we've apparently got (don't know for sure since I've never taken one apart but I think someone on this board reported shim stacks) the shims flex away from the piston in response to increased speed thus increasing orifice size and preventing hydraulic lock. Heck they could probably even handle a million pounds without blowing up!!! ;>