Someone on the internet asked someone else for an easier to understand explanation or translation of my most recent blog post about shock tuning. I feel for that person, but there’s an issue. I want to illustrate the problem with the following quote from Understanding your Dampers: A Guide from Jim Kasprzak, available at kaztechnologies.com:
“In fact in the U.S. they are often called shock absorbers, even though they really don’t absorb shock!”
I’ve heard this a lot. I may have even said exactly the same thing. As an engineer I think, Okay, this is technically correct, but is actually sort of an arrogant way to talk to college kids in an FSAE program, which is the intended audience. (I do understand that starting to teach by challenging assumptions is a valid technique. It tends to wake people up and prepare them for consuming a new idea. I do it all the time, so I don’t want to be too hard on ol’ Kaz. I hope he’ll forgive me.)
Of course shock absorbers do absorb shock in every sense except a narrow technical one. In fact, they turn kinetic energy into heat! Energy gets put into them and it (almost) magically disappears. You don’t have to deal with that energy any more. The spring, which is what Kaz and others want to call the “real” shock absorber, can’t do that. They give back (almost) all the energy they “absorb” and you still have to deal with it, just on a different time scale. Springs are time shifters! There, I created a more accurate description of what springs do. (Didn’t I?)
Anyway, when someone like me assails you with the quote above they’re falling back on a technical idea of “shock” of which the regular person is unaware. That idea is the rate of change of acceleration, technically called jerk.
When you get hit by a truck as you cross the street you get “jerked”. You go from no acceleration to mucho acceleration very quickly. The rate of change of acceleration (the definition of jerk) is very high and that hurts. If you were dressed in a suit of springs the acceleration wouldn’t be so bad. As the springs compress they deliver all that energy of impact to you over a longer period of time. They time shift the energy. You might even survive, at least until those springs give back all that energy and launch you into the next county because only the “shock” was absorbed. You still must deal with all the energy of that impact, one way or another. Unless you have shock absorbers.
When your tire moves up and down for any reason, bumps, potholes, little ripples in the pavement or, God forbid, you turn the steering wheel, all the energy contained, released or transformed by the event goes into the mass of the car despite the springs and make it move.
Unless you have shock absorbers.
The next statement that follows the warning that shock absorbers don’t really absorb shock is usually something like “What shock aborbers really do is damp mechanical motion.” So, we special ones (technologists of whatever sort) have our special, correct word for them. We call them dampers!
When someone says this to the non-technologist has it delivered any useful information to them? I don’t think so. Not much, anyway.
But, wait just a second, you say, filled to the brim as you are with common sense. Don’t springs do that too? When the tire hits a bump doesn’t it get 1) jerked (see, I learned to use that word in a new way, you jerk) and 2) accelerated upward into the car? Doesn’t it start moving very fast and then it slows down and stops? Isn’t it the spring that stops it? Doesn’t the spring provide a force opposing that upward velocity and reduce the velocity and extent of that motion? Didn’t the mechanical motion get damped? So, isn’t the spring a damper too?
Of course it is. Except… oh, never mind!
If you want to understand this stuff then you will have to get beyond the words and develop new, deeper and more accurate ideas. Specifically, you will need to gain some understanding of the theory of mechanical vibration of spring-mass-damper systems, which will be the subject of my next blog post. (I think.) It will take some work. It can’t be “translated” into easier to understand terms because common terms and ideas just don’t contain the necessary concepts and functions. Just like you can’t fully render Shakespearian ideas into the language of a 3rd grade nap-time story. The 3rd graders don’t have the necessary vocab, concepts or life experience. The have to grow up and learn a lot before they can comprehend, much less appreciate, the content of old Bill’s “stories.”
So, buckle up. Springs do more than one thing on a car and so do the shocks/dampers. And to give Kaz his due, he goes on to tell us what those things are. So, maybe you want to go read what he has to say.