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Most of the books on vehicle dynamics tell us that we should be vitally interested in the natural frequency of our springs. I have never figured out why?
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Carroll Smith wrote this and the following sentences in a book published in 1978. I think it's fair to say that racecar engineering has progressed a little bit since then. While his statement was completely reasonable for his time, and probably the next decade and a half, there have been a couple of things that happened that made frequency anaylsis pretty stinkin' important.
First and formost, was data acquisition systems were made small and robust enough to live on a racecar. That was a huge advancement. Secondly, damper technology progressed to the point that we could adjust damping in very specific areas and have the ability to measure with very good accuracy where and how these changes were being applied.
On an aerodynamic racecar, you don't shoot for the 'flat ride' criteria that you do on a street car. If you run the rear at a higher frequency than the front, then as the aero pushes the car down at high speed, it will increase the rake in the car and shift your CoP forward. Not much good is ever said about high-speed oversteer. For this point alone, people tend to run the front quicker than the rear, even if they don't know they're doing it.
The second point is that in most racecars you are constantly looking rear traction at corner exit. Stiff rear springs usually give you all sorts of wheelspin problems. To use a great quote from Carroll, "A car is like a primate, it's got to squat to go."
_The Shock Absorber Handbook_ has a graph on page 61 that is replicated in many other books. No one explains it's massive implications very well, though. It's a graph of transmissibility vs. frequency ratio (impressed freq./natural freq.). We know that if we bounce the system at it's natural frequency the thing bounces out of control. What you should note, though, at higher frequencies the zero damping case is optimum! Who woulda thunk it? We spend all this time on dampers just to find out we'd be better off without them.
So, you might pose the question, what type of a scenario would be good with no damping? Well, the answer is that any road input that comes at the car quicker than Nat. Freq.*(sqrt(2)) will be felt less by the driver if the dampers have no damping. At highway speeds, that means pavement seams and pot holes, but it also means that in your sportscar with its 2Hz ride frequency, anything that takes less than about 1/3 of a second to happen will upset the car less with no damping and more with more damping.
Ever driven an old car with shocks that are shot? You can hit a rail-road track and its ride over the rails deceptively smooth. You get out on the highway and over a little undulation in the pavement wants to bounce into the next lane. This is exactly the case of zero (or very little) damping. In some ways, I like driving bad cars just to feel this.
With a data system on a modern racecar you can look at bumps that the driver complains about and determine what frequency they are coming in at and what direction to go with the dampers to make the car ride them better. You can also see if maybe it isn't a shock problem at all, but the overall spring rate is too quick or too slow for the road input. Adjust the rates accordingly and the problems will sometimes just disappear.
Carroll was a very 'from the hip' sort of guy. He had a huge wealth of experience and would know what to do with the car by using that experience. He wasn't a mathmatics or computer wizz by any stretch. He would flat out tell you that he didn't know a hell of a lot about shocks. He only had to deal with them in his last couple years of race engineering and that meant pretty much delegating the job to other people. I can promise you, though, if I could have sat down and shown him that by knowing the spring frequency and damping ratio the data system will tell me which way to do on the shock clickers, he would have been all over it.
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SuspTestEng, I bet you can pull out a bunch of the low speed damping (where you have it near critical drop down to 0.6 or so) without losing the crispness in the steering, give the car a better overall ride, and better overall grip. I used this exact same reasoning to keep low-speed damping in a racecar. When I finally took it out, we went way faster and never lost any response as long as the front springs were anything above marshmallow level. Piston bleed is your friend.
