Somewhere out there exists the perfect suspension. In theoretical worlds governed by equations and engineering, we're taught that by combining the proper spring and damping rate onto a known mass (like your car), one can create a mechanical system, suspension in our case, that neither under- nor overreacts to inputs. Complex mathematics could be utilized to figure out the exact damping values to make our cars handle like race cars and still be comfortable. At least, I used to think so.
A critically damped suspension is essentially a spring and damper attached to a mass (a car) that almost perfectly absorbs a given input. For example, say you drove your car straight into a curb. If it were an under-damped Cadillac pimp-roach, chances are the front wheels would immediately suck up the bump and continue to compress until bashing into the bumpstops, because there isn't enough compression damping.
Once the front wheels are on top of the curb and the suspension has totally bottomed out, some of that energy transfers to the chassis. The rest of the energy has nowhere to go but to push the wheel back down. Unfortunately, the curb is still there and there's excess kinetic energy not dissipated by the damper. Instead of the wheel dropping just enough to soak up the motion, it drops and bounces off the top of the curb for a second time, because there isn't enough rebound damping. The body is now oscillating at a different frequency. This independent wheel-and-chassis oscillation will continue for several cycles until the under-damped shocks have finally dissipated enough energy.
At the other end of the spectrum, if you had, say, a Civic, lowered on cosmetic street springs with aggressive sport shocks, the opposite occurs. This over-damped scenario would transfer vertical motion into the chassis, causing the chassis to do the majority of the moving and not the wheel. There's so much damping resistance that the wheel is still in its slow upward stroke well past the time you're on top of the curb.
At the same time, so much energy has been absorbed by the damper (and your tailbone) that, as the suspension finally starts to change direction and extend, there is practically no rebound energy left to fully extend the suspension to meet the concrete and support the car's weight coming down, making the drop just about as hard as the jump.
In a critically damped suspension, you get the best of both worlds: enough damping rate to soak up the initial compression bump without hitting the bumpstop and transferring it to the chassis. And sufficient rebound damping to slow the wheel. The oscillations will cancel out within one compression and rebound cycle so you're not still jiggling up and down 10 feet past the curb.
But most people (other than monster truck guys) don't engineer their suspensions specifically to run into curbs or cars. If we ran our critically damped crash-mobile over a series of curbs or, more realistically, speed bumps, at a given speed, we'd still be sitting comfortably with our kidneys intact while keeping the wheels on the ground.
Simple, Right?Not really. A critical damping value only works when the bumps are hit at the right speed. If these bumps were too close together, a critically damped system wouldn't be able to dissipate all the energy from one bump cycle before the next.
What's critically damped at one frequency, isn't at another, so now you're back to trying to dissipate the energy from the first bump, whether in the compression or rebound stroke, when suddenly the wheel hits the second bump, creating another input on top of the first one. Now the whole spring mass system is excited by inputs that are out of phase, which adds up to a big mess.
That's why real-world dampers have different damping rates for low through high speeds. At low speeds, less damping force is generally needed, since there is time for the energy to be dissipated. Speed up and start pounding through the bumps faster and faster, and the forces trying to compress the suspension also increase. The larger the force pushing the suspension, the faster the piston speed will increase. This is why damping forces generally increase with piston speeds.
Problem solved, the world is still beautiful.
It turns out, however, that a suspension isn't really just a mass sitting on a spring and damper. Wheels have mass and tires also have spring and damping rates. So there are two masses jiggling independently of and dependently on each other, with many more modes of vibration. But we can still ascertain the magical critical damping rates, assuming we have the right weights and spring loads. It just takes more math and a computer more powerful than a solar-powered Casio calculator.
In my perfect little nerd world, this would be how suspension tuners and manufacturers go about building exacting suspensions. Reality is far from it. While real suspension gurus know about all this and can dust off some old text books and perform the calculations, most tuners don't even know what a damping ratio (the ratio of your damping coefficient to the critical damping coefficient) is, let alone that it's represented by the Greek symbol zeta. It really doesn't matter anyway.
All the math in the world won't make up for a highly calibrated ass and experience. And as it turns out, most OE street cars are rarely ever critically damped. It's just too hard on the driver and body frame at street speeds.
Add to that the unpredictable slop in OE rubber bushings, and even if you figured out the critical damping force, you wouldn't exactly get what you wanted in the first place. The limited surface area on OE-style twin-tube pistons also make controlling small wheel movements practically impossible. It turns out that OE suspension valving rarely targets a critical damping ratio.
For sport applications, monotube dampers (which have larger piston areas) make a big difference in controlling body roll and small wheel movements. That's one reason why cars like the Evo and STI use such hardware.
The larger pistons will give better and more consistent control over small increments, where little fluid is displaced from one side of the valve to the other. At these low piston speeds, valving control is especially important, since suspension gurus typically overdamp these piston speeds to better control body motions and wheel bounce, hence the modern digressive damping profile.
So the perfect suspension does exist, just not in the form I had once thought. The idea of a critical damping value or ratio is still important. If nothing else, it's a reference point for how to tune a shock at various ranges of piston speed.