Originally Posted by TomsBeast
DW is an interesting topic, and damned frustrating to track down the source of and eliminate. My old slightly lifted leaf sprung Suzuki Samurai suffered it, only after I added a bunch of camping junk to the roof rack just before a trip...
My all stock 1988 F350 2wd Dually came down with a bad case of DW at 80k miles... my local tire store replaced the ball joints, and it went away.
My 2002 Russian Ural sidecar rig suffered what I'd call 'low speed wobble' (as opposed to tank slapping DW) when riding about 5mph over a parking lot speed bump. Cured it with a hydraulic steering damper. Mostly.
My current Samurai, spring over lifted, with longer Jeep YJ leaf springs (a common swap/lift), just experienced high speed DW after I loaded the back with 450lbs of Sterling axle I carried home, white knuckling all the way, having to stay under 50mph.
This rear-steer Bonneville Streamliner I once crewed on, experienced by far the worst I've seen... the first driver, very experienced guy, shut it off at 90mph and pulled off after the butterfly steering wheel tried to break his wrists. Driver #2, experience TF dragster and boat jockey jumped in and tried to 'drive through it', aborted the run at a-buck-forty...the car was cut up for scrap a month later.
All of these experiences seem to have nothing in common, until you dig a little deeper. Everything is a 'system', everything is 'squishy'. DW is nothing more than a lower order frequency coupling. Like feedback between a speaker and microphone (or guitar and amp), disc brake squeal, or for you machinists out there, tool chatter, all higher frequency but the principle is the same Watch a slow speed video of a fat lady running. There's stuff bouncing and jiggling up and down, at different rates. It ain't pretty, but it works. When body parts start bouncing at the same rate and frequency, that's when things go off the rails, and as I witnessed during co-ed softball, this larger and well endowed woman teamate of mine couldn't hold it all together. She was athletic, but big anf jiggly, after getting up to a certain speed, her stuff would bounce in time, and she would fall over on her way to first base. (How's that for a mental picture?). What was happening was 'frequency coupling'.
I contend that the same thing happens with DW, semi flexible things, tires sidewalls, tie rods, the steering shaft take an excitation (bump) start oscillating at the own natural frequency, then by happenstance 'couple', making the oscillation stack on top of another. The very worst Earthquake destruction happens like this, when the earth shaking couples with the natural frequency of a building, it's game over, she's goin' down. Yet the flimsy shack nextdoor, escapes unharmed.
I suggest that it's not coil springs, it's not leaf springs, it's not leading links or caster, it none of, and it's all of those things. That is to say, all of the components combined conspire, or not, to cause DW to occur after a jolt. Since that is based of the systems natural frequency, you can engineer something into the system that raises or lowers it's natural frequency, so the system never goes into uncontrolled oscillation (Like my softball teamate's body) You can also damp it out with something like a steering damper.
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