another DW discussion...
Riddle me this.
I have never replaced my steering stabilizer. It is only there because I don't feel like unbolting the blasted thing.
I have never put a new TRE or a new bushing on my rig.
It was at stock height. I had zero death wobble. I got a flat tire (due to too much fun rallying and a rock getting friendly with my sidewall as a result) and put my spare on. I had occasional death wobble so violent that at one point it literally broke the motor mount casting bosses off the side of the block. I had to replace the motor, there wasn't enough left to bolt the motor mount bracket back on.
While I was replacing the motor I put in the whole new drivetrain and finished the 4wd conversion, and lifted it. The new tires came off my old XJ. They were balanced 2 years and 20 thousand miles ago. Since then they have seen burnouts, severely dented rims, had the beats unseated, had lugs ripped off, had chunks of tread slashed out by fenders, etc. Never rebalanced.
The front lift consisted of two springs. I'm still running the same stock control arms, stock shock absorbers (replaced a few years ago), stock track bar (not even recentered), stock steering (same ZJ tie rod I used at stock height... realigned to same specs as before I lifted it) and I have zero deathwobble now. My camber is horrible because ktm racer did some pretty awesome jumps with this axle before I got it (for the right price) and I wasn't enthusiastic enough with my BFH while bending the inner Cs back into position.
Damn near anything in the frontend can cause deathwobble. A steering damper
will not fix it. It will only weaken it or hide it, and it WILL come back when either the damper wears out somewhat, or the suspension gets looser and the damper is no longer enough to cover it up.
Basically, your suspension is theoretically equivalent to a pendulum, with a very stiff spring holding it in the middle and a shock absorber in parallel with the spring (the spring is actually the equivalent of the springs, track bar, bushings, and to some degree the steering and balljoints.) The shocks and the steering damper are the equivalent of the shock absorber. Every pendulum has a 'characteristic frequency' that it will oscillate at, if 'pumped' by a rhythmically applied force at or near this frequency. Nothing about the damper or shocks will change this characteristic frequency - they can only decrease the 'quality factor' or Q factor, which decreases the peak intensity of the oscillations but widens the range of frequencies the pendulum will almost oscillate at. The real answer to fixing the problem is finding the part (bushings, control arms, track bar, steering links, TREs, balljoints, tire size/weight) that is making the characteristic frequency of the suspension fall within the range of frequencies applied by the road as you drive. With a properly maintained and engineered suspension, all the frequencies that the suspension will sustain oscillation at fall well outside the range of frequencies that can be applied by a road while driving. Unbalanced tires can cause deathwobble because they, like bumps in the road, can apply oscillating force at a frequency that depends on your road speed - and when you reach the road speed that results in the right frequency being applied to the suspension, it 'pumps' it and deathwobble starts up. Bad toe or caster can cause deathwobble because they push the suspension to start oscillating as it is - you can see what happens with crummy caster just by driving a bent shopping cart around and watching one front wheel flap, and bad toe is pretty easy to understand, because the tires are trying to fight each other for what direction you're going. Even bumpsteer is related - if you hit a pothole with the passenger wheel, the suspension can compress enough to cause the passenger wheel to swerve slightly, if it swerves at the right speed it can approximate the characteristic frequency of the suspension well enough to get deathwobble started.
Where things really get strange is how different parts of the road apply vibrations at different frequencies. Ever notice that a bridge expansion joint or pothole that you hit once in a while always seems to start your deathwobble off? That's because sudden impacts are effectively the sudden application of vibrations at a very wide range of frequencies (see also: Fourier transforms, Laplace transforms, Heaviside step function) and generally, your luck is bad and one of the frequencies involved falls in the range that will set off your deathwobble.
Any resonant system (the suspension, in this case) is either underdamped (BAD, deathwobble will result at the right frequency), critically damped (perfect), or overdamped (your steering will respond slowly, or you will get a lot of wheel hop.) You want your suspension to be critically damped at all frequencies but often that isn't possible, so you really want it to be critically damped within the range that it will spend most of its time in, overdamped where that is not possible, and under no circumstances should it be underdamped at any point that a normal vehicle can experience.
Bet you never expected to hear about physics, calculus, and differential equations while talking about jeeps, right? :spin1:
edit: if you're an electrical engineer you probably already know this, but it's exactly the same math that applies to RLC circuits, too.
The only conclusion I can come to from my experience (no deathwobble, swapped on spare tire, got deathwobble really bad) is that my suspension was worn enough that it was close to getting deathwobble, but the mass of the tires and wheels was low enough that the characteristic frequency was too high to be present. The spare tire had more tread and weighed enough that it brought the characteristic frequency of the suspension system down, and suddenly hitting a pothole could produce a high enough frequency vibration for just long enough to produce deathwobble. Putting on the higher-rate aftermarket lift springs and much heavier 33" tires resulted in the characteristic frequency dropping far enough that the suspension only experiences that frequency at low enough road speeds that deathwobble is more of a shimmy that only happens a few times before dying out (overdamped.)
in before "what's the frequency, Kenneth?!"
