To a certain extent this is inherent in the inverted-Y type steering that my Jeep has. Modern vehicles have rack-and-pinion steering, where a cog on the end of the steering column moves a toothed rack to the left and right to steer the vehicle left and right. The problem is that rack-and-pinion steering doesn't do too well with long-travel suspension used for offroad purposes. With rack-and-pinion steering, hitting a rock or bump will cause the suspension to thump upwards, which in turn pushes on the rack as the steering rod pivots upwards on its ball joints and shortens the effective distance between the wheel and the steering column, which then makes your steering wheel twist violently. Inverted-Y type steering instead diverts much of the force of the bump into a toe change -- rather than the steering wheel twisting violently, the other tire toes out. Basically, the more you compress the front end of a Jeep, the more the front tires toe out (point slightly outwards rather than straight ahead). If you lift a Jeep, the wheels toe in (point slightly inwards). Adjusting via the driver's side tie rod is necessary whenever you lift a Jeep, and you want a slight amount of toe-in at rest because aerodynamic forces push the front of a Jeep down at speed and cause the front tires to toe-out.
So why does this cause death wobble on lifted Jeeps? Well, first of all, lifting the Jeep increases the angle of the Y. This means that an upward movement of the suspension causes toe-out to increase by a larger amount than with an unlifted Jeep. So if your passenger tire hits a bump, the driver's tire toes out and tries to steer the Jeep towards the driver's side. This then causes the Jeep to lean towards the driver's side, which loads the driver's side and unloads the passenger's side, which in turn causes the passenger's side to toe out and try to steer the Jeep towards the passenger's side. And so on. This can set up a resonance -- the "death wobble" -- when the natural wiggle overcomes the natural damping of caster, shocks, and steering damper (yes, my Jeep has a steering damper -- a shock absorber for the steering -- to help dampen any such motions).
So my Jeep started doing this at 45mph after my last tire rotation. So what did I do? First thing I did was make sure all my steering components were tight. Death wobble often happens when the track bar is loose, for example. But nothing was loose.
Next thing I did was check all ball joints and tie rod ends for play. They were all tight and play-free.
That having failed, I took my Jeep to the tire store and had them balance my tires. They reported that my back tires were fine and they'd removed some weight from the front tires.
That didn't affect things at all, so the next thing I did was check my alignment. I did this by duct-taping two pieces of angle iron to my front tires, one on each of my front tires, with the front and back of each piece of angle iron exactly 10 inches above the ground (this is necessary due to the fact that my Jeep has exactly 10 inches of clearance under the front control arms at the back of the front tires). I then grabbed a pair of $1.99 Cheap Chinese Tool Store tape measures. I duct-taped the end of the first one to the angle iron on the driver's side in front of the tires and laid the other end on top of the angle iron in front of the passenger side, then did the same with the other tape measure *behind* the wheels. I then pulled both tape measures taut, and found that there was 1/8" difference in measurement between front and back -- I was toed in by 1/8".
I checked my service manual and did a bit of trigonometry and that's well within specs for my Jeep. Still, one thing that helps with death wobble is less toe-in, because if the wheels start out pointing outwards more, they don't turn in more when you hit a bump. So I fiddled with the tie rod and reduced toe-in to 1/16", then headed out to test. I was worried that this would make my Jeep more skittish on the highway, but it wasn't significantly more skittish. But what I *did* find was that my Jeep no longer understeered -- when I cut the wheel to make a turn, instead of fighting me and trying to lift the inside tire, my Jeep now dives into turns with alacrity. Much driving since then has verified this: My Jeep now handles *much* better than with the stock suspension geometry, because a slight bit less toe-in has reduced the understeer that Jeep's engineers built in because, well, that's how American automotive engineers think cars are supposed to handle (i.e., with lots of understeer). Although to be fair, the wider tires on my Jeep, and the lift, also contributed to the understeer effect.
Anyhow, my test verified that the reduction of toe-in didn't make my Jeep wander on the highway (what I was worried about) and made it handle better in the curves. But: It did not eliminate the "death wobble" entirely. It made the death wobble less severe, it showed up as a pulsing in the steering rather than a violent shaking, but it was still there.
At this point I decided I must have a broken belt in a tire. That happens when you drive a Jeep offroad and hit a rock too hard with too much air pressure in your tires. I even know where it probably happened -- on the old toll road up Darwin Wash to Darwin, where it goes over the mountain, where I didn't bother airing down because that's a fairly mild road as far as 4x4 roads go (I could do it in a 2wd pickup truck). So I started taking tires off and replacing them with the spare. I started with the right rear tire, because it has a dent in the rim from whacking into a rock (advantage of steel over alloy -- an alloy wheel would have shattered). No change. Next, I put that tire back on the rear and rolled the spare over to the front right and replaced that tire. And...
So it appears that my new spare has a broken belt that causes a vibration at around 45mph. I'm not sure what I'm going to do about that. Probably just leave it as a spare for now... I only have about 15,000 miles of wear left in this set of tires, and it works well enough to serve as a spare, so if I get a blowout I'll just put it on in place of whatever tire, then head over to the tire store and get some new tires (bigger meats, 33" ones, this time).
So lessons learned?
- The stock settings for toe-in are bunk once you put bigger slower-steering tires and a suspension lift on the beast. You want less toe-in then, because the bigger tires will steer slower than the OEM tires (meaning they don't need as much toe in order to keep the Jeep from wandering on the highway) and the taller suspension will make the inverted-Y add more toe-in when you lean over to do a turn, thus causing more understeer than with a stock Jeep.
- And don't eliminate tires as the culprit even if they are balanced, because there's far more than being out of balance that can cause a tire to add a "thump" that can set up a shimmy. This tire isn't even out-of-round when you spin it with no load on the tire. But once you put weight on the tire, the broken belt shifts and lets the rubber stretch, making it effectively out of round as you roll it down the highway.
-- Badtux the Wrenchin' Penguin