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Sorry, but I have to take exception to several of your statements regarding the causes of death wobble. Yes, caster angle is a very big player when it comes to the problem. Generally speaking, when a lift is installed on a solid-axle vehicle, the angle of the front driveshaft is increased, often to the point of either causing U-joint binding at full droop or a vibration in the shaft. It seems that the usual solution is to turn the housing to raise the pinion higher and reduce this angle. The big problem is that when the housing is turned upward, the caster angle at the steering knuckle is reduced.
The proper solution is to cut the steering knuckles loose from the axletubes, and then re-weld them at the proper angle—not an easy fix. In some instances, an offset ball joint can be used to somewhat solve the problem if the caster angle change is not too extreme. It seems that most people kind of reach a compromise between proper pinion angle and caster angle. On IFS vehicles where the differential is usually lowered, not only to improve driveshaft angle but to lessen the angle of the CV joints on the connecting shafts, caster angle is corrected by use of shims or offset ball joints on the steering knuckle. However, I have never seen a case in where excessive caster angle is the cause of a wobble. It is the lack of caster that contributes to the problem. Land speed-record vehicles and dragsters use an extreme amount of caster angle to help them track in a straight line, and experiencing a wobble at high speeds is “death.”
When trying to solve a wobble problem, a shotgun approach is always needed, especially on a vehicle that has accumulated a considerable amount of miles. One needs to always look at the overall condition of every component that is involved. This does include things such as the condition of the tie-rod ends, steering box play, track bar ends, ball joints or kingpins, and wheel/tire balance. Any worn component can contribute to a wobble, and often it is the culmination of several things contributing to the problem Yes, you’re correct in saying that toe-in does not have any effect on the wobble, but the large tires will have more of a “push” outward, so most experienced alignment shops will add a bit more of toe-in to ensure proper tire wear. The main point here is that larger tires and wider rims will definitely cause a change in everything from scrub radius to the load on steering components.
Let’s take a quick minute here to discuss what is called Steering Axis Inclination (SAI). This is the angle from vertical of the axis about which the front wheels rotate. Look at a front axle and you will see a ball joint at the top and one at the bottom. Draw a line in your mind between the two. This is the SAI. Continue with this line down through the tire to the point where it meets the ground. This is what is called the “scrub radius.” If the point of intersection of this line with the road is in the center of the tire, it is called a neutral scrub radius and is ideal. If it is outside the center of the tire, it is referred to as negative scrub radius. A bit of negative scrub will create a force toward the center of the vehicle, and tends to make the vehicle more stable. If the line hits the road inside the centerline of the tire, we have positive scrub radius. A neutral scrub radius is the ideal, as it gives the best tire wear and the easiest steering. When you go to a taller tire, the scrub radius has a tendency to go negative but usually one also goes with a wider rim and increases the tire width and perhaps a bit less backspacing on the rim for additional clearance, so the scrub radius goes back to neutral. When one reduces the amount of backspacing a considerable amount—such as using a rim designed for a solid axle with, say, 3.75 inches of backspacing on an IFS frontend which originally had a back spacing of, say, 5 inches—all sorts of geometric problems develop.
Due to clearance issues, it is not always possible to maintain the same factory wheel backspacing. Setting a wheel in a wider stance is just like adding a lever arm that causes an exacerbating effect on all the components. I can speak from experience that worn kingpin bearings or out-of-balance tires will definitely cause a “death wobble.” I am not trying to take anything away from your opinion but expressing my view and experiences based on over 50 years of working on 4x4 vehicles.
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