Steering problem question

[Diesel]

Well, I am going to work on it tomrrow so we will see. The main problem is that my Jeep is stuck at a friends house 100 miles away and is my only vehicle so it is hard to work on it. I am not too thrilled to ride Greyhound with a 60lb tool box, but hey, at least no one can make fun of me for not working on my jeep myself.

 

[Eagle]

I was trying to separate the notion that what causes wandering and decreased steering effort shouldn't be associated with what causes death wobble. In my opinion, they are two complete different problems.

You are entitled to your opinion, but if you wish to discuss what I wrote, it would be useful if you didn't misquote me. I never said that low or zero caster causes death wobble. In fact, I always go out of my way to try to clarify that it does not "cause[/i] death wobble. That does not, however, mean that increased caster does not help to mask or alleviate death wobble resulting from other causes. Too many people have reported that increased caster does help for me to ignore it as a factor.

Why? Because of the same reason caster increases steering effort. Think about what caster is. It's the angle between the upper and lower ball joints in the steering knuckle -- what used to be called "kingpin angle" back in the olf days when there was a single rod called the kingpin that held the knuckle to the axle.

When caster is xero, this angle is vertical and it points down to the center of the tires' contact patch on the road. Turn the steering wheel and the front tires rotate around the point where this imaginary line intersects the pavement.

Now dial in a lot of caster. Now the imaginary line is slanted so the lower ball joint is ahead of the upper. The axis of rotation no longer intersects the ground at the center of the tires' contact patch -- it's either toward the front of it, or even entirely ahead of it. What happens now, and why this increases steering effort as well as assisting the steering to return to straight, is that when you turn the steering wheel, in order for the tires to turn you are now forcing the tires to lift up slightly onto one shoulder -- the inside shoulder for the outside tire, and the outside shoulder for the inside tire. The tires' tendency is to run flat on the tread, so when you release the wheel the vehicle's weight makes the tire want to flatten out, and to do that it has to push the tires back to straight.

This same geometry uses the vehicle's weight to dampen and combat death wobble. However, unless something starts a wheel oscillating, there can't be death wobble to worry about. The caster isn't what starts the tire wobbling, even if it does help to control it when it occurs.

In most cases, the actual cause will be found to be tire balance. Other factors, such as worn suspension/steering components, may allow it to get out of hand, but the root cause is almost always tire balance.

 

[jalehman]

I didn't misquote you. You stated that one of the few constants in the death wobble discussions is that people report increasing castor either eliminates castor or reduces it.

I would have to agree on diesels theory that it is the sudden return to center characteristic of lots of castor that might cause the oscillations side to side. The more castor you are running the harder it and faster it will want to return to center. If it returning to center at too fast a rate it will over shoot and then want to return to center the other direction and over shoot and snow ball till you slam on the brakes and have to clean your shorts. Very good theory in my opinion.

With little castor there is nothing pulling the wheels except your hands trying to get the rig to stay in the same lane.

Oh, and by the way, Wed. night the neighbor just shortened his lower control arms to 5* castor from 8* and reset his toe in and it drives fine now with just a slight shimmy right at 55mph. Still needs perfected but definitely will get him to work now.

 

[jalehman]

I meant to say that people report that increasing castor either eliminated death wobble or reduces it in my first statement to eagle.

 

[Diesel]

For those interested, I worked on the Jeep yesterday and today and was able to drive it the 115 miles home with no problems. I re-set the castor to 6 degrees, replaced the lower control arm bushings with rubber ones, but the old poly ones were not worn. All the mounting bolts for the steering box and trac-bar bracket were tight, and i gave the adjusting nut on the top of the steering box about a quarter of a turn which took out some play in the steering. Although it drives good and I didn't have any death wobble, I am not quite convinced it is fixed yet. I guess I will have to just wait and see. Thanks for all your help.