My 06 F350 4x4 does the same exact thing
that all of the other trucks on this forum do with the steering wheel
oscillation and the violent shaking. It is down right scary(as well as
dangerous). I think i may actually know what the underlying problem is
too. My hypothesis (as yet unproven) is as follows:
In my opinion the symptom not-so-fondly named "death wobble", in this
particular instance, would be more appropriately classified as "bump
steer". Bump steer is a very common occurance in lifted vehicles and is
caused by steering geometry that is changed when you drop the front
axle in relation to the frame of the truck. More specifically, it is
directly related to the position of your steering centerlink in
relation to the track bar.
Trac bar - 101. The trac bar is a rigid bar that extends from one side
of the frame of the vehicle down to the axle on the opposite side of
the vehicle. This keeps the axle firmly oriented under the vehicle,
preventing it from shearing off under side loads. Without a trac bar,
the only thing keeping your axle from going side to side are the bolts
on the springs which are woefully under rated for that task. When you
have a rigid track bar in place the axle acually moves side to side
under the
truck as the
suspension compesses.
This is because while the bar holds the axle firmly, it also forces it
to travel in a mild arc. Your front axle actually moves side to side
under the truck as much as an inch (under severe wheel travel) when the
suspension compresses up and down over bumps. Bump Steer happens when
the arc created by the axle differs from the arc created by your
steering centerlink. The steering centerlink acts similar to a trac bar
because as the suspension flexes, it too travels in an arc. When the
axle is moving side to side becuase of the trac bar, the
tires actually
twist from left to right becuase of the arc created by the steering
centerlink. So, if the axle moves an inch, and the steering centerlink
only moves say 3/4 of an inch, this means that the tires are twisting
left to right a few degrees simply because you hit a bump. This is felt
in the steering wheel in mild cases as oscillation and in extreme cases
as this wild violent shaking.
In order to minimize the effects of bump steer, you want to accomplish
two things, one, you want your track bar to be roughly the same lenth
as your steering centerlink, and two, you always want it to be
perfectly parallel to your steering centerlink. Doing this makes the
two travel in in the same arcs and minimizes bump steer yada yada.
Getting back to the case in point. On the 05-06
superduty trucks,
( at least all the ones I have been able to examine) the track bar is
approx. 6 inches shorter than the centerlink and where it mounts to the
frame it is about an inch and a half too high in relation to where the
centerling moints to the pitman arm. This translates into dramatically
different arcs of travel and would seem to me to be a likely cause of
the violent shaking on bumpy roads. I doubt that any amount of part
swapping will do anything to resolve this problem becuase in my
opinion, it is a design issue.
How to fix this problem? One could make a dropped trac bar mount for
the frame side of the trac bar that would move the mointing point down
around an inch and a half to make it parallel to the centerlink. This
would not totally fix the problem, but would likely help. The last
thing to do, and this would be difficult, but if one could adapt the
05-06 superdutys to use the tie rod ends and centerlink that the 99-03
superdutys used, this would mean a shorter overall centerlink and when
combined with a lower track bar mount, it should virtually eliminate
all bump steer.
My idea is based on a few assumptions which may or may not be correct.
1. The largest percentage of vehicles with this problem are 4x4 trucks.
2. Most of the vehicles that do it, do it right off the dealer lot
(even if it doesn't happen isntantly is no reason that it couldn't,
given the right set of bumps)
3. Many owners of 4x4 trucks do experience this problem to varying degrees
4. Dealers often have troubles repeating this problem because the issue
is directly related to wheel travel on the passenger side of the truck.
( if you hit a bump on the drivers side, you will obviously feel the
bump, but you won't have the axle traveling in an arc like you would if
it hit a bump on the passenger side, so no bump steer)
5. Dealers often don't know a lot about steering geometry becuase they are an
OEM shop.
They don't do suspension mods so they haven't had to learn the sciences
behind steering and suspension geometry like a custom 4x4 shop would,
and therefore wouldn't even think to consider geomety as an issue.
6. Lastly, becuase so many people have this issue and none of the
dealers have completely resolved it, to me that would indicate that it
is not so much a faulty component issue, but would more likely be a
design and engineering issue.
I plan on trying these ideas for myself, but until I finish remodeling
my house, the wife says no big truck projects. So............unless
someone else can try my ideas first it will have to wait till after
christmas for me to do some in depth testing and modification to verify
things. Any thoughts anyone?