7/28/2019 Bump Steer
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What Causes Bump-Steer? By Ed A. Stevens
Bump-steer is the result of poor (and sometimes unavoidable)
suspension geometry. Thereaction is described as the vehicle
steering off the intended track, on it's own (withoutdriver input),
when encountering a bump. Sometimes it is described by a driver as
thevehicle needing constant direction correction whenever driving
over a bump, "I hold thesteering wheel straight but the vehicle
steers to the right every time I hit a bump."
The cause is the axle tilt, due to the tire hitting a bump,
changing the length between theaxle draglink mount on a spindle and
the pitman arm on the steering box. During acompression bump this
length is reduced and the spindle must rotate (steer) toaccommodate
the draglink length, or we have to design in something to keep the
axlenearly the same distance from the frame at the steering box --
a pan hard rod or track bar.
Even with a track bar the compression travel (inscribed arc) of
the draglink and the track bar will not be the same unless they are
equal in length and are parallel. If one of the linksis longer than
the other it will scribe an arc with less horizontal travel and
this differencewill be felt as undesired steering.
A short bar link has more horizontal movement for every unit of
vertical movement,compared to a long link, so it will have a
greater impact on steering (side to sidemovement). A steeply angled
bar link will have a larger horizontal travel component fromthe
rest position, compared to a level link, creating a horizontal
movement quicker than thelevel linkage.
The result of a poor combination of linkages is the axle moving
right three inches, in abump, when the axle end of the draglink
moves four. The extra inch of draglink movementpushes the spindle
out an extra inch, steering the tire to the right. This is common
on theXJ when the draglink is extended to center the steering wheel
and the track bar is notadjusted to match.
Tall lift heights compound the bump steer problem due to the
acute angles of the draglink and track bar. These steep angles
require considerable sideways movement of the axleduring a bump.
Sometimes this makes the steering sluggish requiring considerable
effort toturn (like climbing the edge of a large rock with the
right front tire) because you have to
fight both the steering resistance and the horizontal component
of the vehicle weight placedon the trackbar.
Reducing the angles of the linkages, at rest, helps to keep the
immediate horizontalmovement to a minimum. This is where conversion
to a tie rod and draglink over theknuckle helps. The solution also
requires the track bar length and angle to match,requiring the
factory anti-sway bar linkage and steering stabilizer to be removed
andrelocated.
7/28/2019 Bump Steer
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Death Wobble is different from bump steer in that it is unwanted
vibration of the steering,not a simple single event bump. The
vibration amplitude increase during DW becomes solarge that it
feels like the steering wheel will be ripped out of your hands. The
tires andsteering linkages rotate, bend, and flex at a frequency
that is not dampened but increases.A change in speed will usually
reduce the DW (normally easy) but sometimes the reaction
is so severe that the vehicle chooses it's own path before you
can slow down.
There are many factors impacting DW and most near stock vehicles
suffer DW due toworn components. Play in the components will allow
small movement of the steering andaxle increase until the vibration
matches the road speed (usually near 40 to 55 mph), andthen
increase in amplitude with the energy of a quick bump.
Lifted vehicles have the acute steering angle to compound the
problem along with greatertire weight to store more energy. This is
the aspect of DW that most people overlook, thestorage and release
of energy during the DW vibration. When the energy stored
exceedsthe friction dampening of the loose steering component the
released energy impacts the
steering wheel (you feel it wobble).
The factory uses a stabilizer to dampen and reduce the vibration
energy (convert it intoheat). Again, the lifted vehicle usually
results in a steep steering stabilizer mount angle thatreduces the
effect of the stabilizer damper. Keeping the stabilizer level
(in-line with the DWvibration) will help a DW problem. This is not
the fix if other components are loose, but asolution aid if the
components are good. Another element is the stiffness of the tie
rods anddrag link. Heavier parts that provide more resistance to
vibration bending help reduce theDW.
A major contributor to stored energy is tire weight and tire
offset from the spindle bearing
centerline. Greater offset (less rim backspacing) places the
tire energy on a longer moment(lever) arm that quickly amplifies
the tire energy on the steering. A smaller tire with a rimusing
less backspacing (30's on a 3.75 BS rim) will impart greater energy
than a larger tirewith less offset (33 on 5.25 BS rims) masking the
impact of mere tire weight and size. Keepthe rim BS close to stock
and the potential for DW is reduced.
Reducing the caster and toe to zero also reduces the dynamic
energy imposed by the tiresrolling on different paths, resulting in
alignment as a major contributor to DW.
The bending that occurs at the knuckle (or in the wheel bearing)
is unavoidable unless theweak area is reinforced. Another poster's
concern regarding the wheel bearing is valid
because little can be done to change the strength without
changing the design (but what if the real problem is a weak
knuckle/spindle).
This post provides considerable information to chew on, with a
few recommendations, soI'll stop. Be aware that straight axle
Fords, Toyota's, and Dodges all suffer from some DWso you can run
away but not hide from the problem.
