What Causes Bump-Steer?

By Ed A. Stevens

Bump-steer is the result of poor (and sometimes unavoidable) suspension geometry. The reaction is described as the vehicle steering off the intended track, on it's own (without driver input), when encountering a bump. Sometimes it is described by a driver as the vehicle needing constant direction correction whenever driving over a bump, "I hold the steering 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 the axle draglink mount on a spindle and the pitman arm on the steering box. During a compression bump this length is reduced and the spindle must rotate (steer) to accommodate the draglink length, or we have to design in something to keep the axle nearly 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 links is longer than the other it will scribe an arc with less horizontal travel and this difference will 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 side movement). A steeply angled bar link will have a larger horizontal travel component from the rest position, compared to a level link, creating a horizontal movement quicker than the level linkage.

The result of a poor combination of linkages is the axle moving right three inches, in a bump, when the axle end of the draglink moves four. The extra inch of draglink movement pushes the spindle out an extra inch, steering the tire to the right. This is common on the XJ when the draglink is extended to center the steering wheel and the track bar is not adjusted 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 axle during a bump. Sometimes this makes the steering sluggish requiring considerable effort to turn (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 placed on the trackbar. 

Reducing the angles of the linkages, at rest, helps to keep the immediate horizontal movement to a minimum. This is where conversion to a tie rod and draglink over the knuckle 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 and relocated. 

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 so large that it feels like the steering wheel will be ripped out of your hands. The tires and steering 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 to worn components. Play in the components will allow small movement of the steering and axle increase until the vibration matches the road speed (usually near 40 to 55 mph), and then increase in amplitude with the energy of a quick bump. 

Lifted vehicles have the acute steering angle to compound the problem along with greater tire weight to store more energy. This is the aspect of DW that most people overlook, the storage and release of energy during the DW vibration. When the energy stored exceeds the 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 into heat). Again, the lifted vehicle usually results in a steep steering stabilizer mount angle that reduces the effect of the stabilizer damper. Keeping the stabilizer level (in-line with the DW vibration) will help a DW problem. This is not the fix if other components are loose, but a solution aid if the components are good. Another element is the stiffness of the tie rods and drag link. Heavier parts that provide more resistance to vibration bending help reduce the DW. 

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 rim using less backspacing (30's on a 3.75 BS rim) will impart greater energy than a larger tire with less offset (33 on 5.25 BS rims) masking the impact of mere tire weight and size. Keep the 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 tires rolling 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 the weak 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, so I'll stop. Be aware that straight axle Fords, Toyota's, and Dodges all suffer from some DW so you can run away but not hide from the problem.

Happy Trails! 

Ed A. Stevens

Content 1999 North American XJ Association