Bump Steer Explained: Why Lowered Cars Dart Over Bumps
Bump steer is the wheels steering themselves as the suspension moves up and down, with the steering wheel held still. Hit a bump with one front wheel and that wheel toes in or out on its own; the car darts. A small, deliberate amount is designed into most road cars. A large amount, or the wrong direction, is one of the most common side effects of lowering.
Where it comes from
The knuckle is held by the control arms and steered by the tie rod. As the suspension moves, the outer ball joint swings on an arc set by the control arm. The outer tie rod end swings on its own arc, set by the tie rod's length and the position of its inner joint on the rack. If the two arcs match, the knuckle moves up and down without turning. If they do not, the tie rod pulls or pushes the steering arm as the wheel travels, and the toe changes.
For the arcs to match, two things have to be true in front view:
- The tie rod points at the same instant center as the control arms — on a simple layout, it sits parallel to the lower arm.
- The tie rod's inner joint lies on the line between the arms' inner pivots, so its effective length matches.
What lowering does to it
At standard ride height the tie rod and lower arm are usually close to level and the arcs agree around the middle of the travel. Lower the car and both now angle upward toward the wheel, working on a steeper part of their arcs. Any difference in length or pivot position that was harmless near level becomes a larger toe change per millimeter of travel.
| Change to the car | Effect on the tie rod relationship |
|---|---|
| Lowering springs or coilovers | Tie rod and arm work at a steeper angle; bump steer usually increases |
| Roll center correction (extended ball joint) | Moves the lower arm but not the tie rod; needs a matching tie rod correction |
| More caster | Raises or lowers the steering arm slightly relative to the rack |
| Relocated or swapped steering rack | Moves the inner joints; changes both angle and effective length |
| Different knuckles | Changes steering arm height and length |
Measuring it
- Set the car at ride height on stands with the springs removed and the steering locked straight.
- Bolt a flat plate to the hub and set two dial indicators against it, one ahead of the hub and one behind, a known distance apart.
- Move the suspension through its travel with a jack in steps, such as every 10mm, both up and down from ride height.
- The difference between the two indicator readings at each step is the toe change. Plot it against travel.
The result is a curve of toe against travel. Toe that barely moves is the goal. Toe that changes steadily in one direction means the tie rod is at the wrong height at one end. Toe that goes the same way in both bump and droop means its length is wrong for where it is mounted.
Correcting it
- Tie rod end spacers — a bump steer kit replaces the outer tie rod end with a spherical joint on a stud, with spacers to set its height on the steering arm. This is the usual fix on a lowered car.
- Rack height — spacing the rack up or down moves both inner joints together.
- Drop knuckles — lower the car at the knuckle so neither the arm nor the tie rod changes angle.
Any correction that moves the outer tie rod end down places it closer to the inside of the wheel barrel. With extended ball joints and tie rod spacers fitted, small-diameter wheels can foul them, so check clearance inside the wheel as part of the job.
Why some is intentional
Many road cars are designed so the front wheels toe out slightly in compression. In a corner the loaded outside wheel then steers a little out of the turn as the body rolls, which adds understeer and makes the car more forgiving. This is called roll steer. At the rear, toe-in under compression is used for the same stabilizing effect. Removing bump steer entirely makes the car more precise and less tolerant.