What is Bump Steer | Likely Causes & Bump Steer Correction
What is Bump Steer Explained
Maybe the easiest way to describe "what is bump steer" is by the effect: hitting bumps causes your car to steer unpredictably, without input from the driver. What does bump steer feel like? It could be a sudden push or oversteer after hitting that hole in the turn, or a feeling like you’re barely in control on a rough track. Perhaps you find yourself suddenly jumping to the inside or outside lane of the track. Maybe it’s more subtle and you can’t seem to hit your marks, or you notice excess fatigue after the race, like it took more work than it should have. These are all symptoms of bump steer. Bump steer happens on the street, too. You can feel the wheel jerk to one side or the other hitting a pothole or constantly correcting the car on a washboard-like stretch of a straight line. Either way, it’s unsettling and it can be dangerous, especially at higher speeds.
What Causes Bump Steer?
Now you know what bump steer feels like, we need to know what causes bump steer. Bump steer is caused by the suspension components and steering components not being properly aligned as they travel vertically, causing the wheels to steer themselves when the suspension moves up and down, particularly over bumps, but it can happen under hard braking as the front end dives sharply down.
In a properly aligned adjustable suspension, all components in the steering linkage as well as the wheel spindles and upper and lower control arms, move up and down in a synchronized arc, all arcs parallel to each other. However, if the outer tie rod end movement arc is not parallel to the movement arc of the control arms, that force must go somewhere else; in this case since your spindle is also on horizontal pivots, that force gets transferred to push the wheel left or right, whether you wanted it to or not. This unwanted toe in or out is your bump steer.
Let’s show bump steer in action in this example with a Speedway Motors performance G-Comp suspension. Regardless of the type of independent front suspension, the principle is the same. In addition to showing a correct setup, we’ll intentionally introduce incorrect steering geometry in the video to show how bump steer happens.
How to Fix Bump Steer | Bump Steer Measurements
Now you know what bump steer is, and what causes it we need to find out how to fix it. Fortunately, we can diagnose issues with bump steer based on what the wheels do during compression and rebound. Once we know what’s happening with the front wheels, we can make a tie rod adjustment, by either changing length (through our rod ends), or changing height (via shims or spacers) to correct the bump steer problem. Here’s a helpful bump steer correction chart for related symptoms and how to cure them.
Bump Steer Correction Chart
When it comes to how to fix bump steer, we need to know exactly how much is taking place. Even 0.005" of unwanted toe is enough to affect handling. An important tool is a bump steer gauge, like the Longacre 52-79005 Bump Steer Gauge with Billet Plate. A bump steer gauge like this consists of a plate mounted to a frame with a dial indicator on the plate that measures the precise amount of toe in or toe out to determine your bump steer. After removing the front springs and setting the car on block to ride height, you bolt the bump steer plate to your hub. Once the plate is attached to the gauge frame, you raise the front lower control arms with a jack. The plate will move up with the jack, and you'll see if any bump steer occurs on the gauge attached to the plate. Here's how to use a bump steer gauge:
- Determine the position of the lower control arms when the car is at ride height. This can be done by measuring the lower control arm angles, by shock travel, or other method. This will be your starting point.
- Remove the front springs and put the car on blocks to simulate ride height. Put a floor jack under the lower control arm so you can move the suspension up and down.
- Bolt the Bump Steer Plate to your hub. You can use the wheel studs (don’t over-tighten the lug nuts use the included rubber-lined washers) or drive flange holes (bolts not included). Rotate the plate to level. NOTE: If you have Wide 5 hubs with no drive flange or cap bolt holes you will need wheel stud stand-offs #79121. These screw on to your wheel studs (5/8” Coarse only). The plate then mounts to these.
- Set up the Bump Steer Gauge frame as shown so that it leans gently against the Bump Steer Plate. On one side of the frame is a bracket with a roller bearing. This rests against the plate and maintains a constant distance. As you move the suspension, the bump steer will be shown on the indicator. This is much easier to use than dual indicator style gauges where you have to watch the movement of both indicators and subtract the difference.
- Raise or lower the frame so the indicator pointer is at 0.0” on the plate scale. Loosen the black knobs on the inside of the frame to adjust.
- Move the frame slightly as needed so the dial indicator pointer is pointed toward the plate. Rotate the ring on the dial indicator so it reads 0.0”. On digital models simply push the ZERO button. NOTE: This is a special dial indicator designed for this gauge. It reads 0-.050” in both directions. Much easier to use, less chance for error.
- Use the floor jack to raise the lower control arm. This simulates braking when the front of the car goes down. Move until the indicator pointer is at 1.0” on the plate scale. (If you prefer you can measure every .50” or even .25”). Record the bump steer as shown on the indicator and whether it is toe-in or toe-out. You can tell this by which direction the indicator moves. Continue to move the lower control arm and record the bump steer to the max needed. Also, move the lower control arm in the opposite direction from 0.0” and record the bump steer there.
What is a Bump Steer Kit?
The primary purpose of a bump steering kit is to correct the steering geometry so that the wheels maintain their alignment relative to the chassis, even as the suspension moves through its travel. When a car is lowered or its suspension is modified, the tie rods and control arms can end up moving in different arcs, leading to bump steer. A bump steer kit allows you to adjust these components so that they move in parallel arcs, preventing the steering from being affected by suspension movement. A bump steer kit has some or all of these components:
- Adjustable Tie Rod Ends: These are often the main component of a bump steer kit. They replace the stock tie rod ends and allow for horizontal tie rod adjustment, enabling you to align the tie rod’s arc with the control arm’s arc. Also available are Tie Rod Adjustment tools, which are sleeves that screw to your tie rod and rod end allowing you to make precise adjustments to the length.
- Shims or Spacers: These are used to raise or lower the tie rod end relative to the steering arm, which helps fine-tune the steering geometry to reduce bump steer. Often used in conjunction with bump steer studs.
- Bump Steer Studs: These include longer studs (bolts) that replace the stock steering arm studs. They allow for greater adjustability in positioning the tie rod ends, often used in conjunction with spacers.
Understanding and correcting bump steer is crucial for maintaining control and safety, whether on the track or the street. By properly aligning your suspension and steering components, and using tools like bump steer gauges and correction kits, you can ensure that your vehicle handles predictably even over uneven surfaces. For more information on bump steer, check out this helpful piece from Lincoln Tech.
