RC Bumpsteer Explained
What Is Bumpsteer?
Bumpsteer refers to the change in toe angle of the wheels as the suspension moves through its travel (compression and rebound). In simple terms, it describes how much the front or rear wheels steer themselves without any input from the transmitter.
As the suspension compresses (bump) or extends (droop), the geometry of the steering and suspension links causes the wheels to either Toe In or Toe Out. This unintended steering effect is what we call Bumpsteer.
• Toe-in under compression = wheels point inward as suspension compresses
• Toe-out under compression = wheels point outward as suspension compresses
Bumpsteer is most commonly tuned on the front of the car, but it can also exist in the rear depending on the platform.
How Does Bumpsteer Affect Your Car?
Bumpsteer directly influences how your car behaves over bumps, during corner entry and mid-corner stability.
Increased Bumpsteer (More Toe Change)
• Makes the car more reactive over bumps
• Can increase steering on corner entry
• May cause the car to feel nervous or inconsistent
• Can lead to twitchiness on rough tracks
Decreased Bumpsteer (Less Toe Change)
• Improves stability over bumps
• Makes the car easier to drive
• Reduces unexpected steering inputs
• Can slightly dull initial steering response
Real-World Feel
• Too much Bumpsteer: Car feels unpredictable, especially on rough sections
• Too little Bumpsteer: Car feels planted but may lack aggressiveness in tight turns
The goal is to find a balance where the car remains stable but still responds quickly when you need it to.
How Do You Change Bumpsteer?
Bumpsteer is adjusted by changing the angle and position of the steering links relative to the suspension arms.
Common Adjustment Methods
1. Steering Rack Height (Shimming Up/Down)
• Raising or lowering the steering rack changes the steering link angle
• One of the most effective ways to adjust Bumpsteer
2. Outer Tie Rod Height (Spacers at the Knuckle)
• Adding or removing spacers at the steering knuckle changes the link angle
• Fine-tunes how the wheel reacts through suspension travel
3. Inner Tie Rod Position
• Adjusting the inner mounting point on the steering rack (if adjustable)
• Alters the arc of the steering link
4. Steering Link Length
• Small changes can influence geometry, though this is typically used for toe settings first
General Rule
You are trying to match the arc of the steering link to the arc of the suspension arm. The closer these arcs match, the less Bumpsteer you will have.
When To Increase Or Decrease Bumpsteer
Knowing when to adjust Bumpsteer comes down to track conditions and how the car feels.
Increase Bumpsteer When:
• You need more steering entering corners
• The track is smooth with high grip
• The car feels lazy or slow to respond
• You want more rotation in tight sections
Decrease Bumpsteer When:
• The track is rough or bumpy
• The car feels twitchy or inconsistent
• You are struggling with straight-line stability
• The car reacts too aggressively to small bumps
What to Check After Changing Bumpsteer
Whenever you adjust Bumpsteer, you are changing the relationship between multiple suspension components. It’s important to double-check related settings to maintain balance.
1. Toe Settings
Bumpsteer directly affects dynamic toe, so re-check static toe after adjustments.
2. Ride Height
Changes in Ride Height alter suspension angles, which can impact Bumpsteer.
3. Camber Gain
Suspension movement affects Camber and changes in Bumpsteer can influence how the tire contacts the surface.
4. Ackermann
Bumpsteer and Ackermann both influence steering behavior. Adjusting one may change how the other feels.
5. Steering Endpoints / EPA
After changes, ensure your steering throw is still balanced and not overdriving the front tires.
6. Roll Center
Adjustments to shims and spacers can unintentionally affect Roll Center heights.
7. Suspension Arm Angle
Arm angle influences how the suspension moves through its arc, which ties directly into Bumpsteer behavior.
FAQ
Q: What is the easiest way to check Bumpsteer?
A: The simplest method is to remove the springs, cycle the suspension through its travel, and observe how the wheels change toe. Setup tools or Bumpsteer gauges can provide more precise measurements.
Q: Is zero Bumpsteer always best?
A: No. While minimal Bumpsteer improves stability, a small amount can help with steering response. It depends on track conditions and driving style.
Q: Does Bumpsteer affect the rear of the car?
A: Yes, but it is less commonly adjusted. Some platforms allow rear link geometry changes that introduce similar effects.
Q: How much Bumpsteer should I run?
A: There is no universal setting. Most racers aim for minimal toe change, then add slight Bumpsteer if they need more steering.
Q: Why does my car feel inconsistent over bumps?
A: Excessive Bumpsteer is a common cause. The wheels are steering themselves as the suspension moves, making the car unpredictable.
Q: Can Bumpsteer fix understeer?
A: It can help. Increasing Bumpsteer may improve corner entry steering, but it should be combined with other adjustments for best results.
Back to the RC Suspension Tuning Guide.
