opensus
How to reduce bump steer on a double wishbone
To reduce bump steer, move the tie rod's inner pivot: its height sets the slope of the toe curve, and its lateral position (which sets the tie rod's length) sets the curve's bend. Fix the slope first, then the bend. In opensus you can watch the toe curve flatten as you type or drag.
1. Read the toe curve
Open your design (or the example with the button on this page) and go to Kinematics → Heave, range ±40 mm or your real travel. Look at two charts:
- Toe (bump steer): toe against wheel travel. A straight sloped line means a height error; a curve that bends one way at both ends means a length error.
- Bump steer rate: the slope, in degrees per 10 mm. A target of under 0.02° per 10 mm near ride height is good for a race car front.
2. Fix the slope with tie rod height
Select the tie rod inner point (Tie rod inner, tieInner) and change its Z.
On the docs example, each millimetre of height error gives about 0.021° per 10 mm of bump steer: 5 mm too high gives +0.107° per 10 mm (toe-in in bump), 5 mm too low gives -0.107°. So divide your measured bump steer rate by your car's sensitivity to get the height correction, then fine-tune. Raise the inner pivot to make toe go more toward toe-in in bump, lower it for toe-out, on a front-steer layout with the tie rod behind the axle as in the example. On your car, check the direction with a 1 mm test move.
If the inner pivot is on a steering rack, moving it means moving the rack or adding spacers: the Adjusters section can model a shim stack on that point so you can see the range it gives.
3. Fix the bend with tie rod length
If the curve is flat at ride height but bends at both ends, the tie rod is the wrong length: its arc is tighter or looser than the arms'. Move the inner pivot sideways (Y) a few millimetres at a time, refixing the height after each move. For zero bump steer, the inner pivot sits on the line through the two arms' inner pivots in the front view, and the tie rod line passes through the front-view instant center. Turn on View → Roll centres and force lines to see where the instant center is.
4. Check roll steer
Switch to Kinematics → Roll. Toe in roll is a mix of both wheels' bump steer; once heave is flat, roll steer should be small too. If you want a little deliberate roll understeer at the rear, this is the chart to set it on.
5. Check under load
Geometry is half the story: loads also steer the wheel through the links. The Compliance tab shows toe change per kN of cornering force and per 100 N m of aligning torque; see compliance.