Bump steer: your suspension is steering without you
The tie rod swings on a different arc than the control arms, so the wheel steers itself over every bump. Here's the construction that kills it at the front — and why the rear deliberately keeps some.
Your steering rack has been sitting still this whole time. Your car has been steering anyway.
The tie rod is a link like any other: one end on the chassis, one end on the knuckle, swinging on an arc as the wheel moves. The control arms are also links, swinging on their arcs. If those arcs don’t agree, the tie rod pushes or pulls the steering arm as the wheel travels, and the wheel toes in or out. Nobody asked it to. That’s bump steer.
It matters because it happens exactly when you can least afford it: mid-corner over a crest, under braking as the nose dives, on turn-in as the outside wheel loads. The car darts, tramlines, or feels like it’s steering from somewhere behind you.
The construction that fixes it
The tie rod has to swing on the same arc as the rest of the linkage. Geometrically that means two conditions, and both matter:
- Aim: the tie rod, extended, must point at the suspension’s instant center — the same IC from chapter 2.
- Length: its length must place the outer end so it sweeps the same arc as the knuckle point it’s attached to.
Get the aim right and the length wrong and you still have bump steer. Get both right and the toe curve goes flat.
The app draws this for you. In the front view there’s a green guide line labeled “align tie rod ⟶ no bump steer” — that’s the target the inner tie-rod end wants to sit on. Put the tie rod on that line and the toe curve flattens out.
The number to design against is a rate, not a total:
Quoted per 25 mm (an inch) because that’s the convention you’ll meet in setup sheets and on corner-weight scales.
What the cars actually do
Take the strut layout as it comes and sweep it. Toe change from static, positive = toe-in:
| Wheel travel | Toe change |
|---|---|
| −40 mm | −0.163° |
| 0 mm | 0 |
| +20 mm | −0.001° |
| +40 mm | −0.058° |
| +60 mm | −0.173° |
Under a fifth of a degree across the whole sweep, and flattest right around static ride height. That is what a tie rod on the correct line looks like.
Now break it with a single move. Raise the tie rod inner by 25 mm — nothing else touched:
| Toe swing across travel | |
|---|---|
| as designed | 0.17° |
| tie rod inner 25 mm higher | 3.24° |
One point, 25 mm, and the self-steer got nearly twenty times worse — and it now swings both ways, toe-out in droop and toe-in in bump, so the car changes its mind about where it is pointing halfway through a bump. Bump steer is not a property you either have or don’t; it’s a sensitivity, and it is brutally sharp.
Bump steer is a fault at the front and a tool at the rear
On a steered axle, any toe change you didn’t ask for is corruption of the driver’s input. Target zero.
On the rear, the same effect is called roll steer, and manufacturers build it in deliberately. In a corner the outside rear wheel compresses; if it toes in as it does so, the rear axle steers slightly into the corner, which pushes the car toward understeer — the safe, stable direction for a road car. It’s a passive stability system made of nothing but link geometry, and it costs nothing.
So the design rule inverts by axle:
- Front: minimize. Under 0.1°/25 mm is good.
- Rear: choose a value on purpose. A small toe-in on compression (0.2–0.5° over full travel) is a common, deliberate stabilizer. The app’s advisor says exactly this, and warns when the rear layout here overdoes it — too much makes the car wander over bumps and feel inconsistent on turn-in.
Go look at it
Open the strut layout. The tie rod is highlighted for you.
- Find the green “align tie rod ⟶ no bump steer” guide line in the front view. That’s your target.
- In the SETUP ADVISOR, find Bump steer. It reports the toe swing across the car’s full travel — a larger number than the ±40 mm figures in the table above, because it spans the whole stroke.
- Break it deliberately. Drag Tie Rod Inner (Rack) upward by 25 mm. Now hit ▲▼ Bump and watch Toe in the metrics panel. It swings visibly — you’ve just built a car that steers itself over bumps.
- Now fix it. Drag that same point back toward the green guide line. Watch the Bump steer advisory improve as you approach it, and the Toe sweep collapse toward flat.
- Try the other error. Put the inner end back on the line but drag Tie Rod Outer (Knuckle) inboard 15 mm to change the tie rod’s length. Correct aim, wrong length — and the toe curve is bad again. Both conditions have to hold.
- Now the rear. Open the wishbone layout and run ▲▼ Bump with Toe in view. It swings hard, toe-in on compression, and the advisor calls it Bump toe (roll steer) rather than a fault — because at this end it’s a choice.
What it costs you
Getting bump steer to zero is a packaging fight, not a calculation. The tie rod wants to be on a specific line, and that line is usually where the driveshaft, the brake caliper, the steering rack mounts or the subflame already are. Real cars compromise.
The compromise most likely to bite you is ride height. Bump steer is a curve, not a constant, and it’s normally tuned to be flattest around static ride height. Lower the car 40 mm and you move onto a steeper part of that curve — the geometry hasn’t changed, but where you’re sitting on it has. This is the single most common reason a lowered car tramlines and darts, and it’s why bump-steer correction kits exist: they move the tie-rod end back onto the right line for the new ride height.
The other cost is that fixing bump steer moves the tie-rod inner end, which usually means moving the steering rack, which is one of the harder things to move on a finished car. Which is the honest argument for doing this on a screen before anyone welds anything.
Rules of thumb
- Front, road car: under 0.1° per 25 mm of travel. Under 0.05° is excellent.
- Front, circuit car: as close to zero as packaging allows, checked across the actual travel used, not just around static.
- Rear roll steer: 0.2–0.5° of toe-in over full travel is a deliberate, common stabilizer. Toe-out on compression at the rear is the opposite — it steers the car out of the corner and is genuinely dangerous.
- Rally-raid and desert: the hardest case in this course. With 350–600 mm of stroke, a rate that looks tiny becomes a large total — 0.1°/25 mm over 350 mm is 1.4° of self-steer. Long tie rods that closely parallel long control arms are the standard answer, and it is a major reason these cars have such visibly long links. A beam axle with a drag link has the same problem in a different costume: the drag link must arc with the axle, or the car bump-steers as one unit.
- Always check at your real ride height, not the design one. If you lowered the car, your bump-steer figure is not the one in the brochure.
Try this
- Find the tie-rod inner position that gives the strut layout the flattest toe curve. How far did it have to move from where the preset puts it?
- Lower the Mini by dropping ride height and re-check bump steer without touching the tie rod. How much worse did it get for free?
- On the rear, tune the roll steer down to about 0.3° over full travel. Does the advisor prefer it? Do you?
Next: the steering axis — the line through two joints that decides how your car feels in your hands.
Preset hardpoints are illustrative sketches, not measured factory specs.