Ackermann, and whether you actually want it

The inside wheel is on a tighter circle, so it should turn more. That's been the textbook answer since 1817 — and most race cars deliberately ignore it. Here's the geometry, and the slip-angle argument that overrules it.

Put a car in a corner and the two front wheels are on different circles. The inside wheel is on a tighter radius than the outside one, so to roll without scrubbing it has to be turned more.

That’s Ackermann steering geometry, patented in 1817 for horse-drawn carriages, and it is completely correct — for horse-drawn carriages.

The geometry

For all four wheels to roll cleanly, every wheel’s axis must pass through one common center. Set the geometry up so both front wheels point at the same center and you have 100% Ackermann. The required angles are just trigonometry from wheelbase and track:

\cot(\delta_{\text{outer}}) - \cot(\delta_{\text{inner}}) = \frac{t}{L}

with $t$ the track and $L$ the wheelbase. For the preset used here — track 1683 mm, wheelbase 2467 mm — the difference in cotangents needs to be 0.68, which at moderate lock means the inner wheel turns several degrees more than the outer.

In practice you don’t solve that equation to build it. You get Ackermann by angling the steering arms: point the tie-rod outer ends inboard so they aim at the center of the rear axle, and the linkage produces roughly the right split by itself. The further the tie-rod end sits from the steering axis, and the more it’s angled, the more Ackermann you get.

Why race cars throw it away

The 1817 answer assumes wheels roll without slip. Real tires don’t — they generate lateral force by running at a slip angle, a few degrees between where the wheel points and where it actually travels. That single fact undoes the whole argument.

Two things follow:

The inside tire is lightly loaded. In a hard corner, weight has transferred outward. The inside front might carry a third of the load of the outside front, and a lightly loaded tire reaches peak grip at a smaller slip angle than a heavily loaded one. Ackermann steers it further — pushing it past its best slip angle, where it makes less grip and more drag.

Peak slip angle is broad and shared. Both tires want to sit near their own peak. At racing speeds those two peaks are closer together than pure geometry suggests, so the geometric split simply isn’t what the tires are asking for.

So circuit cars commonly run parallel steer (both wheels turn equally, zero Ackermann) or even anti-Ackermann (the outside wheel turns more), which sounds perverse until you remember the outside tire is the one carrying the load and doing the work.

The strut layout in the app reads about −0.4° of steering toe at a 25 mm rack input — very slightly anti-Ackermann. The app reports it as Steering toe (Ackermann), positive meaning the inner wheel turns more. That figure is a property of where the preset happens to put the steering arm, not a claim about any production car; treat it as the starting point you’re about to change.

So who wants full Ackermann?

Road cars at parking speeds, where slip angles are essentially zero and the 1817 model is exactly right. Get it wrong and the tires scrub, squeal and wear on every roundabout and in every parking space.
Autocross and low-speed tight courses, where corners are slow enough that the geometric argument still dominates.
Rally-raid and desert racing, and this is the case people forget. These cars combine very low-speed, full-lock maneuvering in ruts and sand with high-speed sweeping stages. At low speed and full lock on a loose surface, scrubbing a tire costs traction you cannot spare and generates enormous steering loads. Positive Ackermann is generally wanted, and it interacts with the very long steering links these cars need for bump-steer reasons (chapter 6) — the geometry is a genuine three-way fight between Ackermann, bump steer, and packaging around 350 mm of travel.

Formula and circuit cars, spending their lives at 2–10° of steer with big slip angles, are the ones that gain from parallel or anti-Ackermann. It’s a choice about where on the speed range the car earns its living, not a right answer.

Go look at it

Open the strut layout.

In the SETUP ADVISOR, find Steering toe (Ackermann) — about −0.4°, slightly anti-Ackermann. Positive would mean the inner wheel turns more.

Switch to the TOP VIEW panel, which is where steering geometry is visible at all — this is the one chapter of the module that doesn’t live in the front view.

Hit ⟲ Steer and watch the two front wheels through a lock-to-lock sweep. Look at how the inner and outer angles differ as lock builds.

Now change it. Drag Tie Rod Outer (Knuckle) rearward (inboard along the steering arm) and re-run ⟲ Steer. Moving the outer end changes the arm’s angle, which is the physical control over Ackermann. Watch the advisory figure move.

Push it the other way to build clear anti-Ackermann, then ask yourself which you’d want for the car you’re actually building — and at what speed it spends its time.

The inside wheel is on a tighter circle, so pure rolling needs it turned further. True at parking speed; overruled by slip angles once the tires are working hard.

Sweep the steering in top view — Mini front →

What it costs you

Ackermann is nearly free to have — it’s an angle on a bracket — but it’s expensive to change on a finished car, because it lives in the steering arm, which is usually part of a cast upright.

It also collides with chapter 6. The tie-rod outer end position sets Ackermann; the tie-rod inner end and length set bump steer. They share hardware, so tuning one disturbs the other, and on a real car you are solving both at once with about four adjustable numbers. That’s the fight the top view exists to show you.

And a genuine limit worth stating: Ackermann is a low-speed argument being applied to a car that also goes fast. There is no setting that is right at both ends of that range. Every car on the road is a compromise between the parking lot and the corner, and knowing which end you have prioritized is the whole of the decision.

Rules of thumb

Road car: 40–80% Ackermann. Enough to keep parking maneuvers clean without hurting the car at speed.
Circuit / formula car: 0% (parallel) to negative. The outside tire is the one doing the work.
Autocross, karts, tight low-speed: 80–100% and sometimes beyond.
Rally-raid and desert: positive Ackermann, prioritizing the low-speed, full-lock, loose-surface case where scrub costs traction and loads the steering hard.
Sanity check: if the car scrubs and squeals its front tires in a parking lot, you have too little. If it feels like it’s dragging the inside front in a fast corner, you have too much for that speed.

Try this

Set the layout to roughly neutral (0°) steering toe. How far did the tie-rod outer end have to move?
Build clear positive Ackermann, then check bump steer from chapter 6. How much did you break, and can you fix both at once?
For a car that does 90% road and 10% track days, which end of this compromise would you choose — and would you still choose it for a Dakar car?

That’s Module 1 complete. You can now find the instant center, predict the camber curve from it, construct and criticize a roll center, kill bump steer, read the steering axis in both views, build anti-dive, and convert a spring rate into a wheel rate.

Module 2 changes the subject from geometry to what happens over time: springs, dampers, and a road surface that is genuinely random.

Preset hardpoints are illustrative sketches, not measured factory specs.