Ackermann Steering Explained: Why the Inside Wheel Turns More
When a car turns, its two front wheels do not travel the same path. The inside wheel runs around a smaller circle than the outside wheel, so to roll without scrubbing it has to be steered to a sharper angle. Ackermann geometry is the name for how the steering linkage produces that difference — and Ackermann percentage is how much of the ideal difference a particular car actually has.
The ideal: every wheel aimed at one turn center
At low speed, with no tire slip, a car pivots around a point somewhere along the line of its rear axle. For both front tires to roll cleanly, a line drawn through each front axle has to pass through that same point. The geometry that follows depends on only two dimensions: the wheelbase and the distance between the two steering pivots.
Take a car with a 2,570mm wheelbase and 1,480mm between steering pivots. If the inside wheel is steered to 30°, the ideal outside angle works out to about 23.4° — a difference of 6.6°. At small steering angles the two wheels are nearly parallel; the gap opens up quickly as lock increases.
Ackermann percentage
Real linkages rarely match the ideal across the whole steering range, so the usual way to describe a car is as a percentage of the ideal difference at a stated steering angle.
| Ackermann | Wheels at lock | Typical use |
|---|---|---|
| 100% | Inner turns more by exactly the ideal amount | Low-speed maneuvering, tight parking-lot turns without tire scrub |
| 0% (parallel) | Both wheels at the same angle | Some race and drift setups |
| Negative (anti-Ackermann) | Outer turns more than inner | Certain high-downforce race cars with heavily loaded outside tires |
The percentage is always tied to a steering angle. A car can sit near 100% at 10° of lock and fall to 50% at full lock, so a single number without the angle it was measured at says little.
What sets it on the car
Ackermann comes from the steering arms on the knuckles. Seen from above, if lines through each kingpin and its tie rod end meet near the center of the rear axle, the linkage gives roughly full Ackermann. Three things move it:
- Tie rod pickup position on the knuckle — moving the outer tie rod end inboard or outboard changes the arm angle directly. This is what most adjustable or "cut" knuckles alter.
- Rack position fore and aft — moving the rack forward or back changes the tie rod angle in top view, which changes Ackermann without touching the knuckle.
- Steering arm length — a shorter arm gives more wheel angle per millimeter of rack travel and usually changes the Ackermann curve at the same time.
Why drift and race cars often run less
The ideal above assumes the tires roll where they point. At speed they do not: each tire runs at a slip angle, and the outside tire carries most of the load. A heavily loaded tire generally makes its peak grip at a larger slip angle than a lightly loaded one, which is the argument for giving the outside wheel more steering — less Ackermann, or even anti-Ackermann.
Drifting adds a second reason. At large counter-steer angles the leading front wheel does most of the work, and high Ackermann leaves the trailing wheel turned far more sharply than the car's path. That wheel drags, slows the car, and in extreme cases the steering goes over-center. Reduced Ackermann keeps both wheels closer to parallel so they roll more freely at big angle. The cost is tire scrub and heavier steering in ordinary low-speed turns.
Measuring your own
- Set the car on turn plates or two greased steel sheets per wheel, at ride height.
- Steer until the inside wheel reads a round number such as 20°, then read the outside wheel.
- Calculate the ideal outside angle from the formula using your wheelbase and pivot-to-pivot track, and compare.
- Repeat at several angles; the percentage usually changes across the range.