Circuit Modding | The Undercut Wiki

A circuit you draw is a first-class track: the game derives its speed profile, sectors, corner numbering, pit lane, kerbs, grandstands and trees with the same code the procedural generator uses. The only thing you provide is the centre line.

Main menu → Modding Tools → Circuit Editor.

From a drawing to a race

Drawing

Draw — hold the left mouse button and draw the loop. On release the shape is checked: a track that crosses itself is refused (no figure-eights). The Fidelity slider decides how much of your stroke survives — low keeps long sweeping curves, high keeps the exact wiggles you drew. Moving it re-processes your original stroke, so it behaves like a redraw.
Import img… (I) — pick an image of a layout (a dark line on a light background) and the editor traces it into a loop, keeping the picture as a faint calque to redraw over.
Ref img… (B) — put any picture under the canvas without tracing it: a photo, a satellite view, a sketch. You draw over it yourself.
Load… (O) — reopen one of your saved .circuit files, markers, name and ratings included.
Markers (M) — click the track to pin Start/Finish, Pit entry and Pit exit; R goes back to automatic placement (the longest straight).
Preview — the real 2D race renderer, not a schematic. T toggles the textured view and the zone-coloured line with turn numbers.

Whatever you draw, the game enforces a minimum corner radius of 40 m and smooths curvature spikes, so a nervous stroke still produces a drivable track.

Corner shape

Two sliders, Radius and Angle, applied to a scope you cycle with < and >: all corners, one sector, or turn N. Sector and per-corner settings add up. The corners in scope are highlighted with their number on the canvas.

Both operators are local and pinned: a corner’s entry and exit never move, so the loop stays closed and the rest of the track is untouched. The change tapers to nothing at the edges of the bend, which is why a reshaped corner still meets its straight cleanly. Radius tightens or opens the bend; Angle moves the turning towards the apex (a sharp point) or away from it (a constant-radius sweep).

Checking it

Analyse (A) runs a real four-lap simulation with one reference car and reports best lap, top speed, the slowest corner, the average, the full-throttle share, tyre wear per lap and fuel per lap.
Test drive launches your track as a live race against a full grid. Leaving the race returns you to the editor with the layout intact.

Properties

Name, country, city, year, the track character (balanced / high-speed / technical / street), the downforce / tyre wear / overtaking ratings, the visual preset, the target length, and the career placement — a guaranteed calendar slot or the draw pool.

The three ratings are deduced from the geometry and shown for reference; in play the game derives a track’s downforce, tyre wear and overtaking from its character, so that is the field that changes how the track races.

Save (Ctrl+S) writes <your mods folder>/circuits/<name>.circuit — in your user directory, not in the game’s install folder (see where the files go). That one file is the whole circuit.

The `.circuit` file

Plain text, one instruction per line — readable, diffable, and easy to check by eye:

name Monaco 1994
country Monaco
city Monte Carlo
year 1994
target_km 3.3
preset 0                 # 0 = automatic (climate) · 1-6 = a visual template
character street
downforce 88
tire_wear 40
overtaking 15
calendar guaranteed 6    # or: calendar pool
start_finish 0.12        # optional, arc-length fraction 0..1
pit_entry 0.08
pit_exit 0.18
p 512.0 340.5            # centre line, one point per line
p 516.2 338.9
...

The points are in the editor’s own space; the game rescales the loop to target_km, so the absolute numbers do not matter — only the shape.

Getting it into a career

1. Career setup (Custom). New Career → Prime Formula Series → Custom (Advanced). The setup window has a circuit section listing every .circuit the game found; the ones you select are stamped onto the calendar of the career you are creating and are pinned there — the mid-season venue review never swaps them out.

2. Custom circuit pool. Settings → Career → Circuit pool → Custom…. Pick the files, a pool size and how the rest of the pool is filled. Those tracks become world venues: they take the season’s rounds first and then live like any venue — contracts, venue reviews, comebacks. This is the option for “I want my tracks to be part of this world”, rather than “on this season”.

3. A mod pack calendar. Import the files into the Mod Editor’s Circuits page and order them on its Calendar page. A pack carrying a calendar builds the career season from it, round by round, mixing your tracks with autogenerated ones as you like. See mod packs.

Routes 1 and 3 pin their rounds: the track you placed there keeps racing it season after season. Route 2 does the opposite on purpose — a pool venue lives in the world, so it holds a venue contract, can be renewed, can lose its slot at the mid-season venue review, and can come back later.

The .circuit file is self-contained: send it to someone and they drop it in their own mods/circuits/. To publish it for subscription instead, see publishing on the Steam Workshop — a circuit is the simplest possible item, one file in a folder.