CapTrack — swept path

In this version the shipped design-vehicle catalogue has no usable vehicle. All five of its design vehicles are listed in the window — VP, CO, O, SR and RE — but every one of them shows as (incomplete) and cannot be run: only the overall width and length are filled in, and the whole per-axle geometry (wheelbase, overhangs, kingpin distances) is missing. What does run is the two test samples, Sample: VP and Sample: SR, whose dimensions are provisional, not the official values. Treat any result from them as a demonstration, not as a design check. See The vehicle catalogue.

What it does

CAPCAD_CAPTRACK only opens the options window. The window is modeless and single per session: running the command again just brings it back.

CAPCAD_CAPTRACK_RUN is the command that executes. It reads the options from the open window — with no window it tells you to open the dialog first and stops — and then:

1. Asks for the path (Select the path (polyline/alignment)). It only accepts CAD curve entities — line, arc, circle, polyline (lightweight, 2D or 3D), spline and ellipse; anything else is refused on the spot. An infinite curve (ray, xline) passes the filter but cannot be read, and the command reports an invalid path.
2. Samples the path into points: the spacing is a quarter of the shortest tracking length in the chain, clamped to between 0.10 m and 0.50 m. Arcs and curves are turned into straight segments at this stage.
3. Runs the simulation. The command point advances step by step along the sampled path; at each step every unit is pulled by the tractrix and its body is recorded.
4. Builds the output: the track of each wheel point, and the union of all footprints, inflated by the clearance.
5. Bakes the result into the drawing — or hands over the interactive preview, according to the Mode.

The command line reports the outcome: vehicle name, number of steps, how many wheel tracks, how many rings the envelope has and the clearance applied.

The tractrix, in one line

For a rigid unit with command point F (the steering axle of the tractor, or the trailer's hitch) and rear axle R, separated by the tracking length L, when F moves to a new position:

R_new = F_new + L × normalise(R_old − F_new)

The unit's heading is the direction from R_new to F_new. The coupling point sits at a fixed distance from the rear axle, measured along the heading, and it is what pulls the next unit. Repeated along the chain and along the path, that single step is the simulation.

At the start, the whole chain is laid out straight, aligned with the path's initial tangent, each unit set back by its L from the one before. That is why the first step already comes out coherent instead of shaking itself straight over the first few metres.

Workflow

1. Draw (or choose) the polyline that represents the path travelled by the vehicle's steering axle — not the edge of the carriageway, not the road centre line, if they do not coincide.
2. Run CAPCAD_CAPTRACK. The window opens floating.
3. Choose the vehicle in Design vehicle (1). The preview (5) draws the chain in plan, with the wheel points; an incomplete vehicle shows there the list of what is missing, and Run (6) is disabled.
4. Enter the Clearance (m) (2) if you want the margin added to the envelope. Zero is the default.
5. Choose the Mode (3 or 4).
6. Run (6). AutoCAD comes to the front and asks for the path.
7. In Bake mode the result is already in the drawing. In Interactive preview mode, move the cursor along the path to see the vehicle at each station and click to bake — or Esc to discard.

The form

No screenshot. The numbers below follow the order in which the controls appear, top to bottom.

What is drawn

Both layers are created on the first run, with the colour in the table; if they already exist, they are reused with whatever colour they have. These two names are fixed, independent of the active style and of the country profile — see Notes.

Each wheel track carries, in the curve's properties, the unit (tractor, semitrailer, fulltrailer, rigid…) and the axle number, so they can be identified afterwards.

The vehicle, as CapTrack describes it

CapTrack does not know what a truck is: it knows a chain of rigid units, each with four pieces of data.

When the vehicle comes from an engineering catalogue, those four things are derived from the manual's dimensions:

Half the wheel track is half the width minus 0.15 m, because the source gives the body width and not the axle track.

The vehicle catalogue

The default catalogue ships with the product. A catalogue placed in a CapTrack folder in the CapCad installation folder takes precedence over it.

It carries five design vehicles, with the overall width and length from the geometric-design tables of the norm it was built from:

None of the five has its per-axle geometry in this version: wheelbases, overhangs and kingpin distances are not filled in. The five are listed as (incomplete) and the preview shows exactly which fields are missing.

What does run are two samples, with provisional dimensions:

Sample: VP matches the overall dimensions of the catalogue's passenger car (2.10 × 5.80 m); Sample: SR does not match the catalogue's articulated vehicle. Both are provisional.

A catalogue of your own can be written in JSON, either in the engineering format (manual dimensions, derived by CapCad) or in the direct format (tracking length, outline and wheel points already worked out). Both schemas ship in CapTrack/data/ in the product, and the direct format is the practical way to get a usable vehicle today: enter the tracking length, the coupling offset, the body outline and the wheel points of the design vehicle of your own norm.

How the options combine

The simulation step is not the path sampling spacing. They are two different quantities, both derived from the shortest tracking length in the chain:

For Sample: VP (L = 3.40 m), the path is sampled every 0.50 m and simulated every 0.17 m. In other words: the simulation is fine, but it walks over a path that has already been turned into half-metre chords. On a short, tight curve it is the sampling that limits fidelity, not the step.

The discrete tractrix cuts corners slightly compared with the continuous solution — the larger the step, the larger the cut. Since neither the step nor the sampling is configurable in the window, the result is what it is: a representative envelope, fit for checking manoeuvring space, and not a metrological instrument.

The clearance (2) inflates the envelope only. The wheel tracks always come out in their real position.

The interactive preview

In Interactive preview mode the whole simulation has already been computed when the preview starts. Moving the cursor recomputes nothing: the command projects the cursor onto the path, converts the station into a step number and redraws the vehicle outline at that step. That is why it stays fluid even for a combination.

• The wheel tracks appear in cyan and stay still.
• The vehicle outline appears in yellow (ACI 2) and follows the cursor.
• All of this is transient graphics: it is not in the drawing, it does not join a selection, it is not plotted.
• The click bakes — and it bakes the whole result, the same one Bake mode would produce. Where you stopped the cursor changes nothing about what is drawn.
• Esc discards, and the command line reports the cancellation.

Remembered between sessions

Nothing is written to disk. While the window stays open, the vehicle, the clearance and the mode stay as you left them; closing it discards everything, and the next opening returns to the defaults (first vehicle in the list, clearance 0, Bake mode).

The vehicle catalogue is re-read every time the window opens.

Notes

• The two output layers are fixed. CAPTRACK-ENVELOPE and CAPTRACK-WHEELPATH do not follow the active style, do not appear in CapCad's layer manager and are not renamed by country profile.
• The envelope and the tracks are not erased between runs. Running twice leaves two overlapping envelopes on the same layer. Erase the previous one, or freeze the layer, before repeating.
• A Civil 3D alignment is not accepted as a path. The command only accepts CAD curve entities; convert the alignment to a polyline first.
• The path is the route of the steering axle, not the edge of the carriageway nor the road centre line. A path taken from the centre line makes the whole vehicle run offset.
• It is a low-speed model. There is no tyre slip, no superelevation, no speed-based widening. For checking manoeuvres at junctions, roundabouts, yards and accesses it is the right model; for a path at speed it is not.
• The simulation is planar. The path is read in X and Y only — a 3D polyline is flattened.
• Every command-line message is in English, in any product language: the path prompt, the incomplete-vehicle warning, the final summary. The window's own labels are translated.
• The window opens centred on the parent window and may appear over the drawing area; drag it aside before picking the path.