CapTrack — swept path

All five catalogue vehicles can be run: VP, CO, O, SR and RE. See The vehicle catalogue.

What it does

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

The window's Run and Run with the mouse buttons run the same command, which reads the options from the window and then:

1. Asks for the path: a CAD curve (line, arc, circle, polyline, spline, ellipse) or, in Civil 3D with CapCad's Civil 3D module loaded, an alignment. The run starts at the end of the path nearest to where you clicked: to drive the other way, click near the other end. With Reversing ticked, the path is that of the last unit's rear axle. Any other entity is refused with a message and the prompt repeats.
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. A progress window shows the steps and then the building of the envelope; Cancel stops the run and nothing is drawn.
4. Builds the output: the track of each wheel point, the union of all footprints inflated by the clearance, the parts of the obstacles inside the envelope (when obstacles are selected) and the swept width sections (when the Options page has an interval).
5. Bakes the result into the drawing (Run) or hands over the preview that follows the mouse (Run with the mouse). If the same vehicle already ran on the same path, from the same end and the same way (forward or reversing), the previous result is erased and replaced; any other combination adds to the drawing.

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; how many entities of the previous run were replaced; the obstacle check; the largest swept width, with the table of sections; and, when reversing, a warning when the vehicle folds past 90° (jackknife).

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 or alignment that represents the path travelled by the vehicle's steering axle (when reversing, by the last unit's rear axle), not the edge of the carriageway nor 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. To check kerbs, edges, poles or islands, click Select next to Obstacles (3) and select the curves. They apply to every following run, until Clear.
6. Click Run (6) to draw the result straight away, or Run with the mouse (4) to move the vehicle with the mouse. The CAD window comes to the front and asks for the path: click near the end it starts from.
7. With Run, the result is already in the drawing. With Run with the mouse, 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 window has the CapCad side panel with icons on the left and five pages: Vehicle (controls 1, 3 and 5), Options (2, 8, 10, 11, 12 and the vehicle code in the layer names), My vehicles (see Your own vehicles), Signage (not available in this version) and Learn (videos, samples and the Learning Zone). Drive with the mouse (9), Run with the mouse (4), Run (6) and Close (7) stay at the bottom on every page, with an orange Reset link beside them on every CAD: it erases what the last run drew and brings back what it replaced. One Ctrl+Z undoes a whole run. Between sessions the window remembers the vehicle, the type filter, the clearance, the swept width, the vehicle code in the layer names and reversing.

What is drawn

The layers are created through CapCad's layer catalogue (the CAPTRACK family), which gives the colours in the table, or the active style's when it redefines them, and the style's prefix (CAPCAD_CAPTRACK-ENVELOPE with the CapCad style); layers that already exist are reused as they are. With Vehicle code in the layer names ticked on the Options page (the default), the vehicle's code is added at the end, CAPCAD_CAPTRACK-ENVELOPE_SR and CAPCAD_CAPTRACK-WHEELPATH_SR, so each vehicle's paths sit on their own layers and can be frozen, compared or erased apart. Characters a layer name cannot hold are replaced: a space by _, a decimal comma by ., a slash by - (AASHTO P/T gives _AASHTO_P-T).

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.

Obstacles

Obstacles are curves in the drawing (line, arc, circle, polyline): kerbs, edges, poles drawn as circles, islands. Each run clips every obstacle by the envelope (with the clearance) and draws the parts inside it on the CAPTRACK-CONFLICT layer. The command line says how many obstacles were hit and how many metres of them are inside, or that none touches the envelope. Obstacles selected in another drawing are not checked.

Swept width

With an interval on the Options page, CapTrack draws, every so many metres from the start of the path, a line square to the path from the right edge to the left edge of the area the vehicle swept, without the clearance, and writes the width beside it (text height: a fifth of the vehicle width, at least 0.25 m). The command line gives the largest width, where it occurs and how much it exceeds the vehicle width (the widening), and the table of sections with the distance, the width and the parts left and right of the path.

Reversing

With Reversing, the path is that of the last unit's rear axle, driving backwards, facing away from the direction of travel. Each unit is solved from the one behind it: the last unit lies tangent to the path, a unit with its fifth wheel over the axle follows the direction its coupling travels, and a unit with the coupling off the axle follows the tractrix at that distance. Nothing limits the steering: a path that asks more than the vehicle can do shows as a large angle between units, and the command line warns when it passes 90° (jackknife).

Drive with the mouse

Drive with the mouse asks for no path. Click the start (when reversing, where the last unit's rear axle is) and the direction the vehicle leaves in; then, as you move the mouse, CapTrack shows the path to the cursor, the wheel paths and the vehicle where it would arrive. Each click fixes a point and the path carries on from there; Undo drops the last point, Enter finishes and Esc cancels. Between the points the path is straight, and each corner becomes an arc of the tightest radius the vehicle makes, worked out from the outer front wheel's minimum radius (when reversing, the last unit's rear axle's). A point that asks for a tighter turn, or leaves no room for the arc, shows in red and the click is refused with a message. A vehicle with no minimum radius joins the points with corners. When you finish, the path stays in the drawing as a polyline on the CAPTRACK-PATH layer, to edit and run again with the other buttons, and the run is drawn as Run draws it, with obstacles and swept width. Reset erases the path too.

Start and end of a run

With Whole vehicle on the path at the start and at the end ticked (the default), the run starts once the whole vehicle is on the path: driving forward, with the last unit's rear axle at the start of the path; reversing, with the first unit's front axle there. It ends with the axle that follows the path at its end, the vehicle still on the path. So the start and the end look alike. Unticked, the vehicle starts straight, with the axle that follows the path at its start and the rest of the vehicle behind it, off the path. A path shorter than the vehicle runs whole and the command line says so. Remember too that a run starts at the end of the path nearest to where you clicked: clicking near the other end reverses it.

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:

Every unit that pulls another gives its coupling in couplingOffsetFromRearAxle (a tractor may use the older name, kingpinOffsetFromRearAxle); without it the vehicle is (incomplete) and the preview names the missing field. So a B-double, a bi-articulated bus, a tractor + semitrailer + trailer or a road train are just more units in the list. A drawbar trailer is described as a dolly (the drawbar and front axle) followed by a semitrailer (the body, turning on the dolly).

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:

All five have their per-axle geometry (wheelbase, overhangs, kingpin) from the dimensioned vehicle drawings of the norm. On SR, the kingpin position and the front of the semitrailer are not dimensioned in the norm and were measured at the drawing's scale; the dimension printed as 7.90 was taken as 8.00 so that the chain adds up to the printed 16.80 m. RE follows its drawing's chain (0.90 / 3.35 / 6.05 / 2.85 / 6.05 / 0.60 = 19.80): a tandem tractor, a semitrailer and a drawbar trailer with its own front axle. What the drawing does not dimension was measured at its scale: the kingpin over the tractor's rear tandem, the semitrailer front 1.55 m ahead of the kingpin, the drawbar eye 0.65 m behind the semitrailer axle (a 2.20 m drawbar) and the trailer front 0.60 m ahead of its front axle. Simulated at the norm's radius, RE gives an inside radius of 6.97 m, against the norm's 6.90 m. Each vehicle also carries the norm's minimum radius of the outer front wheel (7.30 m for VP, 12.80 m for CO and O, 13.70 m for SR and RE), which draws its turning template.

An AASHTO catalogue follows, with 19 design vehicles of the 2004 Green Book (metric): P, SU, BUS-12, BUS-14, CITY-BUS, S-BUS11, S-BUS12, A-BUS, WB-12, WB-15, WB-19, WB-20, the double and triple combinations WB-20D, WB-30T and WB-33D, MH, P/T, P/B and MH/B, each listed as AASHTO <symbol>, with the Green Book's dimensions and minimum design turning radius. Tandem axles are taken at their centre. In the doubles and the triple, each rear semitrailer rides on a converter dolly hitched to the rear of the semitrailer ahead. Simulated through a 180° turn at that radius, every one of them passes within 0.25 m of the Green Book's minimum inside radius, except WB-33D, which gives 4.21 m against 4.5 m. On WB-20D the printed overall length (22.4 m) does not match the sum of the figure's own dimensions (22.03 m); the geometry uses the dimensions.

A further catalogue carries specific manufacturer vehicles, listed after those: in this version the Mercedes-Benz OF 1726L/59 bus at 11 m (body dimensions from a bus builder's turning template, wheelbase 5.95 m from the manufacturer's data sheet) and at 13.2 m (the same wheelbase, the extra length split between the overhangs in the same proportion; a first estimate that can change), both with a 9.80 m minimum radius (half the data sheet's 19.6 m kerb-to-kerb turning circle).

There are also 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.

Your own vehicles

The My vehicles page adds vehicles that are not in the catalogue, for example a specific bus or truck from the manufacturer's drawing. New clears the fields; Copy from... lists the catalogue vehicles and starts a new vehicle with the chosen one's dimensions, named "copy of ...", to change and save (RE and the AASHTO doubles and triple, which have a dolly, are not in that list; a vehicle whose units differ in width comes with the widest). Type a Name, choose the Type (Rigid or Tractor + semitrailers) and enter the width, the front overhang, the wheelbase and the rear overhang. For a tractor with semitrailers, also the kingpin position (ahead of the tractor's rear axle, negative when behind it) and the Number of semitrailers, 1 to 12, with no B-double limit: a sugar-cane road train with as many semitrailers as the tractor can pull. In Semitrailer no. choose which semitrailer the fields below show: front to kingpin, kingpin to rear axle, rear overhang and where the next one's hitch is, behind the axle (negative when ahead of it; off on the last one). Raising the number copies the last semitrailer. On a bi-articulated bus, each joint is a "kingpin" and each car a semitrailer. These fields stay off for a rigid vehicle. The outer front wheel minimum radius (half the kerb-to-kerb turning circle of the data sheet; 0 when unknown) draws the turning template; a radius too small for the wheelbase turns red. Enter the dimensions of the finished vehicle, body included, not of the bare chassis: the envelope is drawn from the body. When the fields do not fit the window's height, the list scrolls.

The page shows the overall length and draws the vehicle as you type. A missing name, a name already in the list, or a wheelbase of zero turn the field red and leave Save off, with the reason under the fields. Saved vehicles appear in the Design vehicle list after the catalogue, on every CAD, for the Windows user who saved them (they are kept in %LocalAppData%\CapCad\CapTrack\vehicles.user.json). Delete removes the selected one after a confirmation.

A catalogue of your own can also 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 quickest way to add the design vehicle of your own norm: enter its tracking length, coupling offset, body outline and wheel points.

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

With Run with the mouse, 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 Run 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

The window writes %LocalAppData%\CapCad\CapTrack\settings.json on each run and when it closes: the vehicle, the type filter, the clearance, the swept width, the vehicle code in the layer names, reversing, the whole vehicle on the path, the continuous envelope and the hatch, which come back the next time it opens. The obstacles are not remembered.

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

Notes

• Replacing applies to the same run. The same vehicle, on the same path, from the same end and the same way, erases the previous result; another vehicle, the other end or reversing adds to the drawing. Each vehicle's layers stay apart when the vehicle code is in the names.
• 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.