TURN.LSP Instructions

These are the instructions for TURN.LSP, the free Turning Path Tracker for AutoCAD. They replace the PDF instruction sheets that were packaged inside the AASHTO vehicle block library downloads.

TWO VERSIONS

Everything on this page applies to both, except where a paragraph says otherwise.

2.0.0 drops the User block method. It needs a block with dimensions recorded on it — from BUILDVEHICLE or from the AASHTO library. If you have been dragging your own plain blocks along a path, 1.1.17 still does that, and I would like to hear from you before that method disappears for good.

WHAT TURN NEEDS FROM YOU

Two things:

1. A course — a polyline showing where you want the centre of the vehicle's front axle to travel.
2. A vehicle — either one of the AASHTO blocks from the library downloads, or one you define yourself with BUILDVEHICLE.

TURN then draws every wheel path, the hitch path, and the vehicle and trailer bodies at intervals along the course.

STEP 1 — LOAD TURN

APPLOAD or drag turn.lsp into your drawing. TURN creates the layers it needs the first time you run it:

2.0.0 renames these to fit the National CAD Standard's four-character fields, and numbers trailers rather than assuming one: a second trailer lands on C-TURN-TRL2-BODY, a third on C-TURN-TRL3-BODY, and so on.

In 2.0.0 the names are yours. Put a turn-layers.dat anywhere on your support file search path and set your own names, colours and linetypes, the way CNM uses Layers.dat. It ships with a commented block that restores the 1.1.x names above, if you have drawings or layer filters built on them. In 1.1.17 the names are fixed, so recolour if you like but save a named layer state — reloading TURN recreates the layers and discards your changes.

Your course does not have to be on any particular layer. TURN does not care. The old PDF insisted on C-TURN-TRCK-FLTR-PATH; that was never a requirement, and that layer no longer means anything.

STEP 2 — PLACE THE VEHICLE

Insert the vehicle block you want. If you are using the AASHTO library, insert the drawing for your vehicle — for example 2004_AASHTO_WB-67.dwg.

Rotate it to the direction the vehicle is travelling at the start of the course. TURN reads the block's rotation to know which way the vehicle is pointing, and it reads the block's attributes to get every dimension. It does not use the block's insertion point for position, so the exact spot you drop it does not have to be perfect — but get the angle right, because that it does use.

On the little circle in the AASHTO blocks: the small circle sits at the centre of the front left wheel, because the blocks were drawn for the old convention. Do not line that circle up with your course. Your course represents the front axle centre, which is half an axle width inboard of that circle.

STEP 3 — DRAW THE COURSE

Draw a polyline along the path you want the centre of the front axle to follow. Make all line and arc segments tangent to each other.

Make it long

A trailer is still reacting to where the tractor was a full wheelbase ago, and it takes several rig lengths of travel before the articulation settles into the shape it would really take. A course only about as long as the vehicle shows you almost nothing, and the rig plots as one straight box.

Hundreds of feet. Five times the wheelbase or more, with a straight run at each end to warm up. For a WB-67, whose wheelbase is 65 feet, that is 400 feet and up. Long does not hurt.

Minimum radius

TURN will happily track any path you draw, including one no real vehicle could drive. Checking the radius is your job.

AASHTO publishes the minimum turning radius to the outside front wheel. Your course follows the axle centre, which is half the front axle width inboard of that, so:

minimum course radius = AASHTO minimum turning radius − ½ front axle width

This applies to turns in both directions. The table below works it out for every vehicle in the library. The AASHTO radii and axle widths are the same figures that appeared in the old PDF instruction sheets.

How exact is that last column? Not exact, and it is ours rather than AASHTO's. Two things about it are worth knowing before you lean on it.

The front axle width above is centre of tire to centre of tire. That much is certain: it is what BUILDVEHICLE asks for — "half of maximum axle width to middle of wheels" — and in the AASHTO blocks the small marker circle sits exactly half that distance off the centreline.

What is not settled is the other end of the subtraction. AASHTO measures its minimum turning radius to the outer edge of the outer front tire, not to that tire's centre, so strictly another half tire width should come off as well — roughly half a foot on a truck. The old instruction sheets said the vehicle blocks were built to reconcile that difference, but not by how much, and we have not been able to confirm it. Treat the last column as a working figure good to within a few inches, and leave yourself a margin.

STEP 4 — RUN IT

Type TURN. Then:

1. Method — 1.1.17 only. Choose the Generated vehicle method. The AASHTO library blocks carry full dimension attributes, so they work with this method: they are not "user blocks" in TURN's sense. 2.0.0 does not ask.
2. Select the vehicle block.
3. Select the course, near its starting end. Pick a spot on the polyline away from the vehicle block, or you may select the block by mistake.
4. Calculation step distance. TURN works out a sensible value from the wheelbase. Accept it.
5. Steps to skip between vehicle plots. This controls how many copies of the vehicle get drawn along the course. Accept the default, then raise it if the drawing is cluttered or lower it if the plots are too sparse to read.

WHAT 2.0.0 TELLS YOU

1.1.17 draws and says nothing. 2.0.0 draws, then reports at the command line:

TURN: 2 segment(s), 431 steps.
TURN: course 512.0 long, 7.9 x the rig's 65.0 wheelbase.
TURN: swept area is 18442.6 square drawing units.
TURN: tightest turn this vehicle can make is 45.00 radius at the steering axle.
TURN: manoeuvre achievable. No limits exceeded.

If a limit is exceeded it says which, and finishes with this manoeuvre is NOT achievable as drawn. If the course is short against the rig's wheelbase it says that too. It still draws either way, so you can see what happens; it just no longer lets you believe a drawing that cannot be driven.

The swept-path envelope is the outer boundary of everything the bodies sweep, as closed polylines on C-TURN-ENVL. That is the line most plan sheets actually want — the one you hatch or offset for clearance. No 1.1.x version ever drew it.

The steering-lock and jackknife checks only speak when the vehicle carries real limits. Every vehicle in the shipped library has those fields zeroed, because BUILDVEHICLE never asked for them and wrote a placeholder instead. Zero means "do not check" rather than "no limit", so the analysis stays quiet rather than inventing a verdict. If you know the real figures for a standard vehicle, please send them.

IF THE RESULT LOOKS WRONG

DEFINING YOUR OWN VEHICLE

If your vehicle is not in the library, type BUILDVEHICLE (or BV). It asks for each dimension and then draws an attributed block you can keep and reuse.

Two of its prompts catch people out:

• "Distance from rear axle to hitch (forward is negative)." This is on the tractor, measured from its rear axle to the hitch point. On a semitrailer tractor whose fifth wheel sits over the drive axle, the answer is 0. A hitch placed well behind the drive axle is genuinely unstable, and TURN will faithfully draw the resulting mess.
• "Distance from hitch to front of trailer (forward is negative)." The front of a trailer is ahead of its kingpin, so this answer is normally negative — about −3 for a standard semitrailer.

For any group of multiple axles, use their centroid and model them as one axle.

Back to the TURN.LSP page · How the tracking calculation works