Chapter 25 | Tractor-Trailers
Introduction
Creating tractor-trailers, or any combination of tow vehicle and trailer, is a simple process in Virtual CRASH. In this section, we will walk you through creating a tractor-trailer.
Create Tow Vehicle
First, in the left-side control panel, navigate to the assets > vehicles 3D > medium (learn more). Set your keyword filter to "truck" to display the available tractors in the lower window of the assets browser. Scroll down the list, hold+left-click on the desired tractor image, then drag and drop it into the scene.
Attach Trailer
Next, set the keyword filter to "trailer." Scroll down the list of available trailers. Hold+ left-click on the desired trailer, then drag and drop it onto your tractor. When the prompt appears asking, "Do you want to replace current data?", left-click "No".
When the prompt appears asking, “Do you want to replace current mesh?”, left-click “No”.
You will then see your tractor and trailer connected (note this workflow can work for any vehicle and trailer).
The trailer can also be connected to the tow vehicle by first importing an independent trailer object from the database. With the trailer selected, simply use the “pick leader” feature in the “trailer” menu. Left-click “pick leader,” and then left click the desired tow vehicle (see below). Note, a trailer can also be selected as the tow vehicle in cases of a tractor with multiple trailers.
The trailer can also be connected to the tow vehicle by importing an independent trailer object from the database. After importing, select the trailer, then use the "pick leader" feature from the "trailer" menu. Left-click "pick leader," and then select the desired tow vehicle (see below).
Note: A trailer can also be selected as the tow vehicle in cases where a tractor is towing multiple trailers.
Adjust Trailer Position
With the trailer selected, go to the “trailer” menu. You will now see options to adjust the drawbar length, hitch y-offset, hitch height, hitch y-offset, and hitch overhang. Adjust the properties as needed for your subject vehicle (see below).
Drawbar length is defined as the distance between the hitch joint and the forward-most vertex of the trailer model geometry. Negative values will slide the trailer geometry forward of the joint. Positive values will slide the trailer backward of the joint.
Hitch overhang is defined as the distance between the hitch joint and the rear-most vertex of the leader vehicle model geometry. Negative values will slide the joint forward of the leader vehicle’s rear-most vertex. Positive values will slide the joint backward of the leader vehicle’s rear-most vertex. Note, as the joint slides forward or backward with changing hitch overhang values, the trailer remains at a fixed position relative to the joint.
Just like any vehicle object, a trailer’s CG local x position can be modified in the axles menu via “cog to front axle” (learn more). The local y and z positions can be modified in the trailer’s “size” menu using the “c.g. offset y” and “c.g. height” inputs.
The CG position can also be adjusted via translation control grips by switching to “pivot” selection type [Shift+P] and then pressing [1] at the top of the keyboard.
Breaking the Connection
As with any joint connection in Virtual CRASH, the joint connecting the tow vehicle to the trailer can be broken either by time, force threshold, or toque threshold. Simply enable the breaking option in the trailer menu, and select the desired braking method (see next figure).
Connecting Multiple Trailers
The process of connecting trailers to leaders is exactly the same when simulating a train of multiple trailers. Simply drag and drop each new trailer on top of the trailer that leads it, or import each trailer into the scene and use the pick leader option and manually select each trailer’s leader (see below).
Changing Wheel Textures
Wheel textures can be easily changed by dragging and dropping an alternative texture directly onto the wheel of a given axle. In assets > textures > wheels, you will find useful textures for both tractors and trailers. Remember to set keyword to “.”.
To apply a new texture, hold left-click, drag the texture, and drop it onto the wheel. When prompted with the question, "Do you want to replace the current wheel texture?", left-click "Yes".
This can also be achieved by selecting the vehicle, then choosing the axle from the “axles” menu. Next, left-click on "texture" and load the desired texture file.
Accelerating and Braking with Trailer Attached
Note, the relationship between the acceleration input specified in the sequences menu and the fraction of maximum possible longitudinal force applied at each drive wheel is described in Chapter 8. In the case of the tractor-trailer system, this fraction is calculated for the drive wheels assuming only the tractor is in the system, without regard to any effects of the trailer. Just as in cases when vehicles are turning, traveling up inclines, or are loaded with additional cargo weight, which can all reduce the total vehicle acceleration compared to the user input value, effects due to trailer coupling are not “felt” until the simulation starts.
In addition, the vehicles in the Virtual CRASH database have the drive wheels set to front by default. Setting the tractor drive wheels to rear in the axles menu will allow the increased normal load on the rear wheels due to trailer coupling to increase the overall acceleration rate, generally closer to the intended input value.
When a target acceleration rate is required, it may be necessary to “tune” the acceleration input in sequences upward to counteract the increased loading on the tractor as well as to counteract any terrain-related effects. Rather than using the acceleration input directly, it is preferable to use either the “pedal position” input option or wheels separately to tune the percentage of maximum available longitudinal tire force at the drive wheels. One can then use the “Diagram” graphing tool to read the total resulting vehicle acceleration.
When decelerating a tractor-trailer system, keep in mind the option “use tow car sequences” is automatically enabled in the trailer’s “trailer” menu in the left side control panel. This ensures that the same fraction of maximum allowed longitudinal braking force used for the tractor wheels is also used for the trailer, thereby allowing for the same deceleration rate during braking, despite the increased load from the trailer. Indeed, when “use tow car sequences” is enabled, you will find the tractor-trailer system decelerating at the specified rate shown in the acceleration sequences input. However, when “use two car sequences” is disabled, such as when one needs to decouple the magnitude of tractor braking from trailer braking, one can no longer rely on using the acceleration sequences input. In such cases, again, one should simply use percent "pedal position" to tune deceleration rates, and monitor the resulting tractor-trailer system deceleration using the Diagram tool.
Collision Delta-Vs
Collisions involving tractor-trailers with other vehicle objects will automatically use the Kudlich-Slibar impulse-momentum model. Because this model applies impulses directly to the two single (uncoupled) bodies undergoing direct contact, the true tractor-tractor Delta-V, which includes coupling effects, can take a few integration time-steps to completely determine. As a result, one should not read the EES (or Auto-EES) displayed Delta-V values in such cases, but rather estimate Delta-V directly from the diagram tool or by using the dynamics report data.
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