Knowledge Base

Article Number: 34 | VC3 | VC4 | Post Date: April 29, 2017 | Last Updated: April 29, 2017

My tractor-trailer is accelerating at a much lower rate than I am specifying in the sequences menu. What am I doing wrong here? 

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 given by equation (4) in Chapter 8 of the User’s Guide. 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.

Vehicle acceleration rates can be monitored a few different ways in Virtual CRASH. First, report dynamics can be used to obtain the position versus time data, which can be converted to acceleration versus time through a simple spreadsheet analysis. Another way to monitor acceleration (and other parameters) is to use the diagram tool, shown below.

The diagram tool can be used to graph values such as velocity magnitude versus time (“time-velocity”), distance traveled versus time (“time-distance”), acceleration versus time (here acceleration = Change-in-speed / Change-in-time and is labeled “time-acceleration”), vehicle-frame acceleration components versus time (“time-acceleration x”, “time-acceleration y”, “time-acceleration z”), as well as orientation versus time. 

Checking the time-acceleration x boxes for the three tractors, we see that the tractor only system reports the expected acceleration rate based on the 2 ft/s^2 user input. The front drive wheel tractor-trailer reports a much lower acceleration rate as expected due to increased system weight from the trailer. The rear drive wheel tractor-trailer reports a larger acceleration rate because of the increased normal load on the tandems from the trailer coupling. 

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.

Using pedal position is also recommended for braking tractor-trailer systems in cases where "use tow car sequences" is disabled. This is discussed in Knowledge Base Article 19.

Tags: acceleration rate, tractor-trailer, semi-tractor, accelerating, Diagram, time-velocity, time-distance, time-acceleration, time-acceleration x, time-acceleration y, time-acceleration z, graph, plot, graphing, plotting, charts, data.

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