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|Article Number: 66 | Last Updated: Mon, Dec 7, 2015 12:40 PM|
By: Craig Luker
January 26, 2012
Offtracking - the difference in the paths followed by the frontmost and rearmost axles of a vehicle. i.e. how far the rearmost axle tracks inside or outside the frontmost axle around a curve.
- OT is positive if it tracks inside and negative if it tracks outside.Tractrix - the path followed by a trailing point on a body (i.e. a vehicle) when another point follows a curve and the first point is not allowed to slip sideways.
Tire Cornering Stiffness - a parameter defining how much sideways force is developed for a tire for every degree of sideslip (ex. Lbs/deg).
- Affected by the tire’s construction and materials (large differences between truck tire makes and models) and by the load carried by the tire.Sideslip or slip angle - difference between the direction a tire is moving and the direction that it is facing. Unless the vehicle is going straight, this is never zero, but it is often small.
The physics of a turning truck (whether or not it is articulated) is very complex. The exact path it follows is affected by these math parameter:
A. Kinematic Solution - kinematic means that we do not consider the mass of the vehicle, nor the forces acting on it. Only the geometry is considered. This approach is also called a "zero-spread" model.
In practice this method has been shown to be accurate for situations such as:
This method essentially reduces the vehicle to one wheel on each axle. Despite the simplifications, it has been shown to be accurate in many typical situations.
Simplified kinetic solutions need tire stiffness, friction, and load on each tire.
More complex kinetic solutions need full details of the vehicle’s suspension. Information that is not normally available in an accident reconstruction.
The more complex models do account for speed and can even predict rearward amplification and outbound offtracking.
Effect of Speed
Due to geometry, the trailer will track inside of the steering axle. However, speed tends to make the trailer swing outward and, therefore, outside the steering axle path (due to centrifugal force).
Critical Speed - speed at which the trailer tracks right on the steering axle path (and thus offtrack is zero).
- For typical configurations on pavement this speed is between 32 km/h and 76 km/h.Guidelines for Using FARO HD Articulated Vehicle Calculation
Feel confident in the simulation result when:
- Speeds are low, such as normal driving around corners in the city.
- Driving in a parking lot or loading bay unless the maneuver is so tight that the tractor steering is very sharp and/or any tires would be expected to slide, scrub, or slip.
- Calculating gentle lane changes.
- The actual offtracking toward the inside of the curve when a truck goes around a highway curve will always be less than the offtracking shown by the software. As such, the simulation can be used to give the maximum inward offtracking in these situations.
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