Tandem Axle Angles

Tandem Axle Angles

Geometric centerline of a vehicle is a line
drawn through the midpoints of the front axle and
the rear reference axle.

Centerline of a vehicle

Thrust line is the bisector of the total toe angle
of an axle. It represents the direction the axle
“points” compared to the centerline of the vehicle.

Thrust angle

Thrust angle is the angle formed by the
geometric centerline and the thrust line of an axle.

  Tandem scrub angle is the angle formed by the
  two thrust lines of a tandem axle vehicle.
  In the diagram below, misalignment causes the
  tandem axles to work against each other.
  The steer axle must be turned to offset the “push” of
  the axles and keep the vehicle moving straight ahead.
  This causes every tire on the vehicle to scrub

Tandem scrub angle

  Tire wear from tandem scrub occurs at the
  leading edge of the steer tires, in a pattern called
  “inside/outside” wear. For example, on the front
  axle of this vehicle, wear would occur on the
  outside of the left steer tire and on the inside of
  the right steer tire. Tire wear would occur on all
  drive axle tires.




Trailer Alignment 
and Tire Wear

 The same conditions that cause tandem scrub on
 tractors also apply to tractor-trailer combinations.

Trailer Alignment

Misaligned trailer axles cause tandem scrub,
resulting in rapid wear on all tires.

If the trailer doesn’t track correctly, it exposes
more area to wind resistance. This can affect
handling and fuel economy.


  Alignment Angles Affect
  Rolling Resistance and
  Fuel Consumption

  While the effects of misalignment show clearly in
  tire wear, the effects on fuel consumption are less
  easy to quantify. Fuel consumption is affected by
  many factors.
  However, it is obvious that misalignment must
  increase rolling resistance – and rolling resistance
  is a major cause of fuel consumption.


  Geometric Centerline

 Geometric Centerline

  Geometric Centerline Alignment can be used as a
  reference from which to compute individual toe
  angles. The Geometric Centerline of a vehicle is
  established by placing a line from the midpoint of
  the front axle and the midpoint of the rear-most axle.
  The Geometric Centerline is not based on frame
  rails or cross member reference points.
  The alignment system will establish the
  Geometric Centerline.





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