Hensley Arrow: How does it REALLY work?
There have been several discussions on this hitch that have become somewhat 'controversial'. I am NOT trying to stir this controversy/bashing back up. Instead, I'd like to see a good, intelligent di...
Forum Discussion
How the HA Transfers Force and Moment Between TT and TV
Having made good progress on defining the kinematics of the HA's 4-bar linkage, a next step might be to attempt to define how forces and moments are transferred between the trailer and the tow vehicle. Unlike a conventional ball hitch, the HA is able to transmit moment as well as force. This is critical to understanding how the HA works.
All interaction between TT and TV must take place via the two side links of the 4-bar linkage. Since these links have pins and bearings at both ends, a link can only transfer either a tension force or a compression force and the force will be in line with the centers of the pins at each end of a link. An individual link is not able to transmit a moment.
The easiest case to evaluate is the linkage configuration which exists when the TT and TV are aligned straight ahead. In this case, the two side links are oriented at equal but opposite angles relative to the longitudinal centerlines of the TT and TV. It also is practical to consider only the transfer of lateral force between TT and TV (i.e. pulling or braking forces will not be considered).
When the side links are at equal but opposite angles and only a lateral force exists, the magnitude of the axial force will be the same in both links; but one link will be in tension and the other in compression. For example, if the trailer is pushing to the left, the right link will be in compression and the left link will be in tension. There will be a longitudinal component of force pushing forward on the right front pin and an equal longitudinal force pulling rearward on the left front pin. These equal and opposite forces acting with a separation of 7 1/8" produce a counterclockwise moment on the TV. Also, there will be a lateral component of force acting toward the left on each of the front pins. These lateral forces will produce a clockwise moment on the TV. The magnitude of this clockwise moment will increase with increasing distance forward of the pins.
At some distance forward of the pins, the magnitude of the clockwise moment will equal the magnitude of the counterclockwise moment. This distance, measured along the longitudinal centerline of the TV, will define a point at which the resulting moment is zero. However, the lateral force at this point still will be equal to the lateral force on the hitch.
The distance to the point of zero moment can be determined from a free body diagram analysis of the forces in the side links. The distance also can be determined graphically by treating the forces in the side links as force vectors. At the point where the force vectors converge, there is no separation between them and there can be no moment produced by them. Since the force vectors must be co-linear with the centers of the pins, the vectors will converge at the point which previously has been defined as the Virtual Pivot Point.
If Tim concurs with this analysis, I would hope that he could produce a diagram which would do a better job of explaining .
Ron
Having made good progress on defining the kinematics of the HA's 4-bar linkage, a next step might be to attempt to define how forces and moments are transferred between the trailer and the tow vehicle. Unlike a conventional ball hitch, the HA is able to transmit moment as well as force. This is critical to understanding how the HA works.
All interaction between TT and TV must take place via the two side links of the 4-bar linkage. Since these links have pins and bearings at both ends, a link can only transfer either a tension force or a compression force and the force will be in line with the centers of the pins at each end of a link. An individual link is not able to transmit a moment.
The easiest case to evaluate is the linkage configuration which exists when the TT and TV are aligned straight ahead. In this case, the two side links are oriented at equal but opposite angles relative to the longitudinal centerlines of the TT and TV. It also is practical to consider only the transfer of lateral force between TT and TV (i.e. pulling or braking forces will not be considered).
When the side links are at equal but opposite angles and only a lateral force exists, the magnitude of the axial force will be the same in both links; but one link will be in tension and the other in compression. For example, if the trailer is pushing to the left, the right link will be in compression and the left link will be in tension. There will be a longitudinal component of force pushing forward on the right front pin and an equal longitudinal force pulling rearward on the left front pin. These equal and opposite forces acting with a separation of 7 1/8" produce a counterclockwise moment on the TV. Also, there will be a lateral component of force acting toward the left on each of the front pins. These lateral forces will produce a clockwise moment on the TV. The magnitude of this clockwise moment will increase with increasing distance forward of the pins.
At some distance forward of the pins, the magnitude of the clockwise moment will equal the magnitude of the counterclockwise moment. This distance, measured along the longitudinal centerline of the TV, will define a point at which the resulting moment is zero. However, the lateral force at this point still will be equal to the lateral force on the hitch.
The distance to the point of zero moment can be determined from a free body diagram analysis of the forces in the side links. The distance also can be determined graphically by treating the forces in the side links as force vectors. At the point where the force vectors converge, there is no separation between them and there can be no moment produced by them. Since the force vectors must be co-linear with the centers of the pins, the vectors will converge at the point which previously has been defined as the Virtual Pivot Point.
If Tim concurs with this analysis, I would hope that he could produce a diagram which would do a better job of explaining .
Ron
