Answer to Quiz No. 16



Refer to this Graphic     

Question 1: The pivot point is at point “e”

Question 2: The pivot point is at point “k”

The pivot point is at point “k” because it is the point of trailer rotation where there is the least amount of scrubbing of the tires during turning as tire scrubbing takes the path of least resistance. In the case shown, none of the trailer axles are steerable.

The complete tractor-trailer pivots around point “k”. This is why so much maneuvering room is required to move a trailer sideways a foot or two, i.e., if a rig is backed up to a loading dock but must be moved over 1 foot for better alignment, the driver needs a large clear area to pull out into in order to move the pivot point of the trailer over 1 foot and get it backed up square with the dock.

The next time you are driving down the freeway next to an 18 wheeler and are concerned that it is going to swerve over into your lane, just keep your good eye on the tractor’s front steering tire on your side. If the driver is going to move over in your lane, the front tire will move over quite a bit before the tractor or trailer tires will show signs of moving sideways.

Question 3: The minimum turning radius for the tractor is: 27’- 6”
The min. turning radius for the tractor-trailer combination is: 60’- 0”

Question 4: The maximum turning radius for the tractor is: 42’- 8”
The max. turning radius for the tractor-trailer combination is: 79’- 5”

Question 5: The answer is: NO

The above turning radii are for one particular set of turning parameters and will remain constant only if the tractor is kept at an angle of 25 degrees with the trailer as shown.
In fact, if the road has been laid out with the maximum and minimum turning radii for the tractor-trailer combination as shown, the driver can keep the complete rig on the road but to do so he will have to watch the left trailer tires in his rearview mirror to see that they are kept right on the inside edge of the road and he will have to watch his right front fender to see that it stays on the outside edge of the road. To accomplish the above, the driver will have to adjust the steering angle of the front tires of the tractor somewhat from 25 degrees to maintain the 25 degrees between the tractor and the trailer.
If the driver keeps the front tires turned at a constant 25 degrees for a full 360-degree turn, the turning radii will become progressively smaller. If he continues the turn, the tractor will eventually end up approximately perpendicular to the trailer as shown on the small sketch A.

On the other hand, when considering the tractor by itself, if the driver keeps the front tires turned at a constant 25 degrees, the tractor will turn around at a constant radius and in a near perfect circle.

    


    


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