Dynamics - Crabbing

Crabbing is a type of wheel alignment where all wheels are parallel to each other but nonparallel to the body. This method of steering helps reduce wind resistance by aligning the car’s front with the direction of the wind to minimize air friction caused by side winds. The most relevant example of crabbing being implemented is the BluePoint car designed by the Agoria Belgian Solar Car Team(scroll down to their “steering” header).

Background information(All Wheel Steering)

Before discussing crabbing, a brief visual overview of the three main phases of all wheel steering will be provided:

Diagram

Also known as

counter-phase rotation
negative phase

in-phase rotation
positive phase

zero rotation
neutral phase
front/two wheel steering

For more background information, the 4 wheel steering confluence page is a great starting point.

How can crabbing be achieved?

-still adding to this part

Intentional misalignment of the rear wheels to change thrust angle

Changing the thrust angle of the rear axis will induce crabbing, as shown in figure 2 below. To accomplish this, the rear wheels of the car must first turn together to swing the rear left or right. The front wheels then turn with the car to accommodate this.

Since crabbing requires all wheel steering, it is slightly more complex than traditional four or three wheel systems. To implement crabbing, a method to control the pairs of rear wheels and front wheel(s) independently is needed.

Considerations and Drawbacks

When crabbing occurs unintentionally, it is known as dog tracking. Some problems of dog tracking include:

It is hard to detect from the driver’s perspective
Thrust causes the car to tend towards one side
Reduced steering capabilities in sharper corners (rear end tends to understeer/oversteer)

Based on these drawbacks, crabbing should be used during long stretches of straight roads and when the benefit of reducing drag is greater than the downside of needing to compensate for thrust forces acting from the sides of the car.

Implementation methods

The most consistent method to ensure crabbing can be accomplished as needed would be to create a mechanism that changes from front wheel steering/all wheel counter-phase steering to in-phase steering. For example, GMC implemented a Crabwalk(in-phase rotation/positive phase) feature in their Hummer EV that can be activated and turned off as needed(good example from 0:34 - 0:52):

https://youtu.be/JjhzP-KJ0Qc?t=33

This would require a lot of testing and is expected to be very complex. If crabbing cannot be incorporated safely, it is recommended not to implement it.

TL;DR

How complex is the mechanism to incorporate it?