Dynamics - Crabbing and Positive Phase Steering

Crabbing is a type of wheel alignment where all wheels face a similar direction but are 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(under their page’s “steering” header).

Figure 1. Crabbing to reduce wind resistance(blue) while still travelling along a straight path

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

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?

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.

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.

This method is not recommended because independently controlling the rear wheels impairs the handling of a car.

Positive/negative phase changer

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.

Benefits and Drawbacks

Crabbing offers the main benefit or reducing drag from air resistance. Integrating crabbing would also involve implementing positive phase steering, which would be worth looking into.

Positive phase offers an number of benefits, such as:

improved lane changing ability with reduced wear on tires
better maneuverability in tight spaces with few bends

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.

TL;DR

How complex is the mechanism to incorporate it?