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 “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:
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For more background information, the 4 wheel steering confluence page is a great starting point.
How can crabbing be achieved?
-working on 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 automate the process. For example, GMC implemented a Crabwalk 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?
Requires four wheel steering + a method to control the pairs of rear and front wheels independently
see question 3 below
How does the driver interface with this system?
Independently steering the rear wheels first, then aligning the front wheels to be parallel to the rear(see figure 2)
How is the steering implemented (mechanically or by wire or something else entirely)?
Recommendation is to have a manual on/off switch for crabbing
Sources for . . .
past implementations
https://www.solarteam.be/en/cars/bluepoint
4 wheel steering
Design and fabrication of four wheel steering system for light motor vehicles
Confluence 4 wheel steering 2022
simulation to see how steering angles work(negative/counter-phase 4 wheel steering and ackerman)
https://dynref.engr.illinois.edu/avs.html
general content on crabbing