Crabbing is a type of wheel alignment where all wheels are parallel to each other but 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(scroll down to under their page’s “steering” header).
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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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How can crabbing be achieved?-working on this part
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.
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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 DrawbacksThis 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):
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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:
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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):
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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 all wheel steering + a method to control change the pairs phase of the 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 2buttons and levers (crabbing should not be done using “good handling skills“)
How is the steering implemented (mechanically or by wire or something else entirely)?
Recommendation is to have a manual on/off switch for crabbing
mechanical part needs more research
Sources for . . .
past implementations
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