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I have read that some form of four-wheel steering is a feature on a number of production cars, but I haven't been able to find details of those systems.

I'm particularly interested in whether four-wheel steering has ever been applied for non-rotational, crab-like lane-change maneuvers: I.e., instead of having the rear wheels turn in the opposite direction of the front wheels to increase rotation, can any systems turn the rear wheels in the same direction as the front for emergency lane changes?

I imagine the performance advantages of not introducing a rotational moment in the body at high speeds would be substantial: As we approach the limit of a vehicle's handling: conventional steering overloads one tire, whereas a "crab" lane-change evenly loads both outside tires. Not only does the four-wheel maneuver maintain more tire contact for a given rate of change, I think it will execute the maneuver more quickly and in less distance, because it is only trying to translate the car's mass, not also rotate its momentum.

(Another question that remains is how the car would "know" that the commanded turn is a lane-change instead of a turn, unless the driver is given an additional control to command a "translation" instead of a "turn.")

  • What substantial advantages do you expect? At the highway in normal situations, the car barely rotates. Thus, it also demands little energy. I doubt you'd even feel the difference between four- or two wheel steering. Also take in mind that there are lots, lots of other dynamic effects that play a role while (four wheel)steering a car, especially when taking sharp turns like in an emergency. – Bart Jan 23 '17 at 18:48
  • @Bart: One wouldn't notice the benefits except in aggressive maneuvers. But imagine an emergency lane-change, which is something any car may be called to perform. As we approach the limit of a vehicle's handling: Conventional steering overloads one tire, whereas a "crab" lane-change would evenly load both outside tires. When done so aggressively that conventional steering rolls or spins the vehicle, the four-wheel lane-change would still be under control (and would, I think, execute the maneuver more quickly and in less distance). – feetwet Jan 23 '17 at 19:04
  • I'm still learning about first and second-order processes and vehicle dynamics, but i have the feeling the car won't react as desired when you also use the rear wheel to steer. I suspect the rear of the vehicle to react uncontrollably, i can't support that with facts or calculations though. – Bart Jan 23 '17 at 19:15
  • @Bart: I just amended the question with some of my comments. I'm just working this through in my head, but I don't see any dynamic that would upset the rear. The car's center of mass is simply moved laterally and uniformly towards the outside tires. In the limit I suppose one tire will break loose before the other if fore-aft weight distribution isn't perfect, but that would be under dynamics far exceeding the car's two-wheel steering capabilities. And assuming the vehicle is setup to understeer it would produce the same sort of "plowing" effect since the front would lose traction first. – feetwet Jan 23 '17 at 19:31
  • Some of the Lexus cars have this. 2015+ Lexus RC-F and Lexus RC 350 F sports models. – rana Jan 23 '17 at 19:33
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I know of at least 1 case where that was true. From Autoweek

Honda’s first 4WS system, launched in April 1987 on the Prelude, was all mechanical, using a shaft to connect the front-steering rack to a slider operating the rear tie rods. A planetary gearbox determined rear-steering degree based on steering-wheel angle. At small angles, the rear wheels would steer up to 1.5 degrees in phase with the fronts, but at larger angles (more than 246 degrees of steering rotation), the rear wheels would steer up to 5.3 degrees opposite the fronts, trimming the turning radius about 10 percent.

Edit: Actually, the article cites several examples.

  • Do you know any details of the mapping? E.g., one begins to turn the wheel and up to 1.5° the rears are in phase with the front. As one turns beyond 1.5° ... what happens? They snap back to zero? – feetwet Jan 23 '17 at 20:17
  • @feetwet Sorry I don't. I just remember that the Prelude had that from back in the day. – Mobius Jan 23 '17 at 21:06
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Numerous luxury brands currently employ this technology. Usually the "crab" function only turns on above a certain speed, because it is counterproductive at lower speeds. At lower speeds you want the back wheels to turn opposite the front, reducing your turning radius.

However (and this is just my opinion from experience) in terms of "emergency" lane change situations, I don't think this is really a big deal at all. Most of the time, people don't run into things because the car is mechanically incapable of avoiding them. They run into things because the road is icey, or they didn't have a safe following distance, or they weren't paying attention. I've never had a car understeer when changing lanes. This is a very shallow turn in which understeer should not be an issue.

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