Rear Wheel Steering
 

For some reason, the other day a thought popped into my head about an old commercial for the Yukon Denali. The big selling point was that the rear wheels could turn, making it easier to turn the truck. I was just wondering why, other than a select few, more cars don't have this option, especially larger vehicles. Would this not save from having to need such a large PS pump?

I remember that system, the QuadraSteer. It's still often used in 3-axle trucks and buses back here, but it's not reported to decrease fuel economy so considerably. Might be more due to the extra weight, which is not so much considering the whole vehicle weight. But what made it not so popular was the price tag, as it was an optional feature.

In the late 80's there was a Honda Prelude with 4WS, and I believe Honda is bringing it back in one of the 2014 Acura models (saw a commercial for it a few days ago). I imagine complexity and therefore cost are the reasons we don't see it more.

I remeber the Honda Prelude with 4ws. That system was passive and used the lean of the body to turn the rear wheels. The handling was worse and the system was often defeated by owners seeking good performance.

I'd bet active systems need a *bigger* powersteering pump too as the rear would be all hydraulic unless it is done electric instead. Still higher loads.

This feature is only useful if a tighter turning radius is needed.

The VR-4 version of the Mitsubishi 3000GT had 4-wheel steering, and active aero, back in what, like 1993? It was way ahead of its time.

From Wikipedia

"The first generation model incorporated many of Mitsubishi's contemporary performance-enhancing technologies, such as full time four-wheel drive, four-wheel steering, active aerodynamics featuring automatically adjusting front and rear spoilers, sport/tour exhaust modes and electronically controlled suspension (ECS)."

My mother's kept eating ECU's though. :p

They are fairly common in japan. Since all most all of their cities over there have areas that are more tightly packed than NYC and LA.
In the US there isn't really a need for a turning ability that is that tight or the added complexity of a system that will most likely break down.
We keep our cars a lot longer than they do.

the C3 and sierra denali rear steer systems are/were virtually bullet proof. the real problem was it was a $2000 option and noone wanted to pay

Few people wanted to pay extra money for something that inreased weight and complexity while offering few real-world benefits. I don't see how turning two more wheels would reduce load on the PS pump, either.

On 4 axle trucks and busses (2 front, 2 rear), they seem to have resorted to two steering axles up front in Europe.

A local outfit sells cross-steering trailers for semi's.
Renders, X-Steering

Dunno how bad it is in the US, but here in Europe things can get so tight that when these big guys turn, a triple-axle trailer may actually see one or two of them reverse, and they scrub heavily in the process.

Not good for the tyres, and it certainly takes more fuel - but wether that's visible in the overall FC is doubtful, as they spend most of their time going more or less straight.

AWS is only seen on mobile cranes these days.
Some cars have rear axles that have a bit of flex in them, and they steer along passively.

Definitely cost. Back when most of the Japanese manufacturers had systems, it was used as a marketing tool. Often the cars would have decals on the bottom of the doors with all the acronyms - ABS, 4WS, 4WD - for the features so the customer could be reminded of where the money went.

There were various ways to achieve the rear wheel steering. Some used hydraulics but Honda's was a purely mechanical system with an additional shaft to the back of the car. That operated a worm and peg mechanism. At large steering angles the rear wheels steered in the opposite direction to the fronts, to decrease the turning circle. At small steering angles the rear wheels steered in the same direction as the fronts to improve grip.

The hydraulic systems could be speed based. Some only steered the rear wheels in the same direction as the fronts, some also did the rear wheels out trick.

The now unheralded passive systems rely on the deflection of the (quite sophisticated in design) suspension bushes under cornering forces to alter the angles of the rear wheels.

Thanks for the replies guys. I didn't know that AWS was so prevalent in certain areas of the world. It's interesting to note how many different systems there are for the same end goal (much like the rest of cars' various mechanisms).

The reason I thought you wouldn't need as much PS is because with extra turning from the rear wheels, I thought it would help make steering easier. Is that wrong in thinking? Would 2 extra 'pieces' of friction actually make it harder to turn without PS?