4 replies to this topic

[packapoo]

Saw a TV item on last nights news on a locally built car for city driving, that featured all four wheels swiveling at right angles to direction of travel, for sliding into and out of parallel parks.

Electric, of course - but should that be of course?

Figure that conventional drive and steering would be difficult to achieve this ability, but what about using hydraulics?
Have there been any instances of anything similar for the everyday car? I'm not talking power steering.

[Kelpiecross]

Saw a TV item on last nights news on a locally built car for city driving, that featured all four wheels swiveling at right angles to direction of travel, for sliding into and out of parallel parks.

Electric, of course - but should that be of course?

Figure that conventional drive and steering would be difficult to achieve this ability, but what about using hydraulics?
Have there been any instances of anything similar for the everyday car? I'm not talking power steering.

Slightly beside the point but some of the 3-wheeler Bond-type cars had their Villiers motorcycle engine mounted above the single front wheel and it could be swivelled 180 degrees to drive the car backwards. I suppose you could have a four-wheel car with an engine above each wheel that could drive itself in any direction.

[Terry Walker]

Okay for parking maybe, but what about everyday driving? If it's as good at that as any car in its class AND it can do the parking trick. . . well, interesting.

The thing about electric cars is you can have an electric motor in each wheel hub, which eliminates the need for driveshaft universal joints which can do contortions. Maybe you could have a driveshaft to the bottom of a macpherson strut, a 90 degree bevel gear, then up the strut, then another bevel to take the drive to the hub. Lot of fraffing around, in my view, and unsprung weight added to no good purpose.

Still, I recall the brief fad for 4-wheel steering, now history. The extra costs and complexity, not to mention (I suspect) that the public stayed away in droves, brought an end to them. I don't think they're made any more.

[Canuck]

I want to say that recent GM pick-up trucks had rear-steer engagement in reverse but that that may have been a short-lived fad. In terms of sports cars I can't think of any current production that are.

[NotAPineapple]

Okay for parking maybe, but what about everyday driving? If it's as good at that as any car in its class AND it can do the parking trick. . . well, interesting.

The thing about electric cars is you can have an electric motor in each wheel hub, which eliminates the need for driveshaft universal joints which can do contortions. Maybe you could have a driveshaft to the bottom of a macpherson strut, a 90 degree bevel gear, then up the strut, then another bevel to take the drive to the hub. Lot of fraffing around, in my view, and unsprung weight added to no good purpose.

Still, I recall the brief fad for 4-wheel steering, now history. The extra costs and complexity, not to mention (I suspect) that the public stayed away in droves, brought an end to them. I don't think they're made any more.

Porsche have 4WS on their latest GT3.

4WS does nothing for maximum grip performance. It is used to aid stability and to affect acceleration and yawrate in the tactile feedback range of the driver. A lot of the test drivers I have spoken to who have tested 4WS over the years (its been tested on and off for the last 30 years) have commented on the bad feedback it gives. In my opinion it is affecting the yawrate more than the lateral acceleration and the driver feels this disconnection.

Porsche also use it to give you a tighter turning circle at low speed too. Their marketing material gives a few technical hints:

Ein elektromechanisches Verstellsystem an jedem Hinterrad ermöglicht unterschiedliche Lenkwinkel – abhängig von der jeweiligen Fahr- und Lenksituation und der Fahrgeschwindigkeit.

An elektromachanical actuator on each rear wheel allows different steer angles (left and right) which depend on the driving and steering situation and the driving velocity.

Bei niedrigen Geschwindigkeiten lenkt das System die Hinterräder entgegen den eingeschlagenen Vorderrädern. Das führt zu einer virtuellen Radstandsverkürzung. Der Wendekreis wird verkleinert, das Einparken erleichtert.

At low speeds the system steers the rear wheels in the opposite direction to the front wheels. That make a virtual reduction in the wheelbase. It makes the turning circle smaller and eases parking.

Bei höheren Geschwindigkeiten lenkt das System die Hinterräder in die gleiche Richtung wie die eingeschlagenen Vorderräder. Die Fahrstabilität steigt durch die virtuelle Radstandsverlängerung, die Agilität durch das zeitgleiche Lenken der Vorder- und Hinterräder. Besonders bei Überholmanövern auf der Rennstrecke.

At high speed the system steers the rear wheels in the same direction as the front wheels. This increases the driving stability through a virtual increase of the wheelbase and the agility due to the sycronised steering of both the front and the rear wheels. Especially during overtaking manauvers on the racetrack.

Basically, the active steering does the same things that vehicle engineers do with roll steer and lateral compliance steer on a conventional suspension. On a passive axle there is a phase delay before the lateral compliance steer or the roll steer builds up. The advantage of the actuator is that this delay is eliminated since it steers the rear wheels as soon as the fronts are steered.

Anyway, I'm not a fan of such a system because it gives fake feedback to the driver and doesn't increase the grip performance. On top of that, they typically come with a reasonable weight penalty.