Pelican Parts Forums - Why didn't 4 wheel steer catch on for trucks?

Quote:

Originally Posted by speeder (Post 11675460)

I'm also trying to imagine how the rear wheels turning thee same way as the fronts would be an advantage in a high speed turn in a modern Porsche? :confused:

Basically, momentum keeps the vehicle moving the same direction. The input for all 4 wheels moving the same direction is a vector centered on the vehicle instead of a vector at only the front end. It is for small changes (1.5 degrees maximum!).

From Car and Driver in 1988

https://www.caranddriver.com/reviews/a35287962/tested-1988-four-wheel-steering-benefits/

Quote:

In fact, as the manufacturers know well, four-wheel steering has several important potential benefits. Every 4ws system that we've seen so far turns a car's front and rear wheels in opposite directions under some conditions and in the same direction at other times. Opposite-direction steering makes a car turn more sharply, improving its low-speed maneuverability and tightening its turning circle. Same*-direction steering, in theory, improves handling at higher speeds. To understand why, consider that the front tires of a conventionally steered car develop cornering forces immediately when they are steered, but the rears develop cornering forces only when the body of the car has begun to turn. On a 4ws car in contrast, all four tires develop their cornering forces at the same time. The result is more responsive handling in transient maneuvers, such as entering and exiting corners. Furthermore, since the rear tires of a 4ws car do not depend solely on the car's drift angle to produce their cornering forces, the car's tail doesn't swing out as far in corners or wag as much in quick lane changes. These advantages are particularly pronounced on slippery surfaces, say the advocates of 4ws.

The Honda system, the simpler of the two, is entirely mechanical. A shaft from the front rack-and-pinion linkage drives a rear steering gearbox, which steers the rear wheels in the same direction as the fronts when the steering wheel is turned less than 246 degrees from straight ahead, and in the opposite direction beyond that point. Since turning the steering wheel more than 246 degrees from center is likely only at very low speeds, the countersteer mode typically comes into play only in parking-lot maneuvers. In neither mode do the rear wheels steer very much. Their maximum same-direction steering angle is 1.5 degrees, and their maximum counter-steering angle is 5.3 degrees....

The only unquestionable benefit of 4ws is improved parking *lot maneuverability. The Prelude's system reduces its turning circle from 34.8 to 31.4

(High speed part of the article)

Although 4ws theoretically has no effect on steady-state handling, we began our testing on Chrysler's 300-foot skidpad to see how the cars would compare in roadholding. The Hondas were close, with the 4ws car having the edge, 0.78 to 0.77 g....

With the cones spaced 100 feet apart, the two Hondas no longer felt similar. The 4ws Prelude turned in beautifully and seemed to have tremendous grip at both ends of the car. The 2ws model exhibited much greater tail swinging and required considerably more steering correction. The two bottom lines, however, were identical: both Preludes negotiated the course at 62.0 mph.

I cut the article down a bit and focused on the Honda system. The article also goes over a Mazda 626, but it had a complicated electronic and hydraulic system.

At high speed with small inputs, the reviewers liked the 4WS, but it really made no real world difference. At low speed, it cut the minimum turning circle by about 10%. Again, not a huge difference. But I could definitely notice the difference in a parking lot.

At the time, I was looking at 4 cars for the 1st Mrs. Beard

Honda Prelude 4WS - simple and ergonomic
Mazda 626 4WS - Dash board looked like it was from a 747
Ford Taurus SHO - Dash board looked like the Space Shuttle
SAAB 900 (Drove like a TRUCK)

I spent a lot of wasted hours in 1988/1989 in India and Pakistan reviewing Car and Driver and a lot of other car magazines...