What is the restrictive or limiting factor in the turning angle of the front wheels of an automobile? Why can’t cars for example be designed so that the turning angle is higher, which I think will make it more manoverable?
I think I read somewhere that there have been designs where both the rear and front wheels have been made to turn 90 degrees and the car has parallel parked by virtually ‘sliding’ into the slot, but they have remained largely experimental.
I have heard that for real wheel drive cars, some older (French?) models had a great turning circle in theory, but if you turned the wheel to full lock, the front wheels would be at such an angle that they wouldn’t turn easily, and would try to judder straight ahead, effectively skidding sideways. For FWD, I’d imagine the drivetrain would be a problem at steep angles.
If we ever get electric cars with hub motors, then I’d say they could do a lot more.
There’s not just one factor. The key elements are amount of motion in the steering gear, particular geometry of the steering knuckle, end of travel limits built into the chassis, and the wheelbase (distance between front and rear wheels).
Rear wheel steering can decrease the vehicle’s turning radius, but at the cost of significant additional complexity and potential for problems. The demand for its benefits is rather low, so there’s no compelling need for manufacturers to offer it.
It varies a bit depending on the suspension setup; with wishbones, the theoretical maximum is 90 degrees from center.
In practice, you can’t do this for a couple of reasons:
The greater the angle you can turn the steered wheels to, the deeper the wheelarches have to be. That means less space for the engine and ancillaries.
If you turned the wheels to 90 degrees at speed, they’d probably be ripped off the car.
Cost - active four wheel steering (even if the rear wheels only turn a few degrees) adds a great deal of complexity to the rear suspension setup.
IIRC, that was “the new Big Thing” back in the early 90s. The rear wheels would steer slightly against the front ones for tight manoeuvring, and slightly with them for high speed curves. I think it lasted less than a year. I forget which manufacturer it was now, but I do remember it was pulled for reasons of cost and complexity for little benefit.
The early-90s Honda Prelude, Nissan Skyline and Mitsubishi 3000GT all had active four wheel steering; lots of cars (including all Subarus, I think) have passive four wheel steering, which basically means there’s lots of play in the rear axles which allows the wheels to turn slightly “in sympathy” with the front wheels.
Googling suggests it was available on the 1987-2000 Honda Prelude. Here’s a short video that shows it in action.
Nissan and Mitsubishi were the big(ish) proponents of it. Nissan had it on most of their rear wheel drive cars and Mitsubishi had it on their VR-4 models, which were the Galant/Legnum and GTO/3000GT.
I haven’t heard of this passive steering thing. Are you thinking of the Weissach Axle?
Basically, there are bushings on the rear wheels which allow them to pivot (no more than a few degrees). The force acting on them when the car turns causes them to turn opposite to the front wheels (just like an active rear steering system).
ETA: Yes, it’s the same thing as a loose trailing arm/Weissach axle.
I had a Citroen GS years ago and the turning circle was quite astounding, beat the legendary Volvo hands down and front-wheel drive too.
Here is a somewhat updated version - apart from the suspension check out the steering on this thing.
Some WWII armored cars (the Italian AB41, the German light armored car I-forget-its-designation) had four-wheel steering. The AB41 was unstable at speed, and the German one dropped the four-wheel steering during production as not being worth the hassle.
The Nash Quad truck of WWI had four-wheel steering, too, but was a very low-speed vehicle. And apparently you could get into situations (parking very close to a wall) where you couldn’t get out again.
I do understand that turning the front wheels through 90 degrees is not possible for obvious reasons. My question was why the current limit that we have now. Why can’t it a little more, say another 5 degrees or so?
Again - it could be done. But there is a tradeoff for everything. For any given existing design, adding additional front turning angle will require:
a) Either a wider track & wider fenders, or else a narrower engine compartment, or a combo of each.
AND
b) For front wheel drive, a more sophisticated (read expensive) drive line arrangement, maybe triple CV joints instead of dual.
AND
c) For front or rear wheel drive, altered suspension gear with greater travel, taking up more room. This either makes the car bigger, or further eats into engine compartment or cockpit space, or both.
AND
d) For front or rear wheel drive, altered steering gear with greater travel, and increased leverage at the travel ends. Depending on how tight space is under the hood, this might again requre growing the car or shrinking the engine or cockpit. There are also design standards for how many turns lock-to-lock, and there is a tradeoff between excessive sensitivity near the center (where 98% of driving is done) versus total travel.
Finally, at least for modern construction, there are standards for road size, parking lot lane size, etc., which are aligned with the capabilities of the typical modern car. A standard car can make a 180 turn in a standard residential street w/o hitting either curb for example. Given that the world is sized for cars as they are, how much benefit is there to making a car which turns slightly tighter?
It seems the current manufactuerers don’t believe that people will pay enough extra for them to go to the bother.
Apparently the new London Taxi variant of the Mercedes Vito has rear wheel steering. It needed it to achieve the 25ft turning circle requirement.