Four Wheel Steering, Always The Option, Never The Defining Feature

A couple of weeks ago when it emerged that a new Tesla might have a four-wheel steering capability, our colleague Dan Maloney mused aloud as to how useful a four-wheel steering system might be, and indeed whether or not one might be necessary at all. This is hardly the first time four-wheel steering has appeared as the Next Big Thing on the roads. It’s time to take a look at the subject and ask whether it’s an idea with a future, or set to go the way of runflat tyres as one of those evergreen innovations that never quite catches on.

What’s your dream vehicle? If you’re like me, you have more than one. There in my lottery-winner’s garage, alongside the trail bikes and the mobile hackerspace, the dictator-size Mercedes and the Golf Mk1, will be a vehicle that by coincidence has four-wheel steering. The JCB Fastrac is a tractor that can travel across almost any terrain at full speed, and though I have no practical use for one and will never own one, I have lusted after one of these machines for over three decades. Their four-wheel steering system is definitely unusual, but that makes it the perfect vehicle with which to demonstrate four-wheel steering.

Four-Wheel Steering Where It’s Really Useful

The Fastrac steering in action
The Fastrac steering in action, in a still from the linked JCB Agriculture video.

The Fastrac has two axles, and in four-wheel steering configuration the rear axle has steering pivots as well as the front. It doesn’t have a steering box in the same way as a car, instead steering is entirely hydraulic and fly-by-wire. For this reason the top speed is software-limited by law, but the early ones without this annoyance could do motorway speeds. It is after all the world’s fastest tractor.

This steering system is computer controlled, and as such has a variety of modes. There is a mode for tight turns in which front and rear axles pivot in opposite directions, one for sideways movements in which they pivot in the same direction, and even specialist modes such as one in which both front and rear wheels follow the same track when following tram lines in a crop. This lets a giant tractor with huge wheels nonetheless move around in quite small spaces.

… And Where It Isn’t Quite So Much

The tractor makes a great introduction to four-wheel steering, but outside that specialist environment how would it apply to a more normal vehicle? Back in the 1980s there were a slew of 4WS-equipped cars from major manufacturers exploring the market. Probably the most famous example is the third-generation Honda Prelude on which it was an optional extra. This vehicle had an all-mechanical system with an extra steering box at the rear and was driven by a shaft, which makes it an easy-to-understand exemplar.

It’s pretty obvious how any trick to deliver a tighter turning circle would be extremely useful in a crowded parking garage, so pivoting the axles in opposite directions at low speeds is a desirable trait. Meanwhile at high speeds, changing lanes on a motorway could be made more stable by pivoting the axles in the same direction.

The Honda Prelude 4WS rear steering box. Soyuz72, CC BY-SA 4.0
The Honda Prelude 4WS rear steering box. Soyuz72, CC BY-SA 4.0.

The Honda achieved the feat of delivering both steering configurations in one, by the clever use of an epicyclic gearbox for the rear steering. A slight turn of the steering wheel would set the rear wheels pivoting in the same direction as the front allowing it to drift sideways at speed, while further turns would move them round to the opposite direction from the front in order to reduce the turning circle. The dramatic effect on its turning ability is extremely evident in the video linked above, but as an expensive option on the standard Prelude it didn’t tempt enough customers to be made a fixture on later cars.

The story of the Prelude with its interesting steering system that didn’t catch on is the one that has put four-wheel steering in the scrapyard of automotive ideas for every manufacturer that has toyed with the idea since. It’s analogous to 3D cinema: an idea that comes around every few decades as the Next Big Thing, only to fade away soon afterwards. The Honda and its 1980s cohort remain only as interesting curiosities. Will the Tesla do the same? The answer lies in looking again at the Fastrac, it remains a volume-selling 4-wheel steered vehicle because the steering brings something of use to its customers over a conventional 2-wheel-steered tractor. The challenge facing Tesla, or any other manufacturer tempted to give it a try, is to find that elusive usefulness, otherwise they’ll find out as Honda did that consumers prefer to spend their money on something more substantive to boast about.

Header image: Aconcagua, CC BY-SA 3.0.

67 thoughts on “Four Wheel Steering, Always The Option, Never The Defining Feature

    1. I think the point might actually be to streamline the manufacturing process. Yes, a steered axle is more complex, but if it saves you the hassle of having two independent manufacturing processes (steered and unsteered axels) it might be worth it at scale.

  1. I apologize I cannot find the excellent article I read on GM
    Quadrasteer that featured some info on the third gen (canceled?) Version of quadrasteer that was to be better and more affordable.

    If you’ve ever wanted a long wheelbase truck that can do low speed u-turns and parking lot manuevers loke a family sedan, its quite a good idea. Not to mention parking trailers, which a farm duty or work vehicle might be called on to perform regularly.

    It also works the other way, making lane changes at high speed much more stable.

    Looks like it might come back to silverado EV:

  2. Look up GM Quadrasteer, useful in an extended wheelbase truck. There was some development mules with a 3rd gen system I don’t think was ever offered

    Possibly returning for Silverado EV in 2021

    1. If you ever get a chance to drive one, do so! The difference is astonishing.

      I believe you could only get them on a single wheelbase (extended cab+short box/Suburban), but it would be even more useful on longer wheelbase trucks. The tighter turning circle is great (skip the Y-turn and just do a U), and having the rear tires follow a path much closer to the fronts makes tight corners around curbs much easier to deal with.

      Typical of GM, it was a great idea set up for failure then blamed for low adoption.

      It’s supposed to be 2022(I’m sure that was a New Year typo that we all make), but 2023+ is more likely, considering their (in)ability to deliver EVs. Despite November photos of more than a dozen Hummers on the assembly line, only _one_ was delivered to a customer in 2021Q4. Which fulfilled the promise to start shipping in 2021. And they did get 25(!) Bolts out the door in Q4, too. Those 25 might even be exempt from the recall of every other Bolt ever made. Hopefully.
      Despite all that, I know at least one person to reserve their spot in line, for whenever they’re eventually available.

      Back on topic, AWS is also an option on the Hummer EV.

  3. “For this reason the top speed is software-limited by law” – the reason being that the “steering is entirely hydraulic and fly-by-wire”? Could that system not be safety rated?

    I imagine four wheel steering being easier when there’s a drive per wheel instead of a central motor that has its power delivered mechanically, but I’m no mechanical engineer.

    1. It could be stupid things as laws requiring certain “normal” things. E.g. Dutch motor vehicle regulations say “wheels must have air-inflated tires”, which excludes airless tires like the Michelin Uptis or Mattracks.

    2. Similar to the JCB, Case made a large 4WD four wheel steer tractor in the 70s and 80s with 4 modes : front wheel steer, rear wheel steer, 4 wheel steer, or crab steer. It was hydraulic steering on the front axle, and electric over hydraulic for the rear axle via potentiometers and an analog control unit.

      1. I knew they had one, though I thought it was hydraulic on the rear.

        IH had one that was definitely hydraulic front and rear. Linkage from the front and rear connected to a hydraulic valve. The valve controlled a cylinder moving the rear, causing it to follow the front; the linkage could also be shifted into a neutral position, disabling rear steering.

    3. I was talking to a unit student who worked on EVs and solar powered vehicles and I mentioned “drive per wheel” for electric vehicles and she said putting a motor in each wheel was a really dumb idea. I thought it seemed cool but apparently not.

      I think most EVs have a motor per “pair of wheels” with a tradition differential. Front wheel drives just one driven pair and 4WD have two driven pairs – which is how they can do “ludicrous” mode without loosing traction.

  4. there are so many variations on steering control and more reasons to use any one or combination of types,front axel steering with differential braking on the rear axel is great for
    off road racing trucks and ordinary tractors,center pivot for
    industrial loaders and forsetry equipment,on a tesla car it would
    be for parking and to make driving at speed just a tiny bit smoother,quieter, and could extend tire life and reduce the need
    for rotating tires as wheel “scrub” will be reduced

  5. This isn’t quite the same rear-wheel steering, but Porsche, since the late 1970’s, has been including a rear wheel toe-in compensator that steers the rear wheels inwards under deceleration on some models of their cars. It’s called a Weissach axle, and it’s clever and somewhat helps to prevent their notorious propensity for coming out of turns going backwards.

          1. :-)
            Haha, I eventually guessed something like that must have happened. I stared at [Thinkerer]’s comment and linked article for a while trying to work out what you might be disagreeing with.

  6. Back in ’92 a friend and I took a Mitsubishi GT VR4 for a test drive. 4 wheel steering, 4 wheel drive, active suspension, active aero, traction control, and twin turbos. Couldn’t get that thing to lose traction and fishtail at all, and we tried to.

    1. I had a couple of car guy friends who were super into Japanese cars including Mitsubishis back in the day. Most of the time enthusiasts would do a RWS delete (replacing the actuator with a solid linkage) on 90s Japanese cars, especially once the systems inevitably failed. A pity, but pragmatic. On the plus side, though, the solid linkage gave an easy way to adjust rear toe, which they sometimes set aggressively to get the balance more towards oversteer on the FWD and AWD platforms that RWS was so frequently on.

      1. My 1995 Nissan Altima GXE had “Super Toe Control”, which was a passive 4WS system similar to the Porsche system described in another comment. It engaged based on the amount of body roll in the turn, and you could feel it kick in just enough to tighten the turn and reduce the roll, which would relieve the toe-out and widen the turn, which would increase body roll and start the cycle again. You could feel the body wobble a little during what was otherwise a smooth constant turn.

        I remember reading about a Mazda concept car from the 80s that had electric 4WS. It had buttons to select either crab mode or tight turn mode.

      1. There was a comment in a recent article here about “Right to Repair” that suggested the Mercedes system is software-limited, and that users can’t get the full 10 degrees without paying an extra fee to Mercedes.

  7. It’s not exclusive to JCB, there’s a Manitou telehandler that has 4WS. It’s quite good to drive, mostly where the wheels turn in opposite directions, but I think it’s significant that the driver is sitting roughly in the middle of the vehicle. This way you are sitting on the tangent of the turning circle, which I feel would make a difference.

  8. Don’t think it’ll catch on for consumer cars.

    The thing it’s most useful for in consumer cars is parallel parking.

    But the richer you are, the less likely you are to need to parallel park. In the U.K. at least, middle class upwards tend to have have driveways and don’t park on the road, it’s just old terraced houses which don’t have driveways.

    So the people who most need it can’t afford it, and those who can afford it don’t need it.

    1. It’s a thing. Acura TLX, RLX. Audi A6, A7, A8, Q7. BMW 5, 7. Cadillac CT6. Ferarri 812, GTC4. Lamborghini Aventador S, Urus. Lexus GS, LC500, LS500, RC. Mercedes AMG GT. Porsche 911, Cayenne, Panamera. I know the listicle I got those from is missing many (a bunch of Mercedes models for one).

      It’s good for allowing aggressive turn-in response, benign freeway handling and tight turn radii all in the same car. It is mildly helpful for parallel parking, but they typically only turn a few degrees. It might be more helpful to think of it as dynamic toe.

      1. True – but these are higher-end cars which benefit from it, not typical ones.

        Your average consumer car isn’t going to benefit from it, by my understanding, because lack of 4WS isn’t going to be the limiting feature for handling? And not sure any car benefits at sensible and responsible speeds on public roads? Or is it a factor even for normal usage?

        1. It’s not that lack of 4WS is directly limiting but that the nature of suspension design is to make compromises within limitations, and 4WS gives you another degree of freedom.

          The simplest example is turn-in vs straight-line stability. Almost all modern cars’ suspensions are tuned towards understeer for safety’s reason. Turn-in is an interently understeer-inducing behavior, as the tires both have to transfer force to pull the car into the corner and also influence the cars rotational inertia. Essentially, the force asked of them is leads the rears’ in the time scale. If you steer outwards with the rears, this demand is lessened. This gives the feeling of a livelier car with a much shorter wheelbase and less mass, along with improving performance. Traditionally, you’d do this with toe-out at the rears, but this both increases tire wear because they’re literally skidding down the road, and also compromises grip balance through the rest of the corner. These reasons are the main reason it’s on the 911, that car has gotten pretty large, and they want to maintain its sports-car feel.

          I hear you on the sensible speeds thing, but this behavior can be felt at reasonable speed in responsible manners on public roads.

          A concrete example of suspension geometry compromise is turn radius. If you don’t give much room for the suspension, the component’s width intrudes further into the barrel of the wheel, physically limiting how much you can allow them to turn. If you look at the London Cab’s front suspension, it intrudes almost unbelievably far into where the engine bay would normally be, and has A-arms that neck down to one element pretty far from the upright, which means a lot more material (cantilever vs triangle support), and thus mass and cost. They also have an extra lever linkage on the steering linkage, which is more cost and space taken up, just to bring it in closer to the A-arm, again specifically to get their revered turning circle. Even the fact that it has front A-arms is extremely unusual, MacPherson strut is far more common for cost and space savings reasons. All this makes sense for a dedicated cab! They have a lot of reason to want a tight turning circle. However, it means much (this is really apparent in the pictures) less space for the engine, and would be essentially impossible for the ubiquitous traverse engine of modern cars, which means the engine goes elsewhere, which means you have to either make the car larger for the same cabin or take space from the cabin. There will be knockon effects throughout the design.

          RWS buys you another 5-12ish degrees of steering. That can be huge, especially in larger vehicles. If some servo-actuators are a small cost next to the price of the vehicle, then it can be worth it! Though, as you pointed out, this means that it trends towards more expensive vehicles.

          I could go on, but really it always comes back to compromise budget. An extra degree of freedom in any system is huge. The only question is whether it’s worth it.

  9. I want a car with mechanum wheels. I know they don’t exist. But damn it solves the whole parking problem! Maybe they could be retractable. And then again we have normal everyday cars with things like turbochargers, CVT transmissions, AWD,, etc that used to be considered ‘too exotic’ or ‘too unreliable’

  10. For nothing but a tight turning circle 4WS isn’t necessarily needed. One of the requirements to be licensed as a London Taxi, the turning circle of the car must be below 25ft (8.5m). They turn on a coin even with their fixed rear axle.

    1. True, but suspension geometry is always a compromise. 4WS allows a long wheelbase, wider wheels, aggressive turn-in behavior, gentle highway handling and a tight turning circle all in one platform.

      That’s not to say it’s always (or even often) the right choice, but it allows for more flexibility in suspension design than a solid or statically adjusted rear toe would.

  11. For a consumer car, 4 wheel steering makes sense in a long car, but not in a short hatchback.
    I’d openly admit that if I had to parallel park often with my wagon I’d like it.
    But since I live just right outside city limits, it’d just introduce wear&tear components to the rear suspension and instead be a hassle on that front.
    Already dealt with the multi-link suspension up front (because older Audi) by replacing wear parts with aftermarket heavy duty parts originally designed for amateur rally use, but is now being sold for “bad road drivers”

  12. You don’t need to stop at 4 wheel steering when it comes to farming equipment. Check out this beet harvester, it has 6 wheel steering and can drive across the field a little side-wise.
    Here, the wheels of the three axles are deliberately NOT running all in the same track, but in different tracks to better spread the large weight of the machine and reduce soil compaction.
    Seems like a good idea, according to the manufacturer these things can weigh 32 (metric) tons plus another 30 tons of harvested crops…

    1. This one, has (something like) 96 wheel steering in the shown configuration.
      It’s name is SPMT, which stands for “Self Propelled Modular Transporter” And it’s modularity means you can have as many steered wheels as your budget can afford and you have room to drive the thing around.

      A vehicle like that could not even work if not all wheels were steering (I’m guessing that a defective wheel could simlpy be retracted, instead of halting the whole vehicle)

      There are plenty of reasons for multi wheel steering, but for normal road vehicles the gain dimply does not outweigh the cost. Roads and parking lots are designed around being able to navigate with a “normal” car, and it’s not going to help somebody who can’t park their car under “normal circumstances” much.

  13. My Lottery Garage has a bunch of wonky stuff in it.

    – Gotta have a functional Dark Knight era “Growler” Batmobile. Of course.
    – And also a (hopefully better) working version of the Fast & Furious villain “Ramp Car”. You know the looked like an F1 car, if it had been imagined by a Nine Inch Nails music video producer. Coincidentally it also had 4-wheel steering (manual control of the rear wheels via a lever). But the stunt drivers described it as “hellish” and “truly awful to drive”

    But my weirdest want is a custom electric.
    Each wheel is on an arm.
    Independent 4wd. (1 for for each wheel)
    Fully active independent camber, caster, and toe. Because it’s on an arm …
    Think of one of the buggies from the Borderlands games.

    1. trying to reply once again…

      The menzi muck isn’t too far from your custom want, though I’m pretty sure they are all hydraulic.

      There’d be at least one in my lottery garage.

      Other things in my lottery garage would be

      * a vz-9 avrocar

      If equipment is allowed in the garage wishlist, not just vehicles:

      * a Focused Ion Beam (for semiconductor RE)
      * everything in the workshops of the Applied Science guy and the Breaking Taps guy, and probably a few others
      * a proton beam a la so I can create purty plasma balls in the sky

      (disclaimer: the chances of me actually being able to use those items competently are inversely proportional to their cost)

  14. Err, runflat tyres come standard on (almost) entire makes of car I think. Most everything without a spare uses them as OE.

    I’m not a proponent, as I like my tires to offer up suspension for our winter damaged roads here, but I think most new non-trucks have them.

  15. I have a Kubota G1900s garden tractor. It has an 18 hp diesel engine and four wheel steering. Let me tell you it is awesome when cutting the several acres we have. I can literally turn in it’s own length. It is better than a zero turn because I can use it for a whole lot more than just cutting grass.
    I was skeptical about the four wheel steering at first but now I am a convert. Four wheel steering is not a gimmick in this case.

  16. “set to go the way of runflat tyres as one of those evergreen innovations that never quite catches on”

    What’s bad about runflat tyres? They saved my day with my BMW X5 several times.

    OK, I wouldn’t want them with the leightweight roadster – but that’s a niche of it’s own ;-)

  17. I have seen the Chevy Quadrasteer stuff before. It did look interesting. That got me thinking about if I could add 4 wheel steering to my 4wd truck. It could be doable, even for a hobbyist.

    It looks like there weren’t many Quadrasteer trucks made. So there aren’t a lot of those factory parts laying around in junk yards, and aftermarket components are difficult to find.

    The racing and off-road community have already done most of the hard work with their vehicles with independent rear suspension. That would be necessary, just to make the wheels steerable. Use the same knuckles that the front has, and probably a rack & pinion setup, and you’d have 4 wheel steering.

    From there, it’d just take someone with a bit of engineering experience to add a electric motor to the steering system input, and a way to switch between driving modes. And, obviously, some sensors to determine the actual wheel orientation and stuff. Really, the electronics would be the cheapest and easiest part, even if you used redundant sensors throughout it.

    A quick Googling around found that Corvette based independent rear suspension kits start at about $8,000. Those come with the necessary framework to hold the parts, rather than just trying to weld front end parts on that back. You’d still have to add the steering parts. So probably $10k to $15k or so to do it.

    Being my old beater truck, the parts would exceed the value of the whole truck. And when something does break, I’d have to do like the Quadrasteer people do, and just lock them in place for straight driving. $10k of parts on my $4k 4wd truck would be a poor investment. It would probably lower the resale value by about $5k, unless I happened to find a buyer who wanted to do the same thing, and was pleased with my design choices.

  18. Coming to this late- Ah well… I drove a 4WS JLG 150HAX Boom lift extensively (yes, you can drive it from the basket, when the basket is 150′ straight up (or 70′ and 80′ over, at any angle). Quite fun (for me- the passengers tended to hate it). Base was 18′ square. These had the traditional steering linkage with a single hydraulic cylinder controlling the direction on each end (they were controlled by “bang-bang” valves- full open or full closed. Took some finesse- controlled taps on the valves.

    I had to drive (from the base) with the boom at full vertical for roughly 1km at a time at about 1km/hr (it was actually set to 3/4 mph, but I want to mix my measurements to annoy the purists!), keeping it dead straight, since we had to share the road (a test track at Aberdeen Proving Grounds) with (Yes, for real) M1 Abrams tanks doing endurance tests on the track. So, had to keep it vey close to the edge of the road- but not going off the edge, since it probably would not be catastrophic (as in tip over) but it would ruin the data set we were collecting (synthetic aperture radar data).

    4 Wheel steer was very useful in maintaining orientation- we tried some of the early automation/robotic methods (this was the mid-90’s), but having a human in the loop proved to work best (at the time). If you were off track, crabbing would get you much more quickly back to the edge- in a way that steering just at one end could do, but would take longer, and you tended to have to overshoot with the front to get the rear on track (similar to why you should always start a your parallel park in reverse).

    The system we had was quite dumb- hydraulic hub motors, same hydraulic circuit to front/back wheels, so there was quite a bit of tire scrub on tight turns on pavement (not really a problem for the intended design- which was heavy, slow off road equipment).

    What excites me about the possibilities here is what we can get with modern control systems- a motor for each wheel, a steering channel for each wheel [James Bruton] did some interesting work on this.

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