Skip Navigation
258 comments

  • There's 2 significant inaccuracies in the article and 1 large oversight in the official video.

    1. Differentials are not one wheel drive. They can seem to drive only one wheel when spinning the wheels as one let's loose and the other stays still, but it's not driving one wheel. It's still driving both. The problem is the free wheel is spinning at twice the speed indicated on the speedometer and the other is at 0. The driveshaft puts in a certain number of turns, the wheels, together, must add up to an equal output (multiplied by the gear ratio). If the car is going straight with full traction, then they turn the same. If you floor it in snow, one is probably spinning 40% over it's share and the other 40% under. This is not unique to rwd either as fwd cars still very much have a functioning differential. To throw some numbers at it to help clarify the function, let's say the engine is asking the wheels to spin at 30rpm each in a straight line. In a left turn, the right wheel travels further and needs to spin at 35rpm while the inner spins at 25rpm. It still adds up to 60rpm, same as a straight line. Mash it in the snow and it might be 60rpm in the left and 0nin the right or 0 in the left and 60 in the left. It could be 5/55, 40/20, or any other combo as long as it totals 60.

    PS: differentials are irrelevant when the wheels aren't connected to each other. Individual-motor wheels, as shown in the video, don't need a diff. The non-drive wheels in a 2-wheel drive vehicle do not have a differential on the non-drive axle.

    1. Cv joints are not specific to fwd as nearly all modern rwd cars with independent rear suspensions have CV joints. I don't know of any trucks still using U-joints either since big trucks are solid axle. Cv joints function the same as U joints. The difference is C.V. joints output constant velocity whereas U-joints (what you'll see often under trucks on the driveshaft, two square C shaft ends with an X link between) have lopey output that gets worse with greater deflection angle. If you own a u-joint bit for your socket wrench, I invite you to play with it. Instead of a solid pinned X between the U ends, CVs have free-rolling balls that can roll inboard and outboard to maintain the link between the shaft's cup and the wheel's cone.
    2. The article is inaccurate but the video ignores this part, so I don't fault The writer. The CV joints are said to be a poor design, yet, it ignores the part where the video reinstalls them at 4:20 and 5:10 for the front wheels. This mechanism does not allow angular deflection between the motor and hub, as it's shown, without a CV joint. Lateral displacement, yes, but not angular - as in it can't steer. This may be an overall improvement by reducing how often it needs to bend (only when steering), but it doesn't eliminate it. And even then, the rear suspension is still designed to change camber as it changes ride height. Camber is the angle of the wheel as measured top to bottom, as in what you see from looking at the wheels from the front of the car. It keeps the wheels flat on the ground as you lean the car in a corner. You may see an overloaded car's rear wheels look like /---\ as viewed from the rear or --/ when hanging free on a lift.

    Look, I'm not an engineer at Hyundai (or even a competitor) but this doesn't quite pass the sniff test. Cool idea for sure, but it smells a little like marketing is clamoring for something edgy to display. Even as displayed, the motors and original reduces were already very compact and in close proximity to the wheels compared to a normal engine. The slightly reduced footprint of this uni wheel and slightly increased friction of a bunch of additional gears makes me think this is a fractional improvement in practice rather than a revolutionary improvement.

    • I'd be concerned with the amount of unsprung weight this adds, too. You're basically taking the transmission and adding that mass to the hub. Seems like it would be pretty crashy on rough surfaces.

    • I noticed they conveniently didn't talk a lot about steering..

      The claim of "one wheel drive" I think is meant to highlight what happens if traction is lost. It sounds like something I have heard on 4wd off-road forums. I agree the phrase "one wheel drive" is perhaps not a great way to explain the disadvantages of differentials vs limited slip differentials vs locking differentials vs individually driven wheels.

      The idea of "one wheel drive" as I have seen it used, is that in a vehicle with one powered axle assembly (what we normally call 2wd-- either front or rear wheel drive) is that if you lose traction with either drive wheel, the vehicle no longer moves because all power is diverted to the slipping wheel.

      If you have a limited slip differential, there is a limit to how much power is diverted to the slipping wheel. With a locking differential, you only stop moving if you lose traction to both drive wheels.

      Anyway...

      The design is really interesting.

      You also bring up a good point about how camber changes with suspension position. Also the effective track width changes, such as with my 4Runner which has upper and lower control arms, a Double wishbone suspension. If the motor remains in a fixed position, the wheel will move onboard and outboard relative to the motor depending on suspension location.

      I don't quite get how these two effects are addressed with this new design. Or are the suggesting a different suspension technology that they didn't discuss?

      As for steering, I wonder if the design rotates the motor along with the wheel. In that case no CV is needed but I would guess there are some downsides to such a design.

      I agree the video seems kind of... premature. The mechanism is cool but I don't get the sense that its applications haven't exactly been nailed down yet.

      • Individual motors on each wheel will still slip, just with half the power. So sure, it's an improvement by an unrelated mechanism, but not having the wheels connected with a limited slip means it'll still need a traction control system. And even still, the "half" power is a relative term because every car has a different output. That goes for not connecting left to right as much as it goes for front to back. So, not different than a traditional open diff or 2wd. There have been advances in brake-based traction control so they don't just cut power and apply single brakes like the 00s, they can properly modulate pressure to get equal propulsion.

        That's a good point you've mentioned as well - the wheel will change distance to the motor as it goes through it's motions. The only way to avoid that is to place the motor at the effective pivot point of the suspension which is, in a properly design suspension, inside the other wheel to mimic the level dynamics of a solid axle. That of course defeats the short halfshaft design direction. So something has to allow variation in distance. In the non-steer wheels, maybe this could be as simple as a telescoping spline drive. However, the video shows a small black joint at the same time stamps above on the rear and still has those normal-looking cv boots on the fronts.

        Or maybe they're ditching good handling and going with perfectly vertical suspension travel. Give it hard eco tires and it'll slide before the suspension shows it's flaws.

      1. You are sort of correct about this, but it's irrelevant since everyone moved onto limited slips decades ago.

      As to the rest- you're wrong. Sorry.

      But the real reason this tech won't be very important is because it's a lot more complicated and expensive than a cheap ass cv joint and is minimally more efficient. I can buy both sides of my vehicle for like $80 and don't have to worry about em again for ages. I think this new hyundai stuff could be reliable, but it's going to be a lot more expensive.

      Also, they look like they'd be noisy.

      • you're wrong. Sorry.

        No u. Bam, same level argument right there. Are you going to explain why or just throw out contrarian comments?

        Cost will not be a limiting factor. Just about every feature on a 2023 car already costs more than a 1993 car's version. Did adding a wheelspeed sensor, electronic 4-channel hydraulic brake actuator, and dedicated ecm programming cost too much to implement ABS? Did the complication of 40 sensors (100+ now) and a voodoo box of electronics cost too much to go efi instead of carbs? Did the price of disc brakes stop most cars from ditching rear drums? Did the cost of engineering and testing prevent manufacturers from implementing the following nearly-negligible aero improvements to eek out another 0.1% of fuel efficiency;

        1. aero strakes into mirror shells (prius, escape)
        2. relaminating roof spoilers into every hatchback/suv and even into every pickup bed
        3. Vortex generators on the top surface of tail lights (sonata, chr)
        4. Active grille shutters (fusion)
        5. Full underbody trays
        6. Chin spoilers (splitters) on just about every car to keep air out from underneath
        7. Hood beak splitters to keep grille air off the canopy (Volvo, accord)
        8. Short antennas/glass-embedded antennas to reduce antenna drag
        9. Front fender outlet vents to create laminar flow over the wheels (f150 2015+)

        No, it didn't.

        And I'd be interested to hear why you think helical-cut gears will be "noisy". I'm guessing you don't know why reverse whines in certain cars but not the forward gears

        Edit: also, seriously, go do some shopping. LSDs are on the decline. On top of never being common in the first place, manufacturers at removing to brake-based simulated LSD rather than discrete components. There are incredibly few Fwd cars that ever had LSDs and fwd obviously makes up the majority of North American sales. Even Miatas and Mustangs only get LSD with optional packages.

      • Irrelevant since everyone moved onto limited slips decades ago.

        Lol, what?

        My RSX was made two decades ago in '03, so the newest that would be multiple decades old. It's also a Type-S, the sporty model. It's got an open diff.

        My '93 Subaru Loyale, which is 3 decades old, has two open diffs, with a locking center diff. No limited slip.

        My '04 (almost decades old) Crown Vic PI doesn't have an limited slip. It was an option on Interceptor that the city didn't opt for.

        My '07 (not decades old) Volvo XC70 has no limited slip diffs. It uses the traction control to try to imitate them, but no actual limited slip differentials.

        My partner's '07 (still not decades old) Kia Spectra5 has an open diff.

        The only car in my fleet that has a limited slip is my '02 Subaru Legacy Outback, and it was an option that the person who bought it new opted for, and it's just the rear that's limited slip, the front is still an open diff. Apparently the limited slip isn't even that good either, you can still get stuck with two wheels spinning. I haven't tested that yet, I just got the car.

        If you go out and buy most cars today they'll come with open differentials. The traction control system will likely try to compensate for this, but they do not have limited slip differentials.

  • From a mechanical standpoint, the new bearing saves a nearly negligible amount of space. Splitting the motor up and moving it to the notoriously wasted wheel well space is what clears up the center of the frame. Still very cool. It's basically a single output differential, which is already quite compact. No need to split the rotation for turning since the wheels rotation will no longer be mechanically linked.

  • Setting aside all of the already observed questions in the comments already about mechanical viability, i.e. how this assemblage is supposed to steer. The elephant in the room is whether or not this is equivalently economical to produce compared to an axle with a CV joint in it, and/or if it will acceptably reliable for roadgoing vehicle use, what with having a shitload more moving parts in there.

    The animation shows the geartrain assembly in an open faced housing, which if that's how it's ultimately designed is going to mean that there is now no way to keep the gears in a bath of oil or transmission fluid like is presently done in traditional transmissions and differentials. And yes, even in CV joints which are packed with grease inside their rubber sealing boots. I'll let you in on a big automotive industry secret: There's a reason current transmissions and other geartrain devices are kept suspended in oil all the time. A big one. One that has to do with your transmission not glowing red hot by the time you make it to your destination, or converting itself into glitter within the first mile.

    Even setting aside lubrication concerns -- Maybe the thing is chock-a-block full of sealed ballraces or something, for all I know -- the big open slot they depict for the axle to move up and down in is just begging for a stone, a stick, a stray bolt, or any other show-stopping piece of debris from getting in there and causing you to have a very expensive day. Ditto with the gap around the edge of the sun gear, which is going to need a bitchin' huge mechanical seal on it at the minimum. If the solution is perhaps to put some kind of rubber boot over the opening that moves with the axle, it's going to have to be ridiculously flexible and remain so even throughout all kinds of temperatures and operating environments. Cars, you know, being devices quite infamous for being operated outdoors in the weather and all.

    I mean, I can't imagine Hyundai's engineers haven't thought of this. But I wonder if this is one of those works-in-the-lab-and-test-track things, and they're expecting someone else to figure out the viability challenges.

    • I guess this design would require a few seals to keep the mechanism bathed in oil and keep foreign contaminants out.

      Is there enough oil volume to keep the mechanism cool at highway speeds?

      And how do those tiny gears hold up to the loading? They seem a lot smaller than an equivalent pinion gear in a solid axle, for example And they were rather vague on their stress testing. Seemed like a bit like hand waving and "trust us bro".

    • I’m in full agreement.

      Expanding on your concern about the sun gear is what dirt, brine water/winter treatment will have on the mechanism over time. The best case scenario is this becoming more inefficient over time if it gums up, the worst case is having debris act like sandpaper on the gears, ever-so-slowly weakening the teeth over time.

      Maybe they’ve thought of this but I can’t help but to think this is just trying to get a headline or two.

  • I know Hyundai is Korean and all, but this presentation style where the host pretends to be demonstrating a product "uh, wait a second . . . what if we" and is speaking almost like it's a personal conversation between the two of you is giving me huge Nintendo Direct vibes when they demonstrate unreleased games and play them for you.

    Is this a common sort of business/sales presentation method in SEA?

  • Fascinating, but in the video they very quickly swipe off-screen that the top speed their new system was able to achieve was 120 kph / ~75 mph.

    I imagine something like this would have to be limited to vehicles that never need to approach speeds above that on a highway, so maybe busses or indoor shipping & receiving vehicles.

  • That's neat, I'm looking forward to electric vehicles with the sort of modularity and space they are envisioning due to the extra space.

258 comments