If you’ve ever owned an old front-wheel drive car with high mileage, you’ve probably heard the death clicks of a tired CV joint. They’re the special joints that let a driveshaft move and flex as a car’s wheel moves up and down (and, in the case of the front wheels, as it steers). Unfortunately, these take up valuable space, and so does a gearbox, so to get more room for batteries, Hyundai has developed the Universal Wheel Drive System, or Uni Wheel for short. It’s seriously wacky, and we’re left with many questions.
First, we need to talk about what a typical electric vehicle driveline looks like. Generally, there’s an electric motor bolted to a differential, with some power electronics attached somewhere. This whole assembly is usually at or near the axle centerline, and has one axle shaft going from each of the two differential outputs to each driven wheel. Here, you can see Hyundai’s E-GMP platform below:
Look at all that…stuff. Well, Hyundai just dropped a press release on a technology that it hopes will clear out that area so it can be used for other things like batteries. It’s called the Uni Wheel, and it aims to completely change the typical EV drivetrain design.
Fundamentally, it’s a fancy wheel hub with the reduction gear integrated into the hub itself (i.e. no differential/gearbox needed), using a planetary gear configuration. Power from the motor is sent to the sun gear in the center of the hub. The sun gear is connected to two sets of four pinion gears that each transfer drive to the outer ring gear. The wheel is mounted to the ring gear and thus when the ring is driven, the wheel turns. If all of that sounded confusing, here’s a video:
The magic of this setup is the way Hyundai has configured the sun and pinion gears. The two sets of pinion gears are mounted on a pivoting frame, which allows the sun gear to move up and down in the vertical plane relative to the ring gear. This means that the drive motor can be solidly mounted to the vehicle’s chassis with a straight driveshaft going to the sun gear.
As the wheel moves up and down, the pinion gears move around on pivots to maintain drive from the sun gear to the ring gear, but there’s no need for any flexible CV joints in the arrangement, except for on the steering axle. Just as valuable is the fact that the gearing in the hub provides a gear reduction that makes a separate gearbox/differential redundant, freeing up lots of space. Check it out:
To gain the most potential space savings, it makes sense to use a twin set of motors rather than a single one. The twin motors can be moved out closer to each wheel, leaving a space in the middle of the front end that can be repurposed as desired, such as for more cargo space or more batteries. This has the additional benefit of allowing torque vectoring to each wheel by virtue of having two separate motors.
Hyundai also claims a benefit in efficiency. “A conventional drive system using a regular CV joint suffers from a decrease in efficiency and durability as the angle of drive shaft deflection increases when traveling over bumpy, undulating surfaces,” notes Hyundai’s press release, adding “Uni Wheel can transmit power with almost no change to efficiency regardless of wheel movement, ensuring high durability and ride comfort.”
Again, here’s a typical setup with the Uni Wheel setup below:
Here’s an animation I made:
It’s a nifty drive solution that comes with serious packaging benefits. However, it does come with a certain level of added complexity. There are plenty of gears and moving parts involved, and the complexity is especially increased if two drive motors are used in place of one previously. It’s hard to imagine such a setup being cheaper than a pair of old-fashioned CV joint driveshafts and a single motor.
[Editor’s Note: Are there any noise concerns of all these gears whining? Also, how are you going to keep those gears lubricated? Is it all just an enclosed box with the gears bathed in oil like an old Portal Axle setup? How will you seal it if you’ve got a shaft that’s moving around like that? (Hyundai’s image shows an open system; how are those gears being lubricated?!). Also, how much extra unsprung weight is this going to add to the system? That could cause some handling compromises.
Then there are other questions related to degrees-of-freedom. Look at the gif above showing the wheel moving up and down, fore and aft. As you can see, Uni Wheel can account for that. But what about if the rear wheel turns or tips? “Why would a rear wheel turn or tip?” you ask? Well, vehicle dynamics engineers build camber and toe change into suspension kinematics; you’ve probably heard that automakers “build understeer into a suspension.” They do that by allowing the rear wheels to change toe as a function of travel; as our suspension engineer Huibert Mees pointed out, there appears to be no way that Hyundai’s Uni Wheel can account for that unless there’s some kind of joint in that motor output shaft.
Huibert talks about this in his article “I’m A Former Tesla Suspension Engineer And I Need To Tell You Why The ‘Double Ball Joint’ Suspension Is So Incredible.” Here’s a quote from that:
One thing that automakers tend to do is build understeer into their suspensions as a way to maximize vehicle safety should you overcook a turn (the front of a car is designed to crash into things; it’s generally considered better to understeer into something than to go sideways and possibly roll over). David Tracy (who edited this article) discussed how Ford does this with the Ford Bronco Raptor’s “roll steer,” but I’ll talk about how automakers do it with front suspension bushing compliance. Put simply, one way to do it is to make it such that, when you’re turning, the lateral loads on the tire deform bushings in a way that promotes the tire steering away from the turn.
So the rear wheels will “steer’ to some degree, and also, camber will change. I don’t see where Uni Wheel has the capability to accommodate that motion. Here’s what Huibert had to say when I asked him:
It looks like the suspension Hyundai uses there is a trailing arm which would work, but nothing eliminates all the toe and camber compliance you would get. The suspension would have to be extremely rigid and uncomfortable or mounted on a subframe along with the motors.
Apparently “the first ever uni wheel test vehicle has passed a rigorous performance requirement test,” so presumably Hyundai has addressed all of this. We’ve reached out and will update this story if we hear back. -DT].
Hyundai talks about the design as having great potential to “revolutionise the design of future mobility devices.” Beyond cars, the automaker’s materials note potential applications like robots or other purpose-built vehicles. For now, it states that research is ongoing and that the design will, in the future, be optimized for mass production.
By the looks of things, and with no specific model plans mentioned, it seems like this design might still be a few years away from hitting production vehicles. [Ed Note: I would bet it will never make it to a mass-produced production vehicle. -DT]. If it works out, we could see better frunks on future Hyundai and Kia products with more room for shrimp than ever before. Won’t that be something?
Image credits: Hyundai Motor Group
Additional reporting by David Tracy
Not to mention massive efficiency loss with each set of gears!
First of all, haven’t had time to read through the other comments yet.
Questions from first reading:
How does this work with desirable suspension geometry?
How does this handle axle plunge?
Effective (or necessary) axle length varies with wheel travel. SLA (or strut, which is kinematically sort of a variant of SLA) or whatever suspension design that gives you the dynamic characteristics you actually want in a car all mean that the wheel is rotating along the Y axis as it moves through its travel. You can mount the motor to the suspenson arm so that it rotates with the wheel, too, though then you’ve added half the motor’s mass to your spring weight. But then the motor and wheel are fixed WRT each other and you don’t need that gearbox in the hub, so that’s not what they did. Wheel movement in the GIFs shown appears to be only possible at zero camber (or whatever static camber is) such that dynamic camber would be equal to the roll angle, which obviously isn’t very practical. I don’t doubt that they’ve thought plenty about these questions and answered them somehow – I just haven’t seen how yet.
So, heavier and more complicated wheels that can’t be easily swapped? And that’s still without wheel hub motors.
I like weird and am appreciative when people and companies make such an attempt, and I’m willing to wait to see what, if anything, ever happens with this concept.
That’s a lot of small gears moving fast all the time from high torque motors, I’m sure Hyundai “Master of the Engine grenade”, won’t have any problems.
I’m curious on all the questions everyone’s raising, lubrication, camber/toe, braking setup. Not gonna be a stick in the mud saying CV Joints are the best! Cause, well they have issues, but this seems like it’s possibly moving some issues to a smaller package instead of an actual solution.
So, is all braking done by the motor? Or eventually they will use that strange arrange that some old Alfas used having the disc brakes at the begin of the shaft, close to the gearbox.
It is interesting. It may improve packing. But I see this adding a disadvantage to something eletric cars were an improvement over ICE: less moving parts, leading to less energy loss.
But I am no auto engineer, it is just my opinion. Also, how that would resist over several years hitting the curb in parallel parking, bump roads, etc.
But maybe I am being just too pessimistic. After all, someone once considered madness replacing horses with engines, so this may be a real paradigm shift. But if David believes it will never see the day light, I tend to believe him.
Will I need special tools to remove a wheel at home? This will change how we think about repairing punctures.
When some fucker steals your wheels do you then still own a vehicle (and some bricks)?
It is just the hub, not the complete wheel set (it seems). The wheel would be attached to this, like it is today with a conventional hub. Maybe the wheel design will be a bit different. But don’t worry, if the repairability keep the current trend, sooner then later removing and reattaching the wheel will involving special tools and coding that can be only done at the dealer shop anyway.
Gotcha, thanks
Hey, it’s probably a coincidence, but I’m seeing a TON of arrows and labels in photos since “my friend” asked for them a few days back. So thank you!!
I mean my friend thanks you, of course.
I will put my money on Hyundai making this work just fine.
They can’t get their engines to work, why do you this this will?
I’m having a hard time seeing how this would still have constant velocity as the suspension moves up and down.
It’s really hard to think about but I think it’s because of the 18 trillion different pinion gears.
I haven’t bothered to think about it too much but I think this is a Schmidt coupling redesigned to be balanced.
Reminds me of a Watt’s linkage also, like you occasionally see instead of a Panhard rod on rear solid axle suspensions.
Oh yeah, the mechanical part of it does look kinda like a watts linkage with some gears inside it. I can’t really tell what pivots and what doesn’t though, it seems to at least also have an additional point of rotation in the center?
I get patenting these kinds of things because it looks good for a company to say they have a large number of patents, but why publish this? Looks about as rugged as a 3-speed bicycle hub and that’s ignoring the inherent design issues and questionable problem solving reason for it to exist.
This reminds me of the Triumph Sprung Hub only more complicated.
I once had the driver side CV joint on a Volkswagen Dasher fail, rather catastrophically on the Taconic Parkway. It literally tore the wheel off the hub after shredding the tire and propelled me into the oncoming traffic.
What fun! Fortunately, it was very late at night, and there was not much in the way of oncoming traffic.
“[Ed Note: I would bet it will never make it to a mass-produced production vehicle. -DT]”
I agree.
Part of the reason the packaging looks ‘great’ in these pictures is because something is missing… brakes. You still need regular brakes integrated somehow as the electric motors won’t be able to do 100% of the braking.
And given that it allows the wheel to move up and down and front and back sounds great at first.
But without upper and lower control arms along with shocks and springs, there is gonna be a lot of unwanted movement.
And where the motor is mounted looks like it would be on the lower control arm… so there would be added unsprung weight added from the motor as well as those gears in the hub. Not as bad as a conventional hub motor setup, but definitely worse than a more conventional setup commonly used.
Also consider that now every wheel/hub is its own transmission. I imaging every 100,000 miles or so (like you should do on any differential), you’ll be having lots of fun changing the fluid on two to four times.
Nah… this has ‘bad idea’ written all over it.
When my cousin lost all brake fluid in her Focus, she used to put it in Park to slow down (after even hand brake pads were worn off) for a few weeks. I’m sure Hyunday can make a motor brake and hold the car.
They can even install a motor brake right between motor and drive shaft. Or put brake rotor around the drive shaft (brakes work like that on monster trucks)
They can even put resistors on motors to turn regen electricity into heat if the other parts won’t handle the current.
And EVs cannot have oil changes according to all I read (ICE vs EV) so that is not an issue.
The only option I can think of where brakes would fit would be inboard discs, but at that point you might as well just make everything inboard so you can place it low to control the center of gravity. With inboard discs though there’s a very real chance of stripping the teeth on the gears because the tires will be applying torque closer to the center of rotation than the brakes.
This is going to have shocks/struts, control arms, etc. just like we are used to, they just aren’t showing them for simplicity and to focus on the actual design change. Brakes will also be just like we are used to, again they are not shown for simplicity. After all, portal axles support conventional brakes just fine.
I agree that this may not be simpler than a conventional motor/gearbox/CV axles setup, but the packaging and modularity benefits are awesome to me. That said, David is probably right, but I’ll keep hoping he’s not.
This might end up a great idea for slower moving vehicles or those where suspension isn’t a major concern.
For cars, it’s not looking good for mass adoption.
So they reinvented a more complex version of the planetary hub; a design that generally hates high speeds and is difficult to adapt brakes to.
Seems like a great idea.
Look at the example for current vs uni-hub. The motors in the uni-hub example are each like 6″ diameter, so of course it saves space- magic motors!
Overall I think it is a neat concept. Hyundai may not put this into production on next years Santa Fe, but it could be part of a new generation of “mobility crap” like the video was on about.
Time for inboard mounted disk brakes maybe? The unsung hero of unsprung mass.
I want the manufacturing contract for the boots these thing will require. Absolutely massive (the boots that is, not the contract).
Save the Torchinsky’s! Donate to Jason’s Medical Fund Today.
The unsprung hero
“…but there’s no need for any flexible CV joints in the arrangement, except for on the steering axle.“
“But what about if the rear wheel turns or tips?”
Have you heard of Azipods?
They are common aboard newbuild ships of the last 20 years or so.
The pods, which are mounted to the hull and contains an electric motor that runs a propeller, rotates in the direction it’s needed to move the ship forward, back or sideways. This eliminates lengthy fixed propeller shafts, the need for engine to propeller alignment, rudder mechanisms, as well as separate stern thrusters for side-movements and most needs for tug boats, and sometimes even anchor usage – as pods can be used with bow thrusters to hold a ship in place using GPS coordinates.
Uniwheel is a step towards that direction.
You don’t necessarily need a CV joint for the wheels that steer – You rotate the electric drive motor along with the wheel on an arc instead – probably on a curved track.
You also mount the electric motor for non-steering wheels on flexible mounts to accommodate the intended allowable movements, while eliminating the unintended movements.
But then you either have to have another linkage to move the motor in that arc, or you add the motor to the unsprung weight at that corner (if it’s rigidly connected to the wheel), which seems to be the exact thing they’re trying to avoid. Not too mention with either actuation, you need guides or tracks for that arc, and clearances to all the suspension and body to allow the motor to swing around since the pivot for the front wheels typically passes through pretty close to the center of the tire (accounting for caster and kingpin angles)
[Ed Note: I would bet it will never make it to a mass-produced production vehicle. -DT]
A good take, I think that sums it up nicely.