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
I guess a chain drive could accomplish some of the same things.
This seems like a somehow-more-complicated version of a portal axle system – with all of the drawbacks that entails.
So it’s like portal hubs but worse. Great.
Planetary gear reduction hubs have been around for a long long time on graders, dozers, and certain heavy spec trucks.
How would you keep the motor in the exact same plane with the wheel all the time? what is there is some turn and twist force?
I can see a lot of shrapnel coming out from thos mis-mashing gears 🙂
And how would the steering work? imagine now steering 2 motors with portal gearboxes instead of just the wheel.
Why not put the damn motor inside the wheel alltogether, similar to the one used in washer machines:
https://phys.org/news/2012-07-wheel-hub-motor-concept-hybrid.html
“Why not put the damn motor inside the wheel alltogether”
Because unsprung weight.
Cost/durability/utility aside, I’d really like to see how it would seal. Seems like most of the benefit is moving the reduction unit from the middle of the chassis to the hub to clear space for a pack… Wouldn’t a more conventional planetary gear set out in the hub coupled to the motor with a good old CV would achieve that more elegantly?
Interesting thought exercise. Not really intended for normal “real world” vehicles. This would be concepted and packaged for use on a small urban runabout much like a golf cart. All the obvious questions about unsprung weight, lubrication and sealing are outside the scope of an exercise like this. All my opinions, I was not present in any of the original meetings.
I can see solving one possible problem but creating others. Thr most elegant electric drive train is a hub motor and you can integrate reduction gears but at the cost of unsprung weight.
Does the Uni-Wheel save enough weight over pancake motors, or allow higher power motors?
Also is the complexity worth it over over classic CV joints?
What the hell is with the voice in the video? dripping with either sarcasm or con-man deception. Is it AI generated, or did they just cast the most sleazy-sounding voice-actor possible?
I’ve sold Uni Wheels in Brockway, Ogdenville and North Haverbrook… and by gum, it put them on the map!
Well, sir, there’s nothing on earth like a genuine, bona fide… electrified, ring-gear Uni Wheel.
What’d I say?
Uni Wheel!
What’s it called?
Uni Wheel!
That’s right, Uni Wheel!
Uni Wheel. Uni Wheel. Uni Wheel.
I hear those gears are awfully loud.
It glides as softly as a cloud.
Is there a chance the shaft could bend?
Not on your life, my Korean friend.
What about the gear chain friction?
It got secret lubrication!
Were you sent here by the devil?
No, good sir. I’m on the level.
The ring came off my pudding can!
Take my pen Knife, my good man.
I swear it’s Hyundai’s only choice.
Throw up your hands and raise your voice.
What’s it called?
Once again!
But unspring mass makes rides feel rotten.
Just lower the seat and add a cushion!
Uni Wheel!
I dunno, this is more of a Shelbyville idea
River City
COTD
That is a LOT of unsprung weight to add.
There’s a term for this kind of nonsense.
Reinventing the wheel.
On my frost-ravaged roads, I’m lucky to get more than a few years out of a wheel bearing hub. But they are cheap and easy to replace.
How much is one of these suckers gonna cost? And how easy will it be to swap one out?
Don’t worry, the car will have caught on fire before the wheels wear out.
Wouldn’t hub motors be a simpler solution to the same problem these are meant to overcome? Still have the unsprung weight issues, but fewer moving parts.
I’m not really seeing the problem that this is trying to solve. They claim that all that space is unused, but if you look at a Model S pack (at least the original ones) they were using that space for the front end of the battery pack (at least part of it).
That areas that is “unused” also is pretty handy for deforming in a crash, so, having some open space is going to help as far as NHTSA/Euro NCAP is concerned.
“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.”
If by “old”, you mean a 2022 Hyundai with 69,000 miles on it, then, yes, I am familiar with those noises
Same here, in a 2012 Subaru that had less than 40k on it.
Wow, that’s a ton of miles in two years.
I spread my miles out over three cars, so that’s why that one seems lighter
You have to spread something to get 69,000 miles
That’s why its not 85,000
You have literal seconds before NsaneInTheMembrane appears and starts telling you that Korean vehicles have been consistently high quality for decades now and that yours must just be a fluke.
Damn, I haven’t replaced a CV joint since the late ’90s and that’s somewhere around 850k miles ago over 5 cars and that last one was on a car built in ’89 that had probably 120k on it and it failed for a torn boot I didn’t catch soon enough. Even the Focus ST with 270 lbs/ft at low rpm had no problem when I got rid of it at 180k. I figured it was solved technology.
Is North Korea pumping their pollution into S. Korea?
Being like 3 people there and having little industry I’d bet they have cleaner air that their southern brothers, not worth it though
They also have no environmental regulations, mine and burn lots of coal, and a disproportionately high percentage of the vehicles on their roads date to the 1970s, however, they also have almost no electricity outside the capital, have a low level of mechanization in agriculture, and very low car ownership rates, so, yeah, much of the country probably does have halfway decent air quality. The industry they do have is very dirty by world standards, but they don’t have a lot of it.
There isn’t enough industrial activity in NK to generate pollution
“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.”
Nope. One advantage of driving such a car with a two-stroke is that I don’t have to worry much about hearing what’s going on with the CV joints over the general rattle and clatter of the engine.
Come to think of it, I should probably check those CV joints.
That’s great.
Where do you put the shocks and springs?
Inside the tire.
“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?!).”
Exactly my question. They also like that they show how a true MG unit with inverter is packaged in the car, but then the uniwheel example magically deletes cooling and inverters to a bare motor. Those things still gotta go places. Like so many of these white box animations, it’s all so clear and obvious…if you are talking generally, but a little messier when you actually go to actually design and package it.
also…where the brakes go? and Suspension?
And my final concern – seems like this takes one very well protected unit and makes 2 exposed units. Slide into a curb on a snowy day? Might total your car.
Someone at Hyundai is polishing up their CV.
They’re looking to blow that joint.
These puns really grind my gears.
What I really don’t understand here is how this is the best avenue for development for packaging advances. Like detailed here earlier in the week, the coolant routing and heat pump setups of sooooo many EVs is extremely not ideal, while it’s been proven that a single more compact system can save significant cost, weight, and complexity once implemented. This Uni-Wheel setup, while saving some space, does nothing to reduce cost or complexity.
Also of not, the motor shown in their comparison on “space savings” appears to have an integrated inverter, which would still need to be repackaged and included elsewhere for the Uni-Wheel system as the smaller motors at the corners shown don’t appear to have any components other than a rotor, stator and output shaft.
And my last gripe is that as soon as someone inevitably hits a pothole, takes a speed bump too fast, or hits a curb and bottoms out the range of motion for the uni-wheel axle, you’re going to have an extraordinarily expensive repair on your hands. Normally that impact is a control arm or two, tie rod ends, maybe a ball joint and a CV. Here it’s going to be all the above busted up as well as your entire sun gear system on the hub, an axle, and potentially even a motor, as this rigid driveshaft will likely tear a motor apart internally if overextended.
Yeah. That’s how R&D costs work in late capitalism. If you want new tech, somebody has to pay for it somewhere, and the little guys don’t have as much money to complain with.
I can see this on a golf cart, maybe not a car quite yet
I wonder how this would fair in upper midwest snow, salt, and pothole land.
For someone who really doesn’t need much horsepower I’d much rather have lightweight low horsepower in-wheel hub-motors with “virtual lockers” instead of whatever the hell this is or CV joints.
That’s a lot of added complexity and effort just to avoid using in-wheel hubmotors.
It would be better to take the small unsprung weight penalty that comes with direct-drive in-wheel hub motors in exchange for reduced complexity and increased durability/efficiency, especially since the car is retaining a single speed ratio anyway. This also has the added benefit of reducing wear and tear on brake rotors thanks to more efficient regen. Switched reluctance, synchronous reluctance, and axial flux hub motors are now of such efficiency that heat produced at high currents during hard accelerations is a non issue.
Direct drive hub motors is the route I’m going with for the upgrade of front wheel motors in the 3-wheel microcar. And I’m using inferior PMDC technology since that is what control systems are readily available for. Going to have AWD, FOC controllers, torque vectoring, slip detection, and with it all the possibly the ability to launch like a Dodge Hellcat thanks to what will be in excess of 300 lb-ft of torque at stall in a 100 lb vehicle, and maybe 25-ish peak horsepower on tap in something so slippery 100 mph might be held on flat ground with only 5 of those horses.
Uh oh, Toecutter is starting to sound like an EV OEM. 300 lb-ft of *wheel torque*
Directly applied too, since it will be coming from in-wheel hub motors, with no gear reduction.
Are you running 20″ bicycle wheels?
16×1.5″ DOT rims now, which are roughly the same size as 20″ bicycle wheels(so close 20″ bicycle tires can still fit if needed). I have Mitas MC2 16×2.25″ tires on it.
Someone at Hyundai has spent too much time tinkering with LEGO Technics and got the bold idea to scale up. I don’t see how this is viable in any real sense.