Donut Lab's next-generation in-wheel motors
donutlab.com

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45 points

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klemola

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3 days ago

22 comments

vlachen ・ 3 days ago

I spent a number of years designing outer-runner direct drive permanent magnet motors for industrial equipment and I've got some questions.

Cheifly, bearings. They're not shown in any of the oh-wow images, but these will likely be the most expensive component of each motor. Big bearings are expensive, and to accept the loading of normal wheel operation, these will have to be pretty beefy. That's not even discussing operational life and maintenance.

After you've stuffed a pair of angular contact roller bearings into this "wheel", you're going to want to keep salt water and road grime from entering those bearings, so what do you use as a seal? Whatever you use is going to be big, expensive, and suck up huge amounts of power due to the large contact surface.

Finally, once you've got big ass bearings and big ass seals, how do you have enough room to put a decent amount of copper in there? Power in these things always amounts to maximizing the amount of copper in the space, and I just don't see room for it.

  • anamax ・ 2 days ago

    Does "suck up huge amounts of power" for a seal imply "generate lots of heat"?

    If so, is that heat another issue or is it a "don't care" because the heat is over a large enough surface?

    • vlachen ・ 2 days ago

      It absolutely matters. Every Watt of energy that doesn't become torque at a rotational velocity is just heat.

      Contact seals work by contact and friction, friction generates heat proportional to linear velocity and linear velocity goes up proportionally with radius.

      The motors I designed were intended for food production washdown areas, and if I were designing large motors for use in road environments, I would use a lot of similar methods, including high quality contact seals.

      Teflon seals would probably have the required capabilities, but they will get destroyed by dust and grit. Nitrile seals would do it too with the detraction of a huge power loss at the seal. I wouldn't trust a plain labyrinth seal to do the job.

lelanthran ・ 3 days ago

Maybe they thought of this, but I didn't see any mitigation for the unsprung mass problem that these motors bring.

  • notjulianjaynes ・ 3 days ago

    If the weights are listed on the are to be believed, this shouldn't be an issue. It says 40 kg for a 21" wheel. A quick Google is telling me that a regular old steel wheel from a truck of the same size is around 50 to 80 lbs (23-36 kg).

    That said, I was already thinking 630 kw per wheel was a pretty incredible claim before I realized these are apparently not much heavier than a non-mptorized wheel. These have to be some marketing department numbers or something. 630 kw is roughly 850 horsepower.

    • lelanthran ・ 3 days ago

      > If the weights are listed on the are to be believed, this shouldn't be an issue. It says 40 kg for a 21" wheel. A quick Google is telling me that a regular old steel wheel from a truck of the same size is around 50 to 80 lbs (23-36 kg).

      I'm skeptical; They say 40kg, but I think that's just for the motor, not the entire wheel. I working off the (maybe incorrect)[1] assumption that the 40kg doesn't include the steel/mag rim and the tyre.

      As far as a 21" standard mag wheel, the tyre alone is around 12kg, so quite believable that a standard wheel with tyre would weigh maybe 30kg. However this still means that putting a tyre on a 40kg wheel is going to take it to +52kg.

      [1] Just the minimum metal and rubber needed for a 21" wheel to maintain its shape and structure should be around 30kg. Maybe this motor is structurally round already, so doesn't need any rim to reinforce it?

    • vlachen ・ 3 days ago

      There's a whole lot of marketing speak, finger waggling, and wishful thinking in what I am seeing here.

      Copper is heavy. So is silicon steel. So are high strength magnets.

      I love the idea, but this one's going to have a real world bite in the ass once they get it out of the lab.

  • archontes ・ 3 days ago

    I'm thinking about the motorcycle motor. If they're really lighter than competing motors, I don't see the downside of using them like a traditional motor, taking the weight savings, putting it inside a sealed cavity, and coupling it to a driveshaft. Simplifies some of the other problems with sealing and bearings mentioned elsewhere, avoids unsprung mass.

exabrial ・ 3 days ago

There's a concept called "unsprung mass", which basically destroys handling of all vehicles from race cars to trucks. Basically the greater the unsprung mass, the harder it is to damp the input into the suspension because of the inertia of the moving suspension components themselves. An ideal suspension has zero undamped mass, and all input to the suspension is a direct result of contact with the surface the vehicle is traveling on.

There is zero chance this tech will make it into sports cars unless it can beat the weight of a magnesium or AL alloy rim. Even casual vehicles like minivans have rim weight minimized for comfort.

Not an expert :) just watched enough Donut media on youtubes :P

  • JumpCrisscross ・ 3 days ago

    “The unsprung mass (colloquially unsprung weight) of a vehicle is the mass of the suspension, wheels or tracks (as applicable), and other components directly connected to them. This contrasts with the sprung mass (or weight) supported by the suspension, which includes the body and other components within or attached to it. Components of the unsprung mass include the wheel axles, wheel bearings, wheel hubs, tires, and a portion of the weight of driveshafts, springs, shock absorbers, and suspension links.

    …

    The unsprung mass of a typical wheel/tire combination represents a trade-off between the pair's bump-absorbing/road-tracking ability and vibration isolation.“

    https://en.m.wikipedia.org/wiki/Unsprung_mass

  • westmeal ・ 3 days ago

    One thing to add, while your wheels and tires are a big chunk of the weight here - you also have the uprights/knuckle and the wheel hub, bearing, brakes, rotor etc etc. All of that adds up quick. Most times if you are looking to reduce this on a car you will have the most gains (or losses haha) switching to an aftermarket wheel. No wonder the manufacturers slapped alloys on their cars too!

  • Neywiny ・ 2 days ago

    There's also the inertia problem where it's harder to accelerate mass that's further away from you. The classic spinning ice skater demo. This feels physically disadvantageous. Hopefully I'm wrong though, the more competition in spaces the better.

pbronez ・ 3 days ago

Pretty powerful stuff. The Model Y Long Range AWD has a pair of motors that deliver Peak Power of 286 kW and Peak Torque of 510 N⋅m. Donut's claims their Automotive (21") motor can 2.2x the Power (630kW) and 8.4x the Torque (4300 Nm)

That's high enough that I have to assume it's for all four wheels... if it's for a single wheel, then an all wheel drive vehicle would have 2520Kw Power + 17,200 Nm Torque, which is 1.6x more than the most powerful production car in the world: the Lotus Evija (1,500 kW).

  • pbronez ・ 3 days ago

    This page [0] says "Donut Platform empowers hypercars [...] Delivering 1,500kW and 9,000Nm of total wheel torque, with acceleration from 0-100km/h in under 2 seconds"

    I don't see how you go from a single 286kW/510Nm hub motor x4 gets you 1500kW/9000Nm instead of 2520Kw/17200Nm. Wonder what the limitations are + what nonsense they are trying to pull in their single motor stats.

    [0] https://www.donutlab.com/industries/

dscottboggs ・ 3 days ago

At first I was like "this is crazy, those are going to be crazy expensive, they'll never make it" and then I realized we're not the customers, at least not willingly

https://www.donutdefence.com/

tetris11 ・ 3 days ago

I guess I'm not really following. The engine's in the wheel?

  • klemola ・ 3 days ago

    Yeah. In fact, the article linked below[1] is more informative than the page I linked. TL;DR: there’s a big reduction to the overall weight of an EV using these, but they do add to the unsprung mass of the vehicle (the weight below suspension, as mentioned in another comment).

    [1]: https://www.cnet.com/home/electric-vehicles/this-donut-shape...

numpad0 ・ 3 days ago

Looks like one of many "shovels for gold diggers" unitized motors for robotic dogs and humanoids. There's been a lot of such compact all-in-one motor units from China lately.