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Hub motor/drivetrain (out of main)

Fact Checking

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Aug 3, 2018
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Sounds like it might have a CVT.

Or direct drive? Fewer parts, less weight, lower friction loss. Maybe they figured out how to make a high torque low rpm motor thats still small & light?

https://www.twice.com/product/how-direct-drive-will-revolutionize-washing-machine-market-38075

I'm 90% sure the Plaid powertrain uses direct drive permanent magnet motors, i.e. two "hub motors" integrated directly into the rear wheels, eliminating the fixed gearbox, axles, drive shafts, etc.

(Front motor would still use gear reduction and a differential I suspect.)

Direct drive has numerous advantages, the primary one is to shift the torque curve to the left massively, enabling maximum torque at high speeds as well.

On the Nürburgring high speed acceleration is perhaps the most important factor.

Tesla's drive units are good up to 20k rpm I believe - with direct drive that would support speeds up to 1,000 mph ...

With direct drive there should be flat torque output up to the ~400 mph max speed of the Roadster 2.

Another big advantage would be very good, very direct torque vectoring.

The challenges would be ripple torque at low speeds and cooling: if the motors are part of the suspension then any coolant would have to enter through durable yet very flexible pipes. Plus a lot of challenges I don't know about - if it was simple everyone would be doing it. :D
 

mongo

Well-Known Member
May 3, 2017
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OT
That makes perfect sense for how they landed on a 3 motor system. 1 high RPM lower torque motor with a reduction gear on one axle for lower speed performance, 2 lower RPM higher torque motors direct driving the other axle for higher speed performance. Also plays well with the unique turbine like whine that folks heard the Plaid Model S making at Nurburgring.

Given that, I'd _guess_ that the 2 motors would be up front, and the 1 motor in the rear. Rear can use the geared torque better at launch, and separate motors may simplify suspension slightly up front.

Edit: Also, I had been wondering how they fit 3 motors into the skate. Direct drive is how. Puzzle pieces fit.
Rear would be dual motor for torque vectoring.


I'm 90% sure the Plaid powertrain uses direct drive permanent magnet motors, i.e. two "hub motors" integrated directly into the rear wheels, eliminating the fixed gearbox, axles, drive shafts, etc.

The challenges would be ripple torque at low speeds and cooling: if the motors are part of the suspension then any coolant would have to enter through durable yet very flexible pipes. Plus a lot of challenges I don't know about - if it was simple everyone would be doing it. :D

Hub motors mean your unspung weight is terrible, you have coolant hoses along with HV and control harness undergoing flex, plus lack of protection and increased shock load.

Current gear ratio on S/X is 9.73:1 with 163MPH top speed. Roadster at 250+MPH (400 KPH) would be 2.35x the aero force to overcome. Dual motors with half the gear ratio would achieve the goal for that car. 1.9 second 0-60 requires the most torque available which means more motor torque (amp turns) in proportion to the lower ratio. More windings means larger stator or smaller (lower current capability) wire. More current means more heat and higher requirements on inverter output.

Plaid S will have tamer specs so no need to go that extreme in ratio change. As to packaging, they could fold the motor by adding a wide enough gear set to package the motor on the same side as the powered wheel. Main constraints I see are sufficient distance for CV axle to function properly and access/ support for the non drive end of the rotor.

Direct drive puts you in a bad spot on the power efficency band and adds uneeded stress to the drive electronics and wiring in the low speed high torque range.
 
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StealthP3D

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Dec 12, 2018
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Hub motors mean your unspung weight is terrible, you have coolant hoses along with HV and control harness undergoing flex, plus lack of protection and increased shock load.

That was my first impression as well when the possibility of a direct drive for Plaid was mentioned.

I remember being part of an early Internet discussion about electric car design (this would have been around 1994 or 1995). A number of engineer types were very hot on the idea of hub motors to reduce part count and cost. It sounded so futuristic! I was kind of disappointed when I learned the original Roadster (and later the Model S) would not have hub motors but I think hub motors make more sense on basic low-performance transportation, not a performance car due to the power required and the amount of unsprung weight it would entail. Also the practicality of coolant and high voltage needing to go to the hub. Tesla designs the wheels to be torn off in an accident to increase occupant safety. I don't think they want coolant gushing over the accident scene and high voltage wires ripped from the wheel.

Plaid drivetrain with one or more direct drive motors? Possibly. Hub motors? Probably not.
 

Fact Checking

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Aug 3, 2018
7,517
120,112
Vienna
Hub motors mean your unspung weight is terrible, you have coolant hoses along with HV and control harness undergoing flex, plus lack of protection and increased shock load.

So:
  • Higher unsprung mass, as long as it's not rotating, can be counterbalanced with harder suspension. Increases suspension mass slightly but not unreasonably. Much of suspension mass is total mass dependent.
  • I don't see why protective casing, or a short but still direct and unsprung drive shaft would be a dealbreaker. Avoiding gearing and flexible drive shaft losses is worthwhile.
  • Highly durable flexible hoses and cables are a complication, but they exist.
  • Shock resistance: true, but also a function of how long the fixed drive shaft is. If it's longer (but still unsprung) then wheel level shock gets reduced.
I believe the key to inline hub motors is to reduce rotor mass and complexity - and that's the case with permanent magnet rotors.

There's no steering in the rear, which makes this mechanically feasible IMHO.
 
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Fact Checking

Well-Known Member
Aug 3, 2018
7,517
120,112
Vienna
Plaid drivetrain with one or more direct drive motors? Possibly. Hub motors? Probably not.

Note that I meant "hub motor" as a generic term that includes but isn't limited to inline motors.

I.e. I believe it's a valid design to have a hub motor layout with a fixed, unsprung drive shaft. This moves power transmission entirely to the wheels and into unsprung mass, with no mechanical losses from gears or flexible drive shafts.

This would still allow the motor to be away from the wheels - 20-30 cm distance.

Could be wrong though.
 
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printf42

Active Member
Sep 29, 2018
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I'm 90% sure the Plaid powertrain uses direct drive permanent magnet motors, i.e. two "hub motors" integrated directly into the rear wheels, eliminating the fixed gearbox, axles, drive shafts, etc.

(Front motor would still use gear reduction and a differential I suspect.)

Direct drive has numerous advantages, the primary one is to shift the torque curve to the left massively, enabling maximum torque at high speeds as well.

On the Nürburgring high speed acceleration is perhaps the most important factor.

Tesla's drive units are good up to 20k rpm I believe - with direct drive that would support speeds up to 1,000 mph ...

With direct drive there should be flat torque output up to the ~400 mph max speed of the Roadster 2.

Another big advantage would be very good, very direct torque vectoring.

The challenges would be ripple torque at low speeds and cooling: if the motors are part of the suspension then any coolant would have to enter through durable yet very flexible pipes. Plus a lot of challenges I don't know about - if it was simple everyone would be doing it. :D
I seriously doubt in wheel hub motors, for one reason alone, unsprung mass.

I know Tesla often pulls off unbelievable engineering innovations, but this one is really hard, I don’t see anyone be able to make a high performance hub motor that can be fitted in a high performance car.

Actually anytime I see a new EV emphasizes their “innovative” hub motor, I look away. Those are just too much of compromises that the car would for sure suck.

If Tesla does use direct drive motor, I would bet they still have drive shaft to move that weight away from wheels, maybe even take the brake rotor in board too and integrating it with the motor.

But, who knows, this is Tesla we are talking about, we will know very soon.
 

Silent Ludicrosy

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Mar 14, 2018
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F7CDE66E-5A46-4D4B-82B8-601CDB7612ED.jpeg
CEBF13BA-25F1-40A4-8482-444DC7A1B86E.jpeg
 

Fact Checking

Well-Known Member
Aug 3, 2018
7,517
120,112
Vienna
Hub motors mean your unspung weight is terrible,

I seriously doubt in wheel hub motors, for one reason alone, unsprung mass.

So I don't understand the basis of that objection - higher unsprung mass is not deal-breaking constraint I believe:
  • The only deal-breaker unsprung mass if it's rotating, i.e. in the rotating part of the wheel - but that wouldn't be the case here.
  • For the rest of the suspension system, it's generally not unsprung mass that matters, but unsprung momentum of inertia - i.e. mass multiplied with distance of movement. Much of the hub motor mass can reside in the unsprung mass but move less, because it's closer to the suspension attachment points. It would be connected to the wheel with a single fixed, rigid drive shaft 20-40 cm long.
  • But even if unsprung momentum of inertia increases, it can be counter-balanced by making the suspension system stiffer: which increases its weight somewhat - but suspension mass primarily depends on total vehicle mass - and this is a 2+ ton monster. I'd be surprised if the suspension mass increase would be more than ~1 kg.
Am I missing something?
 
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bhtooefr

Active Member
Apr 16, 2018
1,155
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Newark, OH, USA
This is actually good news:


Because InsideEVs is reporting US sales, the first month of the quarter was usually "bad" for Tesla, due to them dedicating much of Fremont output to international markets.

Since there will be no October and November data, there's one less source of FUD.

Although there's a new source of FUD for a quarter or two:

Moving forward we will only post monthly articles on automakers willing to provide their monthly break downs.

Watch the shorts try to claim Tesla had zero sales for October and November, and then again in January and February.

The logic is to stop people who are buying PHEVs to game the system... e.g. because you get 24% off their sticker price vs. an equivalent ICE car (just from the VAT discount alone... more for the pollution fees), so even if you never plan to plug it in, some people were buying them to drive them around on gasoline.
This is why CARB did the whole "BEVx" shenanigans for the i3 REx, where it had to have shorter or equal UDDS charge sustaining range than its charge depleting range, resulting in the software fuel tank limit, and the thing where it started the engine at a very low SoC only. A US-market i3 REx is a car that will absolutely punish you for never plugging it (sometimes to the point of being unable to maintain freeway speeds).

I'm 90% sure the Plaid powertrain uses direct drive permanent magnet motors, i.e. two "hub motors" integrated directly into the rear wheels, eliminating the fixed gearbox, axles, drive shafts, etc.

(Front motor would still use gear reduction and a differential I suspect.)

Direct drive has numerous advantages, the primary one is to shift the torque curve to the left massively, enabling maximum torque at high speeds as well.

On the Nürburgring high speed acceleration is perhaps the most important factor.

Tesla's drive units are good up to 20k rpm I believe - with direct drive that would support speeds up to 1,000 mph ...

With direct drive there should be flat torque output up to the ~400 mph max speed of the Roadster 2.

Another big advantage would be very good, very direct torque vectoring.

The challenges would be ripple torque at low speeds and cooling: if the motors are part of the suspension then any coolant would have to enter through durable yet very flexible pipes. Plus a lot of challenges I don't know about - if it was simple everyone would be doing it. :D

Direct drive motors without gear reduction operating in the hundreds of RPM have pretty horrendous efficiency and weight from what I've seen in the e-bike space - they just make up for their inefficiency with their weight. (That is, because they're naturally big and heavy, their solution to inefficiency is to just rely on their inherently huge thermal capacity to absorb the heat.) Adding 5 or 10:1 or so internal gear reduction to an e-bike hub motor - despite the added friction and (due to a freewheel to avoid cogging torques on a primitive motor design) losing regenerative braking - greatly improves efficiency, as well as the range of effective operation.

Low-end torque on the direct drive e-bike hubs tends to be pretty poor, too, compared to geared. (However, a lot of them aren't using field weakening, and then have high-speed winds, because the remaining direct drive e-bike designs are illegal motorcycles with pedals that are capable of very high speeds. So, a low-speed wind that's field weakened at higher RPM may not have this problem.)

So, now, there's the rotational and suspension losses. Bigger and heavier rotor means more weight to spin up. If it's unsprung, ride quality will suffer (dampers and springs will be worked significantly harder), and the vehicle will have more sluggish response in general.

...cooling is an interesting one, though. I can think of a way to get coolant flow pretty easily without worrying about hoses, and it basically involves using the suspension A-arms as coolant pipes with banjo fittings. That said, hoses are used on unsprung components in every car: the brakes.

And, regarding moving power and coolant lines, as well as unsprung weight... what we've seen of the Semi is that its pairs of motors are unsprung IIRC, but that's an application where the existing axles in ICE semis have quite a lot of unsprung weight, not a high performance application where handling is paramount.

If they did go hub motors, I'd expect planetary gear reduction in the hub motor, really. Probably something like the 9:1 typical of Tesla. That enables a significantly lighter motor that runs more efficiently.
 
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KarenRei

ᴉǝɹuǝɹɐʞ
Jul 18, 2017
9,619
103,828
Iceland
Why does everyone on the internet keep trying to shove the motors into the hubs? There's very good reasons why this isn't done for anything larger than some e-bikes / electric motorcycles. Just to list some (some that have already been mentioned, others not):
  • Unsprung weight (the lighter the wheel, the less it resists following the contours of the road)
    • Traction (acceleration, braking, cornering, etc)
    • General handling feel
    • Comfort (although it depends on the types of loads involved)
  • Vibration loads (drive unit longevity)
  • Physical size limitations (power/torque limitations)
  • Greater cooling difficulty
CV joints are not the devil's work people; they serve an important role! Let the drive units be along the axles between the wheels! :)
 

bhtooefr

Active Member
Apr 16, 2018
1,155
6,541
Newark, OH, USA
And, for that matter, many e-bikes have gone "mid-drive" (that is, the motor is mounted at the bottom bracket, with the pedals going into the motor assembly) nowadays, and many electric motorcycles have the motor in the frame, too.

Hub motors work on e-bikes because many e-bikes don't even have suspension, and they're cheaper and parts of the e-bike market are extremely cost-sensitive.

However, the primitive motor designs (most are still surface permanent magnet, most of the rest are barely interior permanent magnet, and therefore have zero or negligible reluctance torque) have high cogging torque, which encourages a freewheel especially if there's gear reduction involved, and they don't respond as well to field weakening as more modern designs, which encourages multi-speed gearboxes... and there's already multi-speed gearing for the rider (albeit with a freewheel, but you end up needing that anyway), so a mid-drive just takes advantage of that existing gearing. Additionally, center of gravity is both more centralized (not at the front or rear hub, but rather in the middle) and lower (the bottom bracket is lower than the hubs), useful for handling-sensitive applications (especially mountain bikes, which nowadays are often full suspension).

(There was also a time where some e-bikes used a chain drive from a motor directly to the left side of the hub, but generally this was only either the very cheapest stuff with brushed motors, or companies adapting high-speed-wind direct drive motors to high-torque applications through chain reduction gearing. That stuff is now thoroughly obsolete... and with disc brakes being seen as important on e-bikes, it's not even possible any more (as this kind of thing often used sprockets mounted to the disc brake mount on a hub).)

Motorcycles... it's really just the cheapest ones that go for hub motors, everything else uses a belt or chain drive from a frame-mounted motor. (You do get more room in the frame for batteries with a hub motor, at the expense of acceleration, handling, and with the crap hub motors coming from China, most likely efficiency.) The same center of gravity location and unsprung weight applications as in bicycles apply, but even more so.

Oh, and rotating mass doesn't just affect traction, it affects inertia. This directly affects acceleration and braking. But, considering that people can notice a couple pounds taken off or added to each corner in acceleration, braking, and cornering... yeah.
 
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Antares Nebula

Active Member
Feb 26, 2019
1,152
6,772
Med USA
Hub motors? I don't think you guys are being innovative enough.

Apple wanted spherical wheels for its autonomous car

"One feature being explored, though, could’ve been a real game-changer. One part of the Titan team was supposedly looking at spherical wheels, which would be able to rotate in every direction rather than just forward and back. Such a design would be preferable “because spherical wheels could allow the car better lateral movement,”...

Talk about reinventing the wheel!
 

mongo

Well-Known Member
May 3, 2017
12,894
38,080
Michigan
OT

So I don't understand the basis of that objection - higher unsprung mass is not deal-breaking constraint I believe:
  • The only deal-breaker unsprung mass if it's rotating, i.e. in the rotating part of the wheel - but that wouldn't be the case here.
  • For the rest of the suspension system, it's generally not unsprung mass that matters, but unsprung momentum of inertia - i.e. mass multiplied with distance of movement. Much of the hub motor mass can reside in the unsprung mass but move less, because it's closer to the suspension attachment points. It would be connected to the wheel with a single fixed, rigid drive shaft 20-40 cm long.
  • But even if unsprung momentum of inertia increases, it can be counter-balanced by making the suspension system stiffer: which increases its weight somewhat - but suspension mass primarily depends on total vehicle mass - and this is a 2+ ton monster. I'd be surprised if the suspension mass increase would be more than ~1 kg.
Am I missing something?

Rotating mass impacts acceleration due to rotational inertia. Touching back on a direct drive: Gear ratios act as the square of the ratio so from the normal S motor point of view, the wheel has ~1/94 the inertia if it were a direct drive. You could package a planetary with the motor attached to the hub however:

  • The motor would be direct attached and need sufficent volume to fit. This throws out all the current suspension.
  • If the motor were on a fixed shaft, it still moves with the wheel in its camber arc. This moves the volume of space require inboard to the vehicle structure and chops a big void in it (unless the motor is perfectly at the instant center which won't happen due to suspension compliance)
  • You now need to counter the Netwon reaction torque. So you need a rigid structure back to the vehicle chassis that moves with the motor.
  • Yes, you can adjust the suspension, but your tires are going to take the full brunt of the added inertia of the unspung weight (no low profile for you).
  • Putting the motors towards the wheel hurts your polar moment of inertia also, along with roll.
The only gain is removing the CV joints/ axels.

@Silent Ludicrosy is that is a dryer? I've never dealt with a belted washer, the old types I'm used to had a direct connect transmission to do the agitation and spin/ drain cycles.
 

bhtooefr

Active Member
Apr 16, 2018
1,155
6,541
Newark, OH, USA
What about a modified Koenigsegg Regera direct drive design for Plaid?
OK, thanks for bringing up the Regera, because this actually makes things interesting.

So the Regera's powertrain looks like this:

regera--54fee1ba82b3e.jpg

(Source: Engineering Explained: How The Koenigsegg Regera Hypercar Drives Without A Gearbox)

Those two electric motors actually are direct drive, albeit through CV axles. They're YASA 750s, the current version of which is the YASA 750 R.

Here's the YASA 750 R datasheet: https://www.yasa.com/wp-content/uploads/2018/01/YASA-750-Product-Sheet.pdf

It weighs 37 kg, produces 200 kW peak power at 700 V or 100 kW at 350 V, is rated for 70 kW continuous, and is designed to run from 0 to 3250 RPM. (I'd also like to note that 3250 RPM on a Model S Performance rear tire is 412 km/h.) So, weight isn't actually as bad as I was thinking.

Here's the problem, though.

upload_2019-11-5_7-18-3.png


(Annoyingly, Adobe Reader gave me a black background on that copy, so the text on the outside isn't very readable.)

The motor's about as efficient - 93%+ - as Tesla's PMSRM motors at like 2400 RPM and above. Problem is, 2400 RPM is 304 km/h.

At a more reasonable 130 km/h, you're just about touching 86% efficiency. At city speeds, you're under 80% efficiency. That's just garbage efficiency.

...and those are best case numbers at fairly low torques.

You can quite clearly see where reduction gearing would help this motor out, as the meat of the efficiency band is way up in the RPMs that it simply can't achieve.

The rest of the Regera's powertrain is irrelevant for a Tesla, and the "direct drive" when referred to from the engine's perspective is going through both a reduction gear (as Tesla's existing powertrains do) and a torque converter (which, uh, that basically makes it a really inefficient form of CVT, I can't call that direct drive!)

Oh, and Tesla's claiming the Roadster will have 10,000 Nm at the wheels. On three of these motors that would require about 4.22:1 reduction gearing to achieve.

Edit: Also, note that the thing is current-limited up to like 2500 RPM. That means that you don't get full power until then, and that means that you don't get full power until 317 km/h. I highly doubt that Tesla's ditching the reduction gearing.
 
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