Hub motor/drivetrain (out of main)

[The Blue Owl]

OK, thanks for bringing up the Regera, because this actually makes things interesting.

So the Regera's powertrain looks like this:

(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.

View attachment 473378

(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.

I am not a mechanical engineer, but I am an engineer, and reading the explanations from contributors like yourself, @KarenRei, @Fact Checking, @Krugerrand, @JRP3, @mongo, or the faithful departed @neroden, and many others (apologies for not mentioning everybody) feels to me like connecting to the whole Internet via Neuralink. Just wanted to say thanks for helping me understand better.

(*walking away slowly*).

 

[bhtooefr]

Of course, "think washing machines" could just mean that they're going for a belt drive washing machine motor, ala the Sinclair C5:

(Yes, I know it's not actually a washing machine motor, it's a truck cooling fan motor.)

 

[AMN]

I am not a mechanical engineer, but I am an engineer, and reading the explanations from contributors like yourself, @KarenRei, @Fact Checking, @Krugerrand, @JRP3, or the faithful departed @neroden, and many others (apologies for not mentioning everybody) feels to me like connecting to the whole Internet via Neuralink. Just wanted to say thanks for helping me understand better.

(*walking away slowly*).

Ya, we love our bots.

 

[Silent Ludicrosy]

OT



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.

Those are washers. Most modern consumer washers have been belt drive until direct drive was introduced recently.

I agree with your list of direct drive problems. The other option I was considering is a gearbox mounted in the wheel hub assembly. They could use a planetary gearset to replace the wheelbearing, so it would be a small increase in unsprung mass. So for each side: motor>axle>planetary gearbox in hub assembly>hub>wheel. I like this idea better, but then Soylent’s washing machine comments don‘t make sense to me.

The original hummer used gearbox in the wheelhub, but offset to give additional ground clearance under the axles. So its not a completely new idea.

 

[bhtooefr]

...tbh I actually thought of portal axles, and then thought better of it. (Old Volkswagen Buses also used "RGBs" - basically portal axles - to gear down Beetle equipment to be able to actually move a comparatively big van.)

However, portal axles really only make sense nowadays for offsetting the output shaft from the axle centerline. In a truck application it could be interesting, but in a car, there's not much gains other than to get the motor and axles lower, but you're already close to as low as you can go, and you're adding unsprung weight.

 

[Prunesquallor]

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]

OT

Those are washers. Most modern consumer washers have been belt drive until direct drive was introduced recently.

I agree with your list of direct drive problems. The other option I was considering is a gearbox mounted in the wheel hub assembly. They could use a planetary gearset to replace the wheelbearing, so it would be a small increase in unsprung mass. So for each side: motor>axle>planetary gearbox in hub assembly>hub>wheel. I like this idea better, but then Soylent’s washing machine comments don‘t make sense to me.

The original hummer used gearbox in the wheelhub, but offset to give additional ground clearance under the axles. So its not a completely new idea.

Huh, the older washers I've fixed didn't have belts. Motor to coupler (that fails) to transmission. https://www.searspartsdirect.com/model/f4ybi56nfb-000582/kenmore-11020982991-washer-parts

Gearbox at wheel makes balancing harder on the CVs due to higher speed. Then they need to cool the hub gear box. Hummer was great though, inboard brakes were interesting.

I wonder if the washing machine reference is related to the cooling system and piping. I doubt the entire frunk would become a reservoir, but they could pack in more heat exhanger area.

Other out there possibility is vertically oriented motors with right angle final drive for packaging.

Back to regularly scheduled programming.

 

[jerry33]

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.

Strictly speaking they are commonly used for things much larger than e-bikes. It's how mining equipment works because no transmission can take the stresses. (I realize that's not your point, I totally liked the inboard brakes of the DS-21. Much larger diameter and no issues with water and rust).

 

[StealthP3D]

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.

In your earlier post you said "hub motors" would eliminate drive shafts. Now it appears you are talking about a "hub motor" with a driveshaft (which is not possible according to my understanding of what it means to be a "hub motor"). I also don't understand how a driveshaft could be "unsprung". If the motor is "away from the wheel" and sprung weight, then the driveshaft must be partially sprung weight (as a traditional driveshaft is on a vehicle with independent suspension). For the shaft to be unsprung weight requires the motor to also be unsprung weight. This implies the motor moves the same distance relative to the body when compared to the wheel. That is problematic from a basic physics standpoint since the motor will have considerable momentum and (in your example) be offset from the centerline of the wheel.

This doesn't add up. I think the problem may be your understanding of exactly what it means to be "unsprung weight".

 

[JRP3]

Bollinger is using portal hubs with inboard brakes, no hub motors

 

[GOVA]

I have a feeling that Rimac had figured this out a while ago.

Rimac C_Two powertrain and battery | InsideEVs Photos

 

[MrQc]

Mmmh, I posted this already but seems to have not been posted, somehow???...anyway, if it appears twice, my apologies.
Protean...hub motor, electronics, brake, cooling, all in-wheel...
Homepage - Protean

Mod: incorporated edit "colling"->"cooling". --ggr

 

[FactsBot]

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?

Yes you are. //Again// Unsprung weight shakes the whole car on uneven, bumpy roads. Has to be kept minimum for ride comfort.

 

[MC3OZ]

The Plaid Roadster has Torque Steering, it is a fair bet Model S/X do as well.
IMO this means an independent motor for each rear wheel and fine grained control on the wheel speed using motor RPMs.
For example, for many corners we want the outside wheel travelling faster than the inside wheel.
So it is likely that the 2 motors at the rear and big motors and PMSR motors.
We mostly want Torque Steering at higher speeds, so it is also a fair bet that the rear motors are geared for higher speeds.
By deduction (and it is only my hunch) the front motor is induction and geared for low speed launches. the front motor simply coasts at higher speeds.
Starting slow is probably front motor only, starting faster is all 3 motors with the rear motors spinning slower.. tires are the limit on how fast they can start..
So one question is what is the RPM range of the PMSR motors, if it is a wide range, that is very beneficial.
My final wild hunch is that the recent changes with HOLD mode or all other Tesla cars with PMSR motors may even be a result of Plaid testing at the 'ring.
So in this context I am not sure what difference hub motors would make as opposed to a short drive shaft.
The new Ravens have dynamic air suspension, I'm guess the Plaid models do to and that may help with the ride quality. An expensive car needs good ride quality. It may be that the suspension is even a further improvement on the Raven version..
it is an interesting topic, I am by no means an expert... so I am keeping an open mind on hub motors,.. until I fully understand all the pros and cons.

 

[bhtooefr]

I have a feeling that Rimac had figured this out a while ago.

Rimac C_Two powertrain and battery | InsideEVs Photos

Rimac isn't using direct-drive motors, though.

The front motors each have a single-speed reduction unit, the rear motors each have a two-speed gearbox to broaden their operating range.

 

[JRP3]

That's what I thought but I'm not seeing a gear reduction on the front motors

 

[JRP3]

Mmmh, I posted this already but seems to have not been posted, somehow???...anyway, if it appears twice, my apologies.
Protean...hub motor, electronics, brake, cooling, all in-wheel...
Homepage - Protean

Mod: incorporated edit "colling"->"cooling". --ggr

They've been around a long time, no serious EV has ever used their hub motors that I'm aware of.

 

[bhtooefr]

Could just be a planetary reduction box mounted inside of the motor housing.

 

[MrQc]

They've been around a long time, no serious EV has ever used their hub motors that I'm aware of.

True...

 

[Cyberax]

They've been around a long time, no serious EV has ever used their hub motors that I'm aware of.

MIEV tried it and actually sold some of them: MIEV - Wikipedia - they tried it a couple more times but wheel motors never worked right.