Conclusive proof gearboxes are awesome.

[Arlo1]

Roughly 3 times the diameter and 3 times the volume or weight (if no additional weight reduction changes incorporated) for 8 times the torque and 1/8 the rpm. Side benefit would be elimination of a major reliability issue. Lower pack voltages? Looks to be good trade offs.

Im not the expert, but i suspect if you want to maintain the same total motor power, you would need to keep the same battery capacity ( volts x amphrs) . So unless you upgrade to more amphrs, you would have to retain the voltage.
Also, im not sure adding 500-600 lbs of motor weight would be considered a good trade for the reduction gears.
That would have to hurt efficiency, range, performance and handling etc.

You are not listening to us. YOU ARE NOT ADDING ANY WIEGHT! YOU ARE CHOSSING EITHER THE WEIGHT OF A TRANNY OR (KEY WORD OR) THE WEIGHT OF A BIGGER MOTOR. SO there is not adding its one or the other. If you want you can add 500-600 lbs of motor OR OR OR you can add 500-600 lbs of a Tranny.

But your 500-600 lbs is ridiculous the leaf motor will do 250-300hp continuous and it weighs 120 lbs! Why you think you need 500-600lbs is absurd!

 

[Hillhater]

Luke ,...please explain how those motors with a power density of 1kw/kg, bring anything to this party ?
.Tesla's motor produces 420 kW ( battery limited ! ). from its reported 150kg (350lbs) weight !...2.8 kw/kg !
If we used Thingap's technology you would need a 400+ kg motor for the same power ?
What am i missing ?
Arlo...i think you are the one who is missing the point..we are trying to get down to a DD motor . 1: 1 with wheel speed.
We are not talking about adding a trans, we are trying to eliminate the 175 lb Tesla reduction box.
So how do you make a motor of 3 times the diameter/ volume without a significant weight gain ?

.....500-600 lbs is ridiculous the leaf motor will do 250-300hp continuous and it weighs 120 lbs! Why you think you need 500-600lbs is absurd...

Simple deduction, Talking Tesla...the motor is reported to be 350lbs, basic maths tells us a motor 3 times the dia (and only 1/3 as long) , will weigh 3 times as much if similar technology is used , so 700lbs more.
Deduct the weight of the reduction box, and you have 525 lbs !.......but thats just theory guesswork

 

[Arlo1]

Luke ,...please explain how those motors with a power density of 1kw/kg, bring anything to this party ?
.Tesla's motor produces 420 kW ( battery limited ! ). from its reported 150kg (350lbs) weight !...2.8 kw/kg !
If we used Thingap's technology you would need a 400+ kg motor for the same power ?
What am i missing ?
Arlo...i think you are the one who is missing the point..we are trying to get down to a DD motor . 1: 1 with wheel speed.
We are not talking about adding a trans, we are trying to eliminate the 175 lb Tesla reduction box.
So how do you make a motor of 3 times the diameter/ volume without a significant weight gain ?

.....500-600 lbs is ridiculous the leaf motor will do 250-300hp continuous and it weighs 120 lbs! Why you think you need 500-600lbs is absurd...

Simple deduction, Talking Tesla...the motor is reported to be 350lbs, basic maths tells us a motor 3 times the dia (and only 1/3 as long) , will weigh 3 times as much if similar technology is used , so 700lbs more.
Deduct the weight of the reduction box, and you have 525 lbs !.......but thats just theory guesswork

Tesla's motor weighs 70lbs check your facts. Skip to 42 min
[youtube]PULkWGHeIQQ[/youtube]
https://chargedevs.com/newswire/elon-musk-cooling-not-power-to-weight-ratio-is-the-challenge-with-ac-induction-motors/
And when you make it larger in length or diameter it does not proportionately grow in weight. Think hollow rotor. And if the power is enough then it will weigh the same this is what you are missing. We make It shorter and bigger diameter for more torque and lower rpm but the same power and same weight.

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[moderator edit to add transcript for the hearing impaired]

From the video above, starting at 41:35

"I've got a question about the uh Tesla automobile. I understand uh the drive motor is...on the order of 250 horsepower, and only weighs 70 pounds, which is multiple horsepower per pound. I never see, I worked in the transit industry, never seen, and looked, and other sources, motor other than weighs multiple pounds per horsepower. The opposite way. Uh, so, you have an advantage like, an order of magnitude. Um, some of it can be explained by high speed. Can you explain, uh, how you achieved that?

Elon Musk:...[talks about rocket turbo pump]...but for electric motors, uh if you have a properly designed electric motor, AC induction motor, um, getting high-power to weight ratio, and like a really great uh um response rate, like a low latency and all that, extremely low ripple currents and what-not. It just kind of comes naturally to an AC induction motor. But the bigger challenge is actually um cooling it effectively, and in particular, cooling the rotor. You got a rotor going at like 18,000 RPM, so in the Model S, we coaxially cool the rotor, um in order to have high steady state. So, also, for an electric motor, you can have, its easy to get uh peak power for a short period of time. Um, It's hard to have sustained peak power, and, because you overheat. And then uh it's hard to get high efficiency over a complicated drive cycle. Um, those tend to be the problems you wrestle with more than say the peak power. Like, we can get peak power pretty easily, but sustained power power and efficiency over the drive cycle are hard..."

 

[Ohbse]

Luke ,...please explain how those motors with a power density of 1kw/kg, bring anything to this party ?
.Tesla's motor produces 420 kW ( battery limited ! ). from its reported 150kg (350lbs) weight !...2.8 kw/kg !
If we used Thingap's technology you would need a 400+ kg motor for the same power ?
What am i missing ?

You're missing the very substantial difference between continuous and peak. An automobile has a huge ratio of peak to continuous load. Peaks are currently limited by battery (and sometimes controller) tech, continuous torque rating by efficiency and in turn cooling.

 

[riba2233]

Hillhater I think it is supposed to mean the amount of power the motor can produce per given amount of weight will stay the same so when you are making the motor bigger it is getting heavier but it is able to run more continuous power. ......

Sorry Arlo ( and others) i missread this comment last week.
Somehow i though the discussion was suggesting you could have a 8 times more powerful motor which weighed little more than the smaller motor
..but if this is the case, that motor power is basicly a function of weight, then to me that explains exactly why EV designers all seem to stick with compact , high rpm, motors , and a reduction drive.
There would be little point in Tesla / Nissan using a larger motor with 8 times the torque , if it meant the motor was 8 times heavier !...that would be orders of magnitude greater than a reduction box weighs !
( and yes, i understand in the real world the power/ weight ratios are not that simplistic)

The motor doesn't need to weigh anymore if the materials are utilized the same way as the small motor.

Hence, if you have 1kg of iron in a small stator getting used at 600Hz in an RC motor in the shape of a long skinny 4pole inrunner, or in a huge pancake ring outrunner with 100poles turning so slow you can see it, provided the mass and current density through the copper is the same, and the utilization of the magnetic materials is the same, the power density will have the potential to be the same as well.

Yeah but what about end turn losses

I would like to see that example, it would be nice to have such motor, but it seems that nobody is willing to produce something like that, maybe because mechanical reasons?
I think that pancake motor would still have to be heavier for same cont. power output, but would of course require less gearing.

 

[Hillhater]

]
Tesla's motor weighs 70lbs check your facts. Skip to 42 min

And when you make it larger in length or diameter it does not proportionately grow in weight. Think hollow rotor. And if the power is enough then it will weigh the same this is what you are missing. We make It shorter and bigger diameter for more torque and lower rpm but the same power and same weight.

Yes i have seen the vid before, but not conclusive as there are several different weights quoted around various sources
( how come you thought it was 120lbs also ?)
Do you really believe a 400kw motor weighs 70 lbs.?
However several of those sources agree that the drive package, motor, inverter, gear train, weigh 320-340 lbs
And one solid source on DIY EVs has also weighed the inverter at 30lbs (13 kg actually) so that leaves 300lb fo the motor and gear train...and i dont see the gear train weighing 230lbs, do you ?
The reason no solid weight info exists for the motor, is that it cannot be separated from the gearbox, their housings are welded together !..so no one has been able to weigh them separately.
You keep saying make it shorter and bigger in diameter with the same weight, but you dont seem to grasp the simple relationship between diameter, length , and volume (weight)
Hollow rotor wont cut it i afraid, (especially if its full of coolant) as the diameter/ volume of the windings and housing as well as the end casings, all have to be taken into account.

 

[Punx0r]

That motor might be more than 70lb (which would put it at 12.5 kW/kg peak) but I can't see it being 350lb (160kg). That's ICE weight.

You keep saying make it shorter and bigger in diameter with the same weight, but you dont seem to grasp the simple relationship between diameter, length , and volume (weight)

That relationship applies only to solids.

 

[Hillhater]

And Torroids .
The 350 lbs is the motor, inverter and reduction drive.

 

[Teh Stork]

.....500-600 lbs is ridiculous the leaf motor will do 250-300hp continuous and it weighs 120 lbs! Why you think you need 500-600lbs is absurd...

Simple deduction, Talking Tesla...the motor is reported to be 350lbs, basic maths tells us a motor 3 times the dia (and only 1/3 as long) , will weigh 3 times as much if similar technology is used , so 700lbs more.
Deduct the weight of the reduction box, and you have 525 lbs !.......but thats just theory guesswork

Your basic math is wrong when applied to brushless motors. Brushless Permanent-Magnet Motor Design by Duane Hanselman goes into the math and basics of motors wrt diameter, weight and power. Chapter 1.8 goes into detail of how motor torque can be calculated: T=k*D^2*L
T=Torque, k = constant, D=Diameter, L= Length. As you can see the torque increases linearly with motor length, but by the square of the diameter. It is hard to understand this at first, so go read the book if you disagree.

Using this formula, a motor with 1.73 x the diameter and 1/3 the length will perform equal to a 1 dia x 3l motor - quite far from your 3x diameter estimate.

Lastly, the reduction between motor and wheels are ~10:1 in the Tesla - if anyone wants to entertain futher guesswork on direct drive vs single reduction geared motors.

 

[InterestedinEVs]

To add more fuel to the fire...

http://www.racecar-engineering.com/cars/tu-delft/

I'm sure you've seen these cars before, but how do you get rid of the reduction on a car like this? Assume a reduction of around 12:1 and how can you possibly fit a hub motor with enough torque to go direct drive? Just increasing the size(weight) of the motor by the equivalent weight of the reduction won't get you there, at least with current and readily available motor and inverter technology.

 

[Arlo1]

To add more fuel to the fire...

http://www.racecar-engineering.com/cars/tu-delft/

I'm sure you've seen these cars before, but how do you get rid of the reduction on a car like this? Assume a reduction of around 12:1 and how can you possibly fit a hub motor with enough torque to go direct drive? Just increasing the size(weight) of the motor by the equivalent weight of the reduction won't get you there, at least with current and readily available motor and inverter technology.

The weight of the motor does not change the inverter does not change the motor gets hollowed out and grows in diameter and narrows in length.

 

[InterestedinEVs]

Theoretically, yes.

But you're constrained in diameter and length in these cars due to compromises made for the sake of kinematics. With a 10" wheel you might gain an inch, maaaybe an inch and a half in diameter, about that in length. There are some very, very good engineering justifications for not moving to a 13" wheel and tire, so (as far as I can tell with some very crude calcs) there's no way to get enough torque to ditch the reduction completely.

 

[liveforphysics]

Theoretically, yes.

But you're constrained in diameter and length in these cars due to compromises made for the sake of kinematics. With a 10" wheel you might gain an inch, maaaybe an inch and a half in diameter, about that in length. There are some very, very good engineering justifications for not moving to a 13" wheel and tire, so (as far as I can tell with some very crude calcs) there's no way to get enough torque to ditch the reduction completely.

The rate growing the tire OD penalizes torque is linear with the diameter increase, where torque grows at the square of motor diameter. This means as you get larger, the potential for torque/thrust with a given mass of motor material grows linearly (X^2/X = X).

For a tiny diameter motor that needs huge thrust, it may require using exotic materials to achieve direct drive, or perhaps even not be possible if the wheel is small enough and the torque demand high enough.

Most vehicles fortunately have wheels with a diameter friendly towards non-go-kart track surface use.

 

[InterestedinEVs]

For a tiny diameter motor that needs huge thrust, it may require using exotic materials to achieve direct drive, or perhaps even not be possible if the wheel is small enough and the torque demand high enough.

Most vehicles fortunately have wheels with a diameter friendly towards non-go-kart track surface use.

That's exactly where I was going. I fully agree with your vendetta ( ) against reductions, but there are situations where their use is the best option.

That said, these cars rarely run on extremely smooth tracks. Lincoln, NE is probably the smoothest of any competition, but even it has some fairly large bumps. It would be interesting to compare the relative magnitude of surface irregularities on the average FSAE course to an average proper race track. For the speeds run in FSAE vs. full size cars, it may be similar? Maybe not.

If going to a larger wheel and tire meant the car would be faster, you can be sure that teams would do it. Plenty of teams do run 13" wheels, but few of the fastest electric cars do. The competition rules admittedly favor outright speed over efficiency, but if the benefits of running a larger motor outweighed the compromises (in this competition space), every top team would do it.

 

[Hillhater]

Your basic math is wrong when applied to brushless motors. Brushless Permanent-Magnet Motor Design by Duane Hanselman goes into the math and basics of motors wrt diameter, weight and power. Chapter 1.8 goes into detail of how motor torque can be calculated: T=k*D^2*L
T=Torque, k = constant, D=Diameter, L= Length. As you can see the torque increases linearly with motor length, but by the square of the diameter. It is hard to understand this at first, so go read the book if you disagree.

Using this formula, a motor with 1.73 x the diameter and 1/3 the length will perform equal to a 1 dia x 3l motor - quite far from your 3x diameter estimate.

Lastly, the reduction between motor and wheels are ~10:1 in the Tesla - if anyone wants to entertain futher guesswork on direct drive vs single reduction geared motors.

Well, if we are going to be picky for detail...
1) we are discussing an example of the Tesla motor,..which is an induction motor not BLDC.!
2). The torque relationship is understood and was discussed a few pages back. Not in dispute,
...... but you need to check your calculation i suspect, since we are looking for 9 times the Torque of the standard motor !
3). The 3 x diameter figure was not mine, it was proposed by another poster as an "approximation" for a motor to produce 9 times the torque .It is a sensible figure to base estimates on.
4) actually, the Tesla reduction is not 10:1 or 8:1, or9:1, Its 9.73:1, but we were working with easy numbers as it obvious that some folk dont bother to the read detail in these threads !

 

[speedmd]

I posted the hypothetical 3x diameter scenario as a ball park comparison of a tesla with and without the problematic dual stage gear reduction. A motor that diameter would give you a reasonably close to the same amount of off the bottom torque / acceleration. We all know that the tesla is much faster than a passenger car needs to be off the line, so this can certainly be somewhat reduced to a more reasonable size and see very little downside with the proper control and battery pack config. Hard to argue that the dual stage reduction box has been problem free. It is one of things that will need to be replaced in most all of the early builds and we will see how it holds up in the later models in the coming months and years. If we were building tractors, yes, maybe we would be best off with a reduction system as you would never be looking for top speeds. In a sporty car, the upside for high end speed performance may make it better to go without the relatively high ratio reduction system. Interesting analysis regardless.

 

[Hillhater]

Its pretty clear to me, that there is little to gain by eliminating the reduction drive in the Tesla/leaf designs.(accepting the reliability probs that Tesla has had)
Motor size and weight become issues, and for anyone who has seen the insides of the Tesla Reduction unit will realise that there is no more than 20lbs of gears/shafts inside an alloy housing.(much of which is the motor end casing/bearing, and diff housing)
The heaviest component is the differential (~10lbs ?) which couldnt be eliminated unless a dual motor system is considered.
..but that is another discussion altogether !
So, as a wild guess, i would sugggest eliminating the reduction gearing would gain around 30-50 lbs at most, but at the cost of a much larger /heavier motor that would not fit in the current location !

 

[speedmd]

Not only would you have a more reliable drive line, you would have higher efficiency and torque across a much greater speed range. I don't think the weight estimates are close to correct for the gear box or the motor. Size I don't see as a major issue as it is still compact compared to a ICE. That dual reduction gear box is not cheap either. I bet it and the difi is close to the cost of the motor if not more. I believe it is much like Luke stated, it will take some R&D and someone with the vision that is wanting it to happen.

 

[Hillhater]

Unless you redesign the drivetrain around a dual motor set up, you need some form of differential.
Maybe put the diff inside the hollow rotor of the bigger dia motor ?
But if that motor is 3 x the dia of the current motor,.....that wont work !

 

[Arlo1]

Individual motors would be best. I do not understand induction motors enough to know for sure but I have thought about 2 separate rotors in 1 stator one rotor for the left wheel and one rotor for the right wheel. The induction motor works on slip so each rotor would try to go the same speed at all times but it would be aloud to turn different speeds giving a very good form of limited slip type differential.

 

[speedmd]

Unless you redesign the drivetrain around a dual motor set up, you need some form of differential.
Maybe put the diff inside the hollow rotor of the bigger dia motor ?
But if that motor is 3 x the dia of the current motor,.....that wont work !

We are completely redesigning the motor, why not the drive train while we are at it? You make the motor what ever diameter you need to give you the torque required. The 3 times the gap diameter would give you roughly the same amount of overkill torque that the gearboxed version does now. Was hoping the simple concept would turn some lights on for some folks. It was just a example of how flawed the argument is against dropping the unneeded components. You need a motor, a connecting shaft (at least for a simply integrated easy to steer wheel), and a wheel. First mouse to it gets the cheese.

 

[spinningmagnets]

Luke's contributions to this discussion (and the performance of the deathbike) have persuaded me that...if...the battery can provide more than enough watts, then a multi-gear transmission is un-necessary, if you trade the size and weight of the proposed trans for a motor that is larger (in proportion to the loss of the transmission), then that is better.

That being said, I think some of the posters here are pondering the reduction from the motor to the output shaft to the wheel. Due to the issue of unsprung mass, it is highly inadvisable to use a hubmotor, so a motor is best located centrally, between the drive wheels, in order to keep the wheels' individual weight as low as possible.

Should the motor spin the exact same RPMs as the drive wheels? If that is one of the questions being pondered (and I apologize if I misunderstood), I believe it is best for the motor to be spinning many times more RPMs than the wheels, which requires a single-speed reduction. If that reduction (which it sounds like it is a 2-stage for compactness) is in question, I vote for the reduction to stay, in spite of the weight, bulk, and added friction.

Concerning a 2-speed or 3-speed trans, I feel they have value in systems where the watts and the size of the motor are limited. It's also an issue for a bicycle from several years ago, when battery selection and motor selection was limited. If a company can design a motor from scratch, and can contract with Panasonic to make cells to my new spec, a single speed motor with a ratio-d reduction to the wheel (tesla/deathbike), is not just acceptable, but desirable.

 

[InterestedinEVs]

That being said, I think some of the posters here are pondering the reduction from the motor to the output shaft to the wheel. Due to the issue of unsprung mass, it is highly inadvisable to use a hubmotor, so a motor is best located centrally, between the drive wheels, in order to keep the wheels' individual weight as low as possible.

There are compromises to be made here just like anything else. Using the FSAE cars again as an example, several teams have found the benefits of putting high rpm motors in-hub as opposed to sprung in-chassis far outweigh the negatives. These motors typically weigh around 8lbs, with the reduction under 2. Moving from 13" wheels to 10" wheels gives you around that in savings per corner and if you're a team like Delft able to do custom tires, that can be dropped even further, keeping unsprung mass low. Even scaling up, hub motors aren't automatically a poor design decision.

 

[spinningmagnets]

If you're talking about a race track that's guaranteed to be smooth, then it might be worth the experiment. For a street legal car that hits potholes, I would personally prefer the lightest possible wheel in a large enough diameter to make it more streetable (if that's a real word)?

Just to throw another wrench into the mix, if I was electrifying a 1969 Ford Bronco for street, and occasional off-road rock-crawling...I'd want a 2-speed transmission with a wide ratio, while still embracing a "street" Tesla using a single-speed motor.

 

[InterestedinEVs]

Not all race tracks are baby-bottom smooth

Let's assume a given wheel and tire size so we're forcing hub motors to weigh more than a standard wheel-tire combo. How fast do you have to go before the driver notices an impact in every day driveability of the vehicle? We'll say this is an average grocery-getter, not a performance vehicle...mostly limited to 70mph. Where does extra unsprung mass REALLY hurt you in this scenario?