hubmotor pros/cons

[tomtnt]

It seems most builds here use motor/pulley rather than direct drive hub motors- are there any major disadvantages to hub motors preventing widespread usage?

 

[torqueboards]

Forget about climbing my 25-30 * inclines and/or 240lb users climbing uphill.

180 lbs less and 15 inclines.. I think would be best for hub motors..

However, heavier riders or steeper inclines require more umpfh... belt driven offers the advantage of adjusting gearing ratio.

I'd like to hear that riders are able to climb some real steep hills. I do think direct drive is a lot better but unfortunately not for hill climbing at least not yet.

 

[spinningmagnets]

If you use the same diameter of motor in two comparable builds, with the only difference being that the motor+belt/pulley has a 3:1 reduction, then...the belted motor will be spinning 3 times for every single revolution that the hubmotor makes. That has a huge effect on the amount of torque available and the amount of heat that is generated, if both have the same top-speed.

I'm not saying hubmotors are bad, hub and non-hub each has its strengths and weaknesses. If you are on relatively flat land, a hub could be cheaper and there is no belt to wear out. Not so much that the belts are expensive, but it could get tiring to carry a spare and needing to stop and replace it halfway to a destination...

 

[LEVer]

I think building boards so that you can go 25-30 inclines is missing the point. You would surely deplete your battery when you get to the top or you would need a very big battery pack, very impractical. I'd also like to see a video of this mystical belt-drive board aside from the one from Boosted.

Putting gear reduction is only advantageous on acceleration. It's not good for coasting, braking or pushing. Our eboards are not ebikes with freewheel clutch and a separate braking system. The motor still needs to perform on other aspects, not just acceleration

OP, check my website for answer to the Pros. For the cons:

1. Heavier with comparable belt-drives.

2. Needs a battery that can push out the amps, although it's no amps-guzzler. We get the same Ah/Wh as belt drives but riding it on mostly 25mph.

I think a lot of builders want to keep the smaller wheel diameters. I have one available in 83mm but I've put specific use on it. It's actually a common phenomenon here. People started building light boards with small wheels. Then now, they have heavier boards and bigger wheels.

Also, not all hub motors are created equal. Some use small 50mm motors that lack performance. In the end, it's all about the ride.

 

[torqueboards]

There's not many people who will need to climb 25-30% inclines.

I have quite a bit here in SF.. I'd say more like 20-27% inclines and not so much 30%. I've seen a 30% and it's pretty darn steep.

I do think hub motors will be the future.

 

[saming]

I will agree with torque there hub motors are the future! I personally have a problem with Alien drive hub the screws are too small and always wiggle free but not problems with bigger screws on torques hubs and if you eliminate all the extra parts like belts and brackets you have less of a chance of something failing like screws wiggling free(European size screws are hard to find in the us unless its bough online) or motor plate breaking(First gen Caliber Mounts) which we all have some kind of story about if we been here long enough. Keep this in mind hub motors for skateboards are just in its first year with 2 more years and the Chines to copy our beloved community of such it could be amazing beyond what we have or consider given the tech advancement and the cheap chines factory we have for the skateboard community its a whole new ball park. I personally am waiting for version 3.

 

[Trbt555]

I don't think I' be able to trust hub motors when carving really hard. Isn't there always a risk of the urethane separating from the hub due to the high axial forces when carving ?

 

[LEVer]

I have not heard of any separation from my V2s, and I've shipped quite a few of them. The trick is using real fasteners instead of epoxy. There is an epoxy that will work, but it makes the ride hard and jarring. Using fasteners means you can also replace it easily.

 

[Vanarian]

I don't think I' be able to trust hub motors when carving really hard. Isn't there always a risk of the urethane separating from the hub due to the high axial forces when carving ?

No you can get rid of this fear as long as your hubs are properly machined like the ones made by LEVer (Pedig, it is HARD to call you by this name you know :lol: ) you don't have more risk to lose a wheel than with normal trucks. Explanation is simple : unless you break your polyurethane or deplete it badly, the wheel will hold like a normal one. Difference is : hub motors take some space in place of polyurethane, so it can be less comfortable in some way (less rebound?).

Before talking about hub motors pros/cons, we need to get an accurate sight of the situation.

You define a proper choice of motor Kv with your overall system and not by the motor alone. You need to determine which max RPM you want to reach, how much power you want and how much torque.

E-bikers can run ultra low KV motors for 2 reasons :

-they have more offers in high voltage components and low kv motors

-they can run VERY high voltage.

When both of these conditions are met, they gain ultra high power, top acceleration, top speed and top efficiency all while meeting lowered heat. Everything you hope for.

E-boarding stuff still encounters two main problems in 2015 :

-Vast majority of our components are derived or shared in RC hobby products (most of them directly come from it), which means that apart from some rare products like the VESC, we don't benefit from having products made exclusively FOR e-riding.

-Space! we need compact and somewhat lightweight stuff so we can't use bulky designs.

-Voltage. We are generally limited to a maximum of 12S, 6S being the common ground, 8S being an upgrade, 10S being muscle car and 12S being supercar.

You should know that 150-250Kv is a recommended choice because it is good when mixed with both the low general voltage of 6-8S and also because most people don't use hubs but gearing drives.

With hubs you must think think new! Design properly your powertrain and you can take everything. Think low Kv and high voltage. LEVer's hubs are already a wonderful application based on Turnigy SK3 149Kv. Imagine what he could offer with even better motors and lower Kv. Even ultra steep hills will bow in front of these motors. Keyword is "proper choice of overall system".

I personnally work with 85Kv slim hubs, and even like this I am thinking about negociating lower KV again, tests will tell me if I went low enough or if I need more. Key is high voltage, it allows you to gain any momentum you'd fear to lose when lowering Kv yet retains ability to run great torque at low RPM speeds.

In this regard, if you run higher Kv you need lower voltage but higher Amp draws and you will face more heat. Heat is your enemy in this regard, the more heat you encounter the less reliable or capable your motors will become. And amps produce dramatically more heat than voltage.

So pros/cons?

As pro, hub motors are awesome! But as cons, they need a whole system designed around them.

 

[laurentxr]

Really interesting thread, please keep going, I have no knowledge to input there, but I am highly curious.

 

[vedder]

I don't think I' be able to trust hub motors when carving really hard. Isn't there always a risk of the urethane separating from the hub due to the high axial forces when carving ?

No you can get rid of this fear as long as your hubs are properly machined like the ones made by LEVer (Pedig, it is HARD to call you by this name you know :lol: ) you don't have more risk to lose a wheel than with normal trucks. Explanation is simple : unless you break your polyurethane or deplete it badly, the wheel will hold like a normal one. Difference is : hub motors take some space in place of polyurethane, so it can be less comfortable in some way (less rebound?).

Before talking about hub motors pros/cons, we need to get an accurate sight of the situation.

You define a proper choice of motor Kv with your overall system and not by the motor alone. You need to determine which max RPM you want to reach, how much power you want and how much torque.

E-bikers can run ultra low KV motors for 2 reasons :

-they have more offers in high voltage components and low kv motors

-they can run VERY high voltage.

When both of these conditions are met, they gain ultra high power, top acceleration, top speed and top efficiency all while meeting lowered heat. Everything you hope for.

E-boarding stuff still encounters two main problems in 2015 :

-Vast majority of our components are derived or shared in RC hobby products (most of them directly come from it), which means that apart from some rare products like the VESC, we don't benefit from having products made exclusively FOR e-riding.

-Space! we need compact and somewhat lightweight stuff so we can't use bulky designs.

-Voltage. We are generally limited to a maximum of 12S, 6S being the common ground, 8S being an upgrade, 10S being muscle car and 12S being supercar.

You should know that 150-250Kv is a recommended choice because it is good when mixed with both the low general voltage of 6-8S and also because most people don't use hubs but gearing drives.

With hubs you must think think new! Design properly your powertrain and you can take everything. Think low Kv and high voltage. LEVer's hubs are already a wonderful application based on Turnigy SK3 149Kv. Imagine what he could offer with even better motors and lower Kv. Even ultra steep hills will bow in front of these motors. Keyword is "proper choice of overall system".

I personnally work with 85Kv slim hubs, and even like this I am thinking about negociating lower KV again, tests will tell me if I went low enough or if I need more. Key is high voltage, it allows you to gain any momentum you'd fear to lose when lowering Kv yet retains ability to run great torque at low RPM speeds.

In this regard, if you run higher Kv you need lower voltage but higher Amp draws and you will face more heat. Heat is your enemy in this regard, the more heat you encounter the less reliable or capable your motors will become. And amps produce dramatically more heat than voltage.

So pros/cons?

As pro, hub motors are awesome! But as cons, they need a whole system designed around them.

You are completely wrong in most points. I have explained this quite often before.

For the motor efficiency, the kv does not matter one bit. I wrote a bit about that here:
http://vedder.se/2014/10/chosing-the-right-bldc-motor-and-battery-setup-for-an-electric-skateboard/
What matter for the motor is the amount of copper that can be squeezed onto the stator. The kv you get depends on whether you use many turns of thin wire (low kv) or few turns of thick wire (high kv). You can easily rewind any hobbyking 50 or 60mm outrunner to get 10kv if you'd like. The reason that they aren't wound like that is that it doesn't make any sense unless you run at 100V+. 160 to 200kv is a good choise for a 12s system, and going much higher than 12s on hobby toys is dangerous since you can get electric shocks. So, in general, the motor kv is a parameter that can be changed easily when designing a motor, and what it does is to fit the motor to the electrical system. It easier to design a good electrical system for higher voltage and lower current than the other way. If you want to, you could design an electrical system with thick wires, many FETs and few parallel cells and run the same size hub motors as LEVer at 1000kv. The motors will be just as efficient as before, but the electrical system will become very bulky. For our setups, 50V (12s on a full charge) is a quite good choice for the electrical system. The wires are reasonably thin, the ESC can be made compact and the voltage is not too dangerous regarding electric shocks. There are some ebikes with a peak power of 15kw or more, where it makes sense to go even higher in voltage (even though it becomes dangerous), but personally I don't need that much power on an eboard. 12s and 5kw peak is more than enough for me. My 12s board with two geared 168kv 6374 motors throws me off easily when accelerating and has higher top speed than I can ride comfortably at.

Regarding ebikes: Have a look at a common 1000w hub motor. The weight of just the motor is 6kg. That is not very good power density. A geared motor on an eboard that can output the same continuous power is much lighter than 6kg, and that is because it is easier to get higher power density at higher speed and lower torque. So the general conclusion for eboards is: hub motors are fine, but for a given power output they will be bigger and heavier than a corresponding geared setup.

Hub motor advantages:
* More quiet operation (since the speed is lower)
* No belts that wear out

Disadvantages:
* Lower power density and higher weight than for corresponding geared setups
* Not possible to change gearing based on terrain, rider weight etc.

 

[Vanarian]

You are completely wrong in most points. I have explained this quite often before.

For the motor efficiency, the kv does not matter one bit. I wrote a bit about that here:
http://vedder.se/2014/10/chosing-the-right-bldc-motor-and-battery-setup-for-an-electric-skateboard/
What matter for the motor is the amount of copper that can be squeezed onto the stator. The kv you get depends on whether you use many turns of thin wire (low kv) or few turns of thick wire (high kv). You can easily rewind any hobbyking 50 or 60mm outrunner to get 10kv if you'd like. The reason that they aren't wound like that is that it doesn't make any sense unless you run at 100V+. 160 to 200kv is a good choise for a 12s system, and going much higher than 12s on hobby toys is dangerous since you can get electric shocks. So, in general, the motor kv is a parameter that can be changed easily when designing a motor, and what it does is to fit the motor to the electrical system. It easier to design a good electrical system for higher voltage and lower current than the other way. If you want to, you could design an electrical system with thick wires, many FETs and few parallel cells and run the same size hub motors as LEVer at 1000kv. The motors will be just as efficient as before, but the electrical system will become very bulky. For our setups, 50V (12s on a full charge) is a quite good choice for the electrical system. The wires are reasonably thin, the ESC can be made compact and the voltage is not too dangerous regarding electric shocks. There are some ebikes with a peak power of 15kw or more, where it makes sense to go even higher in voltage (even though it becomes dangerous), but personally I don't need that much power on an eboard. 12s and 5kw peak is more than enough for me. My 12s board with two geared 168kv 6374 motors throws me off easily when accelerating and has higher top speed than I can ride comfortably at.

Regarding ebikes: Have a look at a common 1000w hub motor. The weight of just the motor is 6kg. That is not very good power density. A geared motor on an eboard that can output the same continuous power is much lighter than 6kg, and that is because it is easier to get higher power density at higher speed and lower torque. So the general conclusion for eboards is: hub motors are fine, but for a given power output they will be bigger and heavier than a corresponding geared setup.

Hub motor advantages:
* More quiet operation (since the speed is lower)
* No belts that wear out

Disadvantages:
* Lower power density and higher weight than for corresponding geared setups
* Not possible to change gearing based on terrain, rider weight etc.

Allow me to disagree : what you said didn't contradict what I said.

Sure the more copper you get, the more power you can run run through the motor. But for same amount of power, amperage produces tremendously more heat on your system. Thus adapting your voltage and Kv accordingly to max RPM and max power.

Efficiency is a needed aim in terms of overall power but it does not do everything. A V8 ICM is inefficient to max and still produces tons of torque. And torque is a matter of putting K power at X RPM, the faster the lesser, the slower r the bigger.

You said it yourself you can run the same motor with ultra high Kv as long as Amps follow. But you will have so much more heat losses. What is the point in having efficiency if your motor loses all his power in eddy current heat?

Look at all mighty dragster e-bikes from CR, liveforphysics etc. Look at builds with the HubMonster. E-bikes can run more than 100V , it totally makes sense in their case to go ultra low. It adds to what you said but it also add to what I said.

You can run almighty torque on hubs if properly matched with the system and with proper Kv choice .

I know of the benefits of gearing, these are obvious. I'm just saying that hubs can still be awesome with obvious climbing abilities.

 

[vedder]

You said it yourself you can run the same motor with ultra high Kv as long as Amps follow. But you will have so much more heat losses. What is the point in having efficiency if your motor loses all his power in eddy current heat?

Wrong.

Some formulas:
N = turns
I = current
R = resistance
KV = rpm/V
P = power

P_losses = I*I*R (square relation)
KV = K * N (proportional to N with some factor)
Torque = I * (60 / (2*PI*KV)) (proportional to 1 / kv, meaning that half the kv gives double the torque)

If the motor has higher KV, it will produce less heat for a given current. For example, if the motor has R=1ohm, N=10 turns, KV=100kv it will have P=1*1*1=1W losses at 1A. If you rewind the same motor with 5 turns of twice as thick wire (= the same amount of copper), it will get 0.25ohm (the wire is twice as thick and half as long, thus factor 4) and 200 kv. That motor would produce P=1*1*0.25=0.25W losses at 1A, but only half the torque. To produce the same torque, you need to double the current, which gives P=2*2*0.25=1W losses. As you can see, the losses for the same torque are exactly the same if the motors have the same _amount_ of copper, and the KV does not matter at all. This means that if the electrical system can deliver the required current, it does not matter what kv the motor has as long as it has the same amount of copper. The makes perfect sense, because otherwise you could make "magic" motors that have insane power if you have thousands of ultra thin windings and drive them with tesla coils

Further, regarding the electrical system, the batteries don't care so much either. If you have, say, 2 cells, you can connect them in series and deliver double the voltage at the max current of one cell, or in parallel and deliver double the current at the voltage of one cell. The difference that voltage and current makes in the electrical system is really in the wires and the ESC. A 10V 500A ESC will be bigger and bulkier than a 50V 100A ESC, even if lower voltage FETs have lower resistance than higher voltage FETs, and making a PCB that usually has 35µm thick copper handle 500A is difficult. It also makes a big difference for the wiring of course. But if you have a bulky ESC and thick wires, high kv hub motors will work just fine and don't have more losses than low KV hub motors.

That being said, for hub motors, the KV does not matter for the motor losses. What does matter is magnet strength, the number of poles, the air gap and how much copper can be squeezed in. However, as far as I can tell, hub motors will not get much better than they are now unless you make them much more expensive. And even so, it is not like using the best magnets, the best stator and best tolerances for the air gap will make a huge difference. For hub motors, you have to expect to get less torque for a given motor size.


The reason I point this out so strongly is that I get so many questions about this and there are so many people who have the wrong idea about kv. They think that they can replace mechanical gearing with lower kv, and that simply isn't true. There is also nothing special about low kv motors. Every supplier for hobbyking could make all their motors with 10kv easily if there was a reason to do so.

 

[Vanarian]

Ok I understand better what you mean.

This is right by standards but incomplete from my point of view. About eddy currents and hysteresis you know that eddy currents grow by square where hysteresis is just measured by current at no-load & no input.

Power is also obtained by multiplying voltage per amperage. Power output is the power fed less losses. Power is also limited by your battery. Discharge ability , number of series, parallels, capacity... This limit every build have to bear with it.

Low KV motors are supposed to have bigger internal resistance and high KV lower internal resistance. At fixed voltage your totally right , the high Kv will have less heat with same amperage because there is less IR. But the higher Kv you go the lower torque you get too because the motor will spin faster.

Kv is the determining factor, multiplied by voltage, to know your Max RPM.

You need to alter max voltage and max amperage accordingly to obtain same power limit and similar torque , so you need to factor into your maths eddy currents when you want to get the exact same result with exact same power level.

With gearing you are allowed to just spin faster your motor without lowering voltage. But with hubs your electrical system defines your RPM limit.

This is where I was going.

You can counter the raise in IR by raising voltage, and your total power limit will require less amps by huge margin so you lower amperage . So less heat by huge margin too.

There is always a good middle ground though.
I'll add that low voltage components are cheaper than high voltage ones and again vast majority of RC suppliers don't need to carry human weight across the ground. No need thus to do low Kv motors if there is no offer on ESC or batteries dedicated.

That's why there is no availability on low Kv motors. Show them economical gain and it will pop up.

 

[vedder]

With hubs you must think think new! Design properly your powertrain and you can take everything. Think low Kv and high voltage. LEVer's hubs are already a wonderful application based on Turnigy SK3 149Kv. Imagine what he could offer with even better motors and lower Kv. Even ultra steep hills will bow in front of these motors. Keyword is "proper choice of overall system".

This sounds like you think that LEVers hub motors will generate less heat if you ride up a given hill with a given speed when you wind them with lower kv. Can you explain how you did the math? Because that is not true at all. LEVers motors will generate the exact same amount heat losses under a given torque and given speed, regardless what the kv is as long as they have the same amount of copper. This is what you seem to get completely wrong and what I am trying to explain. They can even be 1000kv if the electrical system is designed for that, and won't get the slightest bit warmer when going up a given hill at a given speed with a given load.

Regarding hysteresis losses and eddy currents, they depend on the motor speed and not the KV. I compared a 10V 500A ESC with a 50V 100A ESC, meaning that I refer to a lower voltage for the higher kv, giving the same maximum RPM.

What you could argue about is whether hub motors will be more efficient on low load, such as when you are riding on flat terrain or downhill, since then the resistive losses might not dominate. The optimal RPM efficiency-wise will be lower the lower the load is. In practice it does not matter so much whether you get 2% extra range with hub motors if you only ride on flat terrain or downhill. When it does matter is when you try to take a steep hill and the motors are too weak or about to overheat - and this is where geared setups are better. You need significantly larger hub motors to take steep hills than you would with geared motors. And lowering the kv of the hub motors does not help one bit in this regard, unless the losses come from the electrical system, which will not be the case if the motors are under 150kv. Therefore it does not make sense to make hub motors with less than 100 kv unless they are huge, since the electrical system won't be the bottleneck.

One application for large hub motors might be if you try to reach insanely high speed and brake a world record or so, because then a geared motor will have problems with eddy currents and hysteresis losses. However, if you have something like 1:2 or 1:3 gearing, 80mm wheels, one or two 50 or 60mm motors and a top speed of 30 to 40 km/h, eddy currents and hysteresis losses won't contribute much to the total amount of losses.

 

[Vanarian]

This sounds like you think that LEVers hub motors will generate less heat if you ride up a given hill with a given speed when you wind them with lower kv. Can you explain how you did the math? Because that is not true at all. LEVers motors will generate the exact same amount heat losses under a given torque and given speed, regardless what the kv is as long as they have the same amount of copper. This is what you seem to get completely wrong and what I am trying to explain. They can even be 1000kv if the electrical system is designed for that, and won't get the slightest bit warmer when going up a given hill at a given speed with a given load.

Regarding hysteresis losses and eddy currents, they depend on the motor speed and not the KV. I compared a 10V 500A ESC with a 50V 100A ESC, meaning that I refer to a lower voltage for the higher kv, giving the same maximum RPM.

What you could argue about is whether hub motors will be more efficient on low load, such as when you are riding on flat terrain or downhill, since then the resistive losses might not dominate. The optimal RPM efficiency-wise will be lower the lower the load is. In practice it does not matter so much whether you get 2% extra range with hub motors if you only ride on flat terrain or downhill. When it does matter is when you try to take a steep hill and the motors are too weak or about to overheat - and this is where geared setups are better. You need significantly larger hub motors to take steep hills than you would with geared motors. And lowering the kv of the hub motors does not help one bit in this regard, unless the losses come from the electrical system, which will not be the case if the motors are under 150kv. Therefore it does not make sense to make hub motors with less than 100 kv unless they are huge, since the electrical system won't be the bottleneck.

Well my post was kinda incomplete (or I messed with my redaction, my english is not all good sorry). I'd love to make proper tests on my own 85Kv variant of GT4020 because there is already a 460Kv version. I could give you results at exact same power levels all within the VESC limits and measure heat and performance, as soon as I can I'll do it. We agree on the variating V/A ratio, I thought the same about LEVer's example.

Though either I misunderstand what you try to explain or I kinda disagree on some points here :

-Eddy currents impact. It is huge and based on copper losses through internal resistance met by current, the stronger current the stronger losses even at low speeds. Hysteresis is a constant in comparison. By definition with classic outrunners stuffed with coppers, this can't be avoided, only limited. The highest Kv you'll go, the strongest current you'll need so you are bound to produce more heat than if you run less current, even with a more important IR inside a lower Kv motor.
This will apply on any situation as long as X power is needed in the given situation. I'll try to prove this later on with numbers, wish me good luck

-Building torque for hills as example. Kv is not the magic solution in my mind, but as on hubs you can't use gearing easily (unless using planetary gears or similar system) it is a solution to slow down the motor yet maintain the mechanical power. This builds torque.

Now talking again only about pure hubs without gearing systems. Hills will of course generate the same load on both high Kv and low Kv motors with same amount of copper. Then if we say that both motors have systems designed to run identical power at identical RPM, one being 50Kv and the other 100kv, will both motors produce the same amount of heat? I think not. Main reason is due to eddy currents, which should already be numerous times higher inside the high Kv motor. Torque should be almost the same (lessened by losses) but heat should prove to be worst on the 100Kv version.

 

[CSN]

it's a bummer hub wheels will not climb well.

What do people think about the prospect of planetary gears to increase torque in the hubwheel?

They would help climbing but also increase drag during coasting?

Maybe planetary gear maintenance would be preferred over belt maintenance?

Can't beat the look of HW's but climbing steep stuff is essential function.

 

[sl33py]

One overlooked con (at least from my quick read of the thread - if i missed it i apologize) is convenience.

Wear items on a standard motor mount are a quick swap. Swapping wheels for any number of reasons - wear, or want a softer 'thane, or to change the color to match your new helmet (ha!). (i have one of those friends who rides motorcycles with multiple matching suits for each bike - helmets and leathers).

Anyway - wear on the motor built into the hub (i'm a big heavy guy and impacts don't seem like they would be friendly with a small amount of 'thane protecting the motor), water, grit, etc.

I've not seen long term reliability and longevity experience from hub motor folks.

Then adding in swapping wheels due to wear or just want to change something.

Add in gearing for heavy guys like me (though i think the planetary gears like stary might be the long term solution) - these are reasons i've stuck with standard belt/motor mount setups. And i like to tinker with my setup so the components don't bother me. I don't have the skills/tools to swap/fix/tinker with a hub motor.

 

[MrDude_1]

It seems most builds here use motor/pulley rather than direct drive hub motors- are there any major disadvantages to hub motors preventing widespread usage?

yeah. they are not cool anymore.

First RC drives were cool (but complex)
Then everyone jumped on hub motors because they were easy...
and now the in thing to do is a mid drive motor, now that they've worked out how to mount them easily. :lol:

 

[torqueboards]

Just like eBikes.. I think there will always be a need for both.

Some people need to climb really steep hills which hub motors can't climb and/or some people need to ride only on flat ground and a hub motor would be perfect.

Although, still not sure if these hub motors can climb a 20-30% incline.. Haven't tried it yet.. Hopefully soon.

 

[Flexscoot]

I try to follow the technical discussion with interest.... and I still have a question : will hubmotors (direct drive) be more adapted to larger diameter motors and wheels (like mountainboard wheels) than for skate wheels

 

[LEVer]

Here's a quote from Inboard FB page:

Inboard Hey Leita! Good question. One of our goals has been to get as much raw torque out of the Manta Drive as possible while still balancing with the frictionless glide we've shown previously. With an in-wheel direct drive, you're only going to get so much torque, as the design favors glide over torque, just like belted motors favor torque over glide. From a standstill, a belt-driven motor will have more hill climbing power than a gearless, in-wheel direct drive motor. How much more is still to be seen, as we continually tune and modify our controllers to get as much torque as possible out of our design. The upside to not using belts or gears is less friction for the in-wheel motor, which increases range, motor and battery durability, and gives a smoother feel through the entire power band. We feel this is an acceptable trade-off for the M1 and that we can minimize the torque loss. Bear with us as we approach the point of having full pre-production samples, when we can disclose more specific and detailed specs for hill climbing, top speed, acceleration and range. For the next update we'll cover our progress on nearly every major component of the M1 and make any updates to our timeline if needed. An update was due last week, however we're still waiting for delivery estimates from a key component manufacturer, which we need to give an accuate estimate to our backers. Thanks Leita!

I agree belt-drives are better at hill climbing and can be lighter than equivalent hub motor setup. But I ask, again, what about coasting and braking? The whole argument for belt/gear drives is that you can use a small, high kv motor and put something like a 4:1 gear-ratio. That will be efficient, light and have a lot of torque relative to the size of the motor, but that's like having a stick-shift car with only 1st or 2nd gear. Ever driven a stick-shift car to 60mph in 1st gear and let off the throttle? Yeah...it's like that. And if you brake hard with that gear-ratio, you're bound to mess up the belt or motor pulley eventually.

On the other hand, making a gear-ratio like 2.5:1 or 2:1 is so close to 1:1, it may make more sense just to go direct drive on it. Then of course you would need a lowish kv motor with higher voltage. Then your setup becomes more like an automatic transmission that's able to tackle most conditions.

There is definitely a need for both. But our boards are not Ebikes, we do not have a freewheel clutch or a separate braking system. My SkatEPods have freewheel bearings and mechanical brakes, but no one would buy it. An eboard is all about simplicity. So we make compromises elsewhere.

 

[LEVer]

I try to follow the technical discussion with interest.... and I still have a question : will hubmotors (direct drive) be more adapted to larger diameter motors and wheels (like mountainboard wheels) than for skate wheels

You would need really low kv motors because of the bigger wheels. This is actually the more mature side of hub motors as hubs like this are easily available for Ebikes, Escooters and even those mini-segways.

 

[bunya]

I think one thing that was overlooked in the above discussion is the practical ability of climbing hills with "different Kv motors at different speeds" I am only relating this to hub motors because there is no option for gearing (yet ).

There shouldn't be any doubt that for a given size motor class and voltage a low Kv wind can climb steeper hills than a high Kv wind, this is because a lower Kv wind produces more torque at lower rpm and can crawl up the hill at a "lower speed". I think using 'given speed' in a hill climb discussion can be misleading because there will be a point where the practical ability of getting up the hill is forgotten. A 200Kv motor may be able to climb a 1 in 10 rise at a 20km/h whereas a 100Kv motor could climb a 1 in 5 rise at 10km/h. The given speed is different but the work done is similar and the hill climbed by the lower Kv motor is much steeper.

Maybe one advantage of a lower Kv wind is that they have a high inductance which makes them more suitable for field weakening. It will be interesting to see what can be achieved with the combination of a lower Kv motor and field weakening, maybe they will have a form of gearing...

 

[bandaro]

I'm no electrics genius like some people above, but I would say the motor kv has little effect in real world riding efficiency. Sure, a lower kv motor can run more volts and less amps which is typically more efficient, but the lower kv also means longer wires in the motor (needs to wind around more times) so the extra copper length reduces the power-weight ratio and increases resistance. I feel you can argue all you want, but I don't think you will see any real difference compared to what softening your startup force will do or altering how the motor is driven. Also, what about delta/wye for speed alteration, or is that a whole new can of worms we don't want to open? It is used in a lot of the machines at my work and works very effectively for startup vs running power.

Hub motors have not been around forever so the long term abilities of the motors are unproven, but from a well crafted motor they are looking very promising indeed.

As for ride quality, I feel one should go a lower duro wheel to compensate for the larger "hub" and therefore less flex in a wheel.

Changing the wheel can be very easy, with the new gen drive systems both LEVer and I are using, the wheel simply slips over the motor and its shielding. The outer half of the hub is in place to retain the same strength on cornering/sliding as it previously had. Or at least, I assume LEVer is using a similar method to me...

Planetary gears can be great, but looking at the geared hub motors for bikes shows a problem that will be the same for us; they have extra weight. This means a lower power density, but the gearing also allows for a higher kv and therefore power density of the motor to compensate for the extra weight of the gearing. For example, a 250kv 6374 makes 3kw, and 150 makes 2kw. Can you gear that 250kv to the 150kv speed without destroying the weight? If not, will the 250kv burn up on a hill because it is simply geared too high? Or do you live in Holland where everything is flat therefore don't need the climbing ability, just want the speed? Or are you in Switzerland and want the reverse??

Bike systems faced this same question, and the results will be similar here I feel. Sure, a BB drive offers gearing and therefore more efficiency, a hub offers no gearing, and therefore more efficiency. Both have their place, depending on how you will ride it. Given no skateboards (yet) have multiple gear options, I'm leaning towards the hub motors for skate, and rc/bb drives for bike. One could look into delta/wye switching if they wanted gears in a board without the weight, but there is probably a reason Ebikes and all the funding that goes that way hasn't made this main stream already.