Geared motor on tough off-road terrain - wrong choice?

Punx0r said:

Saying "less unsprung mass is better, therefore mid-drive is better" is a gross over-simplification. You can't judge the overall suitability of a system based on one axiom.

On technical subjects some people know what they're talking about and some people think they know what they're talking about. The latter tend to draw sweeping conclusions based on a single factor, while the former have the knowledge and experience to arrive at the best compromise that works in a real-world application. This is usually non-ideal in most or all factors. That's what you pay an expert for: google will tell any idiot in 10 seconds that "unsprung mass = bad for handling", the expert can tell you whether or not this is relevant in a given application.

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Bumpy off road riding is near the top of the list of applications where you'll benefit from less unsprung weight. Here's a page put together by an "expert" who does motocross suspension: http://www.mb1suspension.com/#!unsprung-weight/c15fs In particular:

MX site said:

Factory teams spend BIG BUCKS on lessening unsprung weight. Most works bikes have magnesium hubs, titanium linkage bolts, Ti axles, aluminum spoke nipples, tapered and butted spokes, titanium shock springs, carbon fiber chain guides and magnesium (or beryllium) brake calipers.

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Anyway, you don't need to be an expert to understand that, just have some riding experience and spend 20 seconds thinking about a wheel interfacing the dirt. Since the wheel and attached mass are forced to contour directly over the ground they'll be able to do a smoother and more consistent job with less mass. Anyway, even on relatively smooth tarmac tracks race teams have been putting huge effort into making things like carbon wheels and aluminum sprockets.

Should I buy some fragile, expensive carbon wheels and the lightest, skinniest tyres I can find because I will automatically feel the benefit because the unsprung mass will be less?

Punx0r said:

Should I buy some fragile, expensive carbon wheels and the lightest, skinniest tyres I can find because I will automatically feel the benefit because the unsprung mass will be less?

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No. The benefits of superlight wheels are grossly overstated. Make them strong and reliable first, lightweight and/or aerodynamically efficient second.

The lightest and most temperamental wheel and tire combination I'd consider for a high speed (>30mph) e-bike would be something like Velocity Chukker rims fitted with 28mm tires. Carbon fiber plastic materials as currently used have no place on a practical bike or one propelled predominantly by a motor.

Under 30 mph, options open up a lot in terms of what components are robust enough to do the job. Alex DA28 rims with 25mm tires would represent a very light, aero, but not horribly fragile setup for sub-30mph speeds.

Remember that pedal bikes are capable of impressive speeds, but they only reach these speeds in brief bursts. Average speeds on pedal bikes must be correlated to the desired average speed of an e-bike when assessing what components may or may not be applicable.

Didn't read the whole thread so pardon me if this is already mentioned.
IF you are looking for light weight yet need the power for steep climbs in rough terrain and on rough surface I would think a 2 wheel drive system would be right for you. Justin has a new motor a Grin Tech motor with 20mm thru axle that seems perfect for your front wheel. Weight only 4 kilos.

If you put like a 9c+ from Grin in the rear you should have 2 powerful motors at about 10 kilos. No internal gears that might brake.
From memory I think those 9c motors can be pushed close to 5 kw peak if properly cooled. With a heat sink and FF or oil you should get high power yet small chances for over heating.

Match up with decent controllers and you should have a well balanced ride with plenty of power to tackle those steep inclines/hills.

Wow!
I have started this discussion, and look how it developed
In the meantime, I thought of another alternative - and tell me what you think:
If I used too identical DD motors - like H3540 - on the front AND on the rear.
I would have solved all the balancing issues and under-steering, but I will have a terrible unsprung weight. In fact 15kg of it! The bike would be nearly impossible to put on a car-bike-rack
Again, on the other hand, I will have a magnificent 2x2 propulsion on strong uphills, and similar regen that will extend the range and enabling you to go down slowly without wasting your brakepads. (Imagine going downhill a 500m mountain on a steady -13% grades on gravel with sharp curves).
The DD would not care about shocks and can be bullet-proof as long as you don't overheat it.
I wonder if the fabulous grip of 2x2 is sort of countering the unsprung weight effect that deteriorate your grip. (since there is always one propelling wheel touching the ground)

What do you think?

I want to note that I will use such bike not on tough single-tracks with rock climbing, drops, etc... Only on comfortable ones, and especially on 4x4 gravel roads, which we have a lot. I also consider whether such solution can be good for a touring bike - where you can see the new country via the nature instead of going on busy-roads. (Try UK for example... Everyroad is busy). And in the touring regime with the heavy weight, regen becomes even more significant.

I have started this discussion, and look how it developed

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Yup, look at the mess you made! :| :wink:

Anyway, iirc you were concerned about this bike for your father not losing it's 'bike' capability by giving up a chainring? So I hardly see the logic in using a dd hub (or two) with all the cogging, weight, higher gearing, and loss or limits on a front susp.

But hey! Give it a shot, make a masterpiece! A 'crawler' style I think would be cool for tough/extreme off-road terrain, though imo a mid would still be better due to gearing versatility and of course rim/tire and susp/ ride character versatility (you can still pour your wheels full of cement to simulate hubs ). Btw, I think Alan B has builds of both 2wd and a mid. There's probably alot to read about them right in his sig links - and it might help alot!

I upgraded my old DD Mountain bike to AWD with a geared front hub. It climbs much better now, but I had to change the front forks to steel to handle the torque safely, and the front doesn't have a lot of weight so it wants to spin. Who knows how long it will hold up, but I don't do jumps, it is more of a tractor, and the FOC controller is very gentle on the clutch. Lifting onto a bike rack is not a problem - it just takes two hands instead of one.

Two motors is very impressive compared to one. The launch thrust is slightly more than a CroMotor with 120 amps now. But it is not as light or nimble as a good mid drive.

It all depends what you want. One of each is fun.

Stories on these bikes are in my sig, Bonanza2WD is the AWD. RidgeRunner is the mid drive, CroBorg is the CroMotor.

What are the lightest > 5 kw dd hubs today?
Mxus 3000 vlatest is around 9 kilos standard and can be shaved down to 7 kilos iirc, and can do peak of 10+ kw out of the box. Imagine what heatsink and FF will do for climbing abilities and probably peak power as well. Even long slow climbs could be done without overheating. Paired with ie Adaptto or even a more powerful controller I imagine peak power in short burst of 14 kw maybe even 15 kw. And FF and heat sink would remove the heat rapidly without adding significant weight.

The leaf is a 1500 w motor, I think I read someone pushed 5 kw for short burst peak. Same with that one. Should do even more with heatsink and FF.

I am not really familiar with others hubs so I can't really say how hard they can and have been pushed but the 9c is also 5+ kw peak. Fairly light too.

I am sure there are various other light and capable DD hubs that stock put out 2-3 kw peak for short bursts that with additional cooling can do significant more and yet be a lightweight alternative to geared hub motors.

What I am getting at is that from what we can see now from the various testing done so far with the heatsink and FF is that not only do they allow for rapid heat movement out from the hub motor, but at proper cooled hub can do more hi torque low speed climbs and repeated accelerations without the need for doing stops so the hub can cool down. More power to the wheel with less heating issues. I think for many riders geared hub motor might not even be required anymore as a single or dual DD hub setup will do slow riding just a good and bring a dramatic improvement in motor life span over geared hubs.

When do we see <3 or 4 kilos dd hubs that peak for short bursts at >5 kw with added cooling? If we can get that much power out of small lightweight dd hubs I would not be surprised if some mid drive riders would consider a dd hub build for improved durability and ease maintenance as well as ease of engineering/fabricating as there are now a bundle of ready made frame kits specially build for dd hub motors.

Power is not the correct way to evaluate DD hubs for tough off-road terrain. Power comes into play when the back EMF rises at higher speeds, but at these low speeds, you cannot put that power into torque efficiently, and with the terrain these speeds are not transient, they are nearly continuous.

Instead we need to look at torque and heat production at low speed, and the motor current at the first knee in the saturation curve that determines how much motor current we can use efficiently.

For example, a 9C 10 turn hits the first knee in the Kt graph at about 48 amps, (so 480 amp-turns for other windings of these motors). Above that level of current the Kt (torque produced per amp) drops quite a bit. So you don't want to operate above that current level for very long, it is wasteful which translates to lots of extra heat.

With a 200 milliohm resistance and 48 amps we have i squared r heating of 461 watts, and the torque produced is about 90 Newton meters. So if you need more than 90 N-m you need a larger motor (or a second motor), or you need the gearing to make 90 N-m be adequate (smaller wheels, for example). And you need to dissipate almost 500 watts all the time you are producing this torque. That's a fair amount of heat.

Alan B said:

Power is not the correct way to evaluate DD hubs for tough off-road terrain. Power comes into play when the back EMF rises at higher speeds, but at these low speeds, you cannot put that power into torque efficiently, and with the terrain these speeds are not transient, they are nearly continuous.

Instead we need to look at torque and heat production at low speed, and the motor current at the first knee in the saturation curve that determines how much motor current we can use efficiently.

For example, a 9C 10 turn hits the first knee in the Kt graph at about 48 amps, (so 480 amp-turns for other windings of these motors). Above that level of current the Kt (torque produced per amp) drops quite a bit. So you don't want to operate above that current level for very long, it is wasteful which translates to lots of extra heat.

With a 200 milliohm resistance and 48 amps we have i squared r heating of 461 watts, and the torque produced is about 90 Newton meters. So if you need more than 90 N-m you need a larger motor (or a second motor), or you need the gearing to make 90 N-m be adequate (smaller wheels, for example). And you need to dissipate almost 500 watts all the time you are producing this torque. That's a fair amount of heat.

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It's true the speeds would be slow, and I know DD motors are not efficient in low speeds. But just like you can use a DD while going on flat at 15Km/h or so, and it can be efficient at those low speeds because of the low power needed - if there is a second motor that is helping - it wouldn't be enough to allow slow climb together? Each will carry half the load.
Can you explain what is that "knee point" you talk about? And the Kt graph?

Look for the thread by justin_le on motors where he makes those measurements and plots. (Any thread by Justin is worth reading). Kt is torque per amp of motor current, as magnetic saturation begins the torque per amp changes slope. Operating above that kills efficiency. The heat produced by the motor is a function of the torque, from i squared r. Low torque, low heat. If you know the dissipation capacity of the motor you can calculate the continuous torque that it can produce, and the limit of motor current that you can use. Here is the thread:

https://endless-sphere.com/forums/viewtopic.php?f=2&t=14494 Justin's thread on motor torque vs current, actual measurements

It is not the speed that causes the inefficiency, it is the motor torque required. You can toodle along all day at walking speed if the torque needed is low. But as the gradient increases the load the heat generation rises with the square of the torque required.

For a given torque, two motors doubles the heat dissipation capacity, and if each motor delivers half the torque the heat in each motor is 1/4 as much. It makes a particularly huge difference if this torque is near the stall torque of one motor. I did pretty much this exact experiment recently with the Bonanza2WD, and the heat went from nearly melting a single 9C motor (it was very hot for hours) to barely warming a pair (one was a BMC gearmotor, so not precisely the same motor, but the result is the equivalent).

A rule of thumb for DC motors is peak efficiency is at about 10% of motor stall torque (not speed). So you want a lot of torque capacity, but only to use a little of it. It makes for great launches, too.

Alan B said:

Look for the thread by justin_le on motors where he makes those measurements and plots. (Any thread by Justin is worth reading). Kt is torque per amp of motor current, as magnetic saturation begins the torque per amp changes slope. Operating above that kills efficiency. The heat produced by the motor is a function of the torque, from i squared r. Low torque, low heat. If you know the dissipation capacity of the motor you can calculate the continuous torque that it can produce, and the limit of motor current that you can use. Here is the thread:

https://endless-sphere.com/forums/viewtopic.php?f=2&t=14494 Justin's thread on motor torque vs current, actual measurements

It is not the speed that causes the inefficiency, it is the motor torque required. You can toodle along all day at walking speed if the torque needed is low. But as the gradient increases the load the heat generation rises with the square of the torque required.

For a given torque, two motors doubles the heat dissipation capacity, and if each motor delivers half the torque the heat in each motor is 1/4 as much. It makes a particularly huge difference if this torque is near the stall torque of one motor. I did pretty much this exact experiment recently with the Bonanza2WD, and the heat went from nearly melting a single 9C motor (it was very hot for hours) to barely warming a pair (one was a BMC gearmotor, so not precisely the same motor, but the result is the equivalent).

A rule of thumb for DC motors is peak efficiency is at about 10% of motor stall torque (not speed). So you want a lot of torque capacity, but only to use a little of it. It makes for great launches, too.

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Oh, you were talking about the motor constant. I thought it was something else, because Justin told me once that on E-bike motors, in order to reach magnetic saturation, you would need to force so much current that the I2R losses would burn your motor in seconds.
The constant will stay the same, but due to increasing losses and wind-resistance increase, as he shows there - your power output decreases. It has nothing to do with magnetic saturation.

Justin clearly shows the saturation reducing the incremental torque production graphs from his measurements. No speed was involved, and he compensated for temperature effects.

It is true that this current is high and leads to lots of heat, so you cannot operate there for long periods of time. People have put 10kw into these hubs and done it, but they heat up rather quickly if done for very long.

It is also the case that on motors up to at least the 9C size that folks operate in this current regime quite often, for short periods of time. Like on every aggressive launch. For example, 480 amp turns is about 48 amps of phase current on a 10 turn (10T) motor, and this definitely is easy to reach during initial launch. On a 6 turn (6T) motor this is 80 amps, and when the motor is starting out there is no back EMF and it only takes a little battery current to generate 80 amps of phase current.

For example, if you have set your controller for 40 amps of battery and the standard 2.5x phase current multiplication factor then the phase current would hit 100 amps on each aggressive start, long before the battery current hits 40 amps. These are common settings. They generate extra motor heat long before the bike's speed is high enough to have losses from wind. These are merely facts that one might want to consider when setting the maximum phase current value.

In my case the PhaseRunner can easily reach magnetic saturation with the 10 turn motor, and probably with the V4TT BMC as well. So I may not want to allow the max phase current. It depends how much you care about efficiency during launch vs torque, and heat in the motor.

Here is one example of magnetic saturation that he measured:

Sorry for talk off-topic.

But your writing 10 turn instead of 10t or 10T should from now on be mandatory for all E-S members and the moderators should watch for it.
Because i recently searched for 1.5T hub motor (meaning turn) but got lots of ambiguity results for 1.5T and 1.5t (meaning Tesla). https://en.wikipedia.org/wiki/Tesla_(unit)

My propose is to use like ( T3 motor ) as abbreviation for 3 turn motor to prevent ambiguity with Tesla (unit).

I posted it also here https://endless-sphere.com/forums/viewtopic.php?f=13&t=9375&start=100 I hope it gets accepted.

Alan B said:

Justin clearly shows the saturation reducing the incremental torque production graphs from his measurements. No speed was involved, and he compensated for temperature effects.

It is true that this current is high and leads to lots of heat, so you cannot operate there for long periods of time. People have put 10kw into these hubs and done it, but they heat up rather quickly if done for very long.

It is also the case that on motors up to at least the 9C size that folks operate in this current regime quite often, for short periods of time. Like on every aggressive launch. For example, 480 amp turns is about 48 amps of phase current on a 10 turn (10T) motor, and this definitely is easy to reach during initial launch. On a 6 turn (6T) motor this is 80 amps, and when the motor is starting out there is no back EMF and it only takes a little battery current to generate 80 amps of phase current.

For example, if you have set your controller for 40 amps of battery and the standard 2.5x phase current multiplication factor then the phase current would hit 100 amps on each aggressive start, long before the battery current hits 40 amps. These are common settings. They generate extra motor heat long before the bike's speed is high enough to have losses from wind. These are merely facts that one might want to consider when setting the maximum phase current value.

In my case the PhaseRunner can easily reach magnetic saturation with the 10 turn motor, and probably with the V4TT BMC as well. So I may not want to allow the max phase current. It depends how much you care about efficiency during launch vs torque, and heat in the motor.

Here is one example of magnetic saturation that he measured:

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I see.
Anyhow, I run a dual motor system with phaserunners, and both have a low phase current limit of around 40-50A.
I launch with both - so I eliminate all gear stress and all efficiency loss issues that would otherwise heat up the motor. So I guess that issue is none relevant for me.
I only allow 80A peak regen phase current for those pulse braking torques, but that's not common, and anyway the phaserunner won't let you stay there for more than few minutes.

It is hard to heat up dual motors. Methods did it, 10KW each to a pair of 9C's. There's a thread.

The PhaseRunner doesn't have enough parallel FETs to put out that much current.

But they are excellent controllers. I'm cleaning up the wiring and installed a new front brake on my Bonanza 2WD. In tests it out-accelerated a Cromotor Fatbike with 24 FET and 60 amps at 72 volts, I was using 46A at 72V. The fatbike has larger tires and a heavier rider.

In any case the PhaseRunners split the torque nicely with torque throttle input, and operate the motors with buttery smoothness and are very small which helps when mounting a pair of them.

Tried to find that dual motor thread by methods but no luck. Can you remember if he made a separate thread or is it maybe in the "Spun front axle and flew over the bars @ 25mph" thread?

That sounds like the thread. I don't recall a separate one but that one rings the bell.

Methods is still on ES from time to time.

Bringing back some dust over this post:
I just bought a H3525F+ with statorade, and I will mate it with Phaserunner.
I will use it on the front wheel of a full suspension mountain bike, so I can keep the same drivetrain 3x10 gearing, with the option to get help on the front wheel.
It's mostly because there are no options nowadays for a direct drive motor with cassette free-hub.
A front DD motor would provide a very good regen on downhills, when more weight is on the front.
On uphills, the rear wheels and the pedals would do most of the work. It would be only a pedal assist system.
So on one hand, it would weight 7.5Kg, but that weight would improve the traction of the front wheel.
Although it's unsprung weight, but these bikes are going to be used mostly on gentle off roads which any family car can do.
It will be a direct drive, since geared motor would probably not last long at that scenario, and it can't regen.
The bike will be used by my Father, who like to "sit" on the brakes on downhills - so instead of replacing the brake-pads often, he will get some extra range.

What do you think?
Is 7.5Kg on the front wheel not too much for that scenario?

Yes I know I am a late arrival to the party but passing along some personal experience....

I started with a MAC internally geared hub motor because I wanted to ride off road and climb hills...bad idea. You can't get it geared low enough to keep it from overheating...and that is the same for a DD motor.

If you plan to leave the pavement...get a BBSHD, a 14s6p battery from EM3ev so you can pad it inside the triangular case that comes with it..I actually used the packing material that came in the box with my battery.

Gear it so you have a top speed that is appropriate for your riding...I use a 28 Lekkie or 30 Mighty Mini chain ring in the front and something like a 24-30 tooth sprocket in the rear depending on what size tire I am going to be using. I like my bike geared so that the top speed on level ground with no wind is 17 mph. That is about as fast as I can go in between the trees where I ride. It also means the bike pulls like a big dog and never overheats. Unless you need to go a lot faster, you really only need one gear. I have mine set up right now with a 28 front, 26 rear, and a Maxxis FBR 26x4 tire. I also have an 18 tooth sprocket on the rear that gives me a top speed of about 25 mph for getting to and from the truck. If you go much over 25 mph the air drag starts to go up quickly and the power required to go faster increase very quickly.

Like somebody said...forget about light weight and make reliability your first priority. A heavy bike that works is a lot more fun than a lightweight bike that is broken .