2WD (Two-Wheel Drive) FAQ

[Alan B]

John and I overlapped in our analyses postings. Quite Interesting. In any case the power cost of the second motor is not a major loss increase, according to this simulator. In some cases it appears to be a slight gain. That was reported on the Magic Pie forum as well where a number of folks have deployed multiple Pies - no significant loss, possibly a slight gain.

Anyway this is Kingfish's FAQ on running two motors, so we don't want to fill it with disagreements, but with useful information and experience. Perhaps those who have done dual motors it can report on their watt hours per mile consumption. Perhaps we can compare Kingfish's results with two motors this summer to his previous trip, and to Justin's cross Canada trip with one motor a few years ago.

I plan to do two motors. Maybe soon. Thanks for the thread, Kingfish!

[neptronix]

Luke has some great, well researched points.

That being said...
I ran my MAC motor on 4kW the other day and enjoyed the feeling of being shot out of a cannon with all that damn torque.
Then my gears and clutch died 4 days later. HM.... coincidence?

The only way forward would then be to run dual motors at 2.5kW each = 5kW total.

I'll take a hit in efficiency, yes.. but do i have the fabrication skills, space, tools to do a non-hub build? hell no.
For some of us, this is the only option.

[John in CR]

Luke is looking at it wrong, there is no efficiency hit. Lower load means greater efficiency all the way up to peak efficiency.

[Toorbough ULL-Zeveigh]

The only way you could ever simulate an efficiency loss adding the second motor would be if the load for 1 motor is so light that the single motor is running very close to peak efficiency and the lowering the load pushes them past peak efficiency and to the downside of efficiency toward no-load rpm. I submit that this is next to impossible to achieve.

umm, you mean like when the front wheel is slipping or completely up in the air...

To make a PM motor turn, even if the wires aren't hooked up to ANYTHING, you have a substantial energy penalty. >90% of your no-load power is just going to make the motor itself turn (core loss), which happens anytime the motor is spinning, no matter if it's powered or not.

3 ways around that i know of fer sur, possibly more.
1. don't spin the motor by utilizing a freewheel as has been discussed.
2. a mechanism to physically pull the core out of the PM field.
AF is the simplest to implement & commercial versions have been around for at least a decade.
3. flux cancellation.
something like a 'back-iron', except a caliper mounted front-iron to short out the magnetic field.
however a more elegant solution would utilize 5-phase windings to generate an anti-phase flux waveform.
we'll soon know how well that works out, hopefully by the beginning of the new year.

[John in CR]

The only way you could ever simulate an efficiency loss adding the second motor would be if the load for 1 motor is so light that the single motor is running very close to peak efficiency and the lowering the load pushes them past peak efficiency and to the downside of efficiency toward no-load rpm. I submit that this is next to impossible to achieve.

umm, you mean like when the front wheel is slipping or completely up in the air...

I definitely concede Luke's point about the front motor wasted when there's so little traction. I was getting at comparing 1 motor to 2, and if the single motor rig is optimized and running at or near peak efficiency, then reducing the load via a 2nd motor could conceivably push operation to the downside of the efficiency curve.

To make a PM motor turn, even if the wires aren't hooked up to ANYTHING, you have a substantial energy penalty. >90% of your no-load power is just going to make the motor itself turn (core loss), which happens anytime the motor is spinning, no matter if it's powered or not.

3 ways around that i know of fer sur, possibly more.
1. don't spin the motor by utilizing a freewheel as has been discussed.
2. a mechanism to physically pull the core out of the PM field.
AF is the simplest to implement & commercial versions have been around for at least a decade.
3. flux cancellation.
something like a 'back-iron', except a caliper mounted front-iron to short out the magnetic field.
however a more elegant solution would utilize 5-phase windings to generate an anti-phase flux waveform.
we'll soon know how well that works out, hopefully by the beginning of the new year.

I really didn't understand this one. I used throttle activated regen, ie I let off the throttle and regen comes on, so I never coast, and I ride the throttle long enough that I always have some mechanical braking at the end. I never coasted to any significant extent before I had regen, and I never coasted when I rode a motorcycle. AFAIC cogging resistance is just low level braking, not a loss. There is no cogging during operation (except maybe the 1 unpowered phase), because the coils are active, and if I'm wrong about that it's part of each motor's efficiency curve, and doesn't change the fact that efficiency increases with decreasing load all the way up to peak efficiency. That means 2 motors has to be more efficient at all power levels less than double up to peak efficiency, which is generally pretty low power.

[liveforphysics]

Luke is looking at it wrong, there is no efficiency hit. Lower load means greater efficiency all the way up to peak efficiency.

I'm afraid that's not the case my friend.

Efficiency for any electric motor is 0% at both ends of the scale. It's an arched curve with 0% efficiency on both sides of the curve, and the peak efficiency always occurring when copper loss matches core loss, somewhere in the middle.

When you unload a motor to the point where core loss is more than copper loss, you're on the decreasing efficiency side of reducing load. This is pretty much the 95% operating mode for folks commuting with a large hubmotor (or a large type of any PM motor).

In return, while climbing really steep things, and/or when accelerating very hard, the larger motor hopefully has a bunch more copper in it, so they can minimize copper loss under those high load situations.

By all means, if you were going to be operating mostly in very high load situations (playing the waiting game to see if the hill will end before your motor smokes, or trying to maintain extreme high speeds or something), then yes! A larger motor will be more efficient for you, and if you can't find a large enough single motor to do what you need, then you could add a second, and be more efficient during those times of very high load.

I know everybody has a different area they ride, speed they ride, hills they ride, bike weight, rider weight, etc. But I envision 95% of folks spend 95% of there time riding in a situation where the single motor is going to be most efficient.

[liveforphysics]

AFAIC cogging resistance is just low level braking, not a loss. There is no cogging during operation (except maybe the 1 unpowered phase), because the coils are active, and if I'm wrong about that it's part of each motor's efficiency curve, and doesn't change the fact that efficiency increases with decreasing load all the way up to peak efficiency. That means 2 motors has to be more efficient at all power levels less than double up to peak efficiency, which is generally pretty low power.

I used to believe this same thing my friend, but after putting a pair of motors on the dyno and driving one of them unloaded (even with the phase wires not connected to anything at all), it totally changed how I see things. Your cogging torque ripple, and ripple ONLY sums to zero, but you have a substantial net loss as the rotor turns in the motor. This core loss is just a reality of having your magnets turning inside the motor. Your core loss is >90% of your no-load power (which I think was a few hundred watts for your motors if I'm remembering right?) This is just a penalty you have to eat for making the magnets change the field of the iron around as they rotate around inside there. Nothing can avoid it without the suggestions that Toorbough had listed (pulling the rotor out when you don't need that motor, making the rotor freewheel, etc).

[Alan B]

This makes a freewheeling gearmotor even more attractive for the second (front) unit. Use it for hole shot and steep climbs, shut it down over 15mph or so when the rear motor hits the efficient region.

Good info!

[amberwolf]

If anyone wants to see the losses/inefficiency of a second (electrically disconnected) motor, it's easy enough even without putting it on a single bike and riding it.

Take any two ebikes, and flip them upside down.
Put them lined up so that the tire of one motor will be able to friction-drive the other, but not yet touching.
Disconnect the phase and hall wires from the driven motor.
Hook up your wattmeter on the driving bike.
Run the motor at your desired speeds, noting down the (no load) power usages.
Push the bikes together so the disconnected motor will be driven from the other.
Run the motor at your desired speeds, noting down the (now slightly loaded) power usages.

That's approximately how much power will be "wasted" just driving the second motor on the bike, even without it being connected to anything, and even without a power load on the first motor.

If you like, redo the test with the second motor still connected to it's controller and battery, and powered on, but at zero throttle. Power usage on the first motor will probably increase noticeably (maybe not significantly).

Redo it again, this time with the second motor also being monitored for power usage, and set it's throttle to match the first one's speed just enough so that there is no "drag" on the first motor by the second one. Total up the power usage between the two motor systems, and it will still probably come out a net loss. (this test may not work as expected because there is no real load on the first motor).

Now, for the sheer fun of having the second motor, this doesn't matter. Nor does it matter if part of your reason is to have a backup motor in case of system failure on the first.

But if you're trying to make it more efficient, I'd venture that in most cases, for our typical purposes, it probably wont' be.

[AussieJester]

Has anyone mentioned the added weight factor and the extra
Power that is needed because of it? Surely The weight of the
Extra frock and controller would would result in more
power required to move the bike along correct?

KiM

[Alan B]

Already covered, apparently insignificant.

[liveforphysics]

Has anyone mentioned the added weight factor and the extra
Power that is needed because of it? Surely The weight of the
Extra frock and controller would would result in more
power required to move the bike along correct?

KiM

Yes, but it's really surprisingly small compared to the other losses.

What isn't small though, is how awful a front hub bike feels to ride. It's like you have to just crash it into obstacles and things rather than hopping up over whatever you want. And then it just spins out if you dump useful power into it...

[Alan B]

How does the weight of a mountain bike fork, wheel and smallish front wheel gearmotor compare to the weight of a motorcycle front fork and wheel?

[AussieJester]

Okedokie....

Yes, being cracked over in a corner then getting
on the throttle with a front frock would be scary
Front end feel would be terrible surely. I can see use for it
In slow muddy offroad conditions for sure
Would scare the piss out of me riding at any speed
On the road though....good luck to those using
Or thinking of building a trip wheel drive bike...

KiM

[Alan B]

The OP of this thread made a 2500 mile trip a few weeks ago with dual hubmotor drive. Didn't seem to be a problem.

[John in CR]

That's approximately how much power will be "wasted" just driving the second motor on the bike, even without it being connected to anything, and even without a power load on the first motor.

If cogging of an unpowered motor is the same as powered it's minor anyway, and reaches a fixed plateau as Justin analyzed some time back. If it was significant that mean that the Xlytes with the splayed stators, which cog far less would be significantly more efficient than other hubmotors, but they aren't? All losses are already part of the existing efficiency curves anyway, and as LI-ghtcycle proved running his 9C as a mid-drive with a NuVinci, hubmotors don't run anywhere near peak efficiency except for a small time. Sure there may be commuters running WOT on flat roads for long continuous distances, but they aren't the norm, and for them the only significant benefit would be that having a 2nd motor leaves them never broken down.

While riding around at a cruising speed some bikes may approach peak efficiency with one motor, as I've said the second motor will make little difference in that case. That's not what people looking at a second motor are looking to improve. They're looking to climb hills better or accelerate harder. There's also guys running their motors in a stressful manner who are more common than you're giving credit, and who are running at peak efficiency at cruise that is poor due to too large a wheel as shown in the following graph, a 2807 9C in a 26" wheel with a battery sagging to 93V drawing 39A and edging just above 40mph on flat roads. Note the efficiency at cruise of 78.7% .

2807 95V 60A 26in running flats at 78eff.JPG

(118.39 KiB) Downloaded 5 times

That's at cruise on a flat road too, ie the absolute best case for that system, and a second motor is going to increase efficiency to 84% or so at that speed. Then every hill, every headwind, and every acceleration will have even greater improvement.

BTW, my 2nd ebike had a front hubbie running at 3kw peak, and it didn't exhibit any traction or handling issues even in beach riding or on sandy trails either. Sure the weight there was an issue on bumps, but having that front motor pulling instead of pushing it through anything soft or around turns on pavement were a definite plus. As long as the axle torque is addressed, my only real issue with front hubs is the potential for plug braking with a controller failure, so I would stay away from motors with strong regen braking, ie high turn count motors. As far as unsprung weight is concerned, the motorcycle fork assembly I'm using for my 2wd front end had a wheel, brake disc and caliper that weigh about as much as my hubmotor and wheel.

Once I get around to building a dirt ebike, the call of 2wd will be hard to ignore, so hopefully a tough, powerful, and durable geared hubbie becomes available in a 10lb or less size. In the meantime, 4-6kw in the front of my low and long streetbike will work fine and definitely run at greater efficiency and performance than now, since the time I run at fixed speed is seconds, not even tens of seconds. The added weight and better tire, along with the pulling front, will definitely give me more confidence in the curves.

[AussieJester]

The OP of this thread made a 2500 mile trip a few weeks ago with dual hubmotor drive. Didn't seem to be a problem.

Good for the OP... he could of done the same 2500 miles with one motor too DogMaN has proven higher powered
front frocks are dangerous when cornering fast they slip and that was on a high traction Kart track the slick
roads or worse wet roads would be dangerous, i like to ride reasonably fast when i go out for blats about the neighbourhood
i like to feel what the front wheel is doing when i am cornering as i am sure many do, i don't want it breaking lose when i'm mid corner and on a nice lean angle powering out of a corner...(Matt.P has some nice video shot from behind me leaning the bike riiiigth over when cornering on a snakey section of the cycle path on last weekend's group ride ) I have confidence in the front end when i can feel what is happening, i would lose that with a heavy ass frock rumbling about on the front end...If they were any better/safer/advantageous for road use majority of road going motorcycles would be using two wheel drive, been tested by major manufactures in the past, how many mass produced non prototype on road 2wheel drive bikes you seen Alan B?
I dont care one iota about efficiency if i need more range i add more lipo, simple... so personally i see no advantage
in two wheel drive the added complexity, weight and cost just doesn't make any sense to me...

KiM

[Alan B]

This thread is not about front hubmotors. It is about 2WD hubmotors which are quite a different thing. Setting up a balanced 2WD power system on a bike produces a different vehicle than a front or rear hubmotor bike.

There have been attempts at two wheel drive motorbikes but the mechanical problems are difficult. With hubmotors the problems are minimal.

A single rear hubmotor on a heavy cargo bike towing a trailer 2500 miles would not have worked as well for KingFish as the dual setup he used which allowed him to achieve much higher average speeds. He has done a big trip with one hubmotor and with two and you can read about it in his several threads on the subject. He has more miles under more conditions with a dual hubmotor than most folks will get to, so it may be worthwhile to read about his experiences. He is also commuting on wet Washington pavement so he can speak to spinout problems if he has them.

For me a front hubmotor is interesting because I have had overheating and traction problems at 10 to 15 percent grades. California has a lot of steep stuff. Some of the roads I ride my ATV on make the hair stand up on the back of your neck. I didn't appreciate a little hill in Marin causing my ebike to lose traction, fall down and nearly melt my motor - I want more capability but maintain the quiet and stealth of hubmotors. With a strong rear hubmotor plus a smaller front geared freewheeled hubmotor I may be able to achieve that in a simple ebike. When I don't need the front hub I can easily remove it and return to my standard front wheel. Or not, as the small front hubmotor is not that heavy or visible. I agree with you that I don't want the front motor to be high powered, and I don't need it to operate at high speed. So I will configure it to work below say 15 mph and produce about 2/3 the power of the rear motor. Above that speed it will freewheel and not consume much power.

[AussieJester]

Different strokes for different folks i guess...I ride for pleasure not so much commuting
great distances 25km max and im done, but i like to do it quickly and have fun doing it
not limp along ...anyhoot, leave you too your dual frock setups best of luck with them ...

KiM

[amberwolf]

If cogging of an unpowered motor is the same as powered it's minor anyway, and reaches a fixed plateau as Justin analyzed some time back. If it was significant that mean that the Xlytes with the splayed stators, which cog far less would be significantly more efficient than other hubmotors, but they aren't?

Probably true; I was just suggesting the experiment for anyone to see for themselves, if they liked. (assuming they have two bikes and two wattmeters available).

[John in CR]

If cogging of an unpowered motor is the same as powered it's minor anyway, and reaches a fixed plateau as Justin analyzed some time back. If it was significant that mean that the Xlytes with the splayed stators, which cog far less would be significantly more efficient than other hubmotors, but they aren't?

Probably true; I was just suggesting the experiment for anyone to see for themselves, if they liked. (assuming they have two bikes and two wattmeters available).

My motors cog pretty good, but it's really just a matter of getting past the initial low speed stuff. If Luke thinks about it, he'll realize how small it is, because he towed me for a mile or so when I ran out of juice on the road. The tow was accomplished holding onto his shoulder, and that was also overcoming a bit of head wind plus rolling resistance of both tires plus wind resistance at towing speed of 20mph+. Whatever excess was cogging resistance of the motor, which couldn't have been much power being transferred through that "tow connection", especially since my out of cycling shape legs did the work for a half mile or so before Luke realized I was missing. He was quite distracted by the bee that had flown up his nose and stung him while in there, so I was happy he realized I was missing at all.

[el_walto]

I'd just like to say that my 2WD got me to work using the same AH as my 1WD in a shorter time. This is because my 1WD would cog like crazy and get really hot as i live on the bottom of a giant hill.

I used one throttle with two controllers for a couple weeks without any noticeable problems. Maybe this worked because I had 2 batteries, one going to each controller.

[The Mighty Volt]

Does 2WD have application? Yes.

Where?

On extremely heavy broken ground, like rocks or scree, where the bike comes to rest with one of the wheels free of the ground and with no traction.

In that circumstance, power is applied to the grounded wheel to try and free the bike.

Apart from that, I have to go with the conventional wisdom which holds that a single, reliable, rear-wheel drive with good thermal properties remains the best way to go.

[Kingfish]

A little History & Perspective:
Single-wheel drive eBikes are cool; not long ago I started out with a FWD eBike – although that is a half-truth:

The kit for the second eBike (the one I call P0) that I constructed came from eBikes.ca and it was ordered-up as a 2WD: One throttle, two CAs, two controllers (I think they were the 25A 6xIRLB4030 and highly modified), and two (F/R) 9C 2806 hubs. The assembly was arrested cos the curious part of my spirit took over and I disassembled the rear 9C hub to find out how it worked and that’s how my first eBike became a FWD, although secretly I still pined away for a 2WD! The frame of the eBike I ride today (P1) was purchased at the same knowing it was going to become a 2WD – that was back in 2009. I took the FWD eBike (P0) to California on the first road trip as 10S18P LiPo; it had all the batteries over the rear tire which proved to be a mistake as it lead to five broken spokes; the bike and I weighed over 350 lb.s. The value of weight distribution was not lost upon me, nor was the overheating motor.

Advocacy:
A single-wheel (or axle) drive makes good sense for many reasons beyond economy, simplicity, and pleasure: It’s what most of us are used to seeing being driven. Our cars are essentially rear- or front-wheel driven (ignoring the differential effects for the moment) single-axle drive. We understand the principle, it’s easy to manage, fun in the rough, in the turns, and on race day! I have owned many single-driven vehicles: Cars, Motorcycles both On- & Off-Road, a Sand Rail, and even a Honda Odyssey modified for full race (bummer that it had no reverse). I am an advocate of this configuration!

Alternatively, I also enjoy pushing the envelope and wish to explore 2WD bikes and AWD. I own a 4x4 truck which is a godsend in the PNW, although today it’s hoisted on blocks cos in 2009 I overtly made the decision to “go green”. To be honest, I’ve customized my 4x4 to the point that the next mods would have taken it into racing… That’s when the eBike bug bit.

The PNW is a steeply hilly place; the only level stretch of ground it seems is crossing the I-90 Floating Bridge over Lake Washington. We commonly experience blustery winds, two hundred words to describe rain, three words to describe sunshine, sometimes no summer at all (that’s three consecutive days of sunshine between July or August), and much of the time cloudy dark winter. It’s cold here, though due to the maritime clime and two large lakes it rarely drops below 23°F/-5°C for extended periods. That condition however spells for a lot of Black Ice.

I was 18 years old the last time I owned a street motorcycle (a fun story all by itself). Lacking recent experience, I asked my motorcycle-driving peer last month how he deals with crappy Seattle weather: He said “I take the car to the P&R and ride the Bus.” That just blew me away! Why not create a 2WD motorbike with traction tires?

And now you know a bit about my next project. I like what 2WD offers, plain and simple

Three pages of comments and weighing in:
I find it amusing that some of these posts on this thread go to lengths doing out 2WD: Say what you want, but unless these individuals own one and experienced scores and hundreds of miles over those wheels… how can they expect their opinions could have any value? I would appreciate if these young lads with their copious eBike knowledge would go drive one and then weigh in with meaningful experience. Be real: A couple of miles screwing around down a dirt track do not add up as “experienced”. If the intent is to derail or debase 2WD et al then I suggest they start another thread focused on just that subject because this one is not arguing which is better: I am not interested in that debate because it’s already predefined that we’re here to share, discuss, and improve our 2WD experiences.

Once again: The purpose of this FAQ is to demystify the concepts, point to relevant topics, and provide concise answers. I do not claim or imply that 2WD is the panacea for the masses. I do not claim or imply that “2WD is more energy-efficient than single-wheel“ and vice versa, although I do believe this is a crude statement that requires qualification. Can we talk instead about practical applications and solutions?

Finally, it is with good fortune that I have survived my recent 2WD trek. I have much to say about it objectively, and I am here to contribute. Let’s work together and share the knowledge!
Thanks, KF

[Rassy]

Well stated KF. Back in 2007 when I joined the forum I had already put together my second multi-drive unit, a delta trike with a front hub and a two hub pusher trailer. I don't remember the details, but there were some quick comments concerning it could always be done better with just one "proper" motor. Then there were numerous incidents where controllers were destroyed and spokes were breaking, etc. due to too much concentrated power and weight. Of course most everything was sla then and "BMS" wasn't part of our everyday vocabulary.

Anyway, more to the point, I think we should all encourage everyone to try whatever they want without being too critical. And when we detail our own experiences it should be in a sharing spirit and not presented as the only way to do it just because it's working good.

If I ever decide to go on a multi-day trip with my mid-drive trike, I plan to pull my BOB trailer with a Bafang in the 16" wheel with it's own battery and separate throttle. This will give me lots of cargo space, extra power for really steep hills, and a redundent system if something breaks on the mid-drive system.

So even though building a 2wd bicycle is no longer a goal, everything else about multi-motor builds still interests me.

Looking forward to your two wheel drive motorcycle build.