Converting a FAT-BIKE to electric for off-road use

Hello,
Just bought a great rigid fat-bike with 4inch+ tires. This beast feels so cool on any offroad!
I then had a thought - the fat tire makes the entire bike mass sort-of-suspended mass, which means a hub motor would be suspended too!
You can actually do a bike-packing with such machine, and stay away from boring and dangerous roads for the entire long distance trip.
Sourcing a rear-hub motor for 190mm axle is a problem, but my front axle is exactly 135mm! Not only this, the 135mm fork is a steel rigid fork, which makes it safe for a hub-motor. (with torque arm)
I know a front motor has traction problem due to lack of weight, but the motor itself add weight plus I plan to have the battery (4-6Kg) mounted on the handle-bar to increase it even further. (and so the battery can be removed easily)
I was wondering how can I calculate what would be my traction force (or motor torque) limit vs the weight I load on the front wheel. The front tyre is 26inch X 4.7inch wide and it has knobby profile. (not semi-slick)
I am aiming for a DD because anyway I would be limited by my maximum torque, plus at downhills the regen can be a great bonus. (and then the motor will have excellent traction!)
Since I don't need to leave space for a cassette, I thought initially to go for the TC4080 with statorade. It would be heavy, and most efficient DD on slow off road speeds thanks to it's low resistance and large magnets.
Then I thought I wouldn't be able to get a traction for this massive torque machine, leaving it's torque abilities unused, so I was wondering if perhaps the rear version of 9C+ (2707RD) would be sufficient?
What do you think?
Is it a good idea, or will I have no climbing traction at all on those dirt/gravel uphills?

And another unrelated question - Did anyone had success with cooling ezee motor? (the newer ezee250rc)
Venting holes will create chaos once the dust from outside mixes with the grease inside, and ATF fluid will increase the cogging torque and will eventually leak outside.

A front hub motor would be possibly the worst configuration for single track trails especially those with any sort of climbs. If you limited yourself to dirt roads it would probably work OK. Having the unsuspended mass of a motor in the steering wheel is not ideal at all for rapid changes of direction, traction would also be limited by the amount of weight you could keep on the front tire while still steering the bike around switchbacks. You also lose the ability to wheelie or even to lighten to front wheel for crossing rocks, logs or other trail obstacles.

thunderstorm80 said:

I am aiming for a DD because anyway I would be limited by my maximum torque

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I'm confused, are you saying you want a DD for the torque? Of all drive systems, a direct drive will give you the least torque

WoodlandHills said:

A front hub motor would be possibly the worst configuration for single track trails especially those with any sort of climbs. If you limited yourself to dirt roads it would probably work OK. Having the I suspended mass of a motor in the steering wheel is not ideal at all for rapid changes of direction, traction would also be limited by the amount of weight you could keep on the front tire while still steering the bike around switchbacks. You also lose the ability to wheelie or even to lighten to front wheel for crossing rocks, logs or other trail obstacles.

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The purpose is not to go on single-trails of course. Without the ability to "wheelie" to bypass rocks and with the insane uphill grades, it would make single-trails a nightmare.
The idea is to use it as a "cycle-cross" bike-packing, or on all of those offroads where a regular family-car can drive but slowly slowly.
Do you think it can fit?

t_tberg said:

thunderstorm80 said:

I am aiming for a DD because anyway I would be limited by my maximum torque

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I'm confused, are you saying you want a DD for the torque? Of all drive systems, a direct drive will give you the least torque

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I want a DD drive since it's more mechanically reliable than geared, considering the application, and because it allows to regen.
The lower torque ability of a DD compared to geared is not a problem, since anyway a front wheel can't provide it.

Wow you're doing it comically wrong.

First, you don't want a FAT bike, you want a full suspension mountain bike. The fat bike actually sucks ass for this type of riding and has horrible handling.

Second, you DON'T WANT A FRONT WHEEL DRIVE. What the hell man, are you purposely going out of your way to make the bike suck?

Third, PUT A MID DRIVE ON IT not a hub motor. The bike will handle way better and if it's a BBS you can use all your ratios.

At the least, use a rear wheel geared motor. I have never heard of too may issues with those. I heard they use nylon gears, but you could replace them with metal ones I would think. I don't know much about geared ones myself, but I am going to try one out soon as an experiment. I have Direct Drive motors on mine. If you want torque, get a mid drive that is the best way.

The reliability issue you bring up is a myth.

flat tire said:

Wow you're doing it comically wrong.

First, you don't want a FAT bike, you want a full suspension mountain bike. The fat bike actually sucks ass for this type of riding and has horrible handling.

Second, you DON'T WANT A FRONT WHEEL DRIVE. What the hell man, are you purposely going out of your way to make the bike suck?

Third, PUT A MID DRIVE ON IT not a hub motor. The bike will handle way better and if it's a BBS you can use all your ratios.

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I plan on going on gravel roads only or off roads where every family car (not 4x4) can go.
With full-suspension you can't carry too much. Seat-post racks are limited to 10Kg or so, and regular metal to frame racks are impossible. Even if you use Thule's frame racks they are wobbling, and they are unsprung mass. The fat-tire makes your entire bike sprung mass.
But I get your point and the others - It is a bad idea...

flat tire said:

[...]you DON'T WANT A FRONT WHEEL DRIVE.

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It's a normal bicycle at the moment, which means that putting a front wheel drive on it makes it ALL WHEEL DRIVE. That is, assuming the rider isn't just a useless sack of fail.

Yes, the big tires do give some cushion but you have huge tradeoffs in tuning your spring rate (pressure) and little to no ability to tune your damping. Plus, the ~2" travel leaves a lot to be desired. Good MTB tires + front and rear spring and damper will be a world away in terms of handling rough terrain.

Honestly, you can use a hub motor off road, and I do it all the time as my bike is multi-purpose and focused more on speed than handling. BUT, a mid drive is a lot better if you have the option...and since you're planning this build from the ground up, might as well get the best parts for the job.

Full susp can still carry a lot, but you will need to be creative with things like attaching your rear rack to the seat post. You can also use a front basket / frame mounted rack.

Chalo said:

It's a normal bicycle at the moment, which means

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Technically true but practically useless info that's completely beside the point. Keep it up!

One you venture more than a short walk from your car, full suspension bikes cause more problems than they solve. They're toys, with lots of nonstandard and consumable parts to let you down when you actually need them. Fat low pressure tires are not a perfect substitute, but they don't have the many foibles of mechanical suspension.

I know I would not go adventuring in the backcountry with a suspension bike, or an e-bike to be honest.

Chalo said:

One you venture more than a short walk from your car, full suspension bikes cause more problems than they solve. They're toys, with lots of nonstandard and consumable parts to let you down when you actually need them. Fat low pressure tires are not a perfect substitute, but they don't have the many foibles of mechanical suspension.

I know I would not go adventuring in the backcountry with a suspension bike, or an e-bike to be honest.

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Higher quality FF bikes are very reliable. I own one, and it's not cheap.
But for bike-packing with the requirement for luggage space than yes, rigid frame would be better.
That's why I thought of a Fat-Bike for that reason as I am very interested about bike-packing.

flat tire said:

Yes, the big tires do give some cushion but you have huge tradeoffs in tuning your spring rate (pressure) and little to no ability to tune your damping. Plus, the ~2" travel leaves a lot to be desired. Good MTB tires + front and rear spring and damper will be a world away in terms of handling rough terrain.

Honestly, you can use a hub motor off road, and I do it all the time as my bike is multi-purpose and focused more on speed than handling. BUT, a mid drive is a lot better if you have the option...and since you're planning this build from the ground up, might as well get the best parts for the job.

Full susp can still carry a lot, but you will need to be creative with things like attaching your rear rack to the seat post. You can also use a front basket / frame mounted rack.

Chalo said:

It's a normal bicycle at the moment, which means

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Technically true but practically useless info that's completely beside the point. Keep it up!

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Again, I do not plan on going on too rough-terrain. The roughest off-road for such bike-packing trip + luggage + front motor would be those light offroads that any family car can penetrate by going slowly. It would be pointless and dangerous to go on single-trails with switchbacks, rocks, etc...
That's why I think it's a Fat-Bike territory for the job, and I am into DD hub-motor since it can regen: When you descent those long stretches of offroads, going slowly with all the weight you carry - regen is a great bonus not only to save your brakes, but also to extend your range. Mid-Drive would also mean I have to sacrifice my triple crank for a single speed chainring. (And I want to do most of the pedalling work. The motor is to assist, and to brake/regen)

OK, that all seems reasonable. To address your first post:

Don't worry about torque calculations, with FWD any halfway decent motor and controller will be able to spin your wheel from a dead stop. At the 1 kw powerlevel it's not a big deal at all and won't intimidate even a total beginner. I would personally go with RWD, but for your use I'll agree front motor can work. Actually, your question is almost irrelevant anyway because even for a consistent tire and power output the grip level available changes massively depending on type of surface, how it's groomed, how fast you're going and how wet it is among other things. Oh, and body position. That will make by far the biggest difference in grip bar none.

--How long are your uphill sections, and how steep? My opinion is that if you're thinking about having to cool your motor just to handle these sections you've got the wrong motor / gear ratio. If the hills aren't very long, or very steep, you can probably get away with a simple direct drive hub motor as long as it's wound for torque not speed (low kv).

I have 2- 9c 2810 motors. Low speed winding. It would be good if you could find one of the 2812 windings. even slower. Dogman had one I think.

flat tire said:

OK, that all seems reasonable. To address your first post:

Don't worry about torque calculations, with FWD any halfway decent motor and controller will be able to spin your wheel from a dead stop. At the 1 kw powerlevel it's not a big deal at all and won't intimidate even a total beginner. I would personally go with RWD, but for your use I'll agree front motor can work. Actually, your question is almost irrelevant anyway because even for a consistent tire and power output the grip level available changes massively depending on type of surface, how it's groomed, how fast you're going and how wet it is among other things. Oh, and body position. That will make by far the biggest difference in grip bar none.

--How long are your uphill sections, and how steep? My opinion is that if you're thinking about having to cool your motor just to handle these sections you've got the wrong motor / gear ratio. If the hills aren't very long, or very steep, you can probably get away with a simple direct drive hub motor as long as it's wound for torque not speed (low kv).

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What do you think about the 9C+ series Grin sells? They are very cheap and reliable. I have the front 9C+ 2706.
I thought to buy the rear version of this family, with the slowest winding - 2707. It has a 135mm axle, so it will fit perfectly on the front steel fork.
The cost is only 185$.... (before shipping)
Of course I will not expect the motor to take me off on gravel uphills or to provide powerful torque. The main power will still be my legs, with the motor assisting.
That's why I think such a medium direct drive motor is enough. It's 6.2Kg which makes the wheel heavy, but increase the traction as well.

How exactly does a heavy wheel increase traction? I believe it's the other way round.

miuan said:

How exactly does a heavy wheel increase traction? I believe it's the other way round.

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Traction is a function of the friction coefficient times the normal force the wheel experience. The heavier the weight, the greater the static friction force, and so you have more traction. (You can exert more torque on that wheel before it reaches spin-out)
The friction coefficient varies greatly with the tire profile and the road/off road profile.
That's why leaning forward when climbing single-trails on MTB, greatly increase your frontal traction.

I had a similar requirement to yourself. A Panther Hercules One Fatbike, with front suspension and alloy frame, with 190mm rear dropouts to use riding the mainly flat gravel and hard pack trails, plus many kms of poorly surfaced rural roads around my home in NE Thailand.
I didn't need the climbing ability of a mid drive, but rather the reliability of a DD rear hub motor.
I struck gold, when I got in contact with Jack Chen, of ChangZhou SuRing Motor Technology Company Ltd via Aliexpress. I explained my requirements which included needing a 190mm axle, A wheel rim matching the style and colour of my OEM Thai bike wheels, motor laced to rim with 12g black stainless spokes, 45amp sinewave controller etc, and after clarifying everything by email Jack offered me a great deal on a Mxus 3000w V2, custom 190mm axle, and sourced a matching rim to build up for me.
Service was fantastic.. KT Kunteng controller, LCD6 display, KT-D12L PAS sensor, torque arm, twist throttle, various spares.. Everything was delivered exactly as I had ordered and in perfect condition.
I cannot recommend this vendor and the Mxus /KT package highly enough. I built my own battery pack 13s 1p 16000mah LiPo pouch cells, purchased from another excellent supplier on Alibaba. The cells are working incredibly well, and offer great performance and range for my riding usage.
Please PM me if you need any further information on my suppliers, etc
Good luck with your build.. I am delighted with my E Fatbike, hope you will be too..!

Here is my Lipo battery packs, configured to fit closely into my small frame Triangle, and custom battery box. Charging via a Radiolink CB86+ Balance Charger, with 12v 400w DC power supply. Good charger and many useful features for safely charging pouch cells.
Also, now have a Rear Rack fitted, and a passenger seat to carry my 8 year old daughter from time to time..

My Mxus 3000w V2 was ordered with a 4T wind and rated at 8.9kv
It tops out at about 55kph, with my 13s pack charged to around 50v, which is plenty fast enough for me on a Fattie ..!
I normally charge to 3.9v per cell, and discharge to 3.75v. I pedal on PAS 1 most of the time, which gives a comfortable 25kph on the trails, with around 125w battery consumption on the level, so I have plenty of range for my normal leisure rides out from home.

If you're on 2wd dirt roads, you don't have to worry about traction since they won't ever be that steep. Floatation maybe if they're sandy, but a fat bike seems like overkill to me, especially if you want to extend your range by pedaling it. A mid fat bike would be better, or the preferred bike packing bike, a 29+. That would give you 2.8" - 3" tires for cush and a simple rigid bike to deal with.

Google bikepacking images and you'll see what I mean.

E-geezer said:

If you're on 2wd dirt roads, you don't have to worry about traction since they won't ever be that steep. Floatation maybe if they're sandy, but a fat bike seems like overkill to me, especially if you want to extend your range by pedaling it. A mid fat bike would be better, or the preferred bike packing bike, a 29+. That would give you 2.8" - 3" tires for cush and a simple rigid bike to deal with.

Google bikepacking images and you'll see what I mean.

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Plus bikes are very expensive to begin with, while the Fat bike I already have was quite cheap and still carried good drivetrain parts.
I plan to have a rear rack (maybe the Tubus-FAT rack) and load it with panniers so the wider FAT tires are the only option to cushion that and create a sort of sprung mass, which is highly important when you are leaving the smooth asphalt roads. With full-suspension bikes you can't carry too much, unless you are willing to sacrifice your amount of food and shelter big time.
I used them already as pedal only bike, and the rolling resistance everybody warned me about isn't a big deal at all!
Because of the really big tires (26 X 4.7inch), you can go to low pressures as low as 5PSI (and even less), but because the contact patch is so huge - the deflection is small, hence the rolling resistance.
If you are at this pressure on asphalt roads then yes, it would be terrible of course, but then you just inflate them to their max pressure (20PSI), and they roll just like any standard MTB knobby tire.

I think you might be getting more "expert ?" advice than you need. Your plan sounds fine to me and I have logged thousands of miles both on and off road with front DD hub. With the weight of the motor up front you can carry more on the back. You most likely won't be going very fast and once you get the feel for it the handling won't be much of a problem.
On a side note, I recently got the chance to ride a new Rad rover fat bike. It had front suspension and a suspension seat post. I thought the handling was a little strange on pavement but as soon as I got it in the dirt it felt very stable and the ride was Cadalac smooth over stuff that would have rattled your teeth out on other bikes.