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

[thunderstorm80]

Hey everyone,
Today I got the TC4080 crown motor, and I mounted it on the fork and also tried on the rear dropout.
First, I mounted it on the fork, as my original plan was. Since the fork of a fat-bike is 135mm OLD + it's made from rigid steel with 9mm dropouts (Not QR!) - it was perfect for oversized rear motor as the crown:

In this pic you can see how the motor's 140mm axle width is a bit wider than the 135mm OLD.

Widening the fork a bit (with very little effort, and the motor is in!


I can assume such 5mm stretching is nothing for a steel fork, right?
However, I don't have much clearance with extending the rotor (with spacers) towards the caliper, due to the motor-cable-exit interfering.

Then I tried to see if it can fit within the 190mm of the rear dropout:


Note how barely it extends beyond the dropouts. If it was just 2cm more, it would have made a match! (I would have preferred rear motor for offroad, but there is no high power DD for rear fat-bike axles, yet)
I was thinking - if there is any mechanical way, to secure it to the frame within those limits? (is there a way to extend the axle?)
The 2 torque arms can go inside as there is plenty of space. However, the rotor fitting would be a great challenge:

Note how -FAR- the caliper from the rotor's threads. Spacers cannot be used more than few mm's due to the motor's cable preventing it.
However, it was nicely "dished" when spacing it evenly between the dropouts:


This is where I thought - I saw I can actually press the two frame sides so I can reduce the OLD by few centimeters. But, I fear that for aluminium frame, such bending reduces it's strength, and can cause a failure while riding. Is it true, or is it ok to do so ?

I will finish by reminding that even for a front-motor solution, I do not plan on pulling thousands of watts through it! The traction would be limited, but such big DD motor would be excellent assistant at slow climbs (together with the rear leg-power - 2x2), and with the Phaserunner I can very easily limit my max torque on the fly. (together with CA v3.1)

 

[torker]

That will be awesome . 2wd 8) As long as you keep the power reasonable it will do fine. Come up with robust torque arms.

 

[thunderstorm80]

That will be awesome . 2wd 8) As long as you keep the power reasonable it will do fine. Come up with robust torque arms.

It won't be a matter of power but a matter of the maximum allowed torque, which will be dialed differently with the Aux-Potentiometer+CA depends on the terrain.
The reason I chose the TC4080 is because it will work at very low offroad speeds, some in the order of 10-15km/h going steep uphills. Any other DD motor would be very inefficient at those speeds and will overheat in minutes, while this one has a very low resistance combined with good motor-constant. (The price is it's weight...)
I thought at start to pick a geared motor, but even a geared motor will heat up very quickly due to it's small mass and the looong+slow speeds, and I will not have regen.
Regen is VERY IMPORTANT on this scenario, and it's almost silly not to take advantage of regen with a front motor. I can in fact draw way more torque during regen, because of the traction. In the forward motion I will create most of the torque alone by pedalling, so anyway I won't need much front forward-torque. Just a small assistance.

 

[torker]

I love my 2810 = slow winding motors. Just make sure you fab some torque arms. Especially on the front. Regen will make it even more important, working on that axle both ways. Ready to see this machine in the woods. 8)

 

[nutnspecial]

I'm about to try this too. 2wd.
Oops :lol: , except I just ordered the steel walmart (mongoose) fatbike and a standard ebay 1kw rear kit. Sounds like they might not play together, but you've given me some hope thunderstorm (if I can maybe squeeze d/o's a little). Actually, being steel I could probably just mod dropouts too.

I already have a 1kw front kit, so hopefully that fits. Hopefully the wind of these ebay 9c clones is acceptable for low end? Having 2 motors running 12s should help.


I'm wondering how you're going about your wheel build? I planned to use bare motors too, and build them into the stock rims/spokes. Not sure of fat rim specific measurements for spoke calcs.

 

[thunderstorm80]

I'm about to try this too. 2wd.
Oops :lol: , except I just ordered the steel walmart (mongoose) fatbike and a standard ebay 1kw rear kit. Sounds like they might not play together, but you've given me some hope thunderstorm (if I can maybe squeeze d/o's a little). Actually, being steel I could probably just mod dropouts too.

I already have a 1kw front kit, so hopefully that fits. Hopefully the wind of these ebay 9c clones is acceptable for low end? Having 2 motors running 12s should help.


I'm wondering how you're going about your wheel build? I planned to use bare motors too, and build them into the stock rims/spokes. Not sure of fat rim specific measurements for spoke calcs.

First, you can do as I did - take the rear motor and put it up front. The front axle on a fat bike is 135mm, and the rear varies between 170mm to 190mm. If mine was 170mm I could have fit the TC4080 over there as is, but it is 190mm.
I bought Grin's only option of 26 fat rim, which comes at 36h since my stock wheels are 32h. Yours are 36h ?
It's good to keep the original wheels intact and build a new motorized wheel. It's easy to sell a full wheel, and it is good if you want to use them also as regular bikes.
I like to buy all the trio from grin (Motor hub, spokes, and rim), since I can use their spoke simulator and make sure everything fits.

I thought at start to pick a 9C motor, and although I need low power assistande in the order of 200-700W and the 9C is rated to 500-1000W it won't be a good match since I work at very low speeds. The motor's maximum power output (without overheating)is a function of your planned speed. Twice the speed, twice the maximum power output. It's the maximum sustainable torque which matters, and for this application, the 9C is a medium DD motor with low continuous torque capability.
I calculated the 9C performance on the simulator and it will overheat very quickly at those low speeds, not to mention taking twice the battery power to do it's work.
Add the fact that Fat-tires have outer diameter bigger than typical 29er MTB, and your motor torque translates to even lower horizontal acceleration.
This is why I took the biggest crown.

 

[nutnspecial]

faaahhhhhhhh!

outer diameter bigger than typical 29er MTB

I didn't know that.
Assumed '26'. -So it'll spin up to more like 28mph on 12s. Oh bother.
I do realize faster winds to be less efficient (and torquey) @ lower speeds but it should be alright for me or hopefully at least worth a try.

I plan to use their simulator as well and would buy from them first if I need anything.
I'll have to wait for the bike to come to confirm but I'm deducing 36h and ~135mm front d/o. My front motor has ~180mm axle so that should work I guess. Will have to hope I don't have to get nuts with that or especially the rear d/o's lol. Also don't want to have to redrill the frig rims.

For cost? I'd rather utilize the stock rims, spokes, tires etc -IF possible, (and not too much pita). Just gotta pretend I bought a custom ebike that didn't come with spare stock wheels (lol?), but I hear ya it's nice to have that stock set. Hopeful total cost w/battery (16ah lipo) <750$


Thanks for the input- Please share anything wheel building specific (from calc to lacing) when you get into it.
I'll post some build info up to my thread when I get there.

 

[thunderstorm80]

faaahhhhhhhh!

outer diameter bigger than typical 29er MTB

I didn't know that.
Assumed '26'. -So it'll spin up to more like 28mph on 12s. Oh bother.
I do realize faster winds to be less efficient (and torquey) @ lower speeds but it should be alright for me or hopefully at least worth a try.

Assuming the same controller and since the leading external wires in every motor family have the same cross-section (not the windings themselves), faster winds will always be less efficient than slow winds, since they require more phase current to achieve the same torque output. You will have bigger voltage drop&power loss, and your controller, which can output a certain max phase current regardless of which motor you use: Will mean less max torque for the fast wind motor. This is why they call them "fast" or "torquey". Internally, the windings of the fast wind are proportionally thicker, so they all have the same electro-mechanical power conversion ability, but you need all your external system to be "proportional" as well (controller, external phase wires), which usually isn't being taken into account or impossible. (replacing the leading phase wires into the motor with thicker ones can be impossible or seriously hard)
The only reason I will ever buy a fast-wind motor is if my planned max speed can't be reached with the slow-wind motor, and even then - I will first do all what I can to increase the battery voltage. (and making sure the controller can accept it - The Phaserunner works with voltage up to 90V). For 20inch wheels, it's usually impossible without a fast-wind, but over there the diameter of the wheel is also small, making even the fast-wind lower max torque to still be making enough linear acceleration. For those small wheels, this is where the fast-wind shines. For fat-bike tire, which is even bigger than 29er - take only the slowest wind motor you can find.

I plan to use their simulator as well and would buy from them first if I need anything.
I'll have to wait for the bike to come to confirm but I'm deducing 36h and ~135mm front d/o. My front motor has ~180mm axle so that should work I guess. Will have to hope I don't have to get nuts with that or especially the rear d/o's lol. Also don't want to have to redrill the frig rims.

You are deducing 36h? Just read the bike's specs!
It's almost 100% they come with 32h. Just buy a 36h rim so you don't need to drill anything.
180mm front drop out length? It's very unlikely and you probably talked about your fat bike's rear axle length. This will be incompatible with standard rear motor with 135mm axle.
If your motor has 180mm axle length, than just put it on the rear of your fat-bike.

For cost? I'd rather utilize the stock rims, spokes, tires etc -IF possible, (and not too much pita). Just gotta pretend I bought a custom ebike that didn't come with spare stock wheels (lol?), but I hear ya it's nice to have that stock set. Hopeful total cost w/battery (16ah lipo) <750$

You plan on going offroads with that bike? Make sure you protect your LiPo (or gun powder battery as I call it) very well!

 

[thunderstorm80]

Hey,
So I have laced the TC4080 into the 80mm rim (finally received it!) and into the 26x4.7inch Bontrager tire. Here how it looks:


And on the front steel fork:


I attached the torque arm to the disc brake bosses:


I think one torque arm would be enough for a steel fork, especially that on this motor the axle is a tiny bit thicker so the torque arm doesn't have any rotating tolerance at all.

The front spacing for a disc rotor on the 135mm axle of a fatbike, almost fitted the rotor placement of the TC4080. I added V-brake spacers to push the caliper a bit inward and the problem solved. It's now just 2mm from the hub, but that's fine:


I thought at start to take the rotor a bit outward using rotor spacer, but then it touches the wire as it exits not via the axle.

I then did autotune on the Phaserunner, and used the unused brake-signal input it has to map a second left throttle - dedicated for variable regen:
Right throttle to accelerate, Left to brake (full twist=full brake torque). Quite comfortable for the purpose of this fat bike which is off road touring. Holding a steady press on a brake lever+thorttle twist for regen seems less natural here.

You ask about the 9Kg weight on the front wheel? The 4.7inch tire couldn't care less, and in fact - Fat-Bike IS THE WAY TO GO if you want your entire bike mass (including hub motors) to be sprung mass as well. (without damper of course - tire suspension only).
The heavy front motor supplies plenty of traction, and balance really well with the luggage of the rear rack.
Front DD motor is the only way for Fat-bikes (for now) to keep my original (and way better than screw on freewheel) cassette sprockets. With rear motor I would have to revert to a primitive screw on freewheel with limited range.

What do you think?