Motor Advice needed for Fat Cargobike

FatCargo

1 µW
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Sep 27, 2021
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Hello everyone,

as we are in need of a cargo bike at the moment, but because I find the current prices for those a bit steep, I have bought a rather interesting frame set.

It is a fatbike cargo bike with stub axle steering (is that the correct word?), see pictures. Originally built so that a 26" fatbike wheel should be fitted in the back.
Since the bike will mostly be ridden on asphalt or gravel roads, I will probably build a 29er wheel, which should have a similar circumference.

Requirements:
The frame weighs just under 20kg as it is now, plus the various parts and motor, battery, controller etc.
Rider weight not more than 80kg.
The load would be a 40kg dog most of the time.

So, roughly speaking, one would have to move a total weight of 160kg with the bike.

Mainly in the city, to work and back every day, with some gradients because two overpasses have to be crossed.

Originally, a Bafang BBSHD was planned as the motor, but unfortunately it doesn't fit under the bottom bracket.

A front wheel motor is not possible because of the design, so imho the only option is a hub motor on the rear wheel. The dropouts have an inner width of 175mm.

Originally I was thinking of a direct drive motor, but now I am not averse to a geared motor.

Continuous power should be at least 500w, not to use it all the time, but to have adequate power and thermal reserves.

VMax certainly not higher than 25 km/h with such a vehicle.

Basic knowledge of electronics and electrics as well as a fully equipped workshop available, time currently rather sparse, but I'm not in a hurry.


Which hub motor would be suitable for such a bike?

(Please excuse my english)

Best regards
Gregor
 

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FatCargo said:
Since the bike will mostly be ridden on asphalt or gravel roads, I will probably build a 29er wheel, which should have a similar circumference.
[...]
Originally I was thinking of a direct drive motor, but now I am not averse to a geared motor.

Continuous power should be at least 500w, not to use it all the time, but to have adequate power and thermal reserves.

VMax certainly not higher than 25 km/h with such a vehicle.

This seems like a good application for the Leaf 1000W hub motor, because they will give you a custom winding that works for the speed range and wheel size you're using. They use thin stator laminations, so the unpowered drag is low for a direct drive motor.

This is an unusual combination of a large drive wheel and low operating speed, so pretty much any hub motor that doesn't feature custom winding will run too fast, without optimum torque for whatever amount of power you use.
 
Spot on Chalo, as usual. The weight and slow speed, combined with a normal rpm geared motor would be ok for flat ground, but would tend to run hot. And definitely overheat if he went up a long enough hill.

He needs a good strong DD. And to limit speeds, a lower rpm winding. NOT for more torque, but to have it run full throttle without being too fast for its handling characteristics.

I'd be tempted to modify the frame, to accept a 135 mm motor wheel, with 20 inch rim. The smaller rear wheel DOES give you more torque.
 
Spot on Chalo, as usual. The weight and slow speed, combined with a normal rpm geared motor would be ok for flat ground, but would tend to run hot. And definitely overheat if he went up a long enough hill.

He needs a good strong DD. And to limit speeds, a lower rpm winding. NOT for more torque, but to have it run full throttle without being too fast for its handling characteristics.

I'd be tempted to modify the frame, to accept a 135 mm motor wheel, with 20 inch rim. The smaller rear wheel DOES give you more torque. You'd cut the frame, assuming its steel, right behind the seat. Lengthen those stays so the dropout lowers enough to keep the pedals off the ground, and the deck level with 20 inch wheel. And squash it in to 135 mm wide.
 
Thank you very much for your replies.

I also thought about modifying the frame to either fit a smaller real wheel or modify the bb to fit a BBSHD.
Unfortunately, the frame is aluminium, so it would not be impossible, but quite a big deal as I'd basically have to built the whole rear end from scratch. Aluminium doesn't bend too good ;)

Modding the BB would be an option I haven't looked into too much yet, because it would definitely mean welding some reinforcing parts into the frame I am trying to avoid.

The Leaf 1000w hub motor seems to be available only with 135 mm dropout width.

Someone else recommended the Bafang 1000w geared hub motor, which is available for 175 mm dropouts.

Would you strongly advise against it?
 
What is on everyone's mind is; do you have any hills or you mostly on flat ground.
The geared hubs have great Torque but get hotter if ran on longer hills.

by FatCargo » Sep 28 2021 8:17am

Thank you very much for your replies.

I also thought about modifying the frame to either fit a smaller real wheel or modify the bb to fit a BBSHD.
Unfortunately, the frame is aluminium, so it would not be impossible, but quite a big deal as I'd basically have to built the whole rear end from scratch. Aluminium doesn't bend too good ;)

Modding the BB would be an option I haven't looked into too much yet, because it would definitely mean welding some reinforcing parts into the frame I am trying to avoid.

The Leaf 1000w hub motor seems to be available only with 135 mm dropout width.

Someone else recommended the Bafang 1000w geared hub motor, which is available for 175 mm dropouts.

Would you strongly advise against it?
 
FatCargo said:
Someone else recommended the Bafang 1000w geared hub motor, which is available for 175 mm dropouts.

Would you strongly advise against it?

It will run at a speed that's far too fast for your use. You can limit the speed electronically, but it will still be geared too high for its operating conditions.
 
Chalo said:
It will run at a speed that's far too fast for your use. You can limit the speed electronically, but it will still be geared too high for its operating conditions.

Thanks, I think I understand the issue now. I will try to ask leaf if the can make a custom winding on the 175mm hub motor I found their homepage.

If my math is correct my rear wheel wouldn't be spinning much faster than 175rpm @ 25kph / 15 mph .

If that is not possible I'll probably try to mod the bb, because somehow a motor being able to use a transmission with different gears makes more sense to me.

One last resort might be to construct my own mid drive a la Cyclone, with a toothed belt drive down to the crank. But then I probably would never finish that project...
ZeroEm said:
What is on everyone's mind is; do you have any hills or you mostly on flat ground.
The geared hubs have great Torque but get hotter if ran on longer hills.




It would be mostly flat ground, but on a daily commute two rather steep ramps leading up to an overpass. Not long though, probably a couple 100 meters. No longer hills in the area, but it would be good to have at least a bit of climbing power.
 
15 mph would be a great climbing speed, just get on them peddles and help. Should not be a problem. Think about wheel size. When you order the motor windings they will think 26" at 48v. A 29' wheel will not help you they are weaker and gear up the bike.
I know you need to keep your cranks off the ground but a 24" wheel is strong.
I would get the motor to top out kv and volts at 20 mph then limit the speed at 15 mph will get rid of the lag as tq ramps up on hills.
 
Thank you very much to everyone who has participated in the discussion.

I have very likely found a way to get a BBSHD under the bottom bracket after all, so I will try to go that path for now. It makes more sense in my eyes, that way I can use the gears for the motor and.
 
That would work best. But if you can find a lower rpm, large DD motor it will work fine. Odd that you never mentioned hills in reply. Any cheap DD hub motor that fits your frame will work fine on grades up to 5%, with a 400 pound total weight of vehicle, rider and cargo. 48v 1000w will do for mild hills and reasonably moderate weights

Judging by the background of the picture, you should be quite capable of copying the frame you have in steel, allowing it to take a 20" rear DD. Just for the fun of it of course.
 
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