Building a bike for towing heavy loads

[Chris Maluszynski]

Hey folks

After supposedly killing my previous bike (with a 48V battery 17A controller for a BFUN 250W Hub motor), I am now thinking if I should build a bike dedicated for pulling heavy trailers up hills.

I have never built anything from scratch (only upgraded the controller and battery on a stock e-bike) before, so I don't know where to start.

1. What kind of donor frame should I look for?
2. Which type of motor? Hub vs crank for max torque?
3. How much power do I need? (I'm thinking I should be able to pull something like 100kg (200lbs) up a hill without worrying about it.
4. What kind of battery? (I am considering if used lithium cells from cars could be used to build a battery, but maybe these are too bulky?)

Has anyone here built or seen any projects like this?

Thanks

Chris

 

[Nealh]

Which actual hub is used ?
Most Bafang's even the so called weedy ones can take far more punishment then 48v/17a, though the motor winding speed needs to be right for towing. A 201rpm hub or less is ideal with lower stall speed and more low down torque.

I tow a trailer locally (only a couple of km) usually max weight is about 60kg as the weight is a vertical taller load and not so stable.
My hub is Bafang cst 300rpm and uses the same 17a/20a 6 fet KT controller.
I limit the power to PAS1 sometimes PAS 2 for towing with no issues, the batteries are over 6 years old and are Sam 29e dolphins, they are worn and also will cut out using high PAS or throttle. Range is down to less then 14miles as they sag badly, rather then throw them I'm using them locally on flat terrain. PAS 1 use mainly with the odd throttle blip and can get 40 - 45 miles still.

 

[Chris Maluszynski]

Not sure what the motor is exactly. It was on the early Wisper 905 called "Wisper Works". I don't have the bike with me, but will check if it has any details engraved when I get a chance

 

[vfr400]

You can't go by figures. If I wanted a bike for towing, I'd use a code 15 or higher (201 rpm) Bafang BPM motor. A 20 amp controller and 48v battery should give enough torque, or for ultimate reliability, a 12 FET controller set to 20 amps.

For a donor bike, I'd be looking for one of those cheap catalogue bikes with a triangle frame and 7 speed freewheel gears. The local tip or bicycle recycling shops are good places to look, or have a word with the scrap men that come round. They nearly always have bikes on the back of their trucks. I got this one from them for a tenner:

 

[Raboa]

Hi, donor bike, try this police auction site:

Bumblebee Auctions

 

[Wisper Bikes]

Not sure what the motor is exactly. It was on the early Wisper 905 called "Wisper Works". I don't have the bike with me, but will check if it has any details engraved when I get a chance

If it's the original motor it will be a Bafang 250W but would need to check the model. If you are using a 48V (54V max) battery and 17A controller you will be putting a lot of strain on the motor, about double its capability. If you have been using it for some time with a heavy trailer I would suggest that you have either stripped the nylon planetary gears or blown the motor all together, you would need to check out the internals to find out which.

All the best, David

 

[Chris Maluszynski]

Hi David! Thanks for that! I was suspecting that is what may have happened. How would I get at it to check? And if this is indeed what happned, do you think the symptoms described would apply? (First power would cut out at anything above PAS 3, and now it does not run at all)?

I really like the bike, frame, riding position, big seat etc of this bike. I only miss the disc brakes and front suspension. It's my first e-bike, and I've had it for a decade or so, so also sentimental value.

Beside the potentially burnt out motor, I can't get the crank out of it to replace. I've tried everything. And the bearings are busted inside, so when pedalling it jumps around and makes cracking noises... It's ok as long as I don't have to pedal too hard, but now with the motor out of comission, I'm thinking maybe it's time to move on... don't know.

If I were to replace the motor wheel - what would you replace it with? 500w Bafang? or even higher output? I want as much torque as possible.

Thanks again everyone!

 

[Nealh]

You don't know it's the hub, one needs to rule out other faults first like battery and controller. The main controller fault will be blown fets to test.

A guy in Germany over powered his SWKX (Bafang) and melted the nylon clutch gears at about 52v and 25a, he then replaced the nylon gears with metal ones and had no issues. Ultimately he went to 35/40a and 60v by which time he had added motor cooling.

 

[Chris Maluszynski]

You don't know it's the hub, one needs to rule out other faults first like battery and controller. The main controller fault will be blown fets to test.

True.
I still need to figure out if it can be the controller or battery. But is there a way to narrow the fault down to the mosfets/controller without buying a new controller?

 

[Nealh]

True.
I still need to figure out if it can be the controller or battery. But is there a way to narrow the fault down to the mosfets/controller without buying a new controller?

Yes, if you have a multi meter.
One can measure the resistance on the phase wires to deduce whether one or more have failed.
Easy to follow guide.

https://ebikes.ca/documents/BlownMosfets.pdf

 

[Surf_Wonder]

My build handles heavy loads.
I have I When I have both trailer and distance to consider I run 48v 2x14ah, I did have to purchase a motor lead extender so that I could pack the contoller and batteries in the pannier, but my day to day is just using 1x14ah and I place everything in a bike frame bag instead.
Donor Bike was DELIBERATELY a 700c road bike (bike wheel =bigger circumference = greater distance efficiency), and I have a 26in mountain bike for fun /exercise rides.
Controller = 11a continuous rating, motor = 500w, all loads uphill... I'm guilty of needing to peek if the load was still there because I can't feel on level 4 or 5, and yes there's a safety strap in use on the trailer when in use.

 

[Chris Maluszynski]

If it's the original motor it will be a Bafang 250W but would need to check the model. If you are using a 48V (54V max) battery and 17A controller you will be putting a lot of strain on the motor, about double its capability. If you have been using it for some time with a heavy trailer I would suggest that you have either stripped the nylon planetary gears or blown the motor all together, you would need to check out the internals to find out which.

All the best, David

David

See image of motor attached. Do you recognize it?