Converting a fatbike

[dlo2025]

Hello,
I've been enjoying riding a 29" Specialized Stumpjumper with a 750W GoSwiss Drive for 10 years. Before inserting the engine, I really enjoyed the bike as well. It's been a great project, and the built-in torque sensor was taken for granted for a long time in simple, enjoyable commutes as well as forest rides.
I wanted to get a more comfortable low-rider, and got a 20" Chinese fatbike to convert. I want to have a bike that can do 65 km/h, pedal comfortably and respond well to the applied torque. I knew I wasn't going to get that off Amazon, but the product as it is currently is really something to cry about, a poor scooter instead of a powerful bicycle. Some people do not understand what the potential of their product really is.
After wiping my tears, I need any kind of help finding:
- a powerful 3000 W engine that can assist with a specific power instead of a maximum speed (ideally directed by a Cycle Analyst)
- a fitting Schlumpf drive to pedal at a high speed so it doesn't feel too stupid to have pedals on it; it currently sports 46/14 sprockets on 20" wheels, that's far from feasible at high speeds
Any suggestions would be of great help.
Thanks in advance,
Lorenzo

 

[E-HP]

Hello,
I've been enjoying riding a 29" Specialized Stumpjumper with a 750W GoSwiss Drive for 10 years. Before inserting the engine, I really enjoyed the bike as well. It's been a great project, and the built-in torque sensor was taken for granted for a long time in simple, enjoyable commutes as well as forest rides.
I wanted to get a more comfortable low-rider, and got a 20" Chinese fatbike to convert. I want to have a bike that can do 65 km/h, pedal comfortably and respond well to the applied torque. I knew I wasn't going to get that off Amazon, but the product as it is currently is really something to cry about, a poor scooter instead of a powerful bicycle. Some people do not understand what the potential of their product really is.
After wiping my tears, I need any kind of help finding:
- a powerful 3000 W engine that can assist with a specific power instead of a maximum speed (ideally directed by a Cycle Analyst)
- a fitting Schlumpf drive to pedal at a high speed so it doesn't feel too stupid to have pedals on it; it currently sports 46/14 sprockets on 20" wheels, that's far from feasible at high speeds
Any suggestions would be of great help.
Thanks in advance,
Lorenzo

The dropout width(s) are an important factor for what your available options are. Also, are you replacing both motors?

 

[99t4]

I want to have a bike that can do 65 km/h

That's going to take a lot of power for a small-fat-tire ebike. What are the battery and controller ratings?

 

[dlo2025]

I'd like to remove the front motor (it's noisy and spins too hard and slips when accelerating) and have only one big one in the rear.
The rear dropout is 180mm. It currently has a 7-speed cassette.

The bike currently has a Dongguan jing hui 52V brushless DC motor controller with 2x output of 25A, on 20" wheels. It's already quite nice and powerful (45-50 km/h), if only I could regulate the PAS/torque better with the CA - the controller has a weird 10-pin output (like Radwagon) and I can't find any information on how to pair that to a CA.

My hope is to achieve such speed (or almost) with a 72V 3000W rear motor; but I don't know if it's possible.
And also, I'd like it to be quiet

 

[dlo2025]

I didn't get a chance to reply in time, but my reply was:
"You are most certainly right at this moment and might remain right"
(in reply to: you may find this bike to never be (come) better than the previous)

But I have a vision of what it should become, and that used to be the same for the other bike over 10 years ago - I just have to try

 

[E-HP]

I'd like to remove the front motor (it's noisy and spins too hard and slips when accelerating) and have only one big one in the rear.
The rear dropout is 180mm. It currently has a 7-speed cassette.

The bike currently has a Dongguan jing hui 52V brushless DC motor controller with 2x output of 25A, on 20" wheels. It's already quite nice and powerful (45-50 km/h), if only I could regulate the PAS/torque better with the CA - the controller has a weird 10-pin output (like Radwagon) and I can't find any information on how to pair that to a CA.

My hope is to achieve such speed (or almost) with a 72V 3000W rear motor; but I don't know if it's possible.
And also, I'd like it to be quiet

QS273, 175mm dropout version, which would be right around 180mm with the required washers. Speed and power shouldn't be an issue. With the CA, you can pair is with any old cheap dumb powerful 72V controller.

 

[dlo2025]

Thanks, that sounds like a very nice solution! I don't see a QS273 on the Risunmotor website with a 20" wheel, but I guess it'll have to be re-spoked.
I'll check thoroughly for the controller anyways, I was already sorry to learn the CA cannot be used with my stock Rad-like 10-pins cable.

 

[Gika]

Vorrei rimuovere il motore anteriore (è rumoroso, gira troppo forte e slitta in accelerazione) e averne solo uno grande nella parte posteriore.
Il forcellino posteriore è da 180 mm. Attualmente è dotato di una cassetta a 7 velocità.

Attualmente la bici è dotata di un controller motore brushless DC Dongguan Jing Hui da 52 V con 2 uscite da 25 A, su ruote da 20". È già piuttosto potente (45-50 km/h), se solo potessi regolare meglio PAS/coppia con il CA: il controller ha una strana uscita a 10 pin (come Radwagon) e non riesco a trovare informazioni su come abbinarlo a un CA.

La mia speranza è di raggiungere questa velocità (o quasi) con un motore posteriore da 72 V e 3000 W; ma non so se sia possibile.
E poi, vorrei che fosse tranquillo

Ciao... anche quello da 3000/5000 watt sul sito Risunmotor va bene... a 72 volt con un controller da 80/100 A puoi superare i 50 km/h e non di poco.

 

[dlo2025]

Ciao... anche quello da 3000/5000 watt sul sito Risunmotor va bene... a 72 volt con un controller da 80/100 A puoi superare i 50 km/h e non di poco.

Grazie!
I have a lot to learn, I will be reading for a few weeks first... I have to find a good understandable combination of a CA+72Vcontroller+3/5000Wengine.
But good to know that it's possible!

 

[dlo2025]

As I am still new to the entire subject and had most of my advice from this message and long conversations with ChatGPT, I'd like to give anyone here the opportunity to shoot down my project in case the below table of conversions to my above mentioned fatbike would not make sense:

Batterij​	72V 80A BMS Samsung 35E - 31.5Ah + 5A charger​	https://www.theebikemotorkit.com/pr...ain-bike-beach-cruiser?variant=50617365463172​
Motor​	Newdoon 72V5000W170mm + 20x4"rim (4T-720RPM) / OK for 7-speed (11T)​	170/190mm 2000W-5000W motor fat bike kit with High quality tires,20*4.0in size. | eBay Peak Power:6000W. Peak Power:8000W. Rated Speed:80-90KM/H(No load). Rated Power:3000W. Waterproof Grade:IP54. Rated Power:5000W. We will do our best service to help you. www.ebay.com
Controller​	Sabvoton SVMC7280 80A​	Sabvoton Elektrische Fiets Controller 80A 1500W-2000W Motorcontrollerkits met USB-display en Bluetooth SVMC7280 V1 UKC1 voor e-bikes : Amazon.com.be: Sport en buiten​
Torque sensor​	100mm length​	https://ebikes.ca/ebike-parts/control-stuff/torque-sensors/erider100q-5b.html​
Cycle Analyst​	CA-SA (custom shunt)​	https://ebikes.ca/ca-sa.html
DC-DC convertor​		36-72 V naar 12 V 20 A 240 W DC Spanningsadapter Converter Reducer (externe bedrading) : Amazon.com.be: Elektronica

With this, I hope to achieve a nice torque sensor-assisted pedelec ride that can go >60 km/u, and than can also be quickly preset into a street legal mode of 25km/u with walk assist. Thankful in advance for any advice or recommendations.

 

[99t4]

What happened to the Schlumpf drive you proposed?

Have you looked at the QS205 motor? I don't trust the vendor's specs for the motor from that link you posted.

 

[dlo2025]

What happened to the Schlumpf drive you proposed?

Have you looked at the QS205 motor? I don't trust the vendor's specs for the motor from that link you posted.

Thanks for the reaction. Good questions indeed.

The Schlumpf seems great but I concluded it cannot be combined easily with a torque sensor, and I need that. Using a sensor in the cranks seemed complicated. Would there be another way?

The QS205 is only available in 175mm with a single speed. And even if double speed, that only starts at 14T. My current front cog is 58t and that feels ok with 11T, but with 14T in the back it's hard to find resistance in pedaling at high speed. In hindsight a fatbike with a 190cm dropout would maybe have made it possible.

The vendor was thoroughly questioned but yes, I also have my doubts...

 

[dlo2025]

I went through my options again, for my criteria:
- 170-175mm dropout
- fits 11T cogwheel (6-7 speed, so sufficient thread)
- 50H
- quiet
- enough RPM (load) to do >60km/h
- best balance towards torque
...it is either a QS205 with single speed that doesn't work if I want to bike comfortably and have a torque sensor, or QS205 clones that are either not fitting or not powerful enough.
For this "Newdoon" engine in the 4T version (I am waiting for another link), the producers claims it does 720rpm at 72V with 160Nm torque, which I hope will get me to the desired 60+km/h. I can alternatively take the 3T that is 860rpm - I'm still hesitating.
Also really open to any good suggestion out there...

Also, I can apparently get a Sabvoton SVMC7280 controller with a CA-plug. Is that of any added value in comparison to using the standalone CA on the controller?

Thanks in advance for any help