FAQ
TL;DR: At 65 kg, this DIY electric cross pairs a QS165 mid-drive with a Fardriver ND72360, and the builder says "this power is sufficient." This FAQ is for riders comparing custom e-cross parts, cooling limits, wheel specs, and road-legality tradeoffs before building a lightweight off-road machine. [#21071271]
Why it matters: This thread shows what a real lightweight CNC-built electric cross can achieve, where it is limited, and which practical choices matter most before you copy the concept.
| Wariant |
Silnik |
Sterownik / ustawienie |
Masa / zasięg |
Przeznaczenie |
| Opisywany build CNC |
QS165 47 Nm |
Fardriver ND72360, ustawiony na 120A/240A |
65 kg |
lekki off-road, geometria zbliżona do Sur-Ron |
| Drugi build z dyskusji |
QS138 70H V3 |
Votol EM150, 120A z baterii |
108 kg, ~100 km przy 50 km/h |
cięższy build terenowy z dużym zasięgiem |
Key insight: The strongest takeaway is not peak power. It is the balance of low weight, conservative 120A/240A settings, and thermal control, which makes the bike usable off-road without chasing the controller’s full paper rating.
Quick Facts
- The frame was drawn in Corel and CNC-machined from 7075 aluminum: 10 mm main structure and 20 mm swingarm sections. [#21033619]
- The drivetrain uses a QS165 47 Nm mid-drive motor, a Sur-Ron gearbox, and a 58T rear sprocket. [#21033619]
- The controller is a Fardriver ND72360 rated 190A/360A, 13 kW max, but this bike currently runs 120A/240A. [#21033619]
- The builder reported 60°C max motor temperature at 8 kW in the field, with a temperature sensor protecting the motor. [#21034347]
- The bike runs 19-inch wheels with 70/100-19R tires and a total weight of 65 kg. [#21071271]
How was the frame of this lightweight CNC electric cross designed and machined from 7075 aluminum on a CNC router?
The frame was designed in Corel and then machined from 7075 aerospace aluminum on a CNC router. The builder used 10 mm aluminum for the main body and 20 mm for the swingarm, aiming for Sur-Ron-like geometry with a fully custom layout. He also chose simpler shapes because of limited CAD and CNC experience, which kept the project buildable from scratch.
[#21033619]
What is a mid-drive motor, and why use it in a DIY electric cross bike instead of a hub motor?
"Mid-drive motor" is an electric drive unit that powers the bike through the drivetrain, centralizing mass and allowing external gearing, which suits off-road bikes that need torque multiplication and better suspension response than a heavy wheel-mounted motor. This build uses a QS165 mid-drive with a Sur-Ron gear and 58T rear sprocket, so the motor sits inboard rather than in the wheel.
[#21033619]
How do the QS165 motor and Fardriver ND72360 controller work together in this build?
They work as a matched power system where the Fardriver ND72360 feeds controlled current to the QS165 motor. In this bike, the QS165 is paired with a Sur-Ron gearbox and a 58T rear sprocket, while the ND72360 is configured below its headline limit for practical off-road use. That combination prioritizes usable torque and thermal headroom over maximum current.
[#21033619]
Which controller settings were used on the Fardriver ND72360, and why was it limited to 120A/240A instead of the full 190A/360A?
The bike runs the Fardriver ND72360 at 120A battery current and 240A phase current. The builder did not chase the controller’s full 190A/360A rating because he said he had no intention of drawing more than 120A and found the present power level sufficient for his riding. That is a deliberate reliability and control choice, not a hardware limit.
[#21034347]
What does the QS165 motor actually tolerate in practice, and how does the temperature sensor protect it at higher power levels?
In practice, the builder said the QS165 can take 18 kW instantaneously, and he observed only 60°C at 8 kW in field riding. He also said the motor is protected by a temperature sensor, so rising heat acts as the main safeguard when power climbs. The clear limit case is heat, not just current on paper.
[#21034347]
How long does the battery last in off-road riding, and how long does it take to recharge it?
The thread does not give a measured off-road runtime or any recharge time for this specific bike. The only firm battery detail is that a second pack was being built from LG Chem E61v 60Ah NMC cells for longer forest and mountain rides, while the current setup stayed limited to 120A because the builder considered that enough.
[#21034347]
Why is registering a one-off self-built electric cross bike so difficult, and what makes the process expensive?
It is difficult because a one-off self-built bike has no standard vehicle identity, and forum users described registration as either practically impossible or possible only with high cost. The builder said people who had tried to register a single custom design reported that it was nearly impossible, which is why he used it off-road in forests and pits instead of public roads.
[#21033930]
In terms of registration and paperwork, is converting a combustion motorcycle to electric easier than building a fully custom frame from scratch?
Yes, the thread suggests a conversion is easier on paperwork because only the drive type changes, while a fully custom frame starts with no existing vehicle documents. One user explicitly wondered whether a converted combustion bike would be easier to register for that reason. The discussion does not prove the process is easy, only easier than a ground-up custom chassis.
[#21034660]
What is OC insurance in this context, and how does family liability insurance differ from vehicle insurance for a DIY electric bike or cross?
"OC insurance" is liability insurance that covers damage you cause to others, but its scope depends on the policy type, insured object, and legal classification of the vehicle involved. In this thread, the builder relied on family liability cover, while other users warned that vehicle-related claims can become problematic if the machine has no clear registration class or policy match.
[#21033984]
What license category would be needed to ride a powerful electric cross like this on public roads?
A forum participant said riding a machine like this on public roads would require category A or B, not a bicycle card. That fits the thread’s overall view that this build sits far beyond ordinary e-bike territory, especially with a QS165 motor and controller settings up to 120A/240A. In practice, the project was discussed as an off-road machine, not a street-legal bicycle.
[#21037280]
How strong is a 10 mm swingarm axle for this type of bike, and why is that dimension also used in Surron, Talaria, and E-Ride models?
The builder said a 10 mm swingarm axle is standard for this class of bike and used the same dimension here. He specifically pointed to Sur-Ron, Talaria, and E-Ride as factory examples, which suggests the dimension is considered adequate when the rest of the chassis is designed around it. The concern raised in the thread was visual delicacy, not a reported failure.
[#21034275]
What wheel size and tire size were used on this build, and how does 19-inch 70/100-19 rubber affect off-road handling?
This bike uses 19-inch wheels with 70/100-19R tires. The thread gives no formal handling test, but that size places it in the common lightweight dirt-bike range and supports the project’s off-road intent. The hard data here are clear: 19-inch wheels, 70/100-19R rubber, and a complete bike weight of 65 kg.
[#21071271]
Where can I buy 19-inch aluminum rims like these for a DIY electric cross, and how do you lace the wheels yourself?
The builder bought aluminum rims on Allegro, specifically rims sold for older models like Romet and WSK, and then laced them himself. A practical thread-based path is: 1. buy 19-inch aluminum rims from Allegro, 2. choose motorcycle-style sizes like 70/100-19, 3. lace the wheel yourself if you have the hub and spoke setup ready. The thread names the source, but not the spoke pattern or tool list.
[#21092615]
QS165 vs QS138 70H V3 — which motor is a better choice for a lightweight off-road electric build?
QS165 looks better for the lightest build in this thread, while QS138 70H V3 suits a heavier, range-focused setup. The QS165 bike weighs 65 kg and uses a mid-drive plus gearing, whereas the QS138 70H V3 example weighed 108 kg with a 21S15P Samsung 40T battery, about 4.5 kWh, and roughly 100 km range at 50 km/h. Choose QS165 for minimum mass, QS138 for a bigger battery package and touring-style trail use.
[#21035380]
What is the Fardriver ND72360, and what do ratings like 190A/360A and 13 kW max actually mean in real-world use?
The Fardriver ND72360 is the motor controller in this build, and its 190A/360A and 13 kW max figures are headline electrical limits, not the settings the rider must use. Here, the builder set it to 120A battery and 240A phase current instead, then reported that the available power was already sufficient. Real-world use depends on tuning, heat, and traction more than the biggest printed number.
[#21033619]
Generated by the language model.
