A custom e-moto is five components in a system. Get each one right individually and they still might not work together. Get the system right and you have a bike that outperforms anything you can buy off the shelf at the same budget.
The stack
1. Frame — sets geometry, weight class, and power ceiling. A 6061 alloy frame rated to 120 kg handles up to ~10 kW continuous. Carbon fibre saves weight but rarely exceeds aluminium in fatigue life on trail bikes. Chinese CNC frames from suppliers like Bafang, JAXON, and Paxis Engineering represent the best value in the market. Budget: €400–900.
2. Motor — mid-drive vs hub is the first decision. Mid-drive (crank position): better weight distribution, works with gearing, higher efficiency in varied terrain. Hub motor: simpler, cheaper, no chain wear. For trail use, mid-drive is almost always the right answer above 3 kW. QS Motor 138 series and the Bafang M620 are the two most supported mid-drives in the community. Budget: €300–700.
3. Controller — must be matched to motor KV and battery voltage. VOTOL EM-150/200 handles 60–84 V at up to 200 A phase current. Sabvoton SVMC72150 is the alternative with arguably better software tooling. Both support CAN communication for display integration. Budget: €200–400.
4. Battery — see the battery upgrade guide for full detail. For a first build: 72 V 20 Ah minimum, 30 Ah preferred. Specify a quality BMS with 150 A continuous discharge and Bluetooth monitoring. Budget: €500–900 complete with charger.
5. BMS — sometimes sold as part of the battery, sometimes separate. JK BMS and DALY are the two trusted brands in this space. Specify: continuous current ≥ controller max draw, active balancing (not passive), and temperature protection with two probes minimum.
Making them work together
The KV of your motor × battery voltage gives you theoretical RPM. Divide by your gear ratio and wheel diameter to get top speed. Cross-reference this against your controller's phase current limit and motor's continuous watt rating. If the numbers don't align, something runs hot.
Use e-moto.bike to model the component stack before buying anything. The configurator shows thermal stress estimates and predicted range based on cell chemistry — it's faster than a spreadsheet and saves the cost of a bad purchase.