Case

Case Highlights

An Asian OEM producing 3–5 kW electric scooters for city commuting wanted to upgrade the transmission between motor and rear wheel. Their existing spur-gear reducer generated noticeable gear whine at 40–60 km/h and showed wear after long hill-climb usage.

DD Gear designed a small-module helical reduction gear set that fits into the existing mid-drive housing. After validation on bench and road tests, the OEM measured around 2–3% drivetrain efficiency improvement and a clearly lower gear whine band at typical cruising speeds, while maintaining the same overall gear ratio. Well-designed helical gears are widely used in EV transmissions because they distribute load over a larger contact area, reduce impact, and help lower noise compared with spur gears.

Customer Background & Project Scope

• Customer industry: Asian manufacturer of electric scooters and light e-motorcycles for urban mobility.

• Vehicle segment: 3–5 kW mid-drive platform targeting daily commuting and food-delivery fleets.

• Application: Two-stage reduction between high-speed BLDC motor and rear wheel via chain drive.

• Project goals:

  • Improve mechanical efficiency of the reducer to support longer range on a fixed battery pack.

  • Reduce gear whine and vibration at 40–60 km/h cruising, where riders are most sensitive.

  • Keep the existing housing, motor mounting, and final drive ratio to avoid a full platform redesign.

The OEM’s original design used spur gears in both stages. As sales increased and scooters were used intensively in hilly cities, issues related to noise and long-term durability became more visible.

Challenges

1. Gear whine at cruising speeds

   • Riders and fleet users reported a high-pitch gear noise around 40–60 km/h.

   • Spur gears engage tooth-by-tooth, which tends to create more vibration and audible noise at high speed compared with helical teeth. 

2. Thermal and load stress on hill climbs

   • During prolonged full-load hill climbing with passenger and cargo, teeth showed early pitting and polishing.

   • Elevated oil temperature and high motor speed increased stress on small-module teeth.

3. Packaging constraints

   • The customer needed a “drop-in” solution: same shaft centers, bearing positions, and external mounting, because the scooter frame and swingarm were already in production.

DD Gear Solution

1. Helical Gearset Design

• Switch to helical teeth on the main stage

  • DD Gear replaced the primary spur pair with small-module helical gears, increasing contact ratio so that more than one tooth pair carries load at the same time.

  • Helical gears are commonly used in EV gearboxes because gradual tooth engagement reduces impact, vibration, and noise while maintaining high load capacity and durability.

• Ratio and geometry optimization

  • Maintained the overall reduction ratio to keep vehicle performance (top speed and acceleration) unchanged.

  • Adjusted helix angle and face width to balance torque capacity, contact stress, and axial force within existing bearing limits.

  • Applied microgeometry corrections (profile and lead modifications) to minimize transmission error over varying loads, a known lever for gear NVH control. 

• Optional planetary second stage (for higher-end variant)

  • For a premium scooter version, DD Gear proposed a compact planetary second stage to increase torque density in the same volume.

  • Planetary gearboxes are widely used in e-bikes and e-mobility drives because they combine high torque, high efficiency, low backlash and low noise in a very compact form.

2. Materials & Heat Treatment

• Carburizing steels for high contact fatigue strength

  • Selected 20CrMnTi / SCM420 carburizing steels for pinions and gears to achieve a hard, wear-resistant case with a tough core. Such steels are standard in automotive and EV transmissions.

  • Case depth set around 0.8–1.0 mm, depending on module and tooth size.

• Heat treatment & finishing

  • Carburizing + quenching followed by precision gear grinding to achieve low surface roughness and stable contact patterns. This approach is consistent with modern EV reducer practices to raise mesh efficiency and reduce micropitting risk.

  • Target surface hardness HRC 58–62, with controlled core hardness to resist bending fatigue and shock loads from potholes or aggressive riding.

3. Manufacturing & Quality Control

• Small-module precision

  • Modules in the m 1.0–2.0 range require tight control of tool wear, alignment, and heat-treatment distortion.

• Accuracy grade

  • Gears manufactured to ISO / DIN Grade 5–6, which is appropriate for high-speed EV and e-scooter reducers where noise and efficiency are important. 

• Validation testing

  • Bench durability tests representing >25,000 km equivalent use, including repeated full-load hill-climb cycles.

  • On-road tests instrumented with sound measurements at rider position, comparing legacy spur design vs. new helical set.

Results

After adopting the DD Gear solution, the OEM reported the following:

• Efficiency & range

  • Drivetrain efficiency improvement of about 2–3 percentage points in the most frequent operating range, based on dynamometer measurements.

  • In internal simulations and fleet tests, this translated into noticeably better real-world range per charge. (For reference, other EV gearbox studies show that optimized multi-stage gear design can yield a few percent energy benefit over conventional arrangements.

• Noise & comfort

  • Clear reduction in gear whine in the 40–60 km/h band; rider feedback described the scooter as “quieter” and “more premium”.

  • Overall acoustic profile became more dominated by wind and tire noise, which is a typical target for well-designed helical EV gearsets.

• Durability

  • After full bench cycles and initial fleet mileage, no pitting or scuffing was observed on tooth flanks within the test window.

  • Fleet operators experienced fewer complaints related to drive noise or abnormal vibration.

• Integration

  • Because DD Gear kept shaft centers and housing interfaces unchanged, the OEM could implement the new gears in production with minimal tooling changes and no frame redesign.

Typical Technical Specifications

Case Summary

By moving from spur gears to small-module helical gears with automotive-grade materials and grinding, DD Gear helped this e-motorcycle OEM:

• Improve drivetrain efficiency and riding range.

• Reduce gear whine at typical city speeds, improving rider comfort and brand perception.

• Maintain existing packaging, allowing a low-risk upgrade of a successful platform.

This approach is ideal for manufacturers of e-scooters, e-mopeds and light e-motorcycles who want better efficiency and NVH without a full redesign.

Planning a new e-scooter or e-motorcycle platform?
Talk with DD Gear’s engineers about custom small-module helical gear solutions for your mid-drive reducers.