Overview
The Dahon TR7 is a 24-speed folding tourer (8-speed cassette paired with a 3-speed internal rear hub, no front derailleur) built on a chromoly frame and fork. Converting this platform to electric assist is fundamentally different from converting a standard diamond-frame bicycle: the hinge mechanism, short wheelbase, and weight distribution create unique constraints that drive motor selection, battery placement, and cable routing decisions.
This reference consolidates field experience from two completed Dahon conversions (one rear-hub with rear-rack battery, one BBS01B mid-drive with rear-rack battery) and applies drivetrain-stress math to evaluate front-hub, rear-hub, and mid-drive topologies on a short-wheelbase folder with front pannier racks and a 24-speed combined drivetrain.
Motor Topology Comparison
Three practical motor placements exist for a folding-bike e-conversion. Each interacts differently with the existing 24-speed drivetrain, the hinge mechanism, and the loaded weight distribution.
| Parameter | Front Hub Motor | Rear Hub Motor | Mid-Drive (BBS01B-class) |
|---|---|---|---|
| Drivetrain stress multiplier | ~1.0x (motor bypasses drivetrain) | ~1.0x (motor bypasses drivetrain) | ~3-4x (motor torque adds to rider torque at chain) |
| Chain life vs unassisted | Up to 3x longer | Up to 3x longer | Reduced; chain/cassette wear accelerated |
| Wheelie tendency on folder | Low | High (short wheelbase + rear weight) | Moderate (depends on BB position) |
| Folding-friendly | Yes (battery on front panniers) | Poor (cable + weight on hinge) | Moderate (battery on rear rack, no hub wires) |
| Uses existing gears | Independent of drivetrain | Independent of drivetrain | Yes (replaces bottom bracket, drives chain) |
| Traction on loose surface | Reduced (less front weight) | High | High |
| Front-derailleur compatibility | Yes | Yes | No (typically 1x only) |
| Cable routing across hinge | Main power cables cross hinge | Throttle/display cables short | Display/throttle only (low current) |
Drivetrain Stress Analysis
Mid-drive motors apply torque directly to the bottom-bracket spindle, which then drives the chain through the existing cassette. The drivetrain therefore sees the sum of rider torque and motor torque at the chainring.
Using representative values for a 250 W-class mid-drive climbing assist:
- Rider average torque: 30 Nm at the crank
- Motor continuous torque: 95 Nm at the crank (typical BBS01B-class)
- Combined peak torque at chainring: 125 Nm
- Baseline (no motor): 30 Nm
- Stress multiplier: 125 / 30 ≈ 4.17x
This is the source of the rule of thumb that a mid-drive can load the drivetrain up to 4x relative to unassisted pedaling. Consequences for a folder with a 24-speed combined drivetrain:
- Accelerated chain stretch (typical KMC/Z-chain life of 3,000-5,000 km on a folder can drop below 1,500 km)
- Cassette sprocket hook wear on the 8-speed block
- Internal 3-speed hub planetary gear wear (replacement parts are proprietary on older Dahon / Sturmey-Archer units)
Front or rear hub motors decouple the motor from the drivetrain. The motor's torque is delivered directly to the wheel, and the rider's chain only carries rider input. With pedal-assist cutoffs at 25 km/h (EU) or 32 km/h (US Class 1), chain load remains near baseline, which is why chain life can extend to 3x a non-assisted folder.
Battery Placement & Weight Distribution
A folding bike has a short wheelbase and a center of gravity that is already compromised by the hinge. Adding a 2-3 kg battery and 2-3 kg hub motor shifts weight in ways that affect handling.
| Configuration | Wheelbase effect | Wheelie risk | Steering feel |
|---|---|---|---|
| Rear rack battery + rear hub motor | Rearward bias, short WB | High (saddle over rear axle) | Light front, twitchy |
| Rear rack battery + front hub motor | Rearward bias, longer effective WB | Low-moderate | Slightly heavier steering |
| Front panniers + front hub motor | Forward bias | Very low | Heavy steering, stable |
| Rear rack battery + mid-drive (BBS01B) | Centralized mass at BB + rear rack | Moderate | Balanced |
| Frame-integrated battery + mid-drive | Centralized | Low | Near-stock |
For a folding tourer with front pannier racks already fitted, mounting the battery in the front panniers plus a front hub motor gives the most balanced loaded configuration and avoids running high-current power cables across the hinge.
Cable Routing Across the Folding Mechanism
Every cable that crosses the main hinge is flexed on every fold. Cable failure modes at the hinge are:
- Conductor fatigue (copper work-hardening, strand breakage) - typically 2,000-5,000 fold cycles for 10 AWG silicone-jacketed cable.
- Insulation abrasion against the hinge plate - mitigated by split loom or PET cable wrap.
- Strain-relief pull-out at the connector - mitigated by anchor points within 50 mm of each side of the hinge.
For a Dahon-class folder, the practical routing options are:
- Rear rack battery + rear hub motor: No high-current cables cross the hinge. Battery cables go straight down to the rear hub. Cleanest folding.
- Rear rack battery + front hub motor: Battery + phase cables (typically 10-12 AWG) must cross the hinge. Not recommended for frequent folding.
- Front pannier battery + front hub motor: Battery + phase cables stay on the front side. Clean folding, balanced weight.
- Mid-drive + rear rack battery: Only display/throttle cables cross hinge. Acceptable for moderate folding.
Bottom Bracket Compatibility (Mid-Drive Conversions)
The BBS01B and BBS02B mid-drive units clamp into the bottom-bracket shell in place of the existing cartridge BB. Before purchasing, measure the Dahon BB shell width and spindle standard. Dahon folders typically use:
| Model era | BB shell width | BB standard | Compatible BBS unit |
|---|---|---|---|
| Pre-2010 Dahon | 68 mm | BSA / JIS square taper | BBS01B / BBS02B with correct spindle |
| 2010-2015 Dahon | 68 mm | Hollowtech II (some models) | Not directly compatible - adapter required or front-hub route |
| Current Dahon | 68 mm | BSA threaded | BBS01B / BBS02B / BBSHD with BSA spindle |
Wheel, Rim & Tire Considerations
Factory Dahon 20-inch wheels (often Kinetix Pro or similar) are typically single-wall aluminum and limited to ~1.5-inch tires. For e-bike use, a wider tire (1.75-2.0 inch) improves comfort and traction, but requires:
- Double-walled rims rated for the higher motor torque (front hub motors can deliver peak torque spikes of 40+ Nm to the rim)
- Spoke count of 36 (rather than 28) for hub-motor builds
- Rim eyelets to prevent nipple pull-through under braking + motor torque
Replacement 20-inch double-wall rims compatible with Dahon hub spacing (front: 100 mm OLD, rear: 130 mm or 135 mm OLD depending on hub) are available from specialist suppliers. Dahon-specific retailers stock double-walled rims originally intended for Tern but compatible with Dahon hubs after light truing.
Motor Sizing for Flat Commuting
For a flat-terrain commute with a total system weight of rider + bike + battery of ~110 kg, the rolling and aerodynamic loads at 25 km/h are approximately:
- Rolling resistance: Crr x N ≈ 0.005 x 110 x 9.81 ≈ 5.4 N (~5 W at 25 km/h)
- Aerodynamic drag at 25 km/h: 0.5 x rho x CdA x v³ ≈ 0.5 x 1.225 x 0.4 x 0.5 x (6.94)³ ≈ 41 N (~285 W)
- Total continuous power at 25 km/h on the flat: ~290 W (mechanical at wheel)
A 250 W-rated hub motor delivers this comfortably on the flat with margin for headwinds and rider input. A 500-750 W mid-drive is overkill for the same duty cycle and only justified if the route includes sustained >8% grades.
Configuration Procedure
The following procedure applies to the recommended configuration for a Dahon TR7 with front pannier racks: front hub motor + front-mounted battery.
Prerequisites
- 250-350 W front hub motor, 36 V, with 100 mm OLD and 36-hole drilling
- 36 V or 48 V rear-rack or frame-style battery with 10-15 Ah capacity (360-720 Wh)
- Compatible rim, spokes, and brake rotor (if using regen or hydraulic discs)
- Torque arm (mandatory for front hub motors on aluminum forks - a Dahon chromoly fork tolerates hub torque but a torque arm is still recommended)
Step-by-Step Installation
- Remove existing front wheel, tire, tube, and rim strip. Retain the wheel bearings if reusing.
- Lace the new double-wall rim to the hub motor with 36 spokes, brass nipples. Target tension 100-110 kgf using a tensiometer.
- True the wheel: lateral deviation <0.5 mm, vertical deviation <0.3 mm. Use a truing stand.
- Install rim tape, tube, and tire (1.75-2.0 inch for improved comfort).
- Mount the wheel into the Dahon fork. Install torque arm on the right (drive) side dropout, clamping the axle.
- Mount the battery tray or bag on the front pannier rack. Locate it as low as possible to keep CoG low.
- Route motor phase cables (typically 10 AWG silicone) along the fork blade to the controller. Secure every 100 mm with cable ties.
- Mount the controller in a frame bag or behind the seat tube. Keep phase cables short (under 600 mm) to reduce voltage drop.
- Connect display, throttle, and PAS sensor. Use 22 AWG for signal, 18 AWG for headlight if used.
- Program the controller via the configuration USB port if available (e.g., KT-series controllers allow power, current limit, low-voltage cutoff, and PAS magnet count settings).
Verification
- Spin test: With battery connected and motor powered off, wheel must spin freely with no rubbing. Any noise indicates hall-sensor or axle bearing issues.
- Brake test: Confirm rim brakes (or discs) clear the motor profile. Some front hub motors are wider than standard rims.
- No-load current: With wheel off ground, throttle applied, current draw should be <0.8 A on a 36 V system.
- Loaded current: At full throttle on flat ground, current should be 5-8 A; on a hill, 12-15 A. Values above 20 A indicate a controller or motor problem.
- Cold test: Check all cable joints for warmth after 5 minutes of operation. Warmth = high-resistance joint = fire risk.
- Fold test: Fold and unfold the bike 10 times. Inspect cable jackets for abrasion at the hinge.
Troubleshooting Matrix
| Symptom | Likely cause | Diagnostic | Fix |
|---|---|---|---|
| Wheel does not spin under power | Hall sensor failure or phase wire break | Check hall sensor voltage with motor spinning manually; check phase resistance phase-to-phase (~0.3-1.0 ohm) | Replace hub or hall sensor board |
| Motor cuts out under load | Low-voltage cutoff, BMS trip, or thermal shutdown | Measure pack voltage under load; check BMS logs if smart BMS | Charge battery; check for cells out of balance; reduce current limit |
| Battery cables warm | Undersized cable, loose connector, or oxidized contact | Infrared thermometer at connector; measure voltage drop across connector | Upgrade to 10 AWG; re-crimp or replace connector; apply dielectric grease |
| Front wheel slips on loose surface | Insufficient front weight + 250 W torque | Rider observation | Add weight to front (cargo, front panniers); reduce power level on loose surfaces |
| Wheelie on acceleration (rear hub setup) | Rear weight bias + short wheelbase | Rider observation | Reduce throttle ramp; shift rider weight forward; consider front-hub conversion |
| Display flickers | Loose signal connector or low battery | Wiggle-test connectors | Reseat connectors; replace display if ribbon damaged |
| Cable failure at hinge | Fatigue from fold cycles | Inspect jacket for cracking; megger test for insulation | Re-route cable with larger service loop; add split-loom protection; reduce fold frequency |
Frequently Asked Questions
Is a front hub or mid-drive better for a Dahon TR7 folding bike?
For flat-terrain commuting on a folder, a front hub motor is generally preferred because it places all high-current cables on the front side of the hinge, preserves the existing 24-speed drivetrain (no 4x stress increase), and balances weight with front pannier-mounted battery. A mid-drive (BBS01B) is preferred only if the route includes steep hills or the rider needs to tow heavy loads.
How much does a mid-drive motor increase drivetrain wear?
A 250 W mid-drive delivering 95 Nm of crank torque combined with 30 Nm of rider input produces ~125 Nm at the chainring, a ~4.2x multiplier over unassisted pedaling. Chain life on a folder can drop from 3,000-5,000 km to under 1,500 km under this loading, with cassette and internal-hub wear accelerating proportionally.
Can I keep the 24-speed combined drivetrain (3-speed hub + 8-speed cassette) with a mid-drive?
Yes. A BBS01B mid-drive replaces the bottom bracket and drives the existing chainring, so the 3-speed internal hub and 8-speed cassette remain fully functional. The motor simply multiplies torque through the existing gearing. Note that this loads the internal hub's planetary gears, which on older Dahon/Sturmey-Archer units can be hard to source.
What battery capacity do I need for a 25 km commute?
For a flat 25 km commute at 25 km/h with ~290 W mechanical demand and ~360 Wh battery (36 V x 10 Ah), expect 50-70% depth-of-discharge, or roughly 2-3 days of commuting per charge. Increase to 15 Ah (540 Wh) if the route includes hills or headwinds, or if you want to charge less frequently.
Do I need a torque arm on a Dahon TR7 front hub motor?
Yes. The Dahon TR7 uses a chromoly fork which tolerates hub torque better than aluminum, but a torque arm is still recommended insurance against axle spin-out, which can cause sudden loss of steering control. Use a single-sided or dual-sided torque arm rated for at least 1.5x the motor's peak torque.
