Pedal Assist System (PAS) Setup for E-Bike Conversion in India

Table of Contents

• What Is a Pedal Assist System?
• PAS Sensor Types: Cadence vs Torque
• Installing the PAS Cadence Sensor
• Wiring the PAS to Your Controller
• Configuring PAS in Controller Settings
• Fine-Tuning for Indian Road Conditions
• Troubleshooting PAS Issues
• Frequently Asked Questions

Pedal Assist System (PAS) transforms an e-bike from a scooter with pedals to a bicycle that feels genuinely better than riding unaided. With PAS active, the motor amplifies your pedaling effort — you pedal, the motor pushes alongside you. This natural feel is what makes quality e-bikes genuinely enjoyable to ride and is a key reason why e-bike adoption is growing rapidly in India as a commuting tool.

Setting up PAS correctly on an e-bike conversion requires understanding how the sensor works, how to install it on your bicycle’s crank, how to wire it to the controller, and how to configure the controller’s PAS parameters for the right feel. This guide covers the complete process from sensor selection through final calibration.

What Is a Pedal Assist System?

A PAS detects that you are pedaling and signals the controller to provide motor assistance proportional to (or triggered by) that pedaling. The controller uses this signal, combined with the selected assist level, to determine how much current to send to the motor.

The practical effect: ride in Assist Level 2, pedal at moderate effort, and the motor contributes additional power — you cover more ground for less physical effort. Ride in Assist Level 1 for light assistance that keeps you within regulatory limits. Use Level 3-5 for steep climbs or headwind conditions. Select Level 0 (no assist) for pure pedal exercise.

PAS is required for builds that want to qualify as electric bicycles under Indian regulations (rather than motor vehicles). A throttle-only e-bike cannot be classified as a bicycle. A PAS-equipped e-bike with appropriate power and speed limits qualifies as a bicycle when configured correctly (see our article on Indian e-bike regulations).

PAS Sensor Types: Cadence vs Torque

Cadence Sensor (Most Common for DIY Builds)

A cadence PAS sensor detects rotation of the crank — specifically, it detects each magnet passing the Hall sensor as the crank rotates. Standard sensors have 5, 8, or 12 magnets arranged in a ring around the crank spindle or bottom bracket. The 12-magnet type is recommended — it provides 12 pulses per rotation (one every 30°), giving the controller more frequent updates for smoother response.

The controller interprets cadence sensor pulses as a binary signal: pedaling (motor assistance enabled) or not pedaling (motor assistance disabled). The actual cadence speed is not used by most standard controllers — any detectable rotation triggers assist. The only cadence-responsive behavior is the delay before the motor cuts after pedaling stops (typically 0.5-1 second, configurable).

Cadence sensors cost ₹200-600 in India and are widely available. They work with all standard e-bike controllers including KT, Sabvoton, and generic brands.

Torque Sensor (Advanced and More Natural)

A torque sensor measures how hard you push on the pedals, not just whether you are pedaling. The controller receives an analog signal proportional to torque and provides motor assistance scaled to that effort. Hard push = strong assistance. Light touch = gentle assistance.

The ride feel with a torque sensor is dramatically different and superior — it genuinely feels like a bicycle amplifying your legs rather than a motor that switches on when you happen to pedal. European premium e-bikes (Bosch, Shimano) all use torque sensors for this reason.

Torque sensors are significantly more complex to install (require specific bottom bracket types or crank compatibility), more expensive (₹3,000-15,000+), and require controller compatibility. Not all controllers support torque sensor input — KT controllers support it with the torque-enabled firmware versions. For most Indian DIY builds, cadence sensors are the practical choice.

Installing the PAS Cadence Sensor

The PAS cadence sensor has two parts: the magnet disk (mounted on the crank) and the Hall sensor pickup (mounted on the chainstay or bottom bracket shell, stationary relative to the frame).

Step 1: Mount the Magnet Disk

The magnet disk mounts around the crank spider or bottom bracket axle on the non-drive side (left side) of the bicycle. Most PAS sensors include multiple adapter rings to fit different crank diameters. Select the correct adapter ring for your crank spindle diameter (typically 22-27mm for standard three-piece cranksets).

Slide the disk over the crank spindle from the left side and position it between the left crank arm and the bottom bracket shell. The disk should spin freely as the crank rotates without contacting the frame or bottom bracket. Tighten the disk’s retention collar so it rotates with the crank without slipping.

For square taper or ISIS bottom brackets: the disk mounts on the axle before fitting the left crank arm. You may need to remove the left crank arm to install the sensor disk. For integrated (external bearing) cranksets: a different disk design that clamps around the crank arm is needed — check compatibility with your specific crankset.

Step 2: Mount the Hall Sensor Pickup

The Hall sensor pickup mounts on a stationary part of the frame, aligned with the magnet disk so each magnet passes within 2-5mm of the sensor as the crank rotates. The ideal mounting location is the chainstay (the frame tube running from the rear axle to the bottom bracket), adjacent to where the magnet disk spins.

Attachment methods:

• Zip ties: Most common for DIY builds. Loop two zip ties through the mounting hole on the sensor bracket and around the chainstay. Ensure the sensor does not slip during riding — add a third zip tie or a drop of thread-locker on the sensor body to the chainstay surface.
• Velcro + zip ties: More secure and adjustable. Wrap Velcro around the chainstay, attach the sensor to the Velcro, add zip ties over the Velcro+sensor sandwich for security.
• Custom bracket: For a cleaner installation, fabricate or 3D print a bracket specific to your frame’s chainstay profile.

Step 3: Set Sensor Alignment and Gap

The gap between the Hall sensor face and the magnets must be 1-5mm. Too far and the Hall sensor will not reliably detect the magnets. Too close and the magnets may contact the sensor when the crank flexes slightly under pedaling load.

Verify alignment by slowly rotating the crank by hand and watching for uniform spacing between the sensor and each magnet as it passes. If the disk wobbles significantly (more than 1mm), check that the disk is properly centered and the adapter ring is correct for your crank.

Wiring the PAS to Your Controller

Standard PAS cadence sensor wiring (3-pin JST SM connector):

• Red: 5V power from controller
• Black: Ground
• Green or White: Signal (Hall sensor output) — pulses from 0V to 5V as each magnet passes

The controller’s PAS connector has a matching 3-pin female JST SM socket. Connect the sensor cable directly to this socket. If the cable is too short (common if you mounted the sensor on the chainstay far from the controller), use a 3-pin JST extension cable — widely available from electronics suppliers in India.

Route the PAS cable along the chainstay and frame tubes using zip ties or cable clips, ensuring it does not interfere with chain movement, wheel rotation, or brake cables. Leave a small loop of slack near the sensor to prevent stress on the connector when the chainstay flexes slightly under load.

Configuring PAS in Controller Settings

After wiring, configure PAS behavior through the controller’s LCD display (for KT controllers) or Bluetooth app (for Sabvoton/Votol):

Key KT Controller PAS Settings

P3 — PAS level count:

• 0 = 3 levels (1, 2, 3)
• 1 = 5 levels (1-5, standard for most builds)
• 2 = 9 levels (1-9, for maximum granularity)

P4 — PAS/throttle interaction: Controls whether the throttle operates independently of PAS level. For legal compliance builds, setting P4 = 2 caps throttle power to the selected assist level.

C15/C16 — PAS start delay: Number of magnet pulses before the motor activates after pedaling begins. Higher values (4-6) create a more natural delay (similar to riding a regular bicycle where you feel resistance at the start of a pedal stroke before the motor adds power). Lower values (1-2) give immediate response.

Assist level power percentages: Each assist level (1-5) corresponds to a percentage of maximum motor current. Factory defaults often have level 1 at 20% and level 5 at 100%. You can adjust this distribution — for a natural feel with KT controllers (C4 and related settings), consider setting levels to 20%, 35%, 55%, 75%, 100% for a progressive feel.

Fine-Tuning for Indian Road Conditions

Indian city riding has specific characteristics that affect ideal PAS tuning:

Frequent traffic light stops: The PAS delay (how quickly the motor responds when you start pedaling from stopped) matters a lot in heavy traffic. Too-short delay causes motor kick when you first start pushing at a red light, potentially causing a surge in traffic. A delay of 3-5 pulses (roughly 1/4 revolution) feels natural and safe.

Uphill starts in traffic: On uphill intersections (common in Pune, Bengaluru, hilly cities), you need motor assistance before you have built up cadence. Consider setting the start delay shorter (1-2 pulses) or use throttle for initial hill starts and switch to PAS once moving.

Monsoon wet roads: On wet roads, sudden motor engagement from PAS at high assist levels can cause rear wheel spin. Use lower assist levels (1-2) in monsoon conditions and engage PAS smoothly by building cadence gently before the motor kicks in at full power.

Speed limit compliance: The controller will cut motor power above the set speed limit regardless of PAS. You will feel the PAS cut out at the speed limit — normal behavior. If this feels abrupt, ensure the speed limit is set correctly and the controller’s speed ramp-down at the limit is smooth (configurable in some controllers).

Troubleshooting PAS Issues

Motor doesn’t activate when pedaling:

• Check PAS connector is fully seated in controller
• Verify 5V on the sensor power pin (red wire) with multimeter
• Rotate crank slowly and check signal wire — should pulse between 0V and 5V as each magnet passes
• If no pulses: Hall sensor gap may be too large, or sensor is defective
• Ensure PAS is not disabled in controller settings (P4 = 0 should have PAS active)

Motor activates briefly then cuts out:

• Check that the magnet disk is spinning consistently — if it slips on the crank, the sensor sees intermittent rotation
• Verify the disk’s retention collar is tight

Motor runs for 2-3 seconds after stopping pedaling:

• This is normal — the cutoff delay is the time between the last cadence pulse and motor cutoff. Adjust C15/C16 if the delay is too long for your preference.

PAS causes jerky, on-off motor feel:

• This is characteristic of cadence sensors with few magnets (5-magnet vs 12-magnet). If using a 5-magnet sensor, upgrade to a 12-magnet unit for smoother response.
• Also check assist level — at high assist levels with sudden motor activation, the jerk is more pronounced. Use PAS start delay setting to smooth the onset.

Frequently Asked Questions

Can I use PAS and throttle simultaneously?

Yes — most KT controllers allow both. The motor responds to whichever demands more power. During PAS assist, adding throttle input supplements the PAS-commanded power up to the motor’s maximum.

My PAS sensor cable is too short to reach the controller. What do I use?

Use a 3-pin JST SM extension cable (same connector type as the PAS sensor). These are available from electronics stores and online suppliers in 0.5m, 1m, and 2m lengths.

Does PAS work while in reverse/motor off?

PAS only activates when the controller is powered on and assist level is above 0. At Assist Level 0, PAS input is ignored — useful for pure pedaling exercise without any motor assistance.