🛠️ Mid-Drive (BBS02 / BBSHD) Install — Step by Step

This is the big one. A Bafang mid-drive replaces your bottom bracket and cranks with the motor unit, then adds a display, throttle, brake cutoffs, and wiring. Work through it in order and don't rush the bottom-bracket removal — that's where first-timers lose the most time.

⚠️ Heads up: connect the battery LAST.

Before you begin

Before you begin: confirm fitment. BBS02 needs a 68–73mm threaded (BSA/JIS) bottom bracket shell; BBSHD also comes in 100mm and 120mm fat-bike widths. Press-fit shells (BB30/PF30/BB86/BB92) need a third-party adapter or won't work. If you haven't already, print the actual-size motor template from our site and check chainstay clearance.

Remove the old drivetrain

Photograph and prep the bike. Shift into the smallest rear cog, photograph your cable routing and lever positions, then put the bike in a stand. Remove the battery from any old accessories and keep your new Bafang battery disconnected and set aside — you will not connect it until the very end.
Remove the drivetrain parts. Break the chain with the chain tool (or drop it off the chainring). Remove both pedals — the left pedal is reverse-threaded (turn clockwise to loosen). Pull both crank arms with the crank puller. If you run a front derailleur, remove it now — a mid-drive is single-chainring, so the front derailleur comes off for good.
Remove the bottom bracket and clean/deburr the shell. Use your BB tool to remove both cups/the cartridge. Both sides — the non-drive cup is often reverse-threaded, so check which way yours turns. Once the shell is empty, remove or trim any plastic cable guide clipped under or through the BB shell, and file down any rough weld ridges or burrs inside the shell. The Bafang axle is a tight fit; even a small burr or an intact cable guide will stop the motor from seating. This deburring step is skipped in most videos and is a common reason a motor "won't fit."

- If the BB is seized (common on older bikes): apply penetrating oil, give it time, and use a breaker bar or long cheater pipe on the tool. If it still won't budge, take it to a shop — a seized BB is the #1 wall first-timers hit, and it's not worth destroying your frame over.

Mount the motor

Seat the motor in the shell. Slide the motor spindle through the shell from the drive (right) side, then rotate the motor body up snug against the chainstay/underside of the frame so it can't spin. Grease the spindle lightly first.
Fit the fixing plate and lockrings. From the drive side, install the triangular fixing plate and its bolts, then thread on the inner M33 lockring with the Bafang wrench. Torque the inner lockring to about 50 Nm and add blue Loctite to the threads. Fit the thin outer locknut if it reaches the threads.

- 73mm bottom-bracket note: on a 73mm shell the motor sits proud, so you need the longer M6 fixing-plate bolts (~20mm) plus a ~5mm spacer/washer stack between the fixing plate and the motor, and the thin outer lockring often can't reach the threads — that's expected. In that case rely on the inner lockring plus Loctite. (See the dedicated 73mm guide below.) - Why this matters: an under-torqued lockring is the single most common install fault. If the motor rotates or "noses down" the first time you apply power, the lockring is loose. Get it tight now.

Install the chainring and cranks. Bolt on the chainring (stock steel, or a Lekkie Bling Ring for a better chainline and lighter weight). Fit the crank arms — the left crank goes on the left, and remember the left pedal is reverse-threaded going back on. Torque crank bolts to about 35–40 Nm.
Re-size and reinstall the chain. With the new single chainring the chain length is almost always wrong. Run the chain and find the correct length (big-big plus a bit of tension, or use your original length as a starting reference from your photos), then join it with a quick link or the chain tool. Check that it doesn't drop or bind across the cassette; aim for a chainline within about 2mm of the middle of the cassette.

Wire the controls

Mount the speed sensor and spoke magnet. Fix the speed sensor to the left chainstay and zip-tie the cable so it can't rub the tire or chain. Attach the magnet to a rear-wheel spoke (T20 Torx screw) and align it to pass the sensor with a 5–10mm gap. This is required — no magnet, or too big a gap, and you'll get Error 21 and the controller will limit power. Bafang's updated speed-sensor magnet looks like this:

Bafang updated speed-sensor magnet, alternate view

Set up the handlebars. Slide off the old grips/shifter as needed to make room. Mount the display, the control pad/button unit, the left thumb throttle, and the brake levers or brake cutoff sensors. Hydraulic-brake users use the magnetic e-brake sensor type. Snug the clamps but leave them slightly loose so you can adjust angles after a test sit.
(Optional but recommended) Install the gear shift sensor inline on the derailleur cable — see the dedicated guide below. Strongly recommended on the BBSHD to protect the drivetrain.
Route the harness and connect the Higo plugs. The motor's main 1-to-4 harness splits to display, throttle, left brake, and right brake. Match the arrows/keyways on each Higo connector and push straight in until it clicks — never force a misaligned plug or you'll bend pins. Route cables cleanly along the frame with a small drip loop at each connector and zip-tie everything so nothing rubs or snags. Connect the battery LAST, after every other plug is seated — connecting the battery while you're still wiring is a classic mistake and can cause sparks or a startled runaway.

First power-on

Power-on test (wheel off the ground). Mount and connect the battery, turn the display on, and confirm no error codes. Set your wheel diameter in the display settings — the screenshots below show where the wheel-size setting lives on the C961, 500C, and DPC-18 displays:

With the rear wheel off the ground, gently test the throttle and each PAS level, then squeeze each brake and confirm the motor cuts out. If you see Error 30, reseat every connector (half-seated plugs are the usual cause on first power-up).

Test ride, then re-torque. Take a short, cautious ride. Note that stock PAS level 1 is often abrupt — that's factory programming, not a fault, and can be tuned later with the USB programming cable. After the first 50 miles, re-torque the lockring, crank bolts, and pedals, and re-check the chainline and connectors. Things settle in on the first rides.

Supplement video: "Step by Step Bafang Mid-Drive BBS02 and BBSHD Motor Kit Installation"