Fat bikes convert well with a Bafang mid-drive, but the decision that matters is made with a tape measure, not from the bike's model name. The number you need is the width of the bottom-bracket shell — the tube the cranks pass through. On the fat bikes in our install records, that shell measured either 100 mm or 120 mm. We have 44 verified fat-bike conversions on file. This article gives you the measured widths, the chainring sizes those builds actually used, the specific models customers converted, and the four clearance checks that catch people out. Another rider's build is a useful reference, not a fit guarantee for your frame. A completed customer fat-bike conversion: Bafang mid-drive at the bottom bracket, battery on the downtube. Photo submitted by an Ebike Essentials customer. On this page Measure the bottom-bracket shell first Measured widths from real conversions Should you order 100 mm or 120 mm? Chainstay clearance and chainring choice Fat bikes customers have converted Four fit checks people miss FAQ Measure the bottom-bracket shell first Three measurements decide whether a kit fits: the width of the shell, its inside diameter, and whether the shell is threaded or press-fit. Our Help Center measurement guide walks through each one on video. Measure the bike in front of you. Frames change between model years and trim levels, so a width you read in a forum thread about the same model name may not be the width of your frame. Print the template. The BBS02 and BBSHD fitment templates are printed at 1:1 scale. Hold one against the frame and you can see whether the motor body clears the chainstay before you order anything. Measured widths from real conversions Shell width follows the frame, not the category label. These are the widths explicitly recorded in our install database: Bike type 68 mm 73 mm 100 mm 120 mm Records with a measured width Fat bike 0 0 6 10 16 Mountain bike 3 7 0 0 10 Two things stand out. No fat bike in our records measured 68 mm or 73 mm, and no mountain bike measured 100 mm or 120 mm. The two categories did not overlap once in 26 measured builds. That is a strong pattern, but it is observed data from our own customers — not a specification that covers every frame ever made. Measure yours. Should you order 100 mm or 120 mm? Order the size that matches your measured shell. Among our measured fat-bike builds, ten were 120 mm and six were 100 mm, so both are common and neither is the safe default. Wider is not automatically better. The bracket size changes where the motor body and chainring sit relative to the frame, which is why it interacts with the clearance checks below. One customer who fitted the 100 mm version told us they would choose 120 mm if they did it again, for clearance reasons — that is one build's experience, not a sizing rule. Of the 44 fat-bike conversions on file, 39 used the 1000W BBSHD. The records show a clear preference for it; they do not tell us why each customer chose it. The BBSHD is also the mid-drive we stock for these frames. The complete kit comes in a 74–100 mm (Fat) and a 101–120 mm (Extra Fat) bracket size. Match it to your measurement. See the complete Bafang 1000W BBSHD kit and battery → Not sure yet? Take the fitment quiz → Chainstay clearance and chainring choice A fat-bike chainstay flares outward to clear a 4-inch tire. That is the whole fitment problem in one sentence: the frame is widest exactly where the motor and chainring need to go. Check two things before you tighten anything — that the motor body clears the chainstay, and that the chain runs at a workable angle in the gears you actually use. The same build up close. The Bafang motor body sits between the chainstays with the chainring and guard outboard of it — this is the clearance you are checking for. Recorded fat-bike builds in our database used 40T, 42T and 44T chainrings most often, with some as small as 28T. Tooth count and chainring offset solve different problems, so a smaller ring is not automatically a chainline fix: Chainring Sizes What it changes BBSHD upgraded aluminum 40T, 42T, 44T Lower gearing for loose ground and climbing; a smaller ring also buys frame clearance BBSHD steel 46T A large offset that moves the chainline outboard; taller gearing A second customer fat-bike build. Note how close the chainring sits to the tire sidewall — that gap is what you are checking before you commit to a chainring size. Aluminum 40T / 42T / 44T chainring → 46T steel chainring → Help Center: chainring alignment, fitment and sizing → Fat bikes customers have converted These are the fat-bike models named in our install records. Treat a model name as a starting point, not a specification — production year, frame version and any previous bottom-bracket work all change what you will measure. Model Builds on record Noted detail Mongoose Dolomite 3 — Specialized Fatboy 2 One build required a PF30-to-BSA conversion Surly Pugsley 2 — Mongoose Argus 1 — Mongoose Kong 1 — Surly Moonlander 1 — Gravity Monster 1 — 9:ZERO:7 1 — A Mongoose fat bike mid-conversion, motor installed. Mongoose frames — Dolomite, Argus and Kong — are the most frequently converted fat bikes in our records. Four fit checks people miss 1. Press-fit shells need an adapter A Bafang mid-drive mounts in a threaded (BSA) shell. One Specialized Fatboy in our records needed a PF30-to-BSA conversion before the motor could go in. Work out whether your shell is threaded or press-fit before ordering — the adapter is a separate part and is not included in the kit. 2. Spacers align the motor; they do not fix a wrong measurement Several customer records mention using bottom-bracket spacers to seat and square the motor on the shell. Fit the motor first, see how it sits, then add spacers if the alignment calls for it. Spacers are not a way to make a mismeasured bracket size work. 3. Rear dropout spacing is a different number Some fat bikes use 175 mm rear dropout spacing. That is a rear-wheel and drivetrain measurement, and it is easy to confuse with bottom-bracket width because both are "how wide is it" questions. It has no bearing on whether you order the 100 mm or 120 mm motor. 4. Check where the battery will actually mount Across every bike type in our records, the downtube is the most common battery position, followed by a frame triangle bag and then a rear rack. Fat-bike frames usually have downtube room, but bottle-cage bolt spacing varies. Check your frame against the battery fit guide and its printable templates before you order. FAQ Which BBSHD bracket size do I need for a fat bike? Whichever matches your measured bottom-bracket shell. In our records, measured fat-bike shells were 100 mm in six builds and 120 mm in ten. Measure the shell rather than ordering from the bike's model name. Can I tell the width from my bike's model name? No. Frames change between years and versions, and a previous bottom-bracket replacement can change what you measure. Our records show fat bikes at both 100 mm and 120 mm. What chainring should I use on a fat bike? Recorded fat-bike builds most often used 40T, 42T or 44T, with some as small as 28T. A smaller ring lowers the gearing and gains frame clearance; the 46T steel ring instead offers a large offset that shifts the chainline outboard. Do I need bottom-bracket spacers? Some frames need them to seat the motor squarely, and several customer records mention them. Decide after you see how the motor sits, not before. My fat bike has a press-fit bottom bracket. Can it still be converted? Yes, with an adapter that converts the press-fit shell to threaded BSA. One Specialized Fatboy in our records used a PF30-to-BSA conversion. The adapter is not included with the kit. How can I be sure it will fit before I buy? Measure the shell width, inside diameter, and whether it is threaded or press-fit; check that the motor body clears the chainstay using the printed 1:1 template; and confirm the battery position. If anything is borderline, send us the measurements and a photo, or use the fitment quiz. Still not certain? Send us your three bottom-bracket measurements and a photo of the crank area and we will tell you what fits. Installation videos, printable templates, wiring guides and troubleshooting all live in the Help Center, and step-by-step build walkthroughs are in our installation guides — including a full mid-drive install on a 73 mm bottom bracket, which covers the same steps on a narrower frame. Take the fitment quiz → Watch the measurement videos →
