Wheel diameter is not enough. A 20-inch fat wheel and a 20-inch standard wheel are different parts, and the number that decides whether a motor physically fits your frame is the dropout width — the gap your axle sits in.
170 mm and 190 mm are not interchangeable. Forcing a motor into a frame that is 20 mm narrower springs the stays, will not clamp the axle properly, and is genuinely unsafe. This is the most common and most expensive mistake in this category.
How to measure it
Take the rear wheel out
Measure the inside gap between the two dropouts Use calipers or a steel rule, measuring face to face where the axle sits.
The number will be one of three
| Measurement | Your frame is | Typical bikes |
|---|---|---|
| 135 mm | Standard | Most non-fat e-bikes, city and commuter frames |
| 170 mm | Fat, narrower standard | Many fat-tyre and folding fat models |
| 190 mm | Fat, wider standard | Wider fat-tyre frames |
Some motors ship with spacer washers that let one unit cover both 170 and 190 mm. Many do not. Confirm before ordering rather than assuming.
The other two numbers to note
Wheel diameter
Printed on the tyre sidewall — 20", 24", 26", 27.5" or 700c. The replacement must match what is on the bike now.
Tyre width
Standard tyres run roughly 1.75–2.5". Fat tyres are around 4". The rims differ by more than double in width, so this is not a detail.
Front wheels
Front forks are commonly 100 mm on standard frames, and wider on fat forks. Measure the same way. Axle diameter is typically 12 mm or 14 mm.
Motors and motor wheels Tell us your three numbers — diameter, tyre width, dropout width — and we will confirm the fit. Not sure what's wrong? Run the three-minute check first — most pedal-assist faults need no new part at all.RideRow sells replacement parts and provides installation information. We do not perform repairs. Installation is carried out by you or a technician you choose.