A hub motor that will not go into your frame is almost never a faulty motor. It is a measuring problem. Three numbers decide whether a motor drops in or sits in a box waiting to be returned: dropout width, axle diameter, and wheel size stated as diameter plus tire width. None of them are printed on your bike. All three take about ten minutes to establish with a rule or a set of calipers.
1. Dropout width: measure the clear inside gap
The dropouts are the two slotted ends of the frame that the axle sits in. What matters is the clear inside distance between them, measured with the wheel removed.
- Take the wheel out. Do not try to measure around a fitted wheel.
- Put the calipers or a steel rule flat against the inner face of one dropout and read across to the inner face of the other.
- Read in millimetres, not inches. The standards are defined in millimetres.
For a rear hub motor the answer will land on one of three numbers: 135 mm on narrow-tire frames, or 170 mm or 190 mm on fat-tire frames. For a front hub motor, 100 mm is the common fork spacing; some fat-tire forks are wider, so measure yours rather than assuming a figure.
The most common error is measuring the outside width of the dropouts, or reading from the outer face of one to the inner face of the other. Either adds several millimetres and can make a 135 mm frame look like a 140-something, which matches nothing.
170 mm and 190 mm are not interchangeable
This is the single most expensive error in this category, and it is worth stating plainly. A 20 mm difference is not something to spring the frame over. Forcing the fit can deform the frame, and an axle that is not clamped square in both dropouts is a genuine safety problem rather than a tolerable annoyance. Some motors ship with spacer washers that allow one build to serve both widths, but not every model does — so treat that as something to confirm before ordering, never as a general assumption.
2. Axle diameter: usually 12 mm or 14 mm
The axle is the threaded shaft that passes through the motor and carries the axle nuts. Measure across the threads with calipers. Two sizes cover most hub motors: 12 mm and 14 mm.
It matters because the axle has to enter the dropout slot, and older or lighter frames were often cut for 10 mm or 12 mm. So measure the slot too: hook the calipers into the open end of the dropout and read the gap. If the slot is narrower than the axle, that motor is not a candidate for your frame. Filing a dropout wider removes material from a part that carries drive torque, and we do not recommend it.
3. Wheel size is two numbers, not one
Diameter alone does not identify a wheel. A 20" narrow-tire wheel and a 20" fat-tire wheel are different parts: rim widths run roughly 25–35 mm for narrow tires against about 82 mm for a 4.0" fat tire, and dropout widths differ with them. Both figures are moulded on the tire sidewall — always quote them together, as in "26 x 4.0".
On a bare motor the diameter is set later, at lacing time — one of the few real advantages of the bare part. Our comparison of motor wheels against bare hub motors covers when each is the sensible buy.
A note on mid-drives
Mid-drive motors are sized by a different dimension entirely: the width of the bottom bracket shell, commonly 68–73 mm, with 100 mm and 120 mm shells used on some fat-tire frames. If your motor sits at the cranks rather than in a wheel, that shell width is the number to measure.
Write the numbers down before you shop
Four figures, on paper: dropout width, axle diameter, wheel diameter, tire width. With those in hand a fit question takes one message instead of three.
Fitting mechanically is only half of it — the connector and pin order still have to match your controller. If you are pairing a motor with a controller you already own, read our guide to phase and hall wiring colour codes first.
Before you take the wheel out
Power the bike off and remove the battery before disconnecting any motor cable, and give it thirty seconds. Separating phase wires with power present can arc and damage the controller output stage. After refitting a wheel, check that both axle nuts are tight and the torque washers are seated in the dropouts, then test at low speed in an open area.
Not sure what you measured?
Send us your four numbers plus photos of your display, its connector, your controller label, and your motor cable, and we will confirm the fit before you order. Current stock is under Motors & Wheels, and the calipers and axle wrenches used for these measurements are in Tools.
RideRow sells parts and provides installation information. We do not perform repairs.