A common way to lose a week on a conversion: a rear hub motor listed as fitting "135–142 mm" arrives, the frame measures 142 mm, and the motor still will not go in. The problem is not width. It is the axle standard — and a bolt-on hub motor cannot be mounted in a thru-axle frame.
Three ways a rear wheel is held in a frame
- Bolt-on (solid axle). A solid threaded axle sits in two open slots at the ends of the frame, and nuts on the outside clamp it in place. This is how most conversion hub motors are built.
- Quick release. A hollow axle sits in the same kind of open slots, held by a thin skewer with a cam lever. Open the lever and the wheel drops straight out.
- Thru-axle. The frame has closed round holes instead of slots. A large-diameter axle slides through one side of the frame, through the hub, and threads into the other side.
Bolt-on and quick-release frames share the thing a hub motor needs: an open slot. A thru-axle frame does not have one.
The 30-second test for your frame
Take the rear wheel out and look at where the axle sat.
- If you only had to open a lever or loosen two nuts, and the wheel dropped down out of a U-shaped or hook-shaped slot, you have open dropouts. Measure the inside gap next, as covered in our guide to dropout width and axle diameter.
- If you had to unscrew an axle and pull it out before the wheel could move, and the frame is left with two closed round holes, you have a thru-axle frame. For a standard bolt-on hub motor, the answer is already no.
Why "142 mm" is a trap
Rear frame spacing comes in several standards: 130 mm, 135 mm, 141 mm quick release, 142 mm thru-axle, 148 mm "Boost" thru-axle, and the wider fat-tire widths of 170 mm and 190 mm. These are not interchangeable.
On a modern frame, "142" usually means 12×142: a 12 mm thru-axle in a 142 mm rear end. That is a closed axle interface, not a wider open slot. A listing that says "fits 135–142 mm" is quoting a width range, not promising that the motor accepts a thru-axle. Ask instead for the motor's published over-locknut width and its stated axle type.
Why a bolt-on motor cannot be made to fit
A hub motor's axle is not a removable part like a thru-axle. It is fixed to the motor, sticks out both sides, and carries the phase and hall cable out through one end. Two things follow:
- It cannot be threaded through a closed hole. A fixed axle needs a slot to drop into.
- It has nothing to grip against. The flat sides on a motor axle, and the tabbed torque washers beside it, are what stop the axle turning in the frame under drive torque. A round hole gives the flats nothing to bear on, and an axle that spins in the frame can tear out the motor cable.
So do not file, bend, or force a standard motor axle into a thru-axle frame, and do not stack spacers to make up a difference. Spreading a frame is not a DIY fix either, especially on aluminum or carbon, where it can cause hidden damage. We have not verified any adapter that turns a thru-axle frame into an open-slot frame for a hub motor, so we do not recommend one.
What you can do instead
- A motor made for your axle standard. Thru-axle frames generally need a hub motor engineered specifically for that standard. Confirm the exact standard with the motor's maker — 12×142 is not the same as 12×148.
- A front hub motor, if the fork qualifies. Run the same test on the fork: it needs open slotted dropouts too, and many bikes with a rear thru-axle use a thru-axle fork as well. Check the fork material and dropout condition, and read our notes on axle washers and torque arms first.
- A mid-drive. A mid-drive mounts at the cranks, so the rear axle standard does not decide whether the motor fits. The bottom bracket shell width does, and our sizing guide covers that measurement.
Before you order a hub motor
A bolt-on hub motor is built for a nutted axle in open dropouts, not for a thru-axle frame. If your frame failed the test above, please do not order one expecting a fit.
Not sure which kind of frame you have? A clear photo of the rear hub from the drive side, with the dropout in view, usually settles it. See the 4 photos we need to confirm a part fits, and we will tell you before you order.
If the bike already has a motor, turn it off and remove the battery before unplugging any motor cable or taking the wheel out.
RideRow sells parts and provides installation information. We do not perform repairs.