What rear hub spacing does a hub motor need?

For most hub motors 135 mm is the standard. Older bikes and road bikes often have 130 mm, thru-axle bikes 142 or 148 mm.

Can I spread a frame from 130 to 135 mm?

On steel frames it often works, on aluminium and carbon it does not. It remains an intervention you should only do if you know what you are doing.

Hub Spacing and Axles for an E-Bike Conversion

Hub spacing is the distance between the inner faces of the dropouts that hold the wheel. At the rear, 135 mm is the usual value for hub motors, at the front usually 100 mm. If your bike has a different spacing, you need a matching motor, an adapter, or you leave it.

Common dimensions

Position	Spacing	Typical for
Front	100 mm, quick release	almost all bikes
Front	12 × 100 or 15 × 100/110 (Boost), thru-axle	MTB, newer bikes
Rear	130 mm	road bikes, older bikes
Rear	135 mm	trekking, MTB with quick release
Rear	142 or 148 mm, thru-axle	MTB, newer bikes

Fahrrad XXL names 135 mm as the standard for most hub motors. Front thru-axle standards generally need special motors; that is an assumption Fabio is still checking.

How to measure

Remove the wheel. Without it you can reach the dropouts.
Measure the gap. With calipers or a ruler, measure between the inner faces of the dropouts, with no axle and no disc brake in the way.
Quick release or thru-axle? If the wheel sits with a lever and a thin skewer, it is a quick release. If there is a thick axle with a thread or lever, it is a thru-axle (12 or 15 mm).
Look at the dropout shape. Open dropouts with a slot or dropouts with a safety tab change how the motor sits.
Write down the values. Together with a photo and the frame material.

The hub or axle end sometimes carries a marking such as "135", which helps with cross-checking.

130 or 135 mm: spreading

Does a 135 mm motor go into a 130 mm frame? On steel frames 130 mm can often be spread to 135 mm. On aluminium and carbon it does not work, because they can crack or break. Even with steel it is an intervention in the frame. Do it with feel and in small steps, or have a workshop do it. The frame maker's warranty is usually gone afterwards.

The easier alternative: look for a motor in 130 mm if your dealer offers one.

Axles and dropouts

The axle of a hub motor is flattened, usually 10 mm, sometimes 12 mm. The dropouts must take these flats. If the axle does not fit, there are two ways:

File the dropouts. On steel this sometimes works, on aluminium forks it is risky.
Choose a different motor.

As a rule of thumb: do nothing to a dropout that you cannot undo or check. A cracked dropout can cause a crash.

Torque arm

With aluminium dropouts and with front motors a torque arm is always sensible. It stops the axle from turning in the dropout. More under torque arm.

Thru-axle and disc brake

If you have a thru-axle or a front disc brake, that narrows the choice. For disc brakes the motor needs a matching mount, see brakes. Thru-axle motors exist but are rarer and usually pricier.

Other things to check

Tyre clearance: A hub motor and wide tyres need room in fork and rear triangle.
Wheel size: The rim must match the ETRTO number, see wheel sizes.
Gearing: At the rear most motors need a derailleur setup, see motor types compared.
Frame material: Carbon is not suitable, see frame and weight.

If it does not fit

If neither spacing nor axle fits, a mid-drive is often the more elegant solution, because it does not touch the wheels at all. It has other hurdles though, see measure your bottom bracket. If you find the adjustments cost more than the bike is worth, skip it and buy used.

How reliable is this?

That 135 mm is standard at the rear for most hub motors comes from one source. The other dimensions, spreading steel frames, axle sizes and the statements on dropouts are experience or assumptions that Fabio is still checking.

Does a conversion fit your bike?

Frequently asked questions

What rear hub spacing does a hub motor need?: For most hub motors 135 mm is the standard. Older bikes and road bikes often have 130 mm, thru-axle bikes 142 or 148 mm.
Can I spread a frame from 130 to 135 mm?: On steel frames it often works, on aluminium and carbon it does not. It remains an intervention you should only do if you know what you are doing.