Wheel Sizes and ETRTO for E-Bike Conversions
When you buy a motor wheel, what counts is not the inch size but the ETRTO number on the tyre. The second number, for example the 622 in 40-622, is the rim diameter in millimetres. Inch names such as "28 inch" are ambiguous and often lead to wrong purchases on conversions.
How to read the size
On the tyre sidewall there is a marking such as 40-622 or 37-622. The first number is the tyre width in millimetres, the second the rim diameter. Next to it you usually find inch figures like 28 × 1.60, which help you little. Note the ETRTO number and match it against the motor.
You can also measure the rim itself: determine the inner diameter at the rim shoulder with a tape measure, but that is less accurate. The tyre marking is more reliable.
The table
| ETRTO | Name | Typical bikes | Trap |
|---|---|---|---|
| 622 | 28", 29", 700C | trekking, city, road, 29er MTB | 28" and 29" are the same rim |
| 635 | 28" × 1 ½ (old) | old Dutch-style bikes | does not fit 622 kits although both are called "28" |
| 584 | 27.5", 650B | MTB, gravel | |
| 559 | 26" | older MTB, touring bikes | |
| 507 | 24" | kids' and youth bikes | |
| 406 | 20" | folding bikes, cargo bikes | do not confuse with 451 |
| 451 | 20" (narrow) | some folding bikes, BMX race |
The table is a compilation from our knowledge base. The statement on 635 versus 622 is an assumption that Fabio is still checking.
The traps
28 inch is not always 28 inch. Modern trekking and city bikes have 622 mm. Old Dutch-style bikes marked "28 × 1 ½" often have 635 mm. Both are called 28 inch, but a kit fits only the right size. So always check the second ETRTO number.
20 inch comes in two sizes. 406 is the usual 20-inch wheel, 451 the narrow variant. A folding bike kit for 406 does not fit 451. On folding bikes a second look pays off, because both sizes occur.
26, 27.5 and 29 inch: 26" is 559, 27.5" is 584, 29" is 622. A mountain bike with "29 inch" has the same rim as a trekking bike with "28 inch".
What to check when buying
- Read the ETRTO number off the tyre.
- Order motor and wheel to match the ETRTO. Many dealers sell kits as a ready-built wheel, others only the hub for lacing.
- Cross-check spacing and axle, see hub spacing.
- Brake type: rim brake or disc? The braking surface must fit, see brakes.
- Tyre width: The new tyre must fit in fork and rear triangle.
Hub, rim, spokes
Lacing creates a new wheel. Thin spokes and motor weight are a common cause of broken spokes, see common problems. If in doubt, let a workshop that does it regularly handle the lacing.
With kits that come with a ready-laced wheel you have to trust that the dealer used a suitable rim. Check the ETRTO in the product description and ask if unsure.
What this means for motor types
Front and rear hub motors need the right ETRTO size, while the mid-drive does not depend on it. If your bike has an unusual size, a mid-drive is often the more relaxed solution, see motor types compared and measure your bottom bracket.
How reliable is this?
The ETRTO numbers and names are standard values; the statements on fit and traps (especially 635 versus 622) are based on experience. Fabio is still checking the table.
Frequently asked questions
- Where do I find the ETRTO marking?
- On the tyre sidewall, for example 40-622. The second number is the rim diameter in millimetres.
- Are 28 inch and 29 inch the same rim?
- Yes, both have a 622 mm rim diameter. 700C is the same size too.
Sources
Related articles
Hub Spacing and Axles for an E-Bike Conversion
How to measure hub spacing, quick release versus thru-axle, spreading 130 to 135 mm on steel frames and what to watch with dropouts.
Torque Arm: When You Need One
Why the axle of a hub motor turns in the dropout, when you need a torque arm and how to fit it.
Mid-Drive or Hub Motor: What Suits You?
Ride feel, hills, gears, installation and cost compared, with a clear guide to when a mid-drive and when a front or rear hub motor makes sense.