Bosch Purion: wheel circumference (why it's dealer-only)
The wheel circumference is shown in the Purion's settings menu but on most units can't be changed there. Because it sets when assistance stops at 25 km/h, Bosch locks it in DiagnosticTool for the dealer. Dealer visit needed for a real change.
Step by step
- Switch the bike on and hold the − button for about 1 second to open the settings menu.
- Step with + / − to 'Wheel' (older firmware: 'Wheel circ.'). The value is in millimetres, e.g. 2205.
- On many Purions this value is read-only. On some later firmware you can adjust it a few percent with + / −; if the value does not change, it is locked.
- For any larger change (different tyre size, new wheel), note the correct circumference from the tyre sidewall or a wheel-size table and ask a Bosch dealer to set it with DiagnosticTool.
- Hold − for about 1 second to leave the menu.
Can't find it?
If 'Wheel' is missing or refuses to change, that is normal on the Purion: Bosch only allows the dealer to set it. A wrong circumference usually shows as speed that is consistently 5–10% off; a jumping or zero speed is a sensor/magnet alignment problem, not a setting.
Frequently asked
Why can't I change the wheel size myself on a Bosch?
The circumference determines at what real speed the motor cuts out at 25 km/h (a legal limit). Bosch therefore stores it in the drive unit and only lets dealers change it with DiagnosticTool.
Does a wrong wheel size damage anything?
No, but speed and distance will be inaccurate and the cut-off will happen slightly early or late. Have it corrected at the next service.
I changed tyre width, do I need a new setting?
Only if the circumference changed by more than a percent or two. Going from a 47 mm to a 55 mm tyre on the same rim is worth having set.
Manuals and video
- Bosch eBike Help Center: official error-code index
- Bosch eBike downloads: quick start guides and battery guide
- Bosch eBike dealer search (DiagnosticTool readout)