BSD (Bead Seat Diameter)


rim labelBSD, or beat seat diameter, is the ISO standard way of stating wheel sizes.
When buying replacement tyres, they must be exactly the right size for the wheel. The BSD measurement is a way to be sure they are made to the correct size.

Tyres have a pair of numbers printed on the sidewall, e.g. 28 – 622. The second of these numbers is the diameter in millimetres of the thin steel hoop embedded in the tyre edge. This steel hoop forms a ‘bead’.

Most wheels will have the BSD printed on the rim label (pictured) or occasionally embossed on the rim.

When a tyre is fitted to a wheel rim, the bead sits in a small groove in the rim known as the ‘bead seat’.

Example BSDs

622mm is the BSD for 700c, the wheel size commonly used on road bikes and touring bikes.

559mm is the BSD for 26″ mountain bike wheels.