The clamp diameter of a bicycle fork refers to the top dimension of the steerer tube (ie. where the stem is installed). The crown race diameter of a bicycle fork refers to the bottom dimension of the steerer tube (ie. where it sits against the headset's lower bearings). Bicycle forks are most commonly seen in a straight steerer tube configuration with a diameter of 1-1/8 inch, meaning a 28.6mm clamp diameter and 30.0mm crown race. Similarly, bicycle forks with tapered steerer tubes from 1.5 to 1-1/8 inch diameter have a clamp diameter of 28.6mm and a crown race of 39.7mm. Bicycle forks are available for different front wheel axle standards such as 9mm quick release skewers, 15mm and 20mm thru axles.