Proper helmet fit is vital to the performance of a helmet during an impact. For the helmet to perform properly during an impact, it must stay securely on the head.
Measuring the head is only a starting point for the entire sizing procedure. Due to varying shapes, heads that are apparently the same size when measured by a tape may not necessarily fit the same size helmet.
A small metal tape measure or a cloth tape may be used to make your initial measurement. You can also use a string, which can then be laid against a measuring tape.
Measure the circumference of the head at a point approximately one inch above the eyebrows in front, and at a point in the back of the head that results in the largest possible measurement. Take several measurements. The largest measurement is the one you want to use.
SIZING COMPARISON CHART
HELMET SHIELD SYSTEM
Bell Racing helmets come equipped with one of two shield systems: SRV or Detent. In early 2006 the majority of Bell's helmets were converted to the SRV system. This new system has a slide mechanism that allows the driver to adjust the tension on the shield allowing for multiple opening positions. When properly adjusted and in the locked position, the SRV systems helps reduce the possibility of the shield opening during an impact.
The helmets that feature the SRV system are: Dominator, Vortex, Infusion Top Forced Air, Infusion Side Forced Air, M4 Pro, M4 Pro Extra, M3, M3 Extra, GTX, BR-1, BR-1 Side Air, M4 and the M4 Top Air.
The Bell helmets that are still equipped with the Detent system are:
Vortex Side Forced Air, Vortex Top Forced Air, M2, M2 Forced Air, Sport 3, K1 Sport and the Kart 2 Pro.
When ordering extra shields for your helmet, it is important to specify an SRV or detent shield.