From The Helmet Update - Bicycle Helmet Safety Institute - Vol. 29, #5 -- December 6, 2011
The Snell Memorial Foundation has introduced an Addendum to their B90 and B95 bicycle helmet standards to accommodate the unique design of time trialing or chrono helmets with long aerodynamic tails.
The Addendum defines the tail for those helmets and exempts it from the projections limit in the basic standards. It is on Snell's web page:
Perhaps most revealing, the Addendum has cautionary language about time trial helmets. There is a statement that includes:
"However, these same features may be deemed inappropriate for bicycle helmets intended for street use and for casual riding. In particular, long, tapered tails are considered to be projections which may become loading points for tangential forces and, as such, are in conflict with the sections of the standards concerning general limitations on construction. However, there are no reasonable concerns for helmets used by experienced riders in supervised training or competition under controlled conditions."
and the standard requires a warning label:
"WARNING: THIS HELMET IS NOT INTENDED FOR RECREATIONAL USE. This special use helmet has been designed to provide an aerodynamic benefit through an aerodynamic tail which in a fall or crash may reduce its ability to provide adequate protection. In a fall or crash the aerodynamic tail may cause the helmet to be pushed out of position thereby exposing the head to serious and/or fatal injury. Similarly, in a fall or crash a rider may be exposed to a strangulation and/or choking hazard from the retention system. USE ONLY ON A CLOSED DESIGNATED COURSE IN CONNECTION WITH SANCTIONED TIME TRIALING ACTIVITIES OR COMPETITIVE EVENTS."
We think Snell has captured the issues very well, and our own web page on chrono helmets:
has always cautioned riders not to use them on the street.
We don't know how a test lab can ever test the rear location of a chrono helmet on the hemispheric anvil, since the tail will push it aside no matter how much duct tape is applied, but the real lesson is that time trial helmets belong in time trials and other supervised competition, and are not for use in regular road biking.