Summary: You want your helmet to slide when you hit the pavement, not stick and jerk your neck. Rounder, slicker helmets tend to do that better.

The optimal bicycle helmet for that moment when you hit the pavement has a round shape and a hard, or at least smooth, plastic, shell. Most bicycle helmets do not have a hard shell today, primarily because they were so heavy and so expensive to manufacture. The ones we see in the US market today are mostly skate style helmets. Most of them have minimal vents and are too hot for warm weather bicycle riding.

The thin shell helmets on today's market can be nearly as good in sliding resistance if they have a rounded, smooth shape. Some of them are evolving into slightly harder shells now as the manufacturers try to open up more vent area and rely more on the shell for impact strength.

For sliding, the rounder the helmet the better, so the "aerodynamic" elongated shape is not optimal. In addition to possibly adding sliding resistance if a rear projection digs into the pavement, the tail of the elongated aero helmet could shove the helmet aside when you hit, leaving your head unprotected. Although it has been debated ever since the elongated designs appeared, Professor Hugh Hurt raised this question again in 2005, based on both testing problems and field reports of injury from helmets being pushed aside. Vents are necessary, but make sure they are smoothly faired into the helmet shell, and avoid any helmet with unnecessary fashion ridges on the outside, or snaps for visors, or any other feature that could cause the shell to snag. This is an easy item for a consumer to assess, as long as you keep in mind that you want your head to slide on impact. It should be evident that you don't want to add any accessory or cover to the exterior of a helmet that adds to its sliding resistance. That includes lights and cameras. Most of the ones we see have mounts that are much too strong to let go easily when you need to slide.

We hope to amend the ASTM standard some day to add a requirement to measure sliding resistance of the shell, but that will take time, and will probably be done when we develop a requirement for measuring and limiting rotational acceleration.

We have put up the lab study that established the value of a round, smooth, slick outer surface if you want to see the scientific data. And we have study on Chin Strap Forces in Bicycle Helmets confirming that a shell that does not slide well increases the jerk on the chin strap.

This page was last revised on: January 17, 2008.

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