Sliding Resistance of Bicycle Helmets
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. Double shell construction with a slip-plane might help too.
In a crash you want an interface with the road that is smooth, hard, round and slick. That keeps your head from snagging, adding to the severity of the impact and putting more strain on your neck.
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.
A Swedish company called MIPS is reviving the slip-plane concept, using two layers in the helmet that are held in place by a frangible pin that breaks and lets the shells slip for about 15 mm upon impact. The effect is to mitigate rotational energy momentarily for the critical milliseconds of the impact sequence. The POC brand helmet incorporating MIPS is very round and smooth on the surface as well, so they have minimal sliding resistance to begin with and the advantage of the MIPS slip-plane design. You can find more on POC in our Helmets for 2010 page. In 2012 Lazer of Belgium is incorporating an inner fit cage that is licensed by MIPS as well, on two of their child helmets.
The helmet community has been discussing slip-planes for years, and is cautiously examining the MIPS data to evaluate the advantages. Everyone agrees that mitigating rotational force is important for injury protection, particularly for anti-concussion effects. But there are questions about how much a slip-plane actually helps, given the tendency for the helmet to slide on the users head and to slide on pavement or other impact surfaces. It would almost certainly be a help if the impact were into a material that does not permit sliding. But will the slip-plane continue to perform for the the life of the helmet, after it has been soaked in water or sweat? Are there some areas or angles where the planes would not be able to slip? Can the technology be used in a well-ventilated bicycle helmet? We don't have answers for those questions yet.
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: December 28, 2013.