Vents and Square Lines:
Problems with some designs
Summary: Some helmet designs have too many sharp lines on the shell surface and too many vents. Reasonable vents and a smooth shell should optimize protection.
A major theme in the helmet market since 1997 has been more and larger air vents. All major manufacturers now have hyper-ventilated models following in the footsteps of Giro's 1997 Helios model (now discontinued). Manufacturers tout the number of vents in their helmets, a meaningless parameter. If all else were equal, more vents would be a Good Thing, but as usual, all else is not equal. Unfortunately opening up new vents usually requires harder, more dense foam and squaring off the edges of the remaining foam ribs to squeeze
out the most impact protection possible from the narrower pieces still there. Since we believe that rounder shells and less dense foam are virtues in a crash, we don't recommend hyper-vented helmets unless you can't live without the added ventilation.
Vents are Hot!
Although it may not be self-evident, the normal vents in the good helmets since the mid-1990's have proven adequate for almost all riding by almost all riders in almost all conditions. To provide impact protection with less foam the manufacturers normally have to harden the remaining foam, so that the force of a blow is transmitted to the rider's head with more pressure on one particular spot. There is no unanimity that this presents a safety problem, and only the Australian standard tests for "localized loading," but all things being equal we would prefer to crash in a helmet with wider foam strips in contact with our head than narrower ones, and a helmet with less dense foam.
Opening up more or larger vents often is achieved by molding the EPS part of the helmet with the plastic shell in the same mold. This bonds the shell and expands the foam "beads" into solid foam in one operation. The resulting helmet has almost every millimeter of space under the shell filled with foam (except for any quality control problems), unlike a taped-on or glued-on shell with voids of several millimeters in some spots giving a "beer can effect" when tested with your thumb. In addition, molding in the shell requires the manufacturer to use a better grade shell material, normally a polycarbonate like GE's Lexan, because the heat of the mold would melt the cheaper plastic used for glued-on shells. The shell's bonding and higher quality plastic contribute to the strength of the helmet structure. In addition, manufacturers often add various types of interior reinforcement to hold the thinner foam together.
Most helmets are designed to reliably meet the CPSC or other relevant standard, not to exceed it by very much. So designers use higher quality construction techniques to thin the helmet out and increase vent size. That evens out impact performance so that better construction techniques don't often mean better impact protection, just thinner helmets and more vents. In short, more money will buy you more vents, but not necessarily more safety. Manufacturers are usually designing to meet the standard, and are not using the more expensive construction features to surpass it. Even so, molding in the shell does continue to offer two advantages. First it provides more consistent resistance to cracking and destruction of the helmet in the first impact. And because the shell is molded to the foam, it should show indentations after a crash to remind you to replace the helmet, while a taped-on shell may just pop out again and hide the damage. For those reasons we continue to recommend it unless price is your first consideration.
Hyper-ventilated bicycle helmets are required to meet the CPSC standard. They tend to be expensive, since consumers apparently will pay more for more vents. Bell's Senior Product Manager Candi Whitsel was quoted in the September 1, 1997, Bicycle Retailer and Industry News saying "The idea is to raise prices and get
the consumer to buy up. If you have a helmet at $50 with 500 vents, how are you ever going to sell a $100 helmet?"
The fashion among helmet designers since 1998 has favored squared-off edges of the foam remaining around the vents, and the addition of sharp lines in the exterior plastic just for style. The elongated "aero" shape dates from that era as well. This is not an optimal design for crashing. We believe that the ideal surface for striking a road resembles a bowling ball: hard, smooth and round. Round shells reduce to a minimum any tendency for a helmet to "stick" to the surface when you hit, with the possibility of increasing impact intensity, contributing to rotational brain injury or jerking the rider's neck. They also eliminate the aero tail that can snag, or in a backward impact can shove the helmet aside as you hit, exposing your bare head. This
is such a problem with some models that lab technicians have to use copious amounts of duct tape to keep some helmets on the headform in their test drops, even after they have pulled the straps extra-tight. Dr. Hugh Hurt has asked ASTM to consider modifying its bicycle helmet standard to eliminate the aero tails and elongated designs. His email on this subject is illuminating.
Those Sharper, Squared Off Designs
In the real world people don't use duct tape, and they don't even adjust their straps well. So our advice is to avoid those elongated aero designs. In fact, they don't give you any real aero advantage until you reach racing speeds anyway. For most riders they are not useful.
Here is a very large .pdf file containing a study by Swedish researchers showing the effect of sliding resistance on chin strap forces. And here is a report on another series of lab tests measuring the sliding resistance of various helmet shells. Although neither used today's models, both show that if your helmet does not slide well on the surface you hit, the effects are potentially injurious.
To reduce potential snagging points to a minimum we would prefer helmets with vents and ribs well faired and rounded. Although the swing to "compact" designs has been an improvement, some current models still have a "shelf" effect in the rear that adds to helmet length but also adds a prominent snag point, a feature we would avoid. We would note that none of the world's bicycle helmet standards includes a test for this, despite the studies cited above that have shown with lab tests that helmets that do not slide well can cause higher neck forces, higher chin strap forces and increased g's to the brain from the impact.
Initially the squared-off fashion trend tended to make older rounded designs look clunky and old-fashioned. A decade later the tide began turning again, and now the most elongated designs are looking dated.
It is worth noting that either style has been performing well in the field. That includes the squared-off designs with snag points in the rear. Nobody really plans to crash, but we believe that a round and smooth shell is the optimal user interface with the road.