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.
Vents are Hot!
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.
Although it may not be self-evident, the normal vents in the good helmets of the mid-90'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. In one operation this bonds the shell and expands the foam "beads" into solid foam. 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 which has voids of several millimeters in some spots. In addition, the heat of the mold would melt the cheaper plastic used for glued-on shells, so molding in the shell requires the manufacturer to use a better grade cover, normally a polycarbonate like GE's Lexan. The shell's bonding and higher quality plastic contribute to the strength of the helmet structure. In addition, manufacturers add various types of interior reinforcement to hold the thinner foam together.
Most helmets are designed to reliably meet the 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. In general the manufacturers are designing to the standard (now CPSC), and are not using the more expensive construction features to surpass it. Even so, in-molding does continue to offer two advantages: 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 pook out again and hide the damage. For those reasons we continue to recommend it unless price is your first consideration.
Hyper-ventilated models now are required to meet the CPSC standard. The only one we have seen so far certified to Snell's
more stringent B-95 standard is the Qranc Reaqtor by OGK. All will continue 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?"
Those Sharper, Squared Off Designs
The fashion among helmet designers since 1998 began to favor 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
continues to dominate as well. This is a less than 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 today's helmets that lab testers 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 super-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!
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 long aero designs. In fact, they don't give you any real aero advantage until you reach racing speeds anyway.
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 helmets, 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. Many current designs 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 this is tested for by any of the world's
current bicycle helmet standards, despite studies 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.
Unfortunately, the squared-off fashion trend tends to make older round designs look clunky and old-fashioned
unless they are graphically very well done. The new ones look great, and perhaps that can translate into more
helmet use. We think that these helmets will perform well in the field, but we just do not consider the squared-off designs optimum.
This page was last revised on: June 9, 2005.
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