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Professor Hugh Hurt Weighs In:

Testing Shows Aero Helmets are a Problem

Summary: Back in 2004 when test labs were struggling with testing road helmets with tapered shapes in the rear, Professor Harry Hurt sent this memo to Dave Halstead, chairman of the ASTM F08.53 Subcommittee, about the difficulties in accurately testing wedge-shaped helmets.

Subject: streamlined helmet ejection

To: ASTM F08.53 Chairman: P. David Halstead

From: Hugh H. Hurt, Jr, Head Protection Research Laboratory

During the last couple of years, the technical staff at HPRL has encountered an interesting-and possibly dangerous-problem with the aerodynamic-shaped or streamlined bicycle helmets. These popular helmets have a teardrop design which tapers to a wedge at the rear of the helmet, supposedly reducing aerodynamic drag along with increased ventilation through the many openings in the shell.

The adverse effect of this aerodynamic shape is that the wedge at the back of the helmet tends to deflect and rotate the helmet on the head when impact occurs there. Any impact at the front or sides of the streamlined helmet is no different from other helmet shapes, but any impact on the rear wedge tends to rotate the helmet on the head, probably deflecting the helmet to expose the bare head to impact, and at worst ejecting the helmet completely from the head. Actually, everybody who has tested these streamlined helmets over the past years has encountered the problem of these helmets being displaced during impact testing at the rear wedge. Usually additional tape was required to maintain the helmet in place during rear impact tests; usually the basic retention system alone could not keep the helmet in place during impact testing on the rear of the helmet.

Unfortunately, the implication of helmet displacement and possible ejection in an actual accident impact did not register as a real hazard in previous years of testing, but now there are accident cases appearing that show this to be a genuine hazard for bicycle riders wearing these streamlined helmets. Accident impacts at the rear of these streamlined helmets can cause the helmet to rotate away and expose the head to injury, or eject the helmet completely. The forces generated from the wedge effect can stretch the chinstraps very easily, and even break the [occipital--Prof. Hurt used a trademarked name] retention devices.

We request that F08.53 committee study this problem and develop advisory information for both manufacturers of these streamlined helmets and consumer bicyclists who now own and wear such helmets. There is a definite hazard for displacement or ejection from impact on the rear wedge of these helmets, and bicyclists should be warned of this danger by an authority such as ASTM.

s/Hugh H. Hurt, Jr
Professor Emeritus-USC
President, Head Protection Research Laboratory

s/Christopher B. Swanson
Laboratory Manager, Head Protection Research Laboratory

ASTM discussed the memo at its next technical meeting and did not make changes to its standards, in part based on the lack of field evidence of an injury mechanism.

This has been one of BHSI's principal themes for years. But there are caveats here: lab testing is done with magnesium headforms that have no chin, so the straps can not perform as they would on your face, and the interface of magnesium and helmet does not approximate your scalp, with and without hair, hair gel or sweat. But Professor Hurt indicates that an injury mechanism is becoming clear in crash reports from actual users. We have not seen that develop over the ensuing years, and in fact we did not expect to see the problem ever identified in field data, where EMTs were never instructed to look for it or report it.

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