Bicycle Helmet Safety Institute
The Helmet Update by Email
Vol. 9, No. 2 Late April, 1991
Hodgson Publishes Second
Attached to this Update is a second study by Dr. Voigt Hodgson of Wayne State University in Detroit. This time Hodgson studied thin shell helmets as well as hard shells and soft shells, and bashed them on rough concrete as well as smooth.
Sliding Resistance Study
We thought that getting Dr. Hodgson's findings to you was worth a second April Update. His results draw your attention to one crucial point: thin shells did almost as well as hard shells in low sliding resistance. And one helmet with soft rubber strips on its shell was the "most hazardous" type. The bottom line is that among the non-hard shells the surface covering of the helmet makes a big difference in its sliding resistance. This is not surprising. But the performance of the thin shells vs. hard shells was not self-evident, since thin shells allow the foam to flatten on contact with the road while hard shells tend to hold their shape. Hodgson's study is significant for the children's flocked-surface or velvet-surface helmets shown at the bike shows last fall. They look cute, but will they skid?
Hodgson's data are packed onto pp 6 and 7, and are the only pages of the report that we did not reduce to cut xeroxing costs. They show that at 45 degree impact angles, and at 30 degree angles as well, the hard and thin shell helmets rank consistently better than the no-shells, particularly on rough concrete. One no-shell did well when its cover came off, allowing it to begin its skid on nylon, but this effect would disappear on anything but the smoothest surfaces, since the pointy high spots of rougher surfaces penetrate the nylon and stick into the EPS foam underneath. There is some possibility that the same effect would occur with thin shell helmets in harder impacts or with rougher surfaces. That leads to some questions that could keep you wearing your old hardshell. Would the results hold true on asphalt as well as concrete? Would they hold when speeds are increased above the 8 MPH maximum the test equipment can stand, and the thinshell foam really flattens?
The most interesting columns in the tables are those labeled "My" and "Dur." These show the force exerted on the dummy's neck and the duration of the bending force. Longer durations and somewhat higher peak bending force are characteristic of the no-shell helmets. The columns labeled Lx and Lz show the force from the opposite side: the amount of force exerted on the slab by the dummy's helmeted head. Again we see that the hardshell helmets produced lower readings resulting from their tendency to slide rather than grip.
The critical among you will note that tests #8 and #70 are apparently the same helmet at the same angle, but there are some significant differences in the data. The data on tests #19 and #20 are also the same helmet, however, and the results are very close.
This report is likely to give a boost to thin shells when it gets out to the consumer level. In addition to helping sliding resistance, the thin shells help to hold foam helmets together after the first impact. Hodgson's results should please consumers who want a light helmet that slides well. And we expect a lot of help from manufacturers in getting the word out quickly, since they have higher profit margins on thin shells. This is the market mechanism at its best.
Our thanks to Dr. Hodgson for a first class piece of work, and to Pat Smith of the Michigan Department of Public Health for providing copies.
The full study is now up on the Web
Action Coming on ANSI Standard?
Since our last Update was published, the ANSI Z90.4 Bicycle Helmet Committee secretariat (Snell Foundation) has polled the committee members and is scheduling a meeting of the committee in Dallas on May 15th. We will cover it in the next Update.
Helmet Articles Blooming in Spring
Don't miss the helmet article in the May issue of Readers Digest. You can read it in the checkout line. It has good emotional impact in the opening paragraphs, and some well-digested information follows.
Bicycling's annual helmet review has their ventilation test ratings, which are the only such comparisons available anywhere. The rest of the article is routine, since it was written before Hodgson published.
An article in the French magazine Le Cycle reveals that European professional cyclists are grousing about wearing helmets, even though they provide yet another surface for earning endorsement revenue. Turns out the pros say they are concerned about heat buildup in the mountains, and especially worried that the fans will not be able to recognize their favorites. Perhaps all problems will evaporate when the advertising price is right, but hopefully some manufacturer is designing a helmet to be promoted as the coolest of them all–the one the pros wear in mountain stages.' Again we might benefit from the market mechanism, and all we would have to do is pay buckets of money for the new and cooler model.
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