The Helmet Update
Vol. 9, No. 2 Late April, 1991
All issues index
Dr. Hodgson Publishes Second
Sliding Resistance Study
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.
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.
Did You Return Your Response Card From The Last Issue? Thank you.
The Helmet Update - Bicycle Helmet Safety Institute
Randy Swart, Editor
4611 Seventh Street South
Arlington, VA 22204-1419 USA
(703) 486-0100 (voice)
(703) 486-0576 (fax)
www.helmets.org