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helmets and rotational acceleration
Two papers available online from today, which appear to address one
of the significant uncertainties regarding helmet performance. They also render some previous comments ('no testing ever looks at oblique impacts') untrue, which might require minor revisions of some web sites (Guy?). I'd need to pay for them (so have only read the abstracts), but others with free-er access to these sorts of things might be interested: Oblique impact testing of bicycle helmets International Journal of Impact Engineering, Volume 35, Issue 9, September 2008, Pages 1075-1086 N.J. Mills, A. Gilchrist http://dx.doi.org/10.1016/j.ijimpeng.2007.05.005 Abstract The performance of bicycle helmets was investigated in oblique impacts with a simulated road surface. The linear and rotational accelerations of a headform, fitted with a compliant scalp and a wig, were measured. The peak rotational accelerations, the order of 5 krad s−2 when the tangential velocity component was 4 m s−1, were only slightly greater than in comparable direct impact tests. Oblique impact tests were possible on the front lower edge of the helmet, a site commonly struck in crashes, without the headform striking the ‘road’. Data characterizing the frictional response at the road/shell and helmet/head interfaces, were generated for interpretation via FEA modelling. Finite-element analysis of bicycle helmet oblique impacts International Journal of Impact Engineering, Volume 35, Issue 9, September 2008, Pages 1087-1101 N.J. Mills, A. Gilchrist http://dx.doi.org/10.1016/j.ijimpeng.2007.05.006 Abstract Finite-element analysis (FEA) was performed for bicycle helmets making oblique impacts with a road surface, to evaluate the linear and rotational accelerations of the headform. Helmet rotation on the head was considered, modelling the helmet and retention strap interactions with the headform. The effects of frictional parameters on the response were explored, and parameters selected to reproduce experimental results. Predictions were made for two helmets, for a range of impact locations and tangential velocities. The design method for the peak headform linear acceleration was confirmed; it was hardly affected by the tangential component of the impact velocity. The peak headform rotational acceleration was investigated as a function of the helmet geometry, impact sites and velocities and the contributing mechanisms established. regards, Ian SMith -- |\ /| no .sig |o o| |/ \| |
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#2
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helmets and rotational acceleration
On 11 Aug 2008 12:41:47 GMT, Ian Smith wrote:
Two papers available online from today, which appear to address one of the significant uncertainties regarding helmet performance. They also render some previous comments ('no testing ever looks at oblique impacts') untrue, which might require minor revisions of some web sites (Guy?). I'd need to pay for them (so have only read the abstracts), but others with free-er access to these sorts of things might be interested: Oblique impact testing of bicycle helmets International Journal of Impact Engineering, Volume 35, Issue 9, September 2008, Pages 1075-1086 N.J. Mills, A. Gilchrist http://dx.doi.org/10.1016/j.ijimpeng.2007.05.005 Abstract The performance of bicycle helmets was investigated in oblique impacts with a simulated road surface. The linear and rotational accelerations of a headform, fitted with a compliant scalp and a wig, were measured. The peak rotational accelerations, the order of 5 krad s−2 when the tangential velocity component was 4 m s−1, were only slightly greater than in comparable direct impact tests. Oblique impact tests were possible on the front lower edge of the helmet, a site commonly struck in crashes, without the headform striking the ‘road’. Data characterizing the frictional response at the road/shell and helmet/head interfaces, were generated for interpretation via FEA modelling. Finite-element analysis of bicycle helmet oblique impacts International Journal of Impact Engineering, Volume 35, Issue 9, September 2008, Pages 1087-1101 N.J. Mills, A. Gilchrist http://dx.doi.org/10.1016/j.ijimpeng.2007.05.006 Abstract Finite-element analysis (FEA) was performed for bicycle helmets making oblique impacts with a road surface, to evaluate the linear and rotational accelerations of the headform. Helmet rotation on the head was considered, modelling the helmet and retention strap interactions with the headform. The effects of frictional parameters on the response were explored, and parameters selected to reproduce experimental results. Predictions were made for two helmets, for a range of impact locations and tangential velocities. The design method for the peak headform linear acceleration was confirmed; it was hardly affected by the tangential component of the impact velocity. The peak headform rotational acceleration was investigated as a function of the helmet geometry, impact sites and velocities and the contributing mechanisms established. Can someone confirm my interpretation of this: ======== From these tests, any additional danger of rotational head injury from wearing a helmet is slight or negligible. ======== |
#3
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helmets and rotational acceleration
On Mon, 11 Aug, Tom Crispin wrote:
On 11 Aug 2008 12:41:47 GMT, Ian Smith wrote: Two papers available online from today, which appear to address one of the significant uncertainties regarding helmet performance. Can someone confirm my interpretation of this: ======== From these tests, any additional danger of rotational head injury from wearing a helmet is slight or negligible. ======== I would say you shouldn't conclude anything from the abstracts of two technical papers. Which is why I'd be keen to hear the views of anyone that does have access to the content. If it was someone I trust to be able to read straight, it might even influence my opinion. regards, Ian SMith -- |\ /| no .sig |o o| |/ \| |
#4
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helmets and rotational acceleration
Tom Crispin wrote:
On 11 Aug 2008 12:41:47 GMT, Ian Smith wrote: Two papers available online from today, which appear to address one of the significant uncertainties regarding helmet performance. They also render some previous comments ('no testing ever looks at oblique impacts') untrue, which might require minor revisions of some web sites (Guy?). I'd need to pay for them (so have only read the abstracts), but others with free-er access to these sorts of things might be interested: Oblique impact testing of bicycle helmets International Journal of Impact Engineering, Volume 35, Issue 9, September 2008, Pages 1075-1086 N.J. Mills, A. Gilchrist http://dx.doi.org/10.1016/j.ijimpeng.2007.05.005 Abstract The performance of bicycle helmets was investigated in oblique impacts with a simulated road surface. The linear and rotational accelerations of a headform, fitted with a compliant scalp and a wig, were measured. The peak rotational accelerations, the order of 5 krad s−2 when the tangential velocity component was 4 m s−1, were only slightly greater than in comparable direct impact tests. Oblique impact tests were possible on the front lower edge of the helmet, a site commonly struck in crashes, without the headform striking the ‘road’. Data characterizing the frictional response at the road/shell and helmet/head interfaces, were generated for interpretation via FEA modelling. Finite-element analysis of bicycle helmet oblique impacts International Journal of Impact Engineering, Volume 35, Issue 9, September 2008, Pages 1087-1101 N.J. Mills, A. Gilchrist http://dx.doi.org/10.1016/j.ijimpeng.2007.05.006 Abstract Finite-element analysis (FEA) was performed for bicycle helmets making oblique impacts with a road surface, to evaluate the linear and rotational accelerations of the headform. Helmet rotation on the head was considered, modelling the helmet and retention strap interactions with the headform. The effects of frictional parameters on the response were explored, and parameters selected to reproduce experimental results. Predictions were made for two helmets, for a range of impact locations and tangential velocities. The design method for the peak headform linear acceleration was confirmed; it was hardly affected by the tangential component of the impact velocity. The peak headform rotational acceleration was investigated as a function of the helmet geometry, impact sites and velocities and the contributing mechanisms established. Can someone confirm my interpretation of this: ======== From these tests, any additional danger of rotational head injury from wearing a helmet is slight or negligible. ======== To me the second one seems to say, that linear impacts forces were not affected by the helmet rotating. They also investigated rotational forces, but do not have the results in the abstract. The first one says that extra rotational forces were negligible when the rider hits the ground at 4m/s (about 9mph). |
#5
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helmets and rotational acceleration
On Aug 11, 1:41 pm, Ian Smith wrote:
The peak rotational accelerations, the order of 5 krad s-2 when the tangential velocity component was 4 m s-1, were only slightly greater than in comparable direct impact tests. Am I reading that correctly? 5000 rad per second per second in an 8mph crash? Surely we must be talking microseconds - which I'd have thought is as likely to be an artifact of the modelling as a real acceleration. In particular, I don't see how it's possible for those sorts of accelerations to be transferred to the head from the helmet - or even from the scalp to the head. Oblique impact tests were possible on the front lower edge of the helmet, a site commonly struck in crashes, without the headform striking the 'road'. Data characterizing the frictional response at the road/shell and helmet/head interfaces, were generated for interpretation via FEA modelling. And here we seem to be talking about crashes where the head would miss the ground other than for the helmet. It certainly would be interesting to see the papers - although I suspect I'm not qualified to critique them as FEA is something I've never done. Tim. |
#6
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helmets and rotational acceleration
Ian Smith wrote:
On Mon, 11 Aug, Tom Crispin wrote: On 11 Aug 2008 12:41:47 GMT, Ian Smith wrote: Can someone confirm my interpretation of this: ======== From these tests, any additional danger of rotational head injury from wearing a helmet is slight or negligible. ======== I would say you shouldn't conclude anything from the abstracts of two technical papers. Which is why I'd be keen to hear the views of anyone that does have access to the content. If it was someone I trust to be able to read straight, it might even influence my opinion. I would concur with Ian's analysis. But, for the sake of argument, let's assume your take is a fair one. The possibility of rotational injury aggravation by helmets has primarily been a conjectured mechanism for how they might make things worse. Even if you demonstrate that as a non-issue, it will not affect in any way, shape or form the whole population studies that demonstrate no clear advantage in terms of serious injuries to wearing a cycle helmet. Pete. -- Peter Clinch Medical Physics IT Officer Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital Fax 44 1382 640177 Dundee DD1 9SY Scotland UK net http://www.dundee.ac.uk/~pjclinch/ |
#7
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helmets and rotational acceleration
Ian Smith wrote:
Oblique impact testing of bicycle helmets International Journal of Impact Engineering, Volume 35, Issue 9, September 2008, Pages 1075-1086 N.J. Mills, A. Gilchrist http://dx.doi.org/10.1016/j.ijimpeng.2007.05.005 A quick search of the web also finds this: http://www.asbweb.org/conferences/2004/pdf/357.pdf |
#8
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helmets and rotational acceleration
Ian Smith wrote:
On Mon, 11 Aug, Tom Crispin wrote: On 11 Aug 2008 12:41:47 GMT, Ian Smith wrote: Two papers available online from today, which appear to address one of the significant uncertainties regarding helmet performance. Can someone confirm my interpretation of this: ======== From these tests, any additional danger of rotational head injury from wearing a helmet is slight or negligible. ======== I would say you shouldn't conclude anything from the abstracts of two technical papers. Which is why I'd be keen to hear the views of anyone that does have access to the content. If it was someone I trust to be able to read straight, it might even influence my opinion. Having scanned the two papers (which are related: the physical testing generates paramenter values for the finite element analysis) the main conclusion appears to be that the coefficient of friction between the headform (with anatomically correct scalp + hair) and the helmet is too low to lead to rotational forces in a 'typical impact' which are large enough to cause brain damage. I don't pretend to be either a materials scientist or somebody with experience in analysing this kind of paper, so I'm not going to critique their methodology. It seemed perfectly reasonable to me froman 'outsider' perspective however. They acknowledge that answering the question of whether the headform stiffness makes a real difference to the measured outcomes will require experiments with cadaver heads. Any volunteers? Phil -- http://www.kantaka.co.uk/ .oOo. public key: http://www.kantaka.co.uk/gpg.txt |
#9
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helmets and rotational acceleration
On Aug 11, 2:48 pm, Martin wrote:
Ian Smith wrote: Oblique impact testing of bicycle helmets International Journal of Impact Engineering, Volume 35, Issue 9, September 2008, Pages 1075-1086 N.J. Mills, A. Gilchrist http://dx.doi.org/10.1016/j.ijimpeng.2007.05.005 A quick search of the web also finds this: http://www.asbweb.org/conferences/2004/pdf/357.pdf Perpendicular impacts yielded an average angular acceleration of –506 ± 419 rad/s2, with the negative value denoting counterclockwise acceleration. 30° LF and 30° HF yielded a significantly higher (clockwise) angular acceleration (p0.01) of 5844 ± 265 rad/s2 and 8846 ± 422 rad/s2, respectively (Figure 3). Greater than 10x the angular acceleration using mid point figures. Best case (closest to zero on the error bars) figures, 50x angular accelerations and worst case, 5x However, I'm not convinced even this test tells us anything useful as written[1]. We need to compare the angular acceleration with and without the helmet. I can't even begin to guess which would come out higher. And then, we need to ask how many cases there are where the only reason for a head impact (at speed) is due to the larger head with helmet and this then depends on reflexes etc. Tim. [1] That wasn't the point of these tests which was in order to show how these measurements could be made, not necessarily to get useful data. quote However, it effectively captured the angular acceleration pulse generated during the initial impact. As such, this test addresses the consistent request for consideration of oblique impacts in helmet testing (Mills, 1997) in a simplicity that may be amendable for implementation into existing test standards /quote |
#10
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helmets and rotational acceleration
On Mon, 11 Aug 2008 15:01:34 +0100, Phil Armstrong
said in : Having scanned the two papers (which are related: the physical testing generates paramenter values for the finite element analysis) the main conclusion appears to be that the coefficient of friction between the headform (with anatomically correct scalp + hair) and the helmet is too low to lead to rotational forces in a 'typical impact' which are large enough to cause brain damage. I think this is a bit of policy-based evidence making. Dr Mills is a non-medical doctor with no relevant qualifications who has given important (to one side) testimony in over 100 cases of contributory negligence. So far as he is concerned, it appears that helmets are always effective as the only factor of importance is the 1.5m a cyclist falls vertically when he's hit. That he might be propelled forwards at the same time by a 50 mph car is totally irrelevant. Brian Walker and John Franklin, plus a few solicitors, are currently largely warding off such nonsense but it could be more difficult if he's now coming up with 'proof' that helmets guard against oblique impacts. Note that the 'proof' in this case is largely dependent on the assumption that he is right. Guy -- May contain traces of irony. Contents liable to settle after posting. http://www.chapmancentral.co.uk 85% of helmet statistics are made up, 69% of them at CHS, Puget Sound |
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