Summary A single sample of the Sport Shieldz Skull Cap was tested to determine what additional protective benefit might result from wearing it under a current motorcycle helmet. A series of impacts were conducted on identical helmet samples with and without the cap. All the tests were done under laboratory ambient conditions. Although substantial drops in peak acceleration and the coefficient of rebound were noted when the cap was included in otherwise comparable tests, there were inconsistencies in the comparisons. These inconsistencies are likely the result of slight differences in the impact sites in the no-cap versus cap tests and quite possibly differences in the helmets themselves. In order to eliminate these sources of difference, additional tests were conducted in which a head form, with and without the cap, was dropped at comparable velocities onto MEP pads. The acceleration responses of comparable tests demonstrated excellent repeatability. The no-cap versus cap condition in otherwise comparable tests demonstrated substantial drops in peak acceleration and the coefficient of rebound. An analysis of the acceleration response suggested that the cap managed impact energy continually throughout the loading phase of the impact; that is, the cap appeared to deform continually until the head form s downward velocity went to zero. The Impact Tests A series of tests were performed using the cap in conjunction with three helmets of the same model and size. One of the helmets was tested on a bare head form, another on a the same head form covered by the cap and for the third, the cap was placed on the head form for some of the impacts but not for others. For comparable impact sites and surfaces, the peak shock transmitted through the helmet and into the head form was lower with the cap in place. The eight charts below show the shock pulses versus time and versus displacement for each of the three helmets in four comparable test set ups. The charts on the left show time series accelerometer results captured at 50,000 samples per second. The data was aligned on the time axis according to the intersection of the onset slope. The charts on the right show the accelerometer results cross plotted against displacement as integrated from the acceleration and impact velocity. This displacement is taken as the deformation of the helmet wall and, possibly, the cap material just as the intersection of the onset slope with the time axis is taken as the instant of first contact between the helmet shell and the impact surface. The legend on each chart shows, for each of the three helmets, whether the cap was in place, the peak shock and the peak deformation calculated. Although the impacts with the cap in place yielded lower peak shocks and greater peak deformations than comparable impacts without the cap, in a few cases the peak deformations with the cap exceeded those for comparable drops without the cap by considerably more than the thickness of the cap itself. Some of this might be explained by slight differences in the impact test set up. Even slight differences in helmet orientation on the head form or in the impact site on the shell will yield different results. It is also possible that differences in fit might be operating. The cap is about three millimeters thick which might add Page 1 of 8
almost a centimeter to the head form circumference. Depending on the geometry of the helmet interior, the thickness of the cap might change the conformance between the helmet and head form considerably. Page 2 of 8
In order to eliminate these uncertainties, we performed some tests using a head form with and without a cap but with the helmet and impact surfaced replaced with a Modular Elastomer Programmer (MEP) pad. A well chosen MEP will deform under impact with a head form and transmit much the same shock pulse to the head form as would a helmet against an unyielding shaped surface. The advantage is that the MEP recovers quickly and is remarkably stable; it will transmit the same shock pulse reliably again and again. We used two different MEP pads, a one inch thick slightly domed MEP of the sort often used to validate helmet drop test gear prior to helmet evaluation and sometimes used directly in sports helmet tests, and a three inch MEP which permits drops at impact velocities of seven meters per second and better approximates the flat impact response of motorcycle helmets. Charts showing brow area impact responses for the head form with and without the cap follow. The charts show four drops each of cap and no-cap drops in comparable tests. The chart legends indicate whether the cap was in place as well as the impact velocity, peak shock and peak deformation. The cap condition yielded a 23 G reduction in peak shock and a 2.5 mm increase in peak deformation. Page 3 of 8
These tests were repeated at a lower nominal velocity. For this series at a nominal 4 m/sec impact velocity, the cap condition yielded reduction in peak shock of about 12 G and an increase in deformation of about 1.5 mm. A third series of tests were performed in which the back of the head form was impacted against the one inch domed MEP with and without the cap. For these drops, the cap condition yielded a 34 G reduction in peak shock and a 2 mm increase in peak deformation. Page 4 of 8
Analysis A simple analysis was performed to isolate the contribution of the cap to the interaction between the head form and the MEP. It was assumed that the deformation of the MEP in the cap condition would be the same as the deformation observed at that particular level of shock for the no-cap condition and that by subtracting the deformation of the nocap condition from the cap condition at corresponding levels of shock, we could get the cap deformation versus G throughout the loading phase of the impact, essentially, the deformation versus G curve for the cap from the moment of contact until the instant of peak shock transmission. In the analysis, interpolation was applied to the loading curves for four tests each of the cap and no-cap conditions in order to obtain deformation values corresponding to uniformly spaced levels of shock. The deformation values at each level were then averaged for the cap and no-cap conditions to obtain the traces shown in the first of the two following charts along with a third trace corresponding to the difference between these deformations which is assumed to represent the behavior of the cap itself. The second of the two charts is just this third trace plus two more traces showing the standard deviation of the deformations for the cap and no-cap conditions. This third trace demonstrates some gyrations which are probably artifacts. The traces for the cap and no-cap conditions include some odd features which I believe are due to shock waves initiated by the impact and which rebound through the MEP material. These features are evident in all the tests performed on the three inch MEP and appear to be repeatable and regularly spaced timewise. Page 5 of 8
The next two sets of charts show similar traces for the three inch MEP drops at four meters per second and for the one inch MEP at four meters per second. Page 6 of 8
The last chart above shows G versus crush performance attributed to the cap for all three test conditions. Since the three inch 7 meter per second impacts were performed first followed by the three inch 4 meter per second impacts and then the one inch four meter per second impacts, the progressive shift of the traces to the right suggest that the cap was becoming fatigued. However, by the time the tests series were complete, the cap had gotten a considerable workout and had held up marvelously. A helmet is considered good for only one impact and the cap had stood up well for better than fifty. The irregularities visible for the three inch MEP tests seem likely due to shock waves propagating through the MEP material. If so, the effects would be uniformly spaced in time for both the cap and no-cap conditions but would be shifted relatively in terms of deformation, particularly near the point at which the head form travel reaches a maximum just before it begins to rebound. A separate study of the three inch MEP seems to support this conjecture. A series of tests were performed in which an impactor with a spherically shaped face was dropped onto the three inch MEP and the shock transmitted to the impactor was captured. For each of three nominal velocities, ten traces were time aligned and the accelerations averaged for corresponding levels of displacement throughout the loading phase as shown in the above chart on the left. These averaged loading phases were then fitted to second order polynomials and these fitted polynomials were subtracted from the averaged traces. The chart on the right shows these residuals plotted versus time. The interaction between the between the spherically faced impactor and the MEP is certainly much more complex than might be modeled by a second order polynomial but the subtraction would exaggerate anomalies Reasonably, these residual traces would consist mostly of time ordered anomalies such as might be due to shock waves rebounding through the MEP and, in fact, the chart seems to support this. In the time domain, such shock waves would reasonably be aligned but in the displacement domain, misalignment would increase throughout the event yielding the anomalies seen in the Cap Performance chart. Page 7 of 8
Discussion The Cap Performance chart suggest that the cap compresses about a millimeter almost immediately on impact but resists further compression with five to twenty times the effective stiffness of a reasonably representative helmet wall. Since the cap stiffness is essentially in series with the helmet liner, the combined stiffness might be about 80 to 95% of the helmet worn without the cap and the peak accelerations would behave similarly. Whether these reductions would have any observable effect on injury risks and crash outcomes is, at best, uncertain but surely there would be no increases. These reductions in stiffness and are due to the additional layer of shock managing material the cap provides though, and this layer implies a greater wall thickness and might necessitate larger helmets. It has been suggested, however, that most motorcyclists select helmets which are one to two sizes too large for them so that the addition of a cap might actually improve helmet fit as well as comfort and maybe also provide a little additional impact management. The single sample of cap appeared to hold up remarkably well over a series of more than fifty impacts. Some fatigue might be inferred from the chart of cap performance in three test conditions but the cap stiffness and range of compression seem well chosen in that they allowed the to help manage impact energy throughout the loading phase of the impacts. Although beyond the scope of this brief assessment, the following items may be worth considering. Since the cap material has an open cell structure, however, its properties might change as it absorbs skin oils, perspiration and the like over prolonged use. Since the cap is considered washable, its properties might also change after a number of washings. Page 8 of 8