Anatomy Lesson Head shape classification Anatomical terms for the three types of the head shapes (that we know as, Long oval, intermediate, and round) Dolichocephalic (long head : what we call “long oval” head) Brachycephalic (short head : what we call “ round head”) Mesocephalic (intermediate) The head shapes are classified by Cephalic Index = x/y × 100 (CI = (bipareital diameter or BPD, side to side measurement) / (occipitofrontal diameter or OFD, front to back measurement) ) × 100 FIg.18 click on the picture to see larger image [/url]Hot spot 18, on Flickr CI < 75 (female) 75.9 (male) is Dolichocephalic CI 75 to 83 (female) , 76 to 81 (male) is Mesocephalic CI > 83 (female), 81.1 (male) is Brachycephalic Note : There’s a difference between male and female skull. That's why the numbers are different for male and female. What matters for helmet fitment are Smooth, more vertical frontal bone (forehead) on female skull Male skull has more angled (tilted back) forehead Rounded chin (female skull) vs Square chin (Male skull) The more blocky and massive cranial mass on male skull vs rounder and tapering at the top female skull Male skull have a deeper cranial structure (I should probably explain the difference between caucasoid vs mongoloid skull shape , but let me keep on explaining about round head hot spot problem (pain on temple area) for now.) (There’s also CRANIAL INDEX, which is essentially the same thing except for the fact that it uses the measurement taken from the skull. For the people who deal with live human’s head (such as helmet manufacturers), Cephalic Index (which takes flesh and skin in account) is the one that matters. For the people who deal with the skull (such as forensic artist), Cranial Index (which takes shrinkage of the dried bone in account) is the one to use.) Now, this Cephalic Index would have been really useful if the helmet manufacturers had used this, to indicate the internal shape of their helmets. Helmet for the round head For example, Arai Quantum-X, supposed to be the most “round head friendly” helmet in U.S. market, would only work on the head with CI around 80 to 85. I can tell, because my own head’s CI is 89, which is almost extreme end of Brachycephalic head, and Quantum-X is nowhere near as round head friendly as RX-7 -X (which is the JDM version of Corsair -X.) If your head’s CI is greater than 85, even with Quantum-X (which Arai calls “round oval” helmet), you need to go one size up. Instead of just saying “round-oval”, if Arai had said “Quantum-X ; round-oval shape, fits Cephalic Index 80 to 85”, it would have been much clearer. (Of course, they need to educate their potential customers, by explaining what Ci is. But, this will greatly reduce the hassle of buying a new helmet.) AGV AX-8 Evo has egg shaped internal, and runs a little bit on the large side. But they have a lot thicker, plush crown pad in there. The result is the helmet that works for the wide range of the head shape. (AGV GP-Tech was such a helmet also.) I would say it (AX-8 Evo) works on the head with CI 75 to 85, even up to 90 so long as your forehead isn’t really wide. (If you have wide, flat forehead and the CI is around 88 or over, it wouldn’t work.) But, the hard, cold fact remains. The true round head helmet is virtually non existent in today’s North American market. NON of the helmet that are officially sold in US are designed for the head with CI greater than 85. So, .... is there any option, other than going one size up, for the people who’s head CI more than 85? Yes, there is. Well, .... sort of. ... to be continued.