They come in 3 shapes: Round, intermediate oval, and long oval.
Measuring my head told me I was between M and L. 58.5cm, where M = 57 to 58cm, L = 59 to 60cm.
Intermediate oval M gave a lot of pain at my temples and the left/right sides of my head. This tells me that my head shape was Round.
Intermediate oval L fit nicely with no pain. BUT, i think it might actually be a bit too large. Because with time, it began to feel loose.
For helmets, there are 4 main testing standards: https://www.revzilla.com/common-tread/helmet-safety-ratings-101
DOT, Snell, ECE, Sharp.
In my opinion, sharp is the most thorough testing standard, so I look for 4+ star rated sharp helmets.
Nylon is stronger than polyester (in tensile)
Cordura is a type of nylon. Polyester is more or less recycled plastic bottles: https://www.youtube.com/watch?v=zyF9MxlcItw
Nylon is hard to recycle though.
600D 84T Polyester/PU is stronger than
500D Nylon, which is stronger than
600D 64T polyester/PVC,
all of which are stronger than jeans (16oz or 12oz canvas)
PU vs PVC
"By comparing the two different 600D fabrics in this test, we can see that the backing material makes a huge difference to the strength of the fabrics. The 600D that comes with PVC backing is about 2.4 times stronger than the 600D fabric with PU backing. This is something interesting to note. Regrettably, PVC is being banded by many countries due to product safety concern."
Nylon is also more abrasion resistant than polyester:http://www.orientbag.net/single-post/2016/08/03/Abrasion-Resistance-Test-600D-Polyester-500D-Nylon-1000D-Nylon
Goretex is a membrane (that has teflon)
Teflon is a spray
Summary & lessons learned:
If you're comparing polyester vs nylon of the same denier, get nylon
CE rating is not enough:
CE refers to the armor. But armor or kevlar lining wouldn't be much use if the seams of your pants or jacket bust open upon contact. EN 13595 rating refers to the entire piece of clothing. EN 17092 is similar.
"Until the star rating system is introduced, Grant has suggested that riders check the CE labels carefully, then go online and research which country they are certified in, for what purpose and to what level.
The number they should be looking for is EN13595-1 (level 1 and 2) for clothing where the abrasion resistance for level 1 clothing is 4 seconds and above, and level 2 is 7 seconds and above.
The number for the armour approval standard is EN1621-2 (previously EN1621-1) followed by a letter which corresponds to the location such as S for shoulder, E for elbow, K for knee etc."
A summary of CE standards for motorcycling clothing: