@tgagain so the TRL say that in an ideal world the eps should be a snug fit. However in the real world manufacturers have to make a range of sizes, this will always lead to cases where there is a gap between the shell and the head, this means that your advice is wrong.

NO its not Bikepawl – think about it.  If there is a 4cm step in circumference of the helmet sizing then its only a 1.25 cm step in diameter which is approximately the width of a finger – so if you can get a finger in there you should go down one size in shell in helmets with multishell sizes as if you can get a finger in the gap then a smaller shell will go on your head and thus a smaller shell will be a better fit

Its mainly aimed at the one size fits all helmets yes but equally applies to helmets with multiple shell sizes

Its just a simple rule of thumb  😉  to allow you to assess if the shell is the right size on the head.  thats all it is.

some discussion here about sizing and also explains the point I made about bigger shells being less effective

https://www.whycycle.co.uk/cycling-advice/safety-and-security/cycling-helmets/

Some helmets use a “sizing ring” which adjusts to fit your head. If this is the case, ensure that the helmet shell is fairly close fitting. Do not rely solely upon the snug fit created by the sizing ring. The sizing rings and soft foam pads offer absolutely NO protection. Only the polystyrene shell will absorb any energy and it is therefore very important that this shell be as close a fit to your skull as possible without it being uncomfortable.

I am not going to debate this any further but its obvious if you think about ti tat with 4 cm steps in sizing of circumference that 1.25 cm steps in diameter which is approximately a finger width.  so if you can get a finger in the gap you could go down one size in shell size

Interesting that you are now saying a finger instead of a little finger. In my case if I went for a smaller shell size I would need a summer helmet and a winter helmet as I wouldn’t be able to wear a hat under the smaller size. Even if you are able to get a finger between the helmet and head that space will be at the back of the helmet, so less likely to be impacted.

Any luck finding any of those top of the range manufacturers that do 5 sizes?

The five shell sizes was quoting something I had read.  If its wrong then I am happy to be corrected. I ain’t spending time searching thru.  My fault for relying on secondary not primary sources.

All this is rehashing old ground.  I am sure you agree with me that many folk wear incorrectly sized helmets incortrectly fitted.  the finger test is an easy way of seeing if the shell you have could be sized down.

Positivly my last comment.

The finger test is good at the point of purchase, but in Ianc’s case it is largely irrelevant as long as the cradle holds the helmet on the head, stopping excess movement. Which is why I think your advice is wrong.