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Author Topic: How to solve helmet hot spot problem  (Read 3074 times)
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« Reply #15 on: October 24, 2018, 09:36:44 PM »

Some important tips about “how to choose size” when purchasing helmet from  overseas. (Meaning, you can’t “test fit” the helmet before placing an order.)

The untold story of fitment, shell size and liner thickness.

(I’m going to use round heads (CI ≧ 85) as an example here. But, what I’ll explain here applies to all type of head / helmet internal shape.)

Example story

So, you are in between the sizes, S and M.
You have been advised  by people on the internet forums and by people at the local shop.
“Go for a smaller size, because the helmet (internal) will break in / stretch / mold onto your head.”

You believed that advice and purchased size Small instead of Medium.

You go for a ride happily with a brand new helmet ...

 ... and  an hour later, you get unbearable pain on the side of your head ...

Sounds familiar?

If you have been riding many years already, tried more than a few different helmets and still get the hot spot on the same area, then you are either ...

wearing the wrong shaped helmet

wearing the wrong sized helmet

... or, could be doing both ...

You came across this thread, and measured your head’s front-back, side to side, and it turned out that you have CI 88 head.

So, now you know you have two problems.

The first problem is CI.
The helmets that you can buy locally, or from Revzilla, STG, etc, are all designed for the CI<85 head.

Knowing that JDM Arai / SHOEI are CI 85-86 helmet as a standard (can be tuned for higher CI), you decided to give it a shot.

But now, you are facing the second problem.

You are in between S and M size.

Which one should you order?

For obvious reason, you don’t want to end up ordering the wrong size and shipping it back to Japan, reordering the different size.

How do you decide the size when actually trying it on is not an option?

4-shell size vs 2-shell size helmet

Would you be surprised if I tell you that, with some helmet, the fitting is quite different from size to size?


Let’s take a look at two examples.

SHOEI X-14 VS OGK Kabuto RT-33

X-14  4-shell size

RT-33  2-shell size

First, let’s take a look at X-14.
They come in 4 different shell sizes.
S(S and XS use the same shell), M, L, and XL (XL and XXL use the same shell).

fig. 19

The relationship between the shell, EPS liner, comfort liner, and the available inner space of the helmet is the same all across the sizes, except for XXL.

Except for XXL, all the helmet comes with 9mm thickness comfort liner as a standard, and you have options to go either thicker (+4mm) to make the fit tighter, or thinner (-4mm) to make the fit looser.

Each size of the helmet (except for XXL) has the same amount of tolerance / limitation for irregular bumps and slightly oversized head.

To put it simple, M is the scaled down copy of L, and S is the scaled down copy of M, they all have the same amount of space for fine tuning.

Now, RT-33, which comes in 2-shell size.

It uses the same shell for XS, S, and M. And  L, XL, XXL share the same, bigger sized shell.

Now, look at the picture below, pay particular attention to the liner thickness and the helmet labeled size.

fig. 20

Yes, you guessed it right.

Size XS, S, and M are THE SAME HELMET with different thickness comfort liner installed.
And size L, XL, XXL are the same helmet, the difference is just the comfort liner thickness.

With this type of helmet, it is a little bit tricky to choose the size.

If you happen to be between the S and M size, you should order S, NOT M.

You have 15mm thick  foam surrounding your head, that’s a lot of cushioning, which will tolerate the slightly oversized  head (56.5 cm) without problem, even if your head some irregular bumps, bulges, etc.

BUT, if you are in between M and L, then, you’d better get L.

Why? I don’t want a bigger shell size ! You just said “L” size helmet use the same shell as XL an XXL, that would make me looking like a bobble-head figure!!”

Well, I (sort of) understand the feeling, but take a look at the fig. 20 again.

RT-33 in size M comes with the 2nd  thinnest (9mm) comfort liner. (thinnest being 5mm on XXL).

Size M comes with the least amount of tolerance(extra space), besides XXL,  for irregular shapes and oversized head.

If your CI is 88 for example, and circumference is 58.5 cm, I can almost guarantee you that you are going to get painful hot spot on the side of your head if you wear size M.

IF. your head shape is slightly off (eg: CI 87, 88) BUT the correct size (between 57 and 58 cm), then M will work.

Or, if the head shape is correct (CI 85, 86) but circumference is slightly greater (58.5 cm), them M will work also.

However, when both the shape and the size are off at the same time, you can’t go for smaller size.

So, I hope you now understand the “multiple shell size” is NOT for the looks, but for function.

Also, I hope you understand now, that why some helmets cost more than the others.
(Just making 4 different sized molds for shell, and 5 different mold for EPS liner would easily make  X-14’s production cost more than twice the production cost of RT-33.)

Oh, by the way, the same rule applies for multiple shell sized helmet, when it comes to sizing.

If you happen to be in between the sizes, you can go for a smaller one
ONLY IF your CI is close enough(±1) to the helmet’s target CI.

Otherwise, you’d better order the larger size.

One method that works (for only Arai and SHOEI) for size checking is to use genuine issue boonie hat.
Go to the military surplus store and try them on.

The brand doesn’t matter, as long as it’s made to proper specification which is MIL-H-44105B.

For example, the size 7 is 55.9 cm. If the size 7 feels too tight, but next size which is 7-1/4 (57.8 cm) fits alright, you should get M, instead S.
If 7-1/2 is too tight, you should get XL, not L.

You know, boonie hat does not stretch, so you should be able to tell clearly if it’s too tight.
« Last Edit: October 24, 2018, 09:58:58 PM by S21FOLGORE » Logged
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