One of my riding buds was looking to purchase a new lid. We measured his gourd and found it to be at the upper range for a medium and the lower for a large, 23.5". He was not sure which one to choose. I would have gone with the med since the mushroom effect is not as dramatic. Is there a guideline as far as safety is concerned or is it just which ever feels right? Lp

I have a large long head and therefore wear a large helmet, but you're right about the mushroom affect. I cannot wear a "one size fits all" helmet. Gotta say that the Giro Atmos helmet my wife wears looks great and is not in the least bulky. Gordon Harris Bike New England

I just had the same issue. I was right on the edge between medium and large. I went medium (large wasn't available in the color I wanted), and I have to wear it wide open, but it fits. I probably should have gone large though.

I can't really comment on the mushroom effect between sizes. The medium sits a bit high, meaning there is a slightly larger gap between my ears and helmet, and I can't really see the brim when I ride (manual says you should for proper fit.) The top of the helmet does not stick up higher, it's just there is less on the bottom if you can smell what I'm cookin'.

Overall, while I think it looks OK, wearing it has not made a significant difference in the wife's amorous desires.

I had a similar issue when picking out a helmet. I looked at the Specialized S-Works (the white/blue/black color scheme), mainly because of the looks. However, the design, while not much different than the Propero that I ended up getting, was different enough such that when I tried it on I felt like someone was pinching my head. Was very uncomfortable. I ended up getting the Propero in the same size (Large), and it has worked well for me ever since.

https://www.bhsi.org/checklist.htm

Helmet sizing varies from brand to brand. Going from measurement doesn't always get you the right size. I recommend going to an LBS, and spending some time trying on some lids. I recently spent over an hour at my LBS trying on lids, and was surprised at how much different each brand fit.

If you can't decide between a larger and smaller size, and they both fit well and feel the same, go with the smaller one.

Smaller helmets look better(less bubble-head effect), are lighter, and more aerodynamic... all else being the same.

But never compromise safety, fit, and comfort for style, obviously.