[entee]

This is incorrect. Most cyclists rarely sustain speeds beyond 20mph and will not regularly see speeds beyond 30-35mph. If going downhill on a road bike, amateur cyclists hit mid 40s occasionally on weekend rides. For reference the maximum speed I have ever seen (I have biked >1,100 miles this year) is 55.7mph for one brief moment.

Motorcycles on the other hand consistently travel at 30+mph and regularly see sustained 55-80mph on the highway. It's not the same problem at all.

If your helmet is likely to encounter a 60mph impact you will design for that. Most likely that means your helmet will be stronger, which means it won't break away as easily at lower speeds. This means when your head will take more of the kinetic energy in a 15mph crash than it otherwise would. Not to mention that the helmet will be heavier, which means there's more momentum on impact which may affect the way forces get dissipated.

If you compare with downhill mountain biking that's a very tiny segment of the population. It's true that some downhill mountain bikers may use motocross helmets. That's not what the average bike helmet is for, and it's not what the average bike rider should be using.

Don't use a motorcycle helmet as a bike helmet.

[persistent]

I don't know why you keep arguing this. You said that motorcycle helmets are "never" to be worn by bicyclists. I point out that there is not just one but -several- different companies cross-marketing DOT-approved motorcycle helmets for bicycle sports. Now you are talking about the "average" bike rider. I think you just have a narrow, inaccurate conception of the universe of all cycle sports and you've probably never been anywhere near a motorcycle. Look at these BMX kids, then look at the motocross kids.

https://crazybmxer.files.wordpress.com/2013/09/bmx-racing.jp... https://i1.wp.com/www.njmotocross.com/wp-content/uploads/201...

[entee]

I have in fact ridden motorcycles, you’re still arguing a niche case. Fine, there exists some cyclist who wears a motorcycle helmet. It’s a bad idea for the vast majority of cyclists.