Removable fullface unless its changed in the past few years is a gimmick and should be avoided unless your doing it for looks or JUST tree branch strikes.

Not just a style thing.

The visor for dirt does a FUCKTON if you are doing downhill, you can cover it with dirt or with tree branch hits.

Also, downhill helmets are full faced and extra beefy for a reason since going ass over tea kettle or into a tree\rock at 20mph on the slow side is a bit painful.

Google says hormones, they're highest in the morning..

Or just a result of sleep cycles with healthy nerves and blood flow

Or a change in nervous system during REM sleep..

There doesn't seem to be an agreed upon answer, more just something that happens

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sure the helmet protects your head when you're biking. but what about going up on a ladder? the roller rink? sprinting at 16 mph? getting out of a bathtub. 30,000 people die in cars every year, but no crash helmets for drivers?

People don't wear a helmet for lots of activities. if you need one for riding a dutch bike you should just wear one all the time, like when driving a convertible or walking in winter or descending a long flight of stairs

if you're racing or doing bmx or downhill dirt jumping, sure, wear a helmet, it's not going to prevent a spine injury tho

because these activities have unique safety requirements. Skiing helmets are designed to protect against impacts from falls at high speeds and from collisions with other skiers or obstacles on the slopes. Bicycle helmets, on the other hand, are designed to protect against impacts from falls at lower speeds and from collisions with vehicles on the road. Both types of helmets are important for preventing head injuries, but they are specifically engineered to address the risks associated with their respective activities.

Ack. Dude.

Regarding your last point, some of the mass gain is going to be mitigated from water as a byproduct of hydrocarbon burning, but then also to consider is that a higher average temp is going to correlate with a lower average pressure.

Not sure which factor would win out, I'd tend to think the temp increase would be a larger factor and therefore lower pressure

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Added a note at the top of my post. Thanks for the correction.

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420 ppm is 0.042%, so the change in oxygen is even smaller.

1000 ppm = 1/1000 is 0.1%.

Not exactly an ELI5, but here’s an ELI15. I used to be test engineer for one of the world’s largest helmet manufacturers. We made helmets for bike, snow, and moto. Ultimately, I was the one who understood the test standards and requirements for each type of helmet, and made sure we were passing those requirements with new models we developed.

The primary reason why different helmets exist for bike and snow is due to differences in the impacts that occur in both activities. Bike helmets are designed to meet the relevant test standards for bike helmets, which are based on real world scenarios involving typical cycling speeds and impact surfaces. For example, the CPSC standard for US/North America requires impacts on flat, curb, and hemispherical anvils, all of which imitate surfaces that might be impacted when crashing on a bike and at typical speeds seen when cycling. This standard also requires helmets to be tested in ambient, cold, hot, and wet conditions, to replicate different weather conditions and user choices.

The snow standard differs from this, and uses a different combination of speeds, anvils, and conditionings that are specific to snow sports. For example, there is no hot condition or curb anvil, but there is a penetration test where a metal spike is dropped on the helmet to imitate the impact of a broken tree branch or something similar. Different materials and construction are used to meet the different requirements for these standards.

A few other factors are head coverage, which is often defined by the test standard, and comfort. Snow helmets have full coverage and are designed to keep you warm, while bike helmets are generally designed to keep you cool and may sacrifice coverage for lighter weight and better aerodynamics.

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Don’t bicycle in snow so need a cold helmet. Ski in snow need warm helmet. Both stop head hurting when crashing.

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Always thought ski helmets were a bit silly unless you were doing tricks or skiing through trees. Bought one on clearance because it was dirt cheap and wore it. Will never not wear one again. Keeps head toasty. Keeps goggles in place even when I put them on top of head. Has built in headphones. Amazing all the way around.

The amount of O2 in the atmosphere has decreased, but it's only a fraction of a percent.

I've retrieved a small live fly from inside my eye socket after a scooter drive. Get a visor or glasses.

The CO2 weighs more but has the same volume as the O2 it replaced. However you've missed the face we've also burned a bunch of hydrogen since almost everything we burn is a hydrocarbon. This produces water (H2O) which precipitates out of the atmosphere with a net result of depleting the amount of oxygen atoms in the air. (See my longer answer).

H20 joins the precipitation cycle but due to increasing temperatures there is more H2O in the atmosphere than before on average.

So yes. We do have more atmosphere than before.

Both by mass and by number of molecules.

The energy we've put into the system will eventually go back to the previous equilibrium after hundreds of years. So it's temporary on the earth's timescale at the moment.

Unless we put too much energy in then it releases more energy, methane released, ice caps melted, less heat reflected, and the change becomes more of an earth timescale one than a human timescale one.

... yes? I hit 45mph literally every single fucking day on my bike going downhill.

We turned solid stuff (Hydrocarbons) into atmosphere. CO2 and H20

The volume of both of those things in the atmosphere has increased.

There are literally tons of Carbon released into the atmosphere and while it will come down eventually we do have more atmosphere than before.

I see your logic, and at first glance I was about to defend your statement, however it's not correct because:

1. If you burn pure carbon, you get O2 + C = CO2. That's the same number of gas molecules after burning. 1 gas molecule in the input, 1 gas molecule on the output. A gas's volume is determined by temperature and number of molecules, not the number of atoms(technically an approximation, but a very good one). Yes there are more atoms making up the gas, but once the combustion gas has cooled to ambient, the volume of CO2 is basically the same as the volume of O2 you started with. While the CO2 is heavier, this doesn't equate to more gas, it's just heavier gas. (If I give you a litre of petrol and a litre of water, you've got the same amount, although the water weighs more.)

However this is kind of moot because:

2. All this combustion can actually make he atmosphere weigh less and have a lower volume! Most of the stuff we burn is a hydrocarbon. Hydrocarbons burn to produce CO2 and H2O. The H2O precipitates out of the atmosphere, so there's actually fewer gas molecules in the air because we've removed oxygen and turned it in to water. In addition, the mass of the oxygen precipitated, is greater than the mass of carbon added, so the air is lighter too. Now it really depends what hydrocarbons we've burned. Natural gas (methane) produces twice as many water molecules as CO2 so would have the strongest depleting effect. Liquid fuels like petrol, diesel and oil produce about equal numbers of CO2 and water molecules so more slightly deplete the atmosphere. Coal produces fewer water molecules than CO2 I haven't done the math on whether this is a net mass increase or decrease (I expect it's pretty mass neutral) but it definitely still decreases the air's volume.

While we're at it, I've always wondered why hockey helmets are so different from football helmets. Both are very full contact sports, but hockey has more potential head injury hazards, between the boards, the ice, sticks and pucks -- all at a much higher speeds. Yet hockey helmets are pretty small and flimsy in comparison to football helmets.