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Comment by srejk | Hacker News Reader

When I was commuting 60k/day on my bike in shitty suburban conditions, I used one of these instead - you get limited use per trip, but you can always fill it up with a CO2 cylinder/bike pump:

https://www.hpvelotechnik.com/en/recumbent-trikes-bikes/acce...

It is loud.

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That’s a crappy pressure vessel holding 350ml of 80psi air, for about 100J of stored energy. I’m not entirely sure I’d be comfortable with that, especially anywhere with my face in the line of fire it it fails.

Good point, but I abused it pretty well and it seemed to do OK - was also in a water bottle holder so closer to the legs than anything.

> That’s a crappy pressure vessel

That's a huge assumption, and likely incorrect.

Your bike already has two crappy 80psi pressure vessels, why not three?

Those two pressure vessels are highly engineered and are wrapped with materials with pretty good tensile strength. Also, they’re made out of materials (fabric and rubber) that absorb a decent amount of energy when they tear and that don’t fragment. And the whole assembly usually depressurizes slowly.

Having personally blown up beverage bottles by overpressurizing them (be very very careful doing this!), when they go, they go violently.

I've blown up beverage bottles for fun. Hooking an air compressor to a 2L bottle and exploding it makes a satisfyingly loud boom.

*We had a valve on the air line so we could be at a safe distance when pressurizing. Be very careful. It's unpredictable exactly at which point they'll blow. Sometimes they hold full pressure for a couple seconds and then go.*

i like to use dry ice for pressure, make sure you have a gun to shoot it if it doesn't go off

When we did it, it always went off on its own. It's been a long time since I did it, but I think the longest it took would've been on the order of 30 seconds. Really makes a person jump when it finally goes.

If this is a modern bike, 80psi is way too high. 50psi is sufficient and will give you a more comfortable ride as well as higher efficiency on real-world surfaces.

80+psi is for old-style road bikes with narrow 23mm tires. Modern bikes (even road bikes for racing) don't use these any more; 28mm is the minimum these days.

… I have a modern bike (a Specialized). The tires' rated range is 75psi to 100psi. I usually pump it to around 80–85psi. The tires are 33mm.

You're overinflating your tires. A lower pressure will increase your speed and efficiency unless you're riding in a velodrome. Here's a video about this:

https://www.youtube.com/watch?v=7r8f3w89XeM

(watch out, there's a lot of extra stuff in this video about the machine they use to measure efficiency)

(Well, they should tell the manufacturers…)

The video's result for both tires they tested was peak efficiency at 5 bar. They had a really coarse sampling of a whole bar, so that works out to a pressure of 65–80 psi.

>(Well, they should tell the manufacturers…)

The manufacturer rating is likely a max allowable pressure rating.

Not to be pendantic (but to be pendantic) 80psi is the correct pressure for 28mm tires ridden briskly on good roads. At least according to ye olde Silca tire pressure calculator. Back in the day when folks ran 23mm tires they would typically run above 100psi (though that may not have been optimal...).

That calculator is wrong. Cycling people have been overinflating their tires for ages (as well as using too-narrow tires), with the assumption that the ground is perfectly smooth. Lower pressures yield higher efficiency (and better comfort) on rougher surfaces.

Surface quality is an input in that calculator.

Because the danger posed by a fairly low energy pressure vessel is highly related to it's failure mode. That's why OSHA has rules about what compressed air pipes can be made of--it's not about the pressure resistance, it's about what will happen if one fails.

It's likewise why most military boom is mostly not actually boom. With artillery you obviously need a very tough case, but standard aircraft-dropped iron bombs are mostly that: iron. They don't need that kind of strength except specialized bunker-busters, they're built that way because for a given weight of bomb you'll do more damage by throwing bits of bomb casing from a smaller charge than from a bigger charge without the fragments.

It's a soda bottle - it fits in your water bottle holder, and you can replace it for a couple of bucks if it fails. 80 psi is pretty low pressure (typical narrow tires are 100-120) and the bottle itself is very low mass, so the fabric around the bottle should ensure safety if it bursts.

IIRC these came out in the early-mid 90s; a bike messenger trick at the time was to fasten the horn to your handlebars with velcro, so you could take it off and hold it near a car window when triggering it.

If it fails by blowing the end off toward your face what damage will it do?

I suppose I should maybe not worry about 80 psi so much. An ordinary bottle of soda on a moderately warm day is around 80psi. The energy is 1/2 * pressure * head space (roughly), and head space is minimal. But you can chill it in the fridge, then open it and quickly pour out half, then close it and let it warm up, and you may still be near 80 psi, and I’ve never heard of anyone getting maimed by an exploding soda bottle.

> When I was commuting 60k/day on my bike in shitty suburban conditions

Here I thought my 4.5 mile (7.25 km) bike commute was a bit long...

An hour and a bit each way, took about as much time as public transit and better than a coffee for waking up. A good road bike goes a long way, and the suburbs suck for road sharing but are great for not having to stop at many lights.

The winters were rough though.

I used to bike ~20 mi / day … back when there were offices. Just as fast as public transit.

Aside from idiots with cars, it's relaxing. Only some of my route was on the road, though.

Like the other poster says … The winters were rough though. I just didn't bike, though my coworker kept trying to get me to.

Can confirm, AirZound is great!

Yeah I had something like this for several years. Works really well for cars

I wonder if one of those recently-emerging Chinese electric blowers that sub for canned air would generate enough air volume to sound the horn usefully. Possibly not quickly enough.

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