Silly question about air pressure in a shock/fork

[jugheaddave]

Hi

 

Passably a silly question which I think I know the answer to, but here goes anyway.

 

Dose outside air pressure (ie sea level vs 2000m above sea level) affect shock/fork air pressure?

 

 

 

[MoreTrails]

Shouldn't really, its a sealed unit? but then again i didn't pass science at school - was to busy riding my bike (and chasing birds of course)

[jugheaddave]

lol, thats what I thought?

 

But, will it affect they way a Tyre feels at altitude?

 

[MoreTrails]

Aaah, now there is a question. n Not sure what the 100% answer is but I would say yes as the air chamber can increase/decrease due to flexibility (like let your tires down before putting the bike on the plane...)

 

Not sure if the actual rubber would have anything to do with air temp, that would be more temperature. Planning on riding up Kili or something?

 

[TomvdP]

in theory it should, because it is sealed, although I am not sure whether the difference would even be noticeable. The only way this could would affect you is if you set the suspension up at one altitude, and then moved to another without re-adjusting your shock. The only pressure that matters is the difference between the internal pressure and the outside pressure. Since pumps operate at the outside pressure regardless of where they are, they will always give you this pressure. Hope that made sense...

 

[brussel]

yes it would and it would affect tires too. if you pumped them to 3 bar at sea level and then went to the top of everest they would be 6 bar..

 

 

 

in reality I am not sure how much of a difference it makes at 2000m. probably negligible

[Reghard111]

Less than 0.2 of a bar....Nothing to stress about

[DirtyFrank]

 

 

The internal pressure of the shock/tyres is always measured against the external air pressure and this is reflected on your suspension/tyre pump gauge.

 

 

 

The pressure will increase relative to an increase in altitude(If you inflated the shock at sea level) but this will most probably be offset due to the drop in temperature as the altitude increases. The inverse would apply when traveling down to sea level.

 

 

 

This increase in pressure is relatively minor even when measuring sea level relative to say +3000m.

 

 

 

The increase in temperature that your shock/tyres can experience in a race due to constant friction/compression etc. will probably have a more profound affect on your shock pressure than the change in altitude does.

 

 

 

All the horror stories of tyres exploding in mid flight are mostly nonsense and would probably apply to highly inflated roadbike tyres but even these have such a high safety margin over and above their max indicated pressure that exploding is unlikely.

 

 

 

I am sure an Engineer will elaborate.

 

 

 

 

 

 

 

 

 

 

[jugheaddave]

I was just wonderig about it. Dont worrie, I not oing to try ride up kili....

[Johan Bornman]

Even if you take your shock to the moon, the pressure inside the air chamber will stay the same and apart from gravitational differences, your ride on the moon or in the Freestate will feel exactly the same.

 

However, the reading may differ, depending on how the pump's gauge works. This is a bit like the Schrodinger's cat thought experiment - the very reading of the pressure may affect the perceived pressure.

 

Tyres - the same story. Once a tyre is inflated to a point where more pressure doesn't change its shape, it will still be at the same pressure when you're on the moon.

 

A tyre is not a flexible balloon that inflates indefinitely. It maintains its shape with varying pressure.

 

The notion that tyres explode in the air is absolute nonsense. Aircraft - the type that I travel on in anyway, are pressurised. Also, they have their own tyres on board which hang outside the pressurised cabin and they seem to do just fine there. Animals, and indeed humans can quite happily survive flying in the cargo hold - in fact, rumour has it that you have more legroom down there and it is more comfortable in the hold. Therefore, a bicycle tyre is perfectly safe, even at maximum pressure, inside cargo.  Airlines don't understand this and are not open to reason. I've tried, at some pretty senior levels.

 

 

 

 

 

 

 

[TNT1]

man... I love that cat...

[Capricorn]

JB is quite correct. However, whether the tire explodes or not is entirely dependent on the tire's ability to withstand the pressure difference. Aircraft tires are designed to work at altitude, so it's not a fair comparison in measuring up a bicycle' tire to an aircraft's.

So whilst i wont say it's absolute nonsense, it's simply unlikely.

 

[Johan Bornman]

JB is quite correct. However' date=' whether the tire explodes or not is entirely dependent on the tire's ability to withstand the pressure difference. Aircraft tires are designed to work at altitude, so it's not a fair comparison in measuring up a bicycle' tire to an aircraft's.

So whilst i wont say it's absolute nonsense, it's simply unlikely.

[/quote'] I shouldn't have mentioned the plane's own tyres, nobody can see my tongue in my cheek. However, it is ABSOLUTE NONSENSE that your bike's tyres will/can/may explode in the aircraft's cargo hold. The cabin and hold (same system, same aircon, same everything) is pressurised at, IIRC, pressure found at 6 000 feet above sea level - just a little bit higher than Jhb.

 

 

[jugheaddave]

ok so, If I was to pump a bicycle tyre on top of everest to 3bars, then bring the same tyre and pum/gaguge down to sea level, the reading would be less? laike maybe 1.5 bars, due to the additional air pressure at sea level affecting the guage reading?

 

And what the h3ll dose a cat have to do with it??

 

[Minion]

 

However' date=' it is ABSOLUTE NONSENSE that your bike's tyres will/can/may explode in the aircraft's cargo hold. The cabin and hold (same system, same aircon, same everything) is pressurised at, IIRC, pressure found at 6 000 feet above sea level - just a little bit higher than Jhb.[/quote']

I agree.

 

You can do a simple test to see the effect of throwing an inflated tyre into a full vacuum:

 

1.) Pump the tyre to it's rated capacity.

2.) Pump it up an additional 1bar.

 

Did it explode in step 2? No? You have just shown your tyre is capable of surviving in a vacuum.

 

To simulate a typical aircraft cabin, in step 2, use 0.25bar if you're flying from sea level and 0.05 bar for flights from Joburg.

 

Basicly, unless you've inflated your tyre beyond it's design limit, the extra 0.25bar from the cabin won't cause problems.

 

[Minion]

 

ok so' date=' If I was to pump a bicycle tyre on top of everest to 3bars, then bring the same tyre and pum/gaguge down to sea level, the reading would be less? laike maybe 1.5 bars, due to the additional air pressure at sea level affecting the guage reading?

 

[/quote']

Correct, although the sea level reading can't be lower than 2bars.