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Tyre pressure and Planes??
I'm off to Rotorua next week for some fun, just a little excited !!!!!!
I know I need to let pressure out of shock and fork though how much do you need to let out in the tyres, I run UST at about 27ish psi.
Do I need to completly deflate or is there a certain pressure they can be let down to safely travel??
Cheers Sam
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Of when I traveled last, I think you should let them down completely....
Would you let pressure out of your tyres/shock/forks if you went up to the snowy mountains? No? Didn't think so.
Pressure change is the same in an aircraft cabin.
If I try hard enough I can break mine in only 4 places to make it fit in my hand luggage...
Thought you didn't have to try at all to snap a bike?
Super human black flash strength!!!
They will ask you if you have let down the tyres although not about forks/shocks. I let them down to about 5-10 psi which is just enough to keep them on the bead but allows for plenty on expansion
Cheers that was what I was trying to avoid don;t really want to break the bead and then open the box up at the other end and have the steed covered in Stans
I wouldn't bother, even if you go into space it will make zero difference unless you're running everything at max plus 50%.
It's propbably not a real issue but all of the airlines mandate it. I've always been a goody-goody
I always let them down to say 10psi or something and then leave them.
Good post, I am off to the UK and Switzerland in August to do a marathon and some riding.
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It makes it fit in my bike bag much more easily if I compress the fork. The differences in pressure for altitude are minimal compared to shock pressures.
I was recently discussing this with some friends.
I'm no engineer or anything, but I can't see it being a major problem. However, I think that Atmospheric pressure at 20,000 ft, is about half what it is at sea level, which is 14.7 psi. So if you've got 30psi in yout tyre at sea level, I am pretty sure that the effective pressure at 20,000 ft would double, as the external atmospheric pressure is halved. At 30,000 ft, the pressure is about 35% of that at SL, making it around 5ish psi. So, if your shock/fork has a maximum pressure value and your riding pressure setting is over 1/3 of the maximum, then it might pay to let some of the pressure out, to be on the safe side of the recommended working pressures of the shock/fork.
Please don't take this as gospel, just my understanding of it. Someone with some more knowledge on the subject may be able to correct or expand on my thoughts.
To get the effective pressure in the tyre you have to take off the atmoshperic air pressure. This give 15.3 psi at sea level and 22.65 psi at 20,000ft . Whilst this is a 50% increase, it is nothing the tyre can't handle. The rule is way over the top for tyres in good nick.
For the shock at say 100psi the effective pressure is 85.3psi at sea level but only goes up to 92.65 psi - about a 7% increase(taking off the same atmosspheric pressure as for the tyres) . There is therefore far less argument to let air out of you shock than for tyres
The engineer in me needed to respond...
Pumps measure gauge pressure. This is defined as the difference between current atmospheric pressure and the pressure you are measuring. It's also the difference in pressure that is important when considering the load on the components.
If you pump your tyres to read 30psi on the gauge and atmospheric is 14.7psi then the absolute pressure in the tyre is 44.7psi.
If you flew in a plane at 35,000 feet and it lost compression the atmospheric pressure and new cabin pressure would be 3.46psi.
This would then give 41.24psi difference across your tyres which is what a pump gauge would read.
The pressure would then drop as the cold air which is around -50 degrees C causes the air in the tyres to cool reducing pressure.
Pressure is simply just force divided by area. In this case pounds per square inch.
For any object in the air the atmosphere exerts a force on it equal to the pressure times the surface area. The air inside a tyre also exerts a pressure and a force back on the inner surface of the tyre. When they are equal you have an uninflated tyre. When you have higher pressure in the tyre it is forced onto the rim.
Air pressure is just air molecules moving around and colliding with us and objects. Luckily we are full of holes and have flexible components so we feel no force as our internal and external pressure is the same unless we have gas. When the air molecules get hotter they move faster and collide harder when confined in a restricted space. If unrestricted they move further away from each other and the air expands. Or if we pump more air into a confined space or gas is created in a confined space there is more air molecules bouncing around so the pressure and number of collisions against the wall increases.
It's a bit like a tug of war except pressure pushes rather than pulls. 8 on 8 is equal (if eveyone is same strength) and a flat tyre. 8 on 4 is uneven and the tyre moves into the rim and basically hits the stops (gauge pressure of 4 people). If two walk away (increase in altitude) leaving 8 on 2 the gauge pressure goes up to 6 people and the rope/tyre is forced against the stops even harder.
Summary, differences in pressures with altitude including going into space are not that significant compared to the design range of our equipment. As has already been said, just don't run max plus 50%.
I reduce pressure because they ask and also because my fat tyres fit in bag better as does my bike when the fork is squashed down.
Since they ask we should drop our tyre pressures. But from an equipment point of view the parameters are within the design range.
Who knows, maybe they have sensors which go off when something goes bang? I know on rare occasions old tubes suddenly let go.