Tyre pressure at altitude
- Thread starter Rod
- Start date
I am in Norfolk this weekend, will I be safe with or without socks?
Think you don't want to change tyre pressures with altitude as the tyre simply still has to carry the weight of the bike, the ambient pressure is irrelevant.
But now, typical front tyre pressure is 2.5bar. Or 3.5 absolute. Thats only 3.5kg/cm2. If you brake hard enough to put all the weight on the front wheel then that's easily 300kg (solo & no luggage). You would need 300/3.5=~85 cm2 of tyre in contact with the road?? That would be a some deformation of the tyre
But now, typical front tyre pressure is 2.5bar. Or 3.5 absolute. Thats only 3.5kg/cm2. If you brake hard enough to put all the weight on the front wheel then that's easily 300kg (solo & no luggage). You would need 300/3.5=~85 cm2 of tyre in contact with the road?? That would be a some deformation of the tyre
trainman
Registered user
how will we know?
Fit your bike with the special tyres that aircraft use, it's a little known fact that the rubber used is semi-porous to allow some of the air to escape at altitude so that they don't explode. During the descent, they allow air back in - they're actually made of the same material used in self-inflating camping mattresses, so they reinflate themselves automatically.
ha ha, that's the best i ever heard. I hope you're not serious !!!
freeloader
Registered user
The difference between the pressure in an aircraft tyre at sea level and at 36000 feet is about 4 -5 % depending on the model of aircraft . 737 NG sea level pressure is about 225psi , this changes by about 4% at cruise level , not enough to even bother with .
Piaf
Registered user
Did you also know that the squeal from aircraft tyres when landing is caused by the sudden impact forcing the air through the holes of the pourous sidewalls
Dr Feelgood
Guest
AlanWM
Registered user
An increase in altitude leads to a decrease in ambient pressure which means an increase in internal tyre pressure - but dont worry, the increase of internal tyre pressure is offset by a proportionate decrease of pressure due to the decrease in tempreture with an increase in altitude.
Sorry... I'll get my coat.
Sorry... I'll get my coat.
CuzGS
Registered user
But are you talking pressure or density altitude.........
Oh fleck it I'll get my coat too
Oh fleck it I'll get my coat too
E. Buygum
Registered user
Unless the law of physics have changed recently, Pressure x Volume is still a constant (Boyle's Law). If the tyres are considered entirely rigid i.e. their volume doesn't change, then neither will the pressure inside them assuming constant temperature, irrespective of what's going on outside them. If the tyres aren't considered completely rigid, they may balloon out a bit at higher altitude and the pressure inside the tyre will drop as their volume increases. The fact that the temperature is lower may also decrease the pressure a bit (Charles's Law). Not that any of this matters for riding a bike.
density altitude is only the pressure altitude corrected for the prevailing air pressure . For engine performance , if you were to correct for air density , it would be the density altitude you would be interested in !! Aircraft tyres are filled with dry nitrogen to minimise the change in pressures as well , you could also do this on the bike , I used to do it with racing tyres and it works very well .
CuzGS
Registered user
Sorry pressman, oops wrong answer.
Density altitude is Pressure Altitude corrected for temperature not pressure
The calculation is DA = PA + or - 120 t where t is the temp difference from ICAO e.g. + 15 at SL dec by 1.98 deg per 1000 ft zzzzzzzzzzzzzzz
I'll get my coat again
Oh dull it may be but it's damn important for aircraft performance, particularly hot and high e.g. Ka Sian mountains
Density altitude is Pressure Altitude corrected for temperature not pressure
The calculation is DA = PA + or - 120 t where t is the temp difference from ICAO e.g. + 15 at SL dec by 1.98 deg per 1000 ft zzzzzzzzzzzzzzz
I'll get my coat again
Oh dull it may be but it's damn important for aircraft performance, particularly hot and high e.g. Ka Sian mountains
After spending all weekend in Norfolk wearing socks (except in bed) I returned home safely, therefore I conclude that it is best to wear socks when visiting Norfolk.
trainman
Registered user
probably a more useful and infinitely more understandable piece of advice then most of whats been put on this thread!!
R17GSR
Registered user
Surely at high altitude you need less air in the tyre to keep the same grip on the road,as there is also far less gravity holding you down.
CuzGS
Registered user
I once got piles whilst in Norfolk and was wearing socks at the time thus proving it would have been better not to surely 
I once got piles whilst in Norfolk and was wearing socks at the time thus proving it would have been better not to surely
Did you forget your underpants? I wore mine (except in bed) and I didn't get piles therefore proving that one must also wear underpants in Norfolk
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