# Bicycle Mechanics - Tyre pressure at altitude

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View Full Version : Tyre pressure at altitude

fujitive
05-06-03, 01:58 AM
So far I have done all my cycling at altitudes less then 300 metres but I will be tackling passes over 2700 metres in Europe later this year.

My question is how much does the higher altitudes affect tyre pressure? I understand that bicycle tyres are known to explode in planes which fly at 10,000 metres. (so I will deflate the tyres when flying)

Should I let some air out to compensate for the altitude? I have my tyre pressure at 75psi which is the limit for the tyre I use. (28mm tyre) If I pump the tyres to 75psi at sea level will they still be 75psi at 2700 metres? or is the tyre pressure relative to the outside air pressure?

Thanks
Mat

:beer:

shaharidan
05-06-03, 07:58 AM
i did a quick search on the web, but didnt find anything good. my guess would be to check your tire pressure as you gain altitude to make sure it doesnt go above what you want.
hopefully someone with better knowledge will respond.
have a great time on your trip :)

Rev.Chuck
05-06-03, 08:15 AM
If you have 75psi at sea level you will have a higher pressure at 2700 meters because the air is less dense and is not pushing as hard against the outside of the tire. Since the tire casing is does not have that much give I dont think it will be a big deal. Besides, at sea level air pressure is 14.7psi, so at any altitude you can still breathe at your tires should be OK. For example the pressure at altitude has dropped to 7psi so your tire would raise to 82psi. However your guage should still be accurate,
I hope this makes sense, reading it back it, I don't think I worded it to well.

shaharidan
05-06-03, 08:25 AM
basically that altitude wont have a huge effect, and your tire gauge will work so you can check them and make adjustments as needed?

MichaelW
05-06-03, 08:44 AM
Ive never heard of a tyre exploding inside an aircraft; has anyone else?
Tyres can generally take a lot higher than their lawyer-proof safety guidline.

deliriou5
05-06-03, 08:47 AM
all tire pressure gauges measure RELATIVE air pressure... that is, the pressure difference between the air inside and outside the tire. so a 75PSI tire at sea level will "feel" as hard as a 75 PSI tire at the top of mt everest

That said, I took the liberty of calculating how much the pressure would increase if you were to start your ride at 300m and climb to a 2700m elevation:

a measly 3.6 PSI!!!

so I doubt you have any reason to worry about your tires popping as you climb the mountain :).

pucci
05-07-03, 08:42 PM
After getting used to the pump I keep at my parents' home which reads one bar low, I unthinkinkingly continued to compensate by filling my tires one bar over after I came home.

On the descent from a 750 meter climb. my overfilled rear tire blew out at 70kph. The tubular tire blew right off the rim and locked the wheel. It was a miracle that i managed to stop the bike upright.

I don't think the altitude will make a big difference, but the heat from the rims on long hairpin descents will make a difference. If you overfill the tires they just might blow at high speed.

BigHit-Maniac
05-07-03, 08:55 PM
I beg to differ.

On the way to colorado from kansas 2 years ago.... my rear tire was at 55 PSI .... and it suddenly & randomly exploded as we gained altitude climbing in the mountains... (while it was still in the trailer behind us on the highway).

So... I dunno... heh.

:D

Waxbytes
05-07-03, 10:46 PM
Ok your tire is inflated to 75 PSI at 300 m, and atmospheric pressure is 14.7 PSI at sea level. So lets say you pedal your bicycle up to the International Space Station, you now have virtuallly 0 PSI outside the tire, so your total effective pressure on the tire is 75 PSI plus the "missing" 14.7 PSI atmospheric pressure or 89.7 PSI total. Most 75 PSI tires can handle that kind of overpressure without blowing. I think you need not worry about the reduced atmospheric pressure causing a blow out at any elevation you can breathe enough to pedal.;)

nathank
05-08-03, 03:19 AM
as most of the posters here have said, it will make a slight difference, but it shouldn't be significant.

i do most of my high altitude riding on a mountain bike with more like 40-55psi but i have hardly noticed a difference (last year did about 5 passes over 2700m). just check your tire pressure at the beginning of a ride and you should be fine.

and most of the passes in Europe aren't really that high. there are only a few places where you can ride a mountain bike over 3000m and even fewer on a road bike --- i'll probably be riding the GrossGlocknerHochStrasse some time this summer :)

and i agreed last night to do a roadbike race with a friend in July that goes over Stilfersjoch which is 2700m... (i rode it on the MTB last August)

fujitive
05-08-03, 04:21 AM
Yeah it was the GrossGlockner I was thinking about...I will be up there around September 21 this year. All up I will be tackling about 12 passes over 2000m.

By the sound of it I dont have much to worry about though I do tend to have my tyres pumped close to their limit as all up the weight load is 122kg's. (me, bike, gear, 4 litres water) I guess I will check the pressure as I ascend just to be sure.

Thanks for all the responses.....

:beer:

Hants Commuter
05-08-03, 05:50 AM
I suspect you tires will be alright. I was on the Jungfrau train and saw a packet of crisps (chips) explode, but a tyre is much tougher

The tyres should also be OK in the aircraft as the hold is normally pressurised, however if it was depressurised for some reason (not sure this can be done without affecting the passenger cabin - Any pilots care to answer?) the sudden change in pressure may cause problems.

Enjoy the Grossglockner, Austria is a lovely country - I was there about 9 years ago doing some walking.

nathank
05-08-03, 06:05 AM
hey Mat,

send me a message before you head out this way... maybe i'll be up for riding the HochStrasse with you...

through the end of august i'm pretty booked (the 9-day TransAlp MTB tour i lead is Aug 1-10), but September is still open. i should be SUPER fit by then (just finished a MTB race last weekend in Italy with 3530meters of vertical in 104km in 7hours 13 minutes)

fujitive
05-08-03, 07:08 AM
Hi Nathank,

I think my fitness level would be pretty ordinary against yours and I would probably hold you up somewhat. I start out from Budapest via Vienna, Prague, Salzburg before I hit the big hills so I should be in reasonable shape but I am more of a plodder. (never know I may suprise myself..I have just never tackled huge climbs before...dont know how the thin air will affect my asthma)

I will keep your offer in mind and let you know the exact dates I will be there. I am still very much planning at the moment.

Cheers
Mat

hayneda
05-08-03, 08:07 AM
Originally posted by fujitive
I understand that bicycle tyres are known to explode in planes which fly at 10,000 metres. (so I will deflate the tyres when flying)

Balderdash. This is a common old wives' tale. Even if you shot your bike into orbit, the pressure drop is 15 psi (from sea level). So, unless you already have your tires pumped to the limit, you'll have no problems. (Keep in mind that the manufacturers 'rate' the tire pressure with a healthly safety factor).

Dave

05-08-03, 08:49 PM
Two related, but off-topic anecdotes.

I don't know about planes or high altitude riding, but as pucci says, heat can do it. I used to drive a station wagon and often left my bike in the cargo area. Twice during the summer, I came out from work to find that I had one or two flat tires. The bike shop guy told me it was from heat... deflate the tires if you're going to leave the bike in the car during the summer.

Story 2, further off topic: I had a friend that was out in California, driving up... whatisit, Bishops Valley? On the Nevada side of the Sierras, anyway. Drove up Tioga Pass to Yosemite, and on the way heard a big POP!! Couldn't figure out what it was until that night when the found the exploded bag of potato chips. (I know, I know. Celllophane bags aren't tires, I just thought it was funny.)

D*Alex
05-10-03, 08:04 AM
As a mechanical engineer, I must take this opportunity to clear up a couple of points.
Firstly is the difference between absolute pressure and gauge pressure. The pressure value listed on the tyre is gauge pressure, which assumes an initial (empty) reading of zero. Absolute pressure is simply gauge pressure + atmospheric pressure at any given point. If the difference here is 3.6 psi atmospheric pressure, then the difference in gauge pressure would likewise be 3.6 psig.
Secondly is the relationship between pressure and temperature. Using the equation of PV=mRT that every student learns in chemistry 101, you can easily produce the equation P2=P1(T2/T1). (Tempeatures must be in absolute temperature, by adding 273 to degrees celsius, or 460 to degrees farenheit).
The blowout described by one person was likely caused mostly by temperature increase, due partly to friction on the tread, but mostly by heat from the brakes.

gasnavi
11-04-11, 01:29 AM
Atmospheric air pressure at sea level is about 14.7psi. if you went up to an altitude of 10000ft, the pressure would be about 10psi. So wouldn't the pressure if the tires be multiplied by (14.7/10)? That comes out to 1.47. If your tire was at 100psi at sea level, it would be at 147psi at and elevation of 10000ft. This seems to more accurately explain why road tires pop in airplanes if they are inflated to normal riding pressures. Both of my 26" Primo Comet's popped when I was going through a mountain pass. They were pumped up to 100psi. This would mean that it would have been 1.3 times the pressure at sea level.

My reasoning may be off, but here it is.
If the atmospheric pressure pushing against the outside of the tire decreases to half of what it was originally, wouldn't the tire pressure pushing out effectively double?

I got my pressures from here:
http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

I know this is an old thread, but i'm driving down again and just wanted to clarify.

11-04-11, 06:11 AM
Yeah it was the GrossGlockner I was thinking about...I will be up there around September 21 this year. All up I will be tackling about 12 passes over 2000m.

By the sound of it I dont have much to worry about though I do tend to have my tyres pumped close to their limit as all up the weight load is 122kg's. (me, bike, gear, 4 litres water) I guess I will check the pressure as I ascend just to be sure.

Thanks for all the responses.....

11-04-11, 06:14 AM
Atmospheric air pressure at sea level is about 14.7psi. if you went up to an altitude of 10000ft, the pressure would be about 10psi. So wouldn't the pressure if the tires be multiplied by (14.7/10)? That comes out to 1.47. If your tire was at 100psi at sea level, it would be at 147psi at and elevation of 10000ft. This seems to more accurately explain why road tires pop in airplanes if they are inflated to normal riding pressures. Both of my 26" Primo Comet's popped when I was going through a mountain pass. They were pumped up to 100psi. This would mean that it would have been 1.3 times the pressure at sea level.

My reasoning may be off, but here it is.
If the atmospheric pressure pushing against the outside of the tire decreases to half of what it was originally, wouldn't the tire pressure pushing out effectively double?

I got my pressures from here:
http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

I know this is an old thread, but i'm driving down again and just wanted to clarify.
Tires do not pop in airplanes. The side wall pressure is 1/2 the pressure required to blow the tire off the rim.

Looigi
11-04-11, 06:54 AM
Atmospheric air pressure at sea level is about 14.7psi. if you went up to an altitude of 10000ft, the pressure would be about 10psi. So wouldn't the pressure if the tires be multiplied by (14.7/10)?

WTF? Add or subtract, not multiply or divide. If worked as you say the pressure in anything would go to infinity as the ambient pressure went to zero, as it essential does in space, in which case any space capsule would blow up...especially if made of carbon fiber.

prathmann
11-04-11, 07:01 AM
Atmospheric air pressure at sea level is about 14.7psi. if you went up to an altitude of 10000ft, the pressure would be about 10psi. So wouldn't the pressure if the tires be multiplied by (14.7/10)? That comes out to 1.47. If your tire was at 100psi at sea level, it would be at 147psi at and elevation of 10000ft. This seems to more accurately explain why road tires pop in airplanes if they are inflated to normal riding pressures.
No, if you pump up your tires to 100psi when at sea level with normal ambient air pressure of 14.7psi, then the absolute pressure inside the tires would be 114.7psi since your pressure gauge measures the difference between the tire pressure and the outside air pressure. Take those tires up to an altitude where the ambient air pressure is 10psi and then their absolute pressure of 114.7psi would read as 104.7psi on a pressure gauge - a bit higher but insignificant and well within the safety margins of a tire which should be ok up to almost twice the rated pressure specified on the sidewall.

And no, road tires do not pop in airplanes. I've taken my folding Bike Friday on numerous airline trips in its suitcase with the tires fully inflated without any problems. In fact the rear tire is a Primo Comet with a rated pressure of 85psi that I normally inflate to 100psi which is still not a problem when in the cargo hold of an airliner (which is pressurized to be equivalent to an altitude of 8000' or lower regardless of flying altitude). Even if the plane's pressure system failed and all those oxygen masks came down, the effective pressure in the tire wouldn't go over 110 psi which is still well within its safety margin.

Monster Pete
11-04-11, 07:21 AM
gauge pressure is a difference between inside and outside pressure. If you inflated a tyre to 70 psi, then took it into space, the gauge pressure would increase by just under 15psi, so tyre pressure would effectively be 85psi. Nothing to worry about, even if you tried cycling on the moon. High-altitude cycling should be a non-issue, especially as there is a safety factor built in to the tyre's rated pressure.

Cargo holds in airliners are at exactly the same pressure as the passenger cabin. It's far easier to pressurise the entire aircraft (cylinder shaped) than just the cabin (cylinder with flat edge)

p2templin
11-04-11, 08:03 AM
Take those tires up to an altitude where the ambient air pressure is 10psi and then their absolute pressure of 114.7psi would read as 104.7psi on a pressure gauge
Never mind the fact that you can't get the tire up to that altitude instantaneously. The tire will leak (why else would we inflate our tires before every ride) on the way, and it's just not going to matter.

medevilgirl
11-04-11, 08:13 AM
On the descent from a 750 meter climb. my overfilled rear tire blew out at 70kph. The tubular tire blew right off the rim and locked the wheel. It was a miracle that i managed to stop the bike upright.

Pete In Az
11-04-11, 09:25 AM
On the descent from a 750 meter climb. my overfilled rear tire blew out at 70kph. The tubular tire blew right off the rim and locked the wheel. It was a miracle that i managed to stop the bike upright.
If you were descending, wouldn't the absolute pressure in the tire be going down?

prathmann
11-04-11, 09:39 AM
"... The tubular tire blew right off the rim ..."
If you were descending, wouldn't the absolute pressure in the tire be going down?
Yes, the outside air pressure would have been increasing and tending to reduce the gauge pressure in the tire. But application of the brakes might well have increased the temperature enough to still raise the tire's pressure although it takes a very large temperature increase for this to cause problems. I'm also puzzled by a tubular (i.e. sewup) tire blowing off the rim. Even if the tire had a sudden blowout, it shouldn't have come off the rim to which it has been glued. Is it possible that it wasn't glued securely and rolled off the rim in a turn? If it then got caught against the frame that may have been the cause for the blowout.

p2templin
11-04-11, 10:47 AM
On the descent from a 750 meter climb. my overfilled rear tire blew out at 70kph. The tubular tire blew right off the rim and locked the wheel. It was a miracle that i managed to stop the bike upright.
That's exactly what Pucci said earlier in the thread. What are you trying to tell us?

BCRider
11-04-11, 11:33 AM
On the descent from a 750 meter climb. my overfilled rear tire blew out at 70kph. The tubular tire blew right off the rim and locked the wheel. It was a miracle that i managed to stop the bike upright.

Which, as previously mentioned by another descending rider, was related more to the heat in the rim from all the braking getting to the tire and weakening the rubber of the tire such that it wasn't able to hold the cords that make up the carcase any longer. Yes, the heat would also increase the pressure in the tire but not by a whole lot. However if you were up near the max for the tire then the heated rubber combined with the heat related rise in the air pressure could all add up to a blown tire.

Being that you're saying that this was a tubular it's also likely that the glue softened from all the heat and allowed the tire to roll a bit. This could add to the forces such that the tire may have managed to tear itself as well as then blow out. But it's all still heat related due to the descent and generous use of brakes.

Folks doing serioiusly long descents if they aren't in a race should just stop a couple of times to let the rims and brake pads cool down. Or pack a drag chute to use behind them to control their descent speeds.... :D

JanMM
11-04-11, 11:36 AM
That's exactly what Pucci said earlier in the thread. What are you trying to tell us?

Pucci said that eight years ago!
This zombie thread was awakened from an eight year nap early this morning.:D

dougmc
11-04-11, 11:48 AM
Tires do not pop in airplanes. The side wall pressure is 1/2 the pressure required to blow the tire off the rim.They do occasionally pop in cars, however, at least in Texas heat.

But that's more about the rubber getting soft and the pressure going up somewhat due to the 160 deg F heat -- and I think it's more about the soft rubber. I've had a few tires go flat due to the plastic rim tape getting too soft and letting the tube "creep" up into the hole and pop there -- now, if I leave a bike inside a car during the summer, I let the air out of the tires first.

As for a plane, space is often a premium, so I'd suggest usually letting the air out of the tires -- not to keep them from popping because any air pressure changes are small and the temperature will be kept reasonable, but just because that makes them take up a little less space.

And even eight year old threads deserve a little love ... though it's unfortunate that the person who woke it up was so very wrong in his claims.

JanMM
11-04-11, 12:56 PM
And even eight year old threads deserve a little love ... though it's unfortunate that the person who woke it up was so very wrong in his claims.

This Zombie thread has aged better than most others.

tomecki
11-04-11, 02:26 PM
Since this thread is already pretty nerdy, I'll take it one step further.

Don't forget about the change in temperature with elevation.

So let's assume you're at sea level:
Temperature is 20C (68F, 293K)
Atmospheric Pressure: 101.3 kPaa (14.7 psia)
Tire Pressure:100 psig (689 kPag) or 114.7 psia (790 kPaa)

Then you climb to 2700m.
Data from standard atmosphere calculator at http://www.digitaldutch.com/atmoscalc/
Temperature: 2.6 deg C (37F, 275.6K)
Atmospheric Pressure: 72.8 kPaa (10.6 psia)

If the temperature was still 20C at the top of the climb your tire pressure gauge would read 104.1 psig.

But since the temperature at the top of the mountain pass is much lower than at sea level, we should account for that. Using
P1/T1=P2/T2 we get
P2=P1*T2/T1
P2= 790*275.6/293
P2= 743 kPaa = 107.7 psia

So your gauge would show 107.7 - 10.6 = 97.1 psig

But since you probably started early on your big climb, and by the time you got to the top it had warmed up a bit, to let's say, 10 degrees C (50 F, 283K)

P2 = 790*283/293
= 763 kPaa = 110.6 psia

So your gauge would show 110.6 - 10.6 = 100 psig! So don't worry about it.

Just be careful you don't heat up that rim too much on the way down by doing something crazy like using the brakes ;)

pdlamb
11-04-11, 07:11 PM
I wouldn't go so far as to state categorically that no bicycle tire ever exploded in an airplane; heck, I've read where people's tires exploded sitting in their living room!

But as noted above, there is an insignificant difference in pressure at sea level and 30,000 ft. Even more negligible between 0 and 10,000 ft. Make sure your tires are mounted correctly, and aren't damaged, and you should be fine.

(Come to think of it, that applies all the time!)

Pete In Az
11-04-11, 07:29 PM
Since it has become pare of the discussion, could someone please define a "long decent"? It doesn't have to be too exacting.

feet?
Meters?
Kilometers?
Miles?
How many of what?