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Discussion Starter · #1 ·
OK, I live at 7500 feet with pressure altitude of 11.1 psi. Yet it’s less than a 30 mile ride down to near sea level with a pressure altitude of 14.7 at least once a week. A pressure delta of 3.6 psi. If I set the front at 38 at home I’ll only have 34.4 when down the mountain. If I set the front tire at 38 at sea level I’ll have nearly 42 when home. I generally set both front and backs between 42 and 43 at home splitting the difference.

This pressure delta is NOT something I am overly concerned about and I do NOT stop half way up or down and change. The ride is just too great with lots of twisty and generally little traffic.

Any thoughts from the tire experts?
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Holy cow. And I thought that I had tire pressure problems! I am
blessed. I have no answers but I did live in the Denver area for
several years and rode into high elevation regularly. I didn't even
think about the pressure change and never had a problem. Course
I was riding an FLHS at the time and the main problem at high altitude
was that it wouldn't hardly run at all. Tires were the least of my worries. HA.
 

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John,

I don't think a ΔP of 3.6 psi is anything to be concerned with. Depending on where you do most of your riding, I would set tire pressure at 41psi for that area. In other words, I would set it at 41 for the high altitude. If you spend a length of time (a week or more) at the lower altitudes, then you might want to compensate.

I've rode on my bike for several weeks at 36 to 38 psi with no ill effects. I did however notice a considerable difference when my pressure was at 22 psi. That happened this weekend. Had a nail in the tire and a slow leak. Plugged it, back to 41 and I'm good to go. Fortunately, new tires are in my future within the next several months.

Bartman
 

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Discussion Starter · #4 ·
Pressure Delta is not the biggest Challenge

Just came back up the mountain after spending a few days near the Beaches. The other issue is the temperature differences. It was 83 degrees when I filled up near the base and 30 miles later at 8300 feet, Onyx pass, it was only 44 degrees. Heater grips on full blast and I was getting cold - but then only 15 more miles to home. This is getting near the extreme change I see coming up the mountain, late in the afternoon as the sun is setting. During the afternoon it generally runs 20 to 25 degrees. The ride from the base to the top of the pass is only about 30 minutes if the slower traffic pulls over and lets you by. Generally, most drivers on this road are very good about pulling over and if they don’t I just back off and wait for one of the few passing zones. It’s a great ride with higher speed turns on the upper 1/3 and lots of very tight twists in the bottom 1/3.
 

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Your forgetting that the bike weighs less and so do you at higher altitudes. The most important thing to remember about airing your tires is to do it as listed on the sticker on the inside of the trunk lid or the instructions in the operators manual.
 

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John, After reading your second post Me thinks you are just bragging
about your interesting dailey ride. TWIT galewing
 

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John.....Bartman has good scoop for you. I might add that its a good idea also to reset your altimeter before turning onto final approach or you will be flying thru the trees instead of over them! Meby Bartman will reach out and check his tire pressure on the Skyhawk next time he's at service ceiling. That would give us a real perspective of the issue.
Hmmmm, I know your question was serious, and this is my last hopeless attempt at humor. :D
 

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Ambient temperatures are more important than pressure, so you're looking for density altitude :). IIRC, altitude contribution is too small to worry about unless you're on Mt. Everest.

The specs are set at 80F ambient. A good rule of thumb is ot allow a 1 psi change for every 10 degree change ambient. Example it's 50 F out, so subtract 3 psi (80-50 = 30, and 10:1 ratio yields 3.) Conversely, if it's 100 out, add 2 psi.

Also, if you measure your tires immdiately after riding, all bets are off - they will not read what you hoped. They need to return to ambient and that can take several hours. Boyle would get upset.

David M.
 

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GoldWingrGreg said:
Your forgetting that the bike weighs less and so do you at higher altitudes. The most important thing to remember about airing your tires is to do it as listed on the sticker on the inside of the trunk lid or the instructions in the operators manual.


Air your tires up per the recommended max cold air inflation rate printed on the sidewall of the tires. This comes directly from the tire manufacturer. This also provides for improved handling and tire tread life, as well. The only benefit you get from the operator's manual info is a softer ride, decreased tread life, and poorer handling characteristics.

Something else to remember is that as your tire heats up (even in colder temperatures) while riding, the air pressure inside the tires will increase, somewhere in the neighborhood of 8--10 percent, meaning if you start out with, say 40 lb. tire pressure cold, the pressure can increase from 43.2 to 44 lb. This is taken into consideration as a factor when the max air pressures are printed on the sidewall of the tire. You won't hurt the tire or risk anything safety-wise by following the figure on the sidewall of the tire. My recommendation is to keep your tires inflated for the elevation you spend the most time riding at, and don't worry about lower elevations unless you plan to spend a lot of time down there.
 

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Okay, maybe I am missing something here? What does density altitude have to do with pressure in a sealed container (tires)? The only thing that changes is the pressure outside the container. This may allow the container to expand or shrink thus affecting the pressure but that's all.

Temperature has a lot more effect on tire pressure.
 

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riffraff said:
Okay, maybe I am missing something here? What does density altitude have to do with pressure in a sealed container (tires)? The only thing that changes is the pressure outside the container. This may allow the container to expand or shrink thus affecting the pressure but that's all.

Temperature has a lot more effect on tire pressure.
Actually, since you use a gauge to measure pressure, it is referred to as Pounds Per Square Inch Gauge (PSIG). Actually, there really isn't such a thing as PSIG since gauge simply measures the pressure ABOVE the atmospheric pressure. Gauge pressure plus atmospheric pressure is known as PSI Absolute (PSIA) or the pressure above an absolute vacuum.

So if you use a gauge to measure your tire pressure and it was 41 PSIG and the atmospheric was say 11.1 PSI then this is 52.1 PSIA. Once you dropped to sea level where the atmospheric is 14.7 PSI then your gauge would read 52.1 - 14.7 or 37.4 PSIG. This represents about a 7% drop in pressure reading from a gauge.

On the other hand the equation for changes in pressure due to temperature variation is pretty simple (Temp Deg F + 460) / 520; so at 60 deg F the factor is 1.0 at 70 deg F the factor is about 1.02 at 80 it is 1.04 etc. So each 10 deg F change in temperature represents about a 2% change in pressure. In order to get an equivalent change in pressure due to temperature as you did for atmospheric change above you would have to have a change of about 35 deg F.

Tires experience this kind of change in temperature all the time, based on the SmarTire I run. Don't worry be happy!
 

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So if you use a gauge to measure your tire pressure...
Yup, I was missing something. I didn't think of the effects on the guage.

So, if you were to air up at sea level and ride to 8k feet you could safely assume the absolute pressure in the tire is the same. Well, except for the nails you picked up on the ride. :roll:

I just checked all my vehicle's tires because the morning temps have dropped from 50s to 30s. I found all of them about 3-4 pounds low.
 
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