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I've always considered it common knowledge that a typical compressor won't fill a bike tire past 50 or 60 pounds, regardless of what three-digit number is on the side of the tank.

So a customer came in the shop just now and stated that he uses his home compressor to fill his road bike tires. His thinking was that since he had a 120 psi compressor, and since the tank gauge reads 120 he could just fill till his tire stopped taking air and that was 120.

Here this guy who been riding around with his roadie tires half-flat, and he will absolutely not accept that the only way to properly fill a road bike tire is with a floor pump.

I've filled enough bike tires over the last 15 years to know I'm right, but I couldn't offer this guy a scientific reason why this was the case. So the stubborn moron left content to keep making the same mistake over and over.

So, any engineering types want to explain this? I assume it's because of the volume of the tank compared to volume of the tire.

PS: I don't wanna hear about your hi-zoot compressor that can do it. :) We're talking about the $250 ones that have little plastic wheels and say "Craftsman" on the side.
 

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singletrack said:
I've always considered it common knowledge that a typical compressor won't fill a bike tire past 50 or 60 pounds, regardless of what three-digit number is on the side of the tank.

So a customer came in the shop just now and stated that he uses his home compressor to fill his road bike tires. His thinking was that since he had a 120 psi compressor, and since the tank gauge reads 120 he could just fill till his tire stopped taking air and that was 120.

Here this guy who been riding around with his roadie tires half-flat, and he will absolutely not accept that the only way to properly fill a road bike tire is with a floor pump.

I've filled enough bike tires over the last 15 years to know I'm right, but I couldn't offer this guy a scientific reason why this was the case. So the stubborn moron left content to keep making the same mistake over and over.

So, any engineering types want to explain this? I assume it's because of the volume of the tank compared to volume of the tire.

PS: I don't wanna hear about your hi-zoot compressor that can do it. :) We're talking about the $250 ones that have little plastic wheels and say "Craftsman" on the side.
My cheapish compressor works just fine. Rated to 135psi, will inflate to at least 120psi--as long as you set the pressure regulator (the output pressure) at or above the pressure you want to achieve.

Never had an issue with any compressor that has a pressure tank. If it has a very small tank you may need to have the compressor turned on (duh).
 

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I have dealt with many air compressors in my day, including cheap Craftsman models, and have never had the problem you are talking about. The pressure shown on the tank gauge is the same pressure it will deliver to your tire as long as the regulator on the output is not turned down.
 

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Stay thirsty my friends
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...unless of course you live in Colorado or somewhere 3 miles above sea level.:D
 

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4JawChuck said:
...unless of course you live in Colorado or somewhere 3 miles above sea level.:D
Uh, there's nothing in Colorado 3 miles above sea level, Grand Junction where the OP is at around 4700 ft....and I don't see why that would make a difference inside a compressor anyways...

ps I meant compressor tank
 

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Even my 12 volt tankless compressor will fill a bike tire to 120psi. A compressor that doesn't put out its rated psi unregulated, tankless or not, is usually refered to as broken. My 10 gallon storage tank will fill a bike tire to 175psi(or until the tire blows which ever comes first) when filled.

Bikinfoolferlife is right, the compressor will still put out it's rated psi at high altitude but the volume delivered will be reduced.
 

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4JawChuck said:
...unless of course you live in Colorado or somewhere 3 miles above sea level.:D
Does not make any difference. May just take longer to reach the pressure.
 

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Bikinfoolferlife said:
Uh, there's nothing in Colorado 3 miles above sea level, Grand Junction where the OP is at around 4700 ft....and I don't see why that would make a difference inside a compressor anyways...
Don't go into engineering then!

I also don't remember saying anything in Colorado is 3 miles above sea level?:skep:

Please don't make me post an explanation of why a positive displacement compressor would achieve less efficiency at high altitudes...I am way too lazy to educate on Christmas Eve.:rolleyes:

READ

http://www.cheresources.com/invision/index.php?showtopic=1998
 

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4JawChuck said:
Don't go into engineering then!

I also don't remember saying anything in Colorado is 3 miles above sea level?:skep:

Please don't make me post an explanation of why a positive displacement compressor would achieve less efficiency at high altitudes...I am way too lazy to educate on Christmas Eve.:rolleyes:

READ

http://www.cheresources.com/invision/index.php?showtopic=1998
He was refering to a compressor not being to produce it's rated psi at that altitude. It's fill time will be increased due to a decrease in cfm but not enough to be noticable. Not even sure how the op thought volume had anything to do with a compressors abilty to reach rated psi.
 

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4JawChuck said:
Don't go into engineering then!

I also don't remember saying anything in Colorado is 3 miles above sea level?:skep:

Please don't make me post an explanation of why a positive displacement compressor would achieve less efficiency at high altitudes...I am way too lazy to educate on Christmas Eve.:rolleyes:

READ

And to pompous to do it any other day of the year
 

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yummmmmmmm, thermodynamics on christmas eve. There are a few "givens" that weren't given in the OP.

Is the assumption that the guy is filling the tires directly from the tank without the compressor running? If so, you can approximate by using the universal gas law PV=nRT, which would actually simplify down to P1V1 = P2V2 at steady state conditions (sorry no subscripts).

If you are assuming that the compressor motor is running and ensuring that the reservoir pressure stays constant (does not pump when resevoir is over set pressure, say...... 120 psi), then yes theoretically the pressure in the bike tube should be whatever the compressor output pressure is (the full 120psi). Please keep in mind that efficiencies need to be employed in real world applications.

......that's more engineering than I've done in three years :madman:.
 

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shiggy said:
as long as you set the pressure regulator (the output pressure) at or above the pressure you want to achieve.
+1 I think the OP needs to learn how to use a compressor or get his compressor repaired.

I'm glad I wasn't the customer he tried to set straight with incorrect info.
 

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I guess if providing accurate informed answers to a question...then I am pompous as charged.:thumbsup:

A compressor that can achieve 100PSI at sea level will only be able to achieve approx 80PSI at 5000ft.

The interesting part is the gauge will still read 100PSI.

At three miles altitude your looking at 50-60 psi depending on humidity...the gauge still reads 100PSI.

Of course at the lower atmosperic pressure the tire "thinks and acts" like it would if pressurized to 100PSI at sea level.

I won't go into compressor surging at high altitudes which will limit the output pressure and cause excessive heating of the reed valving causing eventual destruction of a compressor running at 100% rated pressure capacity at high altitudes for long periods.:p

BTW I have a 3HP "Craftsman" oiless compressor in the shed and I doubt it could achieve more than 100psi now that it is used, it used to get up to 120psi when it was new though.

...but at 780 ft elevation above sea level (where I am) thats more like 118PSI.:D

Merry Christmas!

P.S. I still think he forgot to adjust the regulator to 120PSI.;)
 

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Why would anyone think a compressor can't inflate tires beyond 50psi, and now we're arguing about it?

Merry Christmas!
 

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pursuiter said:
Why would anyone think a compressor can't inflate tires beyond 50psi, and now we're arguing about it?

Merry Christmas!
No. This is MTBR. We're not arguing about the original thread subject, but rather someone made a comment that someone else decided to call him out on. Now they're arguing about that. In the mean time, I've set you and I up to argue about what people in this thread are arguing about.
 

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4JawChuck said:
I guess if providing accurate informed answers to a question...then I am pompous as charged.:thumbsup:

A compressor that can achieve 100PSI at sea level will only be able to achieve approx 80PSI at 5000ft.

The interesting part is the gauge will still read 100PSI.

At three miles altitude your looking at 50-60 psi depending on humidity...the gauge still reads 100PSI.

Of course at the lower atmosperic pressure the tire "thinks and acts" like it would if pressurized to 100PSI at sea level.

I won't go into compressor surging at high altitudes which will limit the output pressure and cause excessive heating of the reed valving causing eventual destruction of a compressor running at 100% rated pressure capacity at high altitudes for long periods.:p

BTW I have a 3HP "Craftsman" oiless compressor in the shed and I doubt it could achieve more than 100psi now that it is used, it used to get up to 120psi when it was new though.

...but at 780 ft elevation above sea level (where I am) thats more like 118PSI.:D

Merry Christmas!

P.S. I still think he forgot to adjust the regulator to 120PSI.;)
Is because of the difference is normal air pressure at sea level (~14.6psi) and at altitude or in how the gauge reads the pressure? There is a 20% drop in the air pressure at 5000' but if the gauge assumes "0psi" is 14.6psi would a reading of 100psi at 5000' actually be ~97psi, not 80?
 

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The altitude in Colorado would not account for what the OP is saying.

"a typical compressor won't fill a bike tire past 50 or 60 pounds, regardless of what three-digit number is on the side of the tank."

If what the OP is describing is really happening to him, and assuming that the output regulator is set properly, the best explanation I can think of is that his air line from the tank is creating a restriction. Anytime there is an orifice in a line, there will be a pressure drop. If you have 125 psi on the tank gauge and you are using 50 feet of hose with an inside diameter of 1/16", it would take forever to get decent pressure transferred to the tire.

While what 4Jaw is saying about the pressure gauge reading versus sea level pressure is true, the pressure gauge reading of what is in the tire would also read off due to the altitude. So, in the theoretical example given of 3 miles above sea level, if the tank gauge showed 100 psi, you would have no trouble getting enough air into the tire that a tire pressure gauge check would show pretty close to the same pressure shown on the tank.
 

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nachomc said:
No. This is MTBR. We're not arguing about the original thread subject, but rather someone made a comment that someone else decided to call him out on. Now they're arguing about that. In the mean time, I've set you and I up to argue about what people in this thread are arguing about.
Ha-ha ain't that the truth! Hillarious!

I think the real point of this post is that the noob who walks into a bike shop with a brand new road bike who can't figure out how to check and fill his tires needs a bike with solid tires...maybe something with three wheels and included hockey helmet for cruisin the main street while he claps his hands furiously.

Let go down the steep downhill section John!

 

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As stated before, something is wrong with the OP's compressor .......

I also have a $250 craftsman compressor (crappy "oil free" one even) with little wheels, and it will do 120 psi all day long. It's 15 years old. I even have gage that's been recently compared to a calibrated gage, and it was plenty close enough.

Now if we're talking about the claimed scfm vs. reality cfm, that's a totally different story. The labeled claim is about double what they will really do. Marketing guys gone wild.......
 

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4JawChuck said:
Ha-ha ain't that the truth! Hillarious!

I think the real point of this post is that the noob who walks into a bike shop with a brand new road bike who can't figure out how to check and fill his tires needs a bike with solid tires...maybe something with three wheels and included hockey helmet for cruisin the main street while he claps his hands furiously.

Let go down the steep downhill section John!
]http://kaz-usa.com/Tricycle%20adult%20race%20handicap.jpg[/IMG]
wow, just wow............. on christmas???

having a sick sense of humor myself, I have to admit that I did chuckle.........but still.......give'em a break on christmas.
 
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