# Compressors and why they don't inflate to their rated pressure.



## singletrack (Feb 19, 2004)

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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## shiggy (Dec 19, 1998)

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.
> 
> ...


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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## Moustache rider (Jun 1, 2007)

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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## 4JawChuck (Dec 1, 2008)

...unless of course you live in Colorado or somewhere 3 miles above sea level.


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## Bikinfoolferlife (Feb 3, 2004)

4JawChuck said:


> ...unless of course you live in Colorado or somewhere 3 miles above sea level.


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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## Remy Darke (Mar 18, 2008)

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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## shiggy (Dec 19, 1998)

4JawChuck said:


> ...unless of course you live in Colorado or somewhere 3 miles above sea level.


Does not make any difference. May just take longer to reach the pressure.


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## 4JawChuck (Dec 1, 2008)

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.

READ

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


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## Remy Darke (Mar 18, 2008)

4JawChuck said:


> Don't go into engineering then!
> 
> I also don't remember saying anything in Colorado is 3 miles above sea level?:skep:
> 
> ...


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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## dirtdonk (Jan 31, 2004)

4JawChuck said:


> Don't go into engineering then!
> 
> I also don't remember saying anything in Colorado is 3 miles above sea level?:skep:
> 
> ...


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## FloridaFish (Mar 29, 2004)

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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## gmcttr (Oct 7, 2006)

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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## 4JawChuck (Dec 1, 2008)

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. 

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. 

Merry Christmas!

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


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## pursuiter (May 28, 2008)

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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## nachomc (Apr 26, 2006)

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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## shiggy (Dec 19, 1998)

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.
> 
> ...


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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## 11 Bravo (Mar 12, 2004)

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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## 4JawChuck (Dec 1, 2008)

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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## Manning (Apr 11, 2007)

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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## FloridaFish (Mar 29, 2004)

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.
> 
> ...


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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## Bikinfoolferlife (Feb 3, 2004)

While I'm dense when it comes to this kind of stuff (and obviously I'm no engineer nor at my age do I plan to go that direction) I can understand some of the explanations but the one that I'm not seeing/getting is why the pressure reading for the contents of a metal tank would be affected by atmospheric pressure differences. Or is a metal tank still somewhat elastic like a tire at elevation? Or is the gauge not actually measuring pressure inside the tank? Sorry to you engineers for not understanding this stuff...


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## 4JawChuck (Dec 1, 2008)

Simple explanation, atmospheric pressure at altitude is lower...at 3 miles it almost half of sea level pressure.

Most gauges (unless they are rated at PSIA or Absolute pressure) are referenced to outside pressure as they are typically the bourdon tube type. Therefore at low outside pressure a lower "inside the tank" pressure would register the same.

Also...since the pressure at altitude is less, a tire needs less pressure inside it to inflate it to the same pressure on the carcass.

For these reasons you should not fly with your bike tires inflated to spec, airing them down is very wise or you risk blowing them off the rim. Aerosol cans experience a similar effect although they are designed to handle increases in pressure due to altitude changes.

For example, a tire inflated to 100 psi at sea level would be inflated to 150 psi (approximately) at 3 miles altitude and if you checked the pressure at that altitude the bourdon tube gauge would indeed say the tire is at 150psi.

In our next chapter we will explore the pressures inside a chicken egg inside a 1000watt microwave oven set on high. Please remember to bring your safety goggles for this class.

P.S. You guys didn't care for Johns new three wheeler? C'mon its got offroad turf tires on it and he said hes ready for a little black diamond practice before he goes to Whistler.

This is me dressed for the slopes ready to huck...you know your jealous.


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## shiggy (Dec 19, 1998)

4JawChuck said:


> Simple explanation, atmospheric pressure at altitude is lower...at 3 miles it almost half of sea level pressure.
> 
> Most gauges (unless they are rated at PSIA or Absolute pressure) are referenced to outside pressure as they are typically the bourdon tube type. Therefore at low outside pressure a lower "inside the tank" pressure would register the same.
> 
> ...


But the point in question here is a compressor and tire that is used and inflated at altitude, not filled at sea level and taken to elevation or vise versa.

Is or is not a pressure tank and/or tire filled (and used) at whatever elevation being pressurized to a "proper" operating level as record on the gauge?

I would say yes, it is. I have never seen a tire, fork or any other air filled product manufacturer supply different psi recommendations based on altitude.


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## 4JawChuck (Dec 1, 2008)

Ummm...yes! 

...but being a supermoderator you have not set a good example for the rest of the class by not bringing your safety glasses to the demonstration.

:nono: tsk...tsk...tsk.


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## shiggy (Dec 19, 1998)

4JawChuck said:


> Ummm...yes!
> 
> ...but being a supermoderator you have not set a good example for the rest of the class by not bringing your safety glasses to the demonstration.
> 
> :nono: tsk...tsk...tsk.


I _ALWAYS_ wear safety glasses.:yesnod:

It seems I have managed to sort through the non-relevant information that is confusing the discussion.


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## dan0 (Oct 12, 2005)

shiggy said:


> I _ALWAYS_ wear safety glasses.:yes nod:
> 
> It seems I have managed to sort through the non-relevant information that is confusing the discussion.


as usual
the first sentence of the original question doesn't make sense if you think about it 
"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"

if that were the case then why would you ever buy a compressor that went beyond 60psi? maybe he's confusing the tank size and CFM that allows a higher psi for a time than you would get constantly


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## CdaleTony (Jun 21, 2005)

The OP's post kinda makes me shudder...
*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.*
Would that be a case of the pot calling the kettle black? Sort of I guess, only one pot though...
And as far as confusing PSI with CFM....A bike pump isnt really rated in CFM is it  ?

*Last Activity: 59 Minutes Ago 10:20 PM 
Replying to Thread Compressors and why they don't inflate to their rated pressure. - 59 Minutes Ago*
Odd, no reply actually made it in here. 
I say delete the whole embarassing thread.
CDT


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## ArmySlowRdr (Dec 19, 2003)

I agree --delete the thing OP--I was so befuddled when I read it, I was about to reply but then didn't in case there was someting buried deep within there that I was missing....



CdaleTony said:


> The OP's post kinda makes me shudder...
> *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.*
> Would that be a case of the pot calling the kettle black? Sort of I guess, only one pot though...
> And as far as confusing PSI with CFM....A bike pump isnt really rated in CFM is it  ?
> ...


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## Teemu Kalvas (Sep 8, 2007)

4JawChuck said:


> Simple explanation, atmospheric pressure at altitude is lower...at 3 miles it almost half of sea level pressure.
> 
> [...]
> 
> For example, a tire inflated to 100 psi at sea level would be inflated to 150 psi (approximately) at 3 miles altitude and if you checked the pressure at that altitude the bourdon tube gauge would indeed say the tire is at 150psi.


If the air pressure at sea level is around 15 psi and at 15 000 feet around 8 psi (values from here), won't the pressure differential between the sides of the tire carcass only go from 100 psi to 107 psi? How does a bourdon tube gauge work anyway? Does it somehow measure one side in multiples of the other? In that case that'd make sense.

Anyway, for purposes of possible tire blow-off, the difference in pressure in airplane cargo holds really isn't all that much, because it can't possibly exceed the air pressure at sea level, which is 15 psi. Even increased pressure due to temperature rise is a bigger risk (and that's not much of one either.)


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## pursuiter (May 28, 2008)

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.


I just had to come back and see how this thread was going., it's good to start the day out laughing


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## Ricko (Jan 14, 2004)

*Floor Pump*

I see no reason to turn on the compressor and wait for it to pressure up when I can just grab the floor pump to fill up a bike tire.

FWIW, I can only get about 70psi in a road tire with my compressor and the gauge reads 100. Don't know why, always accepted it for what it is...and always just used the floor pump to fill bike tires.


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## Jim311 (Feb 7, 2006)

This thread makes my brain hurt.


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## dan0 (Oct 12, 2005)

CdaleTony said:


> The OP's post kinda makes me shudder...
> *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.*
> Would that be a case of the pot calling the kettle black? Sort of I guess, only one pot though...
> And as far as confusing PSI with CFM....A bike pump isnt really rated in CFM is it  ?
> ...


I know a hand pump isnt cfm rated, I was just trying to come up with a reason for his compressor problem, a cheap compressor with a low cfm might not be able to output enough air , I know its a stretch, more likely someone set tthe output regulator to 50-60psi and hes looking at the tank pressure and scratching his a**


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## singletrack (Feb 19, 2004)

*Oi vey....*

I have very little interest in arguing.

I have difficulty believing you guys fill ROAD BIKE tires with a compressor and call it good.

Yes, I know how to use the regulator. No, I don't trust it fill skinny tires.

Every road bike that comes through my shop gets inflated with a floor pump because that's how I can be sure it's done right. That's been a standard practice everywhere I've ever wrenched.

Now here's some smillies because this thread needs color.


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## CdaleTony (Jun 21, 2005)

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.
> 
> ...


Theres really no argument to be made, we want a concession. A compressor will fill to dam near the rated PSI _if_ the regulator is used. More so if you check it with a tire gauge. I mean if you dont trust a compressor gauge, do you trust a floor pump gauge? Or even one of those $2 gas station gauges?
Using your language (about a paying _customer _no less!) who is the stubborn moron?

I am going to have to pay attention at the shop, to see what they use. I cant see some guy pumping a way on some old Pista when he has a compressor in the corner....

Then again I am just a lay person customer.


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## singletrack (Feb 19, 2004)

CdaleTony said:


> Using your language (about a paying _customer _no less!) who is the stubborn moron?


OK, sorry for the inflamatory language. Then again, he didn't actually buy anything.


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## singletrack (Feb 19, 2004)

CdaleTony said:


> I mean if you dont trust a compressor gauge, do you trust a floor pump gauge?


It's not really about the gauges. I can continue to inflate with the floor pump past the point where the compressor won't fill.

That was my _question_ after all. Why won't a compressor reading 120 actually fill that high? It's quite common.


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## shiggy (Dec 19, 1998)

singletrack said:


> I have very little interest in arguing.
> 
> I have difficulty believing you guys fill ROAD BIKE tires with a compressor and call it good.
> 
> ...


Actually I do. I use a Problem Solver Air Bob inflator on my compressor. Tank pressure 135, regulator 125, tires inflated to 100-110psi by the Air Bob gauge (which has been checked as accurate against 4 different brands of digital gauges and two different floor pumps). :yesnod:

The only reason I have been given not to use a compressor on road (or any bicycle) tires is they can put out too much volume, too quickly, and blow the tire off the rim. :eekster: Never that it is a problem to get enough pressure in the tires.

Using a floor pump to "be sure it's done right" is fine, but it is very different than claiming it is because compressors can not achieve the pressure required.:thumbsup:


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## singletrack (Feb 19, 2004)

shiggy said:


> Using a floor pump to "be sure it's done right" is fine, but it is very different than claiming it is because compressors can not achieve the pressure required.:thumbsup:


Of course there are compressors that can do it. Good that yours is one of them. Most can't.

Ours has a tank pressure of 120 and the regulator reads 110. A road tire will probably hit 70 when the compressor ceases to fill.

I think if you looked at the average home-use compressor, you'd find most similarly incapable of properly filling a road tire.

Thus I will continue recommend that road tires be filled with a floor pump.


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## pursuiter (May 28, 2008)

Maybe one of your gauges is metric?


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## Jim311 (Feb 7, 2006)

singletrack said:


> Of course there are compressors that can do it. Good that yours is one of them. Most can't.
> 
> Ours has a tank pressure of 120 and the regulator reads 110. A road tire will probably hit 70 when the compressor ceases to fill.
> 
> ...


Maybe your regulator is screwed up, or your gauge is off. OR your floor pump gauge is off.


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## Bikinfoolferlife (Feb 3, 2004)

For those who say this thread makes their brain hurt or they want it closed....go read another thread. This thread beats hell out of the typical what bike should I buy thread or what should I upgrade first thread or what tire should I use thread or my disc brakes squeal thread etc. OK, well _ I_ find this interesting, it has some entertainment value and am learning something about compressors and gauges (maybe)...I'm also away from home watching over my dad while he recovers from his new replacement hip surgery, he's pretty much deaf so conversations around here are minimal. This northwest Oregon weather is pretty screwed so pretty much anything works for me right now since riding my bike isn't an option...

I've used compressors to fill road bike tires no problem (like Shiggy says, the problem can be too much pressure), but that's with a shop compressor of unknown specs (to me) and at sea level. While I can see a tire woud be affected by atmospheric pressure (and have experienced it), it's harder for me to understand the contents of a rigid steel tank would be; I've seen lesser containers be affected by atmospheric pressure but I couldn't imagine the steel tank, so the gauge thing I'll go read more about now...

It still confuses me that a hand pump can deliver more pressure than a compressor at altitude...


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## singletrack (Feb 19, 2004)

Bikinfoolferlife said:


> For those who say this thread makes their brain hurt or they want it closed....


My OP was bike-snobbish, that's the real issue I think...

As for compressors, I can't see altitude affecting anything, plus our compressor in Pittsburgh sucked too. 

I just think most compressors are assembled from a lot of cheap junk that can't be relied upon. That's my own personal scientific explanation.

As for a hand pump delivering more pressure, that's just because my arms don't turn off at some randomly-determined psi.


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## shiggy (Dec 19, 1998)

singletrack said:


> Of course there are compressors that can do it. Good that yours is one of them. Most can't.
> 
> Ours has a tank pressure of 120 and the regulator reads 110. A road tire will probably hit 70 when the compressor ceases to fill.
> 
> ...


I disagree with your contention that most compressors can not do the job. Most can. Even with a 1 gallon pressure tank and the power turned off 120psi is plenty to completely fill two 700x23 tires to at least 100psi. Just topping off from 50-60psi is a snap.

My experience with compressors in shops is they tend to have long air lines from the compressor to the work area (mainly for noise control), and several outlets in the lines. These systems are rarely totally air tight and any leak will cause the pressure available at the end to drop. With a home system you will rarely see more than 10-15 feet of hose between the tank and the air chuck. Pressure loss is minimal.


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## Bikinfoolferlife (Feb 3, 2004)

shiggy said:


> I disagree with your contention that most compressors can not do the job. Most can. Even with a 1 gallon pressure tank and the power turned off 120psi is plenty to completely fill two 700x23 tires to at least 100psi. Just topping off from 50-60psi is a snap.
> 
> My experience with compressors in shops is they tend to have long air lines from the compressor to the work area (mainly for noise control), and several outlets in the lines. These systems are rarely totally air tight and any leak will cause the pressure available at the end to drop. With a home system you will rarely see more than 10-15 feet of hose between the tank and the air chuck. Pressure loss is minimal.


I think you've got a winner here, this makes the most sense so far...having used a leaking system in Dville (3000' FWIW) last year for a bit until it got fixed up (but never tried a road tire there before or after).


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## RoyDean (Jul 2, 2007)

shiggy said:


> I would say yes, it is. I have never seen a tire, fork or any other air filled product manufacturer supply different psi recommendations based on altitude.


Thats because what creates the proper stiffness in the tire is the pressure DIFFERENTIAL between inside the tire and outside the tire!

If you were to take your bicycle tire into space, what you pumped it up to be 100psi at sea level will now be ~115 in the vacuum of space (as measured with a typical differential type pressure gage, like a cheap bourdon tube).

So the manufacturers (either through an educated assumption or just plain pure ignorance) are relying on you to use a cheap (non-absolute, differential style) pressure gage, as doing so will provide the optimum actual internal air pressure of the tire, regardless of altitude or ambient air pressure! 

And to the OP, I agree... your compressed air system sucks. My compressor at home has no problem outputting 100% of what the internal pressure gage reads... Although thats AFTER I changed out the cheapo original pressure gage for one that was slightly more expensive.


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## 11 Bravo (Mar 12, 2004)

singletrack said:


> It's not really about the gauges. I can continue to inflate with the floor pump past the point where the compressor won't fill.
> 
> That was my _question_ after all. Why won't a compressor reading 120 actually fill that high? It's quite common.


It has been hit on a few times here. If your output pressure regulator is set and functioning properly, then there is a restriction in the delivery system (the hose or fittings).

If the compressor is pumping the tank up to 120 psi, then it has noting to do with failure of the pressure to transfer to the tire. When the tank cut off pressure is reached and the compressor shuts off, it has done it's job. Even a small compressor tank of 20 gallon size would fill fill several road bike tires to full pressure before the compressor needs to start up and build more pressure in the tank.


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## Anonymous (Mar 3, 2005)

FloridaFish said:


> 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).
> 
> QUOTE]
> 
> ...


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## gmcttr (Oct 7, 2006)

singletrack said:


> Of course there are compressors that can do it. Good that yours is one of them. Most can't.
> 
> Ours has a tank pressure of 120 and the regulator reads 110. A road tire will probably hit 70 when the compressor ceases to fill.


The tire will cease to fill when it reaches equilibrium with the pressure allowed by the output regulator. If the regulator reads 110 *and it is correct*, then it will fill the tire to 110. There is something wrong with your system if it will only fill to 70 psi when reading 110 psi.

My cheap compressor and my even cheaper compressor will both fill to the regulator reading. Your premise that "Most can't" is incorrect.


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## 4JawChuck (Dec 1, 2008)

There is another reason why a tank pressurized to 120 psi will not pressurize a tire to 120 psi unless you wait an extremely long time for the pressures to equilize.

As a tires internal pressure comes closer to tank pressure air will flow more slowly into the tire, this effect is known a _*differential flow stagnation*_.

Simply put, as the pressures begin to equalize the flow rate naturally reduces, the closer the two inflated containers are in pressure the less air will flow.

Eventually a point is reached where fluid flow becomes restricted due to increasing boundry layer thickness inside the connecting tube, you can look at this boundry layer as an ever increasing restriction to fluid flow.

Try it sometime, this effect is exaggerated the smaller the connecting tube is.


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## CdaleTony (Jun 21, 2005)

4JawChuck said:


> There is another reason why a tank pressurized to 120 psi will not pressurize a tire to 120 psi unless you wait an extremely long time for the pressures to equilize.
> 
> As a tires internal pressure comes closer to tank pressure air will flow more slowly into the tire, this effect is known a _*differential flow stagnation*_.
> 
> ...


awesome , science!
But could you _*quantify*_ the effect in a real world situation? Like for a 1/4" ID hose x say 15 feet? Lay people want to know!


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## pursuiter (May 28, 2008)

4JawChuck said:


> There is another reason why a tank pressurized to 120 psi will not pressurize a tire to 120 psi unless you wait an extremely long time for the pressures to equilize.....


The time constant of this system is measured in fractions of a second, 4-5 time constants is more than enough time to equalize the pressure. So 2-3 seconds is more than enough time to negate this effect, bike tires don't have much volume.


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## 4JawChuck (Dec 1, 2008)

pursuiter said:


> The time constant of this system is measured in fractions of a second, 4-5 time constants is more than enough time to equalize the pressure. So 2-3 seconds is more than enough time to negate this effect, bike tires don't have much volume.


Not in my experience, the effect is magnified when the connecting hose is longer. Not to mention the small diameter orifice at the chuck end which further restricts flow.

I would test the theory but my compressor is in the shed and its -20C here and there is no way I am going out there with a stop watch to do the testing.

I seem to remember trying this once last summer by setting my regulator at 35psi and trying to fill my tires to pressure, it took so long to equalize I eventually went and jacked it back up to 90 psi to finish the job quickly.

I am sure there is a formula in one of my texts here somewhere but there would be so many variables...and I am just f-in lazy. BTW my fill hose is at least 30ft long and 3/8" ID so that doesn't help.

Try it sometime, it is not practical to set a regulator to the pressure required and let the two volumes equalize, this effect is the main reason why if staged tanks are used in industry the connecting plumbing is extremely oversize and usually 12-20 times (or more) rated CFM flow for the system.

Stagnation pressure is also affected by objects in the airflow plane, such as a jiggle valve as is typically found in the end of an air chuck. This device is there to prevent air from leaking out of the tank when the chuck is not passing air into a tire. In a flow system such as that encountered when air is flowing it forms a stagnation point where air must make a right angle turn to go around the obstacle. Anywhere air must make a turn there will be stagnation flow which causes severe restriction and at low flow rates complete blockage to air flow.

The phenomena is complicated but its effects are simple, the less differential between the two volumes and the more restrictors in the flow path you can reach a poijt where no air will flow.

It is akin to the problem of trying to reaching a wall and taking a half of a half step towards the wall each time, with each step...your steps become smaller and you will never reach the wall in theory. In this example there is no incline or force required to make the step but if you add in this factor the result is an exponential increase in time required to reach the wall.

During our next class we will explore the discovery of sulfur and its effects on the production of raw steel and the development of the common tire. We will explore how the discovery of sulphurs effects on steel "cold shorting" caused the sinking of the Titanic and how it has a beneficial effect on tire vulcanization which was discovered accidentally.


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## gmcttr (Oct 7, 2006)

OK...I have read all of the scientific info posted here and understand and except the principals. Let's put them to a test in the real world and see how they relate to bicycle tires.

-The compressor - small cheap Menard special (about $80). 50 feet of coiled 1/4" hose.

-Presta to Schrader valve adapter. (plenty of restrictions with the small hose, adapter and presta valves)

-Timex stop watch.

-2.3 Hutchinson Spider MB tire - regulator set to 35 psi - flat to 31 psi in 30 seconds. (An additional 30 seconds brought it to 32 psi.)

-700x25 road tire - regulator set to 100 psi (as high as it would go) - flat to 105 psi in *20* seconds.

I used the gage on my topeak floor pump to take the "after" readings. Obviously there are some inaccuracies in the two gauges and/or regulator as expected.

IMHO, my tests trump the science in relation to filling bicycle tires with a cheap compressor.

I still maintain that if the OP could only fill a road tire to 50-60 psi with a regulator set at over 100, he has a problem with his system.

With all of that said....I use the floor pump to fill my tires.


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## wormvine (Oct 27, 2005)

So how does my nail gun work when I set the regulator on the compressor to 90psi as recommended? The guns need 90psi in the chamber at the gun to work and they need it fairly quick. Like 1/4 second. 
I've used 3 framing nailers on a 100' supply hose that split to 3-50' hoses and you could fire those guns pretty fast with a gas powered 4 gallon compressor. But I guess I couldn't fill a tire to 70psi??????


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## FloridaFish (Mar 29, 2004)

gmcttr said:


> IMHO, my tests trump the science in relation to filling bicycle tires with a cheap compressor.


NOTHING TRUMPS SCIENCE!. :madman:


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## 4JawChuck (Dec 1, 2008)

gmcttr said:


> -2.3 Hutchinson Spider MB tire - regulator set to 35 psi - flat to 31 psi in 30 seconds. (An additional 30 seconds brought it to 32 psi.)


Sounds like an exponential time to reach pressure if you ask me.:thumbsup:


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## gmcttr (Oct 7, 2006)

4JawChuck said:


> Sounds like an exponential time to reach pressure if you ask me.:thumbsup:


And the road tire to 105psi in 20 seconds?


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## 4JawChuck (Dec 1, 2008)

gmcttr said:


> And the road tire to 105psi in 20 seconds?


That is what is known as a scientific anomaly since the tank was set to 100psi...you throw that one out. We really should get NASA involved, this is ground breaking research!

I need popcorn...


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## gmcttr (Oct 7, 2006)

Scientific anomaly? Nope...just a cheap compressor with cheap gauges and a cheap regulator.

You just can't admit that all of your science just doesn't keep one from rapidly inflating a bicycle tire with a compressor. 

I also wonder if the OP has any additional thoughts.


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## Teemu Kalvas (Sep 8, 2007)

gmcttr said:


> IMHO, my tests trump the science in relation to filling bicycle tires with a cheap compressor.


Tests ARE science. Maths is just... maths.


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## 4JawChuck (Dec 1, 2008)

...since I like to keep a good thing going, lets discuss the chaotic behaviour of the vortices at the "wall" junction through the inflator chuck and its influence on the restriction seen by the fluid as it passes the interface.

http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.fluid.33.1.415?cookieSet=1&journalCode=fluid

Please review the attached linked article (you do have a subscription right?) and have ready for next class an example matrix that shows the differences between transonic flow and subsonic flow at the interfaces and explain the reasons for the different fluid behaviour and how it relates to fluid density.

Extra credit will be awarded to the papers that links stagnation flow to the exhausting medium pressure.

..and yes this is a test that counts towards your final mark.

You will grade each others test and mark according to the test model we will assemble using a "crappy" air compressor and air nozzle assembly encountering a static junction model using colored smoke to show flow variations. Papers with calculations will be graded higher even if they are wrong than those that are right with no calculations.

In your calculations determine the proper nozzle sizes and flow rates to achieve both subsonic flow and transonic flow with the following criteria.

-Crappy compressor volume/flow rate = 5 gallons/2.3 cfm
-Nozzle size = ?
-Minimum connecting hose size for both flow rates = ?
-Fluid density = Air at room temperature (20C) and air at -60C. Negate humidity in all equations
-Ambient sea level pressure/temperature to be used for all equations for the exhausting medium.
-Flat junction plate of 4"X4"

Nozzles should be designed to have no convergence or divergence angles

P.S.Actually I am amazed that this thread has garnered so much interest when its quite obvious the noob didn't set the regulator correctly. I'm even more amazed I still remember this problem from class, it still makes my head hurt.


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## gmcttr (Oct 7, 2006)

4JawChuck said:


> P.S.Actually I am amazed that this thread has garnered so much interest *when its quite obvious the noob didn't set the regulator correctly*.


That's all I wanted. Thanks.


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## Maida7 (Apr 29, 2005)

4JawChuck said:


> P.S.Actually I am amazed that this thread has garnered so much interest when its quite obvious the noob didn't set the regulator correctly.


Yes, the OP is very wrong in thinking that compresors won't fill past 50 or 60 psi. If a compressor is rated 120psi then it will fill a tire very close to 120psi. If it doesn't then it's not set up correctly (regulator or leaking hose) or it's broken. That is where this whole thread should end.


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## dave54 (Jul 1, 2003)

4JawChuck said:


> ... We really should get NASA involved, this is ground breaking research!...


We need someone better than NASA. I am sending this to MythBusters!


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## 4JawChuck (Dec 1, 2008)

> We need someone better than NASA. I am sending this to MythBusters!


Hey now thats a great idea!

I would like to see if they could duplicate my experience with a Bontrager kevlar bead tire.

Heres the scenario;

It seems with Stans in the tire I could not get enough flow rate from my hand pump to seat the tire to the Bontrager tubless strip equipped rim.

Since it was getting late in the evening I decided to take the whole thing to the gas station for filling. Once there I filled the tire and seated the bead with no problem, but since I forgot my pressure gauge at home I didn't know how much air I had in the tire.

I drove home with the tire in the passenger seat quite satisfied I solved the problem so quickly and admiring the new assembly seated beside me.

As I pulled into the driveway and shut the car off there was a loud explosion that at the time I could only describe as the sound of a shotgun blast inside the car.

The air inside the car was filled with a white smoke or powder (hard to tell my ears were still ringing) and my brand new tire was now around my neck.

I stepped out of the car to review the situation and realized the tire blew completely off the rim and the white smoke/powder was actually Stans fluid that had aerosolized inside the interior of the car during the explosion, most surfaces were completely covered including myself.

Investigation revealed that the kevlar bead snapped causing rapid decompression and my new necklace, the tires bead had broken catastrophically at some unknown pressure. I eventually used the brand new broken beaded tire as my 5 place bike trailer rear bumper...had to use it for something...it was brand new.

Try explaining that to your LBS.

Perhaps Mythbusters could duplicate using methane gas inside the tire and a lit cigarette hanging from the dummies mouth to see if they can get the windows to blow out of the car, that would be the only way that could get more embarrassing.:winker:


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## MTK (Feb 18, 2004)

*That was funny*



4JawChuck said:


> Hey now thats a great idea!
> 
> I would like to see if they could duplicate my experience with a Bontrager kevlar bead tire.
> 
> ...


4JawChuck,with every sentence tears were streaming. You told the story so well. 
I could see you all happy and wham! WTF? Great thread,great read.

Last,have any of you ever had a 18 Wheeler tire Blow right next to you with your window
Down doing 60mph in a small car? Man oh Man did that scare me! I must say,I was very
proud not to stray out of my Lane. I did jerk the wheel,but was in control. May you
never have that happen to you 4JawChuck,or anyone for that matter. Peace.

MTK


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## FloridaFish (Mar 29, 2004)

4JawChuck said:


> Hey now thats a great idea!
> 
> I would like to see if they could duplicate my experience with a Bontrager kevlar bead tire.
> 
> ...


sounds like you wrapped up that FRACAS investigation rather quickly. good work.  

MTK -- had that happen twice to me so far; once while on the side of the road checking a boat trailer we had a big rig blow a tire and rain hellish melting rubber pieces and wire bead everywhere, and once while along side a truck a tire blew and put a couple real nice scratches in my side panels........not a pleasent experience.


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## gotboostedvr6 (Sep 1, 2008)

gmcttr said:


> +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.


+1 :nono:


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## gotboostedvr6 (Sep 1, 2008)

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.
> 
> ...


where did you go to school? :madman:


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## MTK (Feb 18, 2004)

*Twice?*



FloridaFish said:


> sounds like you wrapped up that FRACAS investigation rather quickly. good work.
> 
> MTK -- had that happen twice to me so far; once while on the side of the road checking a boat trailer we had a big rig blow a tire and rain hellish melting rubber pieces and wire bead everywhere, and once while along side a truck a tire blew and put a couple real nice scratches in my side panels........not a pleasent experience.


FloridaFish,you got me beat bad on both of those stories. I must say,the 2nd must
have been worse. Although there was hellish melting rubber pieces and wire bead 
everywhere,you were busy checking that boat trailer? Dam Re-Tread!

Now on this second story,you were just walking along and Boom! I'll take Story number
2 Bob for a 1000. Thank's for sharing.

MTK{wonders which one was worse for FloridaFish?}


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## FloridaFish (Mar 29, 2004)

MTK said:


> FloridaFish,you got me beat bad on both of those stories. I must say,the 2nd must
> have been worse. Although there was hellish melting rubber pieces and wire bead
> everywhere,you were busy checking that boat trailer? Dam Re-Tread!
> 
> ...


1st time was kinda cool.

2nd time. I was driving in left lane trying to pass a semi in the middle lane that was doing about 80 mph, when I was about halfway one of the forward drive tires (3rd axle) blew up and threw a nice piece out that hit the front quarter-panel and passenger door. This was all in a construction zone with no shoulder (cement barriers). Happened fast so i didn't swerve or nuthin. Good times in the old XJ. not so cool.


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## screamingbunny (Mar 24, 2004)

*why yes*

3 days after returning from Iraq while I was in the pasanger (TC) seat with my wife driving, did I mention I had been hit by 2 IEDs on convoy during that rotation. before I new it I was reach for the radio and pulling the seat cushin out of my crack:thumbsup: ....good times.


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## FloridaFish (Mar 29, 2004)

screamingbunny said:


> 3 days after returning from Iraq while I was in the pasanger (TC) seat with my wife driving, did I mention I had been hit by 2 IEDs on convoy during that rotation. before I new it I was reach for the radio and pulling the seat cushin out of my crack:thumbsup: ....good times.


okay, I think you win. :eekster:


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## MTK (Feb 18, 2004)

*Ummm yeah,*



FloridaFish said:


> okay, I think you win. :eekster:


We can all sit down now. Hush little baby dont you cry.

MTK{glad screamingbunny is ok}


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## 4JawChuck (Dec 1, 2008)

gotboostedvr6 said:


> where did you go to school? :madman:


I went an excellent high school G-E-T. Lots of great memories from G-E-T High although I don't remember most of it.

http://www.getschools.k12.wi.us/contact/g-e-thighschool

After HS I went to an excellent college, we affectionately called it U-H0M0. We used to have a great time in the dorms with "Nude Towel Snap" Fridays and "Fudge Packing" day just before spring break when we would go home for a few weeks....ahhhh the good ole days.

http://hf.org.pl/ao/index.php?id=7

Have a Happy New Year!:winker:


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