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Hello to all,

I purchased a Bontrager Full Size air pump for home a few weeks ago and been airing up everyday before I ride. I always notice the short "ppppst" of air that escapes when removing the connector from the tube. Today, I decided to try estimating how much pressure is actually escaping so I experimented several times airing them up to 120psi and removing the connector the checking tire pressure.

Result is a loss of about 5psi.

My question is: Am I using the pump wrong thus allowing air to escape?

There is a lever on the connector that when connected is perpendicular to the connector while parallel to the connector when not connected.

Procedure I use:

1. Unscrew the little valve on the tube to allow air to be pushed into tube.
2. Press the air pump connector to the valve then pull the lever to perpendicular position.
3. Fill with air to watch gauge to 120psi.
4. Push lever on connector to parallel position and remove it from valve
5. close valve stem

I did search for post on this subject, but didn't find any.

Thanks in advance.
 

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POGUE MAHONE
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1,111 Posts
sounds normal to me. I believe it's just the high pressure of the tube that makes it impossible for the valve to close immediately when you remove the pump. I use the same pump for my mtb tires and have found no air will escape until I pump over 65 or 70 lbs.
I just give it one extra shot to make up for the 5 lb air loss.
 

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Resident Curmudgeon
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13,390 Posts
What/how did you ck the pressure after you removed the pump from the stem? If you used a seperate tire gauge you should know that + - 5 psi variance between gauges is nothing. The air you hear escaping comes from the pump, not your tube. The pump has a small resevior. When you remove the pump head, the pressure built up in that escapes. It's not coming out of your tube...honest.

BTW, how often are you checking / inflating your tires? I ride between 5000 - 7000 miles per year. I check & inflate, if needed, once per week. Before I ride, I give'em a finger pinch. Unless it feels funny, it's good. Folks can get really anal about tire pressures. Well, OK, lots of other stuff too. I tend to run my tires a bit below their max inflation. There are 5 reasons why I do this:
1. I get fewer flats
2. Better traction
3. Smoother ride
4. Tires last longer
5. There's no increase in rolling resistance. (no kidding here - you will NOT go faster with harder tires)
 

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haole from the mainland
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Mr. Versatile said:
The air you hear escaping comes from the pump, not your tube. The pump has a small resevior. When you remove the pump head, the pressure built up in that escapes. It's not coming out of your tube...honest.
I can attest to this. Some pumps, like a Joe Blow I had, have a pressure release button. When you press the button, air is released from the pump and hose & then it's absolutely silent when the pump head is pulled of the tube valve.

The Joe Blow pump broke & I've decided I like the little air assist I get for removing the pump head. I always move my valves to a top position so that gravity helps pull the pump head off. Before I was anal about doing this, I would sometimes ruin the tube at the base of the valve pulling the dang head off.
 

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Mr. Versatile said:
I tend to run my tires a bit below their max inflation. There are 5 reasons why I do this:
1. I get fewer flats
2. Better traction
3. Smoother ride
4. Tires last longer
5. There's no increase in rolling resistance. (no kidding here - you will NOT go faster with harder tires)
How much is "a bit" below max inflation? Are you talking 1-2 psi, or more than that?
 

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NeoRetroGrouch
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6,493 Posts
tystevens said:
How much is "a bit" below max inflation? Are you talking 1-2 psi, or more than that?
Here is a chart from Michelin based on your weight.

BTW, I agree on you procedure of filing before each ride. Mine are normally 90-95psi when I fill them (ride at 100psi, 700x23). If they are <85 (not something that I can feel), I better check what is wrong or I will be fixing a flat in some mosquito-filled ditch half way though the ride.
 

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Resident Curmudgeon
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What I mean by a bit below max pressure is:

The tires I use have a max pressure of 120 PSI. I run the front tire at 95-100 psi, and the rear at 110 psi. Like I said, I check'em once per week, although I ride pretty much every day throughout the year.
 

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Harder tire will result in less effort. Think about it....why not ride at 75psig instead of 110psig if softer is more comfortable (which I agree with) and does not require more effort? Everything has a trade off...less pressure is more comfortable but requires more effort due to larger contact area with ground and higher pressure is less compliant but has less contact patch with road resulting is less energy required.
 

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Resident Curmudgeon
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MCF said:
Harder tire will result in less effort. Think about it....why not ride at 75psig instead of 110psig if softer is more comfortable (which I agree with) and does not require more effort? Everything has a trade off...less pressure is more comfortable but requires more effort due to larger contact area with ground and higher pressure is less compliant but has less contact patch with road resulting is less energy required.
Sorry, that's not the way it works. Like Andrea said, a harder tire will require less effort ONLY if the surface is glass smooth. A harder tire will actually resist forward motion because of the way it reacts to irregularities in the road surface. Hard and soft are, of course, relative terms, but keeping that in mind I'll use them here just for the purpose of comparison's sake.

When a hard tire contacts an irregularity, it "bounces", and resists rolling over it. As the hard tire hits a small 5-6 mm bump, rather than absorb the impact, it will try to repel it. The road surface is harder, so instead of repelling the bump, the bump exerts a force on the tire that is in the opposite direction of its travel. A soft tire better absorbs the irregularity and keeps going with less bouncing/resistance. Force in the opposite direction is still exerted, but some of that force is absorbed by the tire, helping the tire to continue in its intended path. The resistance is actually less. The contact patch difference is so small that it's not a significant factor unless you use a much larger tire. 25mm and even 28mm tires don't have much, if any, more rolling resistance than 23mm. They're also noticeably more comfy. In addition to less rolling resistance and a more comfortable ride, another benefit to wider and/or softer tires is that they're less puncture prone, and wear longer.

Re: your question about why not ride at 75psi instead of 110psi if it's more comfortable and doesn't require more effort? Good question. Optimum inflation (not maximum) is based mostly on rider weight. You need to have the tire inflated enough to prevent "pinch" or "snake bite" flats. This happens when the tire is not sufficiently inflated to prevent the tube being pinched between the road and the rim, when a bump, railroad tracks, pothole, etc. is encountered. For example, a 200 pound rider would need to have more psi in his tires than would a 170 pound rider.

So, you might ask, why do all the racers use 23mm tires. Well, for one, they're racers. For another, the tires are lighter. Another reason is racers aren't particularly concerned with tire wear or punctures. When they get a flat, the team car pulls up behind them with a mechanic, who installs a new wheel in a few seconds.

If you search for the topic, you'll find quite a lot of info on this subject here on RBR.
 
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