Just curious, Schrader valve
 

Not a big deal but I was just curious when I was topping up my tires.

When you remove the pump from the schrader valve, how much psi is lost when when you get that hsss?

Basically none, the air's coming from the pump hose.

I've wondered this myself (I'm like that). It's going to depend on the pressure and tire volume. I'm guessing around 0.5 psi.

Mark

This.

You guys make a good point - that must be the case.

Mark

That is true only for a Presta valve. Prestas are check valves that only allow air flow in one direction. Schrader valves have to be held open to fill. Any hissing is air being lost from the hose and tube. How much air is lost from the tube would depend on a number of factors and would be difficult to quantify. It would also vary widely.

Not necessarily, if the pump head is set up properly the plunger which depresses the valve should be shorter than the seal around the valve. This way the valve only gets depressed once the seal is made around the valve.

Easy to check and if your pump head is getting sloppy might be worth it to check with a gauge after inflating which will also catch gauge inaccuracies.
A clean Schrader value can work as well as a Presta in limiting the air escape as demonstrated by their use in high-pressure hydraulic systems on Aircraft.

A lot will depend on the volume of the tire you are pumping. Most bicycles with tires that use schrader valves are high volume/lower pressure tires. With these tires very little pressure is lost and the pressure loss is of very little consequence

Not enough for it to matter, anyway.

My old Fuji S-10S came with a screw-on hose for the factory Primus frame pump. I couldn't get that hose off the Schrader valve quick enough, losing lots of pressure while doing so. Soooo.... I ditched the Primus pump and got the Zefal HP with a quick-release chuck withn the first month or so. That old Fuji still wears a Zefal HP today, albeit it's fourth one for various reasons...

BTW, even back then, I was inflating my 27x1-1/8 tires to ~100psi. WITH Schrader valves!

I suppose it depends on what type chuck you use for Schrader. However many that lock on to your valve stem probably release the pin almost as soon as you release the locking lever. Just like prestas, what's being heard is air escaping from the chuck, hose and sometimes the pump.

However if you have a 60 plus year old floor pump like I have with a chuck that you screw onto the Schrader valve, then yes there is some air loss while you race to unscrew it quickly. However I'd really be surprised if they make air chucks for bicycle floor pumps like that anymore.. Besides, it had no gauge on it so I still had to check with a gauge after I removed the chuck to see if the tire was full. So how much air is lost wasn't an issue since I had to check the pressure any how.

That pump is pretty much not used ever anymore. I only keep it around because it's been here as long as I've been here, maybe longer.

Most pump heads I’ve seen and used for Schrader have a plunger that is nearly flush with the outside of the head. The pin of the plunger engages the valve almost as soon as the chuck is placed on the stem. The seal isn’t that tight around a Schrader valve either needing either a thumb lock or screw on mechanism to hold it in place. Flipping open the thumb lock or unscrewing the chuck results in some pressure loss (how much is difficult to quantify) because the pin engages the stem’s valve until it is clear of the stem.

If you hear hissing from a Schrader valve, air is being lost from the tube as well as the hose. With a Presta, air is being lost only from the hose.

I bought a Lezyne floor pump a while back that came with their "ABS-2" chuck. Before I got it, I thought that I would probably replace the chuck with a lever-lock version, but I was pleasantly surprised how well the ABS-2 chuck works. On the Schrader valves I have used it on so far, there just isn't much drama when unscrewing the chuck from the valve, and not much air lost.

For sure for most cheaper pump heads. The thread on Silca Schrader pump head works as I described with zero air loss as do some of the other premium heads I have used in the past.

Hiss duration is a factor. How long do you hear the hiss? If you have to fumble around for a few seconds to get the pump nozzle off completely, while it's hissing the entire time, you're likely losing more air pressure from the tire than you want to.

i would bet that the air lost using a gauge to check pressure after filling is more than lost removing the air hose.

The only way to tell is with a separate pressure gauge. The issue with that is the pump gauge may not be accurate.

You could use the pump on a car tire and then compare the pump reading with the pressure gauge. There is so much volume that a small amount probably won’t impact the reading. It will also indicate if your floor pump gauge is off.

I have an inexpensive floor pump that is off by 3psi. Not a big deal at 75psi, but more so at 30psi.

John

Swiping a saying about a man with two clocks...

"A cyclist with one gauge knows what the tire pressure is. A cyclist with two gauges does not."

:)

I'm a schrader user

related: when I check my tire pressure with a hand held gauge, a little air escapes. so when I'm deflating the tires on my Jeep, when I get close to the pressure I want, I use the gauge repeatedly to get it perfect. meaning the more I check the pressure, the lower it goes. because bike tires are so low in volume, checking the pressure may change the pressure more than one wants. so when you get it where you want it, leave it alone

Pump 5 psi higher than you want, and stop worrying!

gm

It must be very little because we use Schrader valves on mountain bike shocks. The pressure is about 80psi but the volume inside the shock is tiny. So if really any amount of air came out you would lose a lot of pressure.

Fun fact: the Schrader valve was patented in April 1893 by George Schrader of New York City, New York, USA.

Safety bicycle --> pneumatic tire --> Schrader valve :thumb:

0.37724 grams of air per second

According to my calculations, you are 74.67% accurate.