CosmicF

So, I decided to get my mountain bike back in shape for days when my wife isn't riding with me, so I can do more adventurous riding. My friends talked me into a few upgrades, one being tubeless tires. I get my kit and successfully throw the tubes away. Like the house falling from the sky and all the munchkins standing around in amazement and wonder, I circle these new-to-me Presta valves.

If I am not to completely trust the gauge on my floor pump (since it only measures the pressure inside the barrel), my frame pump having no gauge at all, and my stand alone tire gauge seemingly defunct now, how am I to really know my tire pressure, especially since the pressure seems to need to be so exact?

Do I have to get converters? Which seems stupid to me, if Presta valves are supposed to be superior (in some people's minds), I would have just looked for the other valves if I had known. How do you measure air pressure with your Prestas?

CliffordK

How "exact" does the pressure have to be?

I always just go by the pump gauge. Or, once one gets a feel for the pressure you like, the "thumb gauge".

trailangel

Spin Doctor Tire Pressure Dial Gauge

drlogik 

Well, you could buy a presta gauge but it's really not needed but it might be nice to have some day. Any gauge will be close enough as long as you don't go all the way up to the maximum pressure.

Once upon a time ago pumps didn't have built-in gauges. We relied on "thumb pressure" as was previously suggested. That is based on checking the pressure with a presta gauge and then squeezing the tire to get the "feel" of how hard it is. From then on, we squeeze the tire; or in my case, I thump'flick it with my finger and listen to the sound. Yep, I check the pressure with a gauge on a particular tire and then flick it with my finger. Like the thumb test, it gets it close enough.

cyccommute 

First, that's the way that all pump gauges work independent of the type of valve being used. They measure the pressure of the system not just what's in the tube or tire. They may or may not measure the pressure in the barrel since there is a check valve at the bottom of the barrel which should keep air from flowing back into the pump barrel. It's not a perfect system however.

If you don't have a pump gauge, you use a separate pressure gauge that is designed for presta valves. There are any number of them around. My personal favorite is the Zeal Twin Graph. It can be used for both presta and schrader valves, it requires no batteries and it has a stop on the gauge needle so that you can more easily read the pressure.

Presta valves have some feature that are superior to schrader. A presta valve is a check valve. When you put air into the tire, the valve closes and doesn't let the air back out. A schrader valve has to be held open when allows air to leak back out of the valve at all times. If your connection isn't tight and you hear air hissing with a schrader valve, you are losing air out of the tire. The same isn't true for a presta. Any air leakage you might hear from the pump/valve connection is just coming out of the hose.

gsa103

For mountain bikes, ideally +-1 psi, maybe +-2 psi.
Keep in mind that many mountain bikes run ~20psi, so a 2 psi difference is equivalent to 10 psi on a road tire.

Most floor pumps aren't accurate enough, especially in the lower end of their range. Floor pump gauges work alright, as long as you don't trust the absolute number. They tend to be "precise" but not "accurate". So if you always use the same pump they can work reasonably well.

CliffordK

That may be the key. Decide what works for the individual, and one's own pump should give reasonably repeatable results.

+/- one or two PSI sounds like an awfully tight range. Do most analog pump gauges even give much more than about a 5 PSI scale?

SquidPuppet

MTBs, and motorcycles should use low pressure gauges that top out at 30lbs. That allows them to fine tune in small increments. On my road racing motorcycles I could feel a significant difference between 30 and 28 in the front. Like worthlessly hard and slippery to sublimely soft and sticky. I've never ridden a fat bicycle but I've heard that those guys adjust in ~1/4 lb increments because they run so little pressure.

AnkleWork

You can think up lots of excuses to not ride.

Most people just ride until they notice the tire(s) a little low, add some air, and keep riding.

CosmicF

Thanks, lots of things to mull over in the thread. Once I get used to the "feel" of a tubeless tire, I will most likely use the thumb method. But, I will get me a gauge to go by until I get used to what it should feel like.

My back tire seems to lose air on my rides, but never too far beyond half sqishy. I'm just used to riding tight tubes. I'm working on getting head around tubeless, ha ha!

Thanks again. Great info, that I couldn't find online.

AlexCyclistRoch

I've had a dial presta gauge for years-works great on larger presta tires.

Bezalel

There is nothing wrong with measuring the pressure inside the barrel. At any point in time (when disconnected from the pump) the pressure in the tire will be equal or less than it was the last time it was pumped up (assuming equal temperature and atmospheric pressure). All you need to do is get the pressure in the barrel equal to what you want your tire pressure to be.


There is no need to be exact.