alank2

So I picked up a Bell Floor pump with gauge and I find it a little odd to use, but maybe I'm not familiar with 70 psi bike tires. I connected the end to it and it has a tab that locks it into place. My rear tire was at 60 so I decided to pump it to 70. The pump however is odd. I can get it about 70% of the stroke without much force, but it doesn't pump any air into the tire. Then the last 30% of the stroke takes some muscle to push, but it does increase the pressure. It doesn't take much air at all to go from 50psi to 70psi in the tire, maybe just one pump. Is this normal?

djb

sort of normal for the extra pressure at end, not so normal to go from 50 to 70 in one stroke.
who knows, get a real person to check it out, pretty darn hard on the internets to know.

alank2

The pump I bought is this:

https://www.instacart.com/landing?pr...CABEgJ0bPD_BwE

Perhaps the problem is "high volume". Maybe I need a skinnier pump that has less volume...

Chuckles1

Ditto.

Iride01 

Push it all the way to the bottom. You are compressing air. As you get to higher pressures in the tires, what you feel in the pump is going to get harder. There might be check valves and such that the way they are designed you have to over come both the relative air pressure on both side of the valve as well as the spring tension or compression.

Think about it. Just to make it easy, lets say your pump piston has a surface area of 1 square inch. With a tire that is at 79 PSI, you will at the bare minimum to put more air in the tire you will have to exceed 79 pounds of force on pump handle with your body.

cyccommute 

Not as much as you might think. Going from 70°F at 100 psi to 90°F causes the pressure to rise 4 psi. Pressure rises with temperature but it’s just not that much of a change. If you wanted to double the pressure, you’d have to heat the tire to 500°F. The tires will likely burst into flames before that temperature is reached.

A bit of a science lesson is in order. A pump is nothing but a column of air in a tube. It’s at about 14 psi at sea level. If the plunger in the pump is 1 square inch in area, it’s experiencing 14 pounds of force. Now you push the plunger down and the pressure in the tube increases. That means that the force pushing back on the plunger increases so more weight is needed to overcome that force. At the very bottom of the stroke, the pressure is equal to the pressure in the bicycle tire. If the pressure is 70 psi and the plunger is 1 sq in in area, it needs 70 pounds of force to push the air into the tube. It’s not that you need more “muscle”, you need more weight. Many people stop pumping when they feel resistance in the pump handle and pull it up. That will never get air into the tube. You have to push the plunger all the way to the bottom of the pump.

You might notice that you can’t get more air in the tire than your weight if the plunger is 1 square inch in area. Higher pressure pumps use smaller plungers but put less air into the tire per stroke.

alank2

I appreciate the explanation - this was exactly the issue - I would get to the last 30% of the stroke and have to overcome the pressure to add more. What I didn't notice on the packaging was "high volume" which is probably great if you are inflating a mattress, but not at all what I want to get my bike tire up to 75 psi. I bought the $5 cheaper pump here:

https://www.walmart.com/ip/Bell-Floo...iABEgId2fD_BwE

Now I takes about 6 or 7 pumps to go from 60psi to 75psi no problem.

alank2

Spot on - a smaller tube made all the difference.