# Does the inner tube affect the maximum PSI?

If I have road tires that have a recommended pressure of 110 PSI and maximum of 120 PSI can I safely pump my tires up to that level or does the inner tube effect the maximum?

I currently have the tires pumped up to 90 PSI however the question What Pressure Should I Run My Road Bike Tyres At seems to indicate I should go for the full 110.

It feels when I'm pumping that it would be very hard to pump above the 90 mark and I am wondering if the quality of the inner tube (or a inner tube max PSI) could be the reason. Should I just go for it and try to pump it up to 110 PSI or am I best off leaving my tires at 90 PSI?

The inner tube has no significant effect on max pressure. The valve stem area is the only area sensitive to pressure, and in general the valve can withstand 200 PSI or better. (And if you exceed whatever limit there is the result is "catastrophic" failure, not simply the inability to pump in more air.)

What it sounds like you're noticing is the geometry of the pump. A piston pump has a compression ratio, just like a cylinder in an engine, and the max pressure that the pump can generate is that compression ratio multiplied times the 14 PSI of atmospheric pressure. In general, pumps are designed either to move a lot of air at low compression ratio to fill low pressure tires quickly, or to move less air at a higher compression ratio to be able to fill high pressure tires at all (though more slowly).

Good quality pumps will be advertised with a stated maximum pressure, and you can fill tires reasonably fast to 80-90% of that max pressure, after which progress will slow to a crawl. So when you shop for a pump you need to pay attention to that number.

• I have a pump that will go to 120 psi, I managed to get the wheel to the recommended 110 with it, just was worried when the pump started having trouble. Thanks. Aug 17, 2012 at 8:03
• Keep in mind that when you start with an empty tire each stroke is only injecting air at 14psi. As the pressure goes up, the volume of air goes down proportionally (the air compresses), so each stroke injects less. You will notice as the pressure gets quite high that you only hear air entering the tire during the last inch or two of travel of the pump piston, since most of the pump stroke was spend simply compressing the air to match the pressure in the tire. Aug 19, 2012 at 19:08

The most likely cause of the increase in difficulty at higher pressure is the mechanics of the pump. You have to work harder to squeeze more air in the tube. The tire rating is the controlling factor in constraining the tube. The tire construction supports the tube to allow it hold the air at high pressure. Pressures higher than the tire rating cause the tire to fail. Without the tire to support the tube it would pop like a balloon.

In addition to the tire limiting PSI, rim tape is also a factor. Cheap rim tape can prevent higher pressures since they, and the tube, will dimple into the spoke holes of the wheel. Over time, this can cause the tube to fail.

But most riders would probably think 110 pounds as excessive unless they are very heavy. The ride will be more comfortable and maybe more efficient five or ten pounds lower.

• Depends on the bike ofcourse; I myself prefer about 120 pounds on my road bike, even though I'm considered to be light. On an MTB however you could go as low as 40 depending on the terrain and for a commuting bike I prefer 70 to 90. Sep 25, 2012 at 15:17