Bicycle Mechanics - How much air does one lose in a tire with no leaks?

I was losing air, about twenty pounds a week or more. I placed the tube (inflated) in water and could not find any leak. I changed tubes, my LBS caries Sunlite tubes. They are very small in dia. like for a road bike. I have 700X35 tires. I still get that twenty pound a week or more loss. I am going to replace the rim tape (three months old) tomorrow with another tube. This is only experienced in the front tire. The back tire gives me a loss of about five pounds a week.

Any opinions on what is wrong, or is this normal? I ride a Connandale H400.

You're within the range of normal.

The rate of air loss depends on pressure, and the ratio of surface to volume of the tires. Higher pressure bleeds faster, as do skinny tires vs. fat ones. But all tires bleed the same way balloons lose air over time. It isn't a leak, but a matter of porosity to the small gas molecules. Tubes also vary, with some doing better than others even within the same brands, due to variations in wall thickness.

My 105psi 25mm road bike generally loses about 10% of tire pressure per day. OTOH my commuter using 1.9" tires inflated to 60psi loses about 15-20% per week.

I top off the road bike every morning before riding, whereas the commuter is a Sunday chore along with putting out the trash.

The rate of air loss depends on pressure, and the ratio of surface to volume of the tires. Higher pressure bleeds faster, as do skinny tires vs. fat ones. But all tires bleed the same way balloons lose air over time. It isn't a leak, but a matter of porosity to the small gas molecules. Tubes also vary, with some doing better than others even within the same brands, due to variations in wall thickness.

That is one of the best, most easily understood statements/posts I have ever read. Thank you for the factual and informative answer.


...whereas the commuter is a Sunday chore along with putting out the trash.

This is where your wrong...

Stepping out to the garage to (fill in the blank) on your bike is no chore...it's a labor of love. I always am finding an excuse to go out there and tinker on my bike!

I lose about the same amount of air as FBinNY. I find myself toping off my tires every ride. I go from 110psi down to 85-90psi within 48 hours on my 700x23. You would notice quite a bit more air loss if there was actually something wrong.

The thickness of the tube plays a huge role in how quickly air leaks through it. Most tubes you find in a shop are the thin racing types. On request, you can pick up a much thicker touring-type thorn-resistant tube. The thickness of the tube on the outside tread-side can be 3-4x thicker than a racing tube. The other rim-side will still be 2x thicker. This will keep the tyre up to pressure for much longer, easily less than 5psi loss per week. Heck, on my ride across the U.S. in 1995, if it wasn't for the 2 flats I got, I may have topped off my tyres at most 4 times during those 6-weeks.

Don't forget the material either: latex tubes loose pressure much faster (http://schwalbe.com/gbl/en/technik_info/schlauch/?gesamt=58&flash=1&ID_Land=38&ID_Sprache=2&ID_Seite=144&tn_mainPoint=Technik) than butyl tubes.

Stepping out to the garage to (fill in the blank) on your bike is no chore...it's a labor of love. I always am finding an excuse to go out there and tinker on my bike!

It may be for you, but after 45 years it's just a chore. A very minor one to be sure, but a chore none the less. I'm in the bike biz, and there's not much romance left.

But even before I was in the bike business, (to me) bikes were strictly for riding. I never got excited in just owning some shiny, beautiful, exotic, fancy bike. I always had top quality bikes but never let myself worry or obsess over them. I've ridden through floods, on the worst of dirt tracks (road bike, mtb wasn't invented yet), across farm pasture, on frozen lakes. I've tossed my bike onto just about every kind of vehicles.

I have a one-way selfish relationship with my bikes, they're there to serve me, not the other way around. My goal is to have the highest possible or riding time to maintenance time, and pumping tires messes with that.

Yes and the skinney racer boy tube will lose pressure faster for two reasons. The pressure is higher, and the volume is smaller.

As I have posted before while they tell me that there are only 3-4-5 tube manuf in the world, some leak down less than others. Personally I have found that Bontrager tubes seem to leak down less than other brands.

...my LBS caries Sunlite tubes. They are very small in dia. like for a road bike. I have 700X35 tires.
Are you saying you are using tubes designed for 23mm tires in your 35mm tires? If this is the case, you are stretching the tube pretty thin which will result in faster air loss.

I have a one-way selfish relationship with my bikes, they're there to serve me, not the other way around. My goal is to have the highest possible or riding time to maintenance time, and pumping tires messes with that.

I appreciate efficient too. You manage it nicely in your posts as well. When people start getting into theory and then arguing about it, it tends to go the other way, and I drift off to simpler places.


Are you saying you are using tubes designed for 23mm tires in your 35mm tires? If this is the case, you are stretching the tube pretty thin which will result in faster air loss.

I was wondering about that too. I never go above the recommended tire size for tubes. This one is enough of a stretch that I'm surprised it didn't pop.

Heavier tubes stretch out the time between top ups with the pump..

But even with a set in my Touring bike with a load I got clues of needing topping up every other
day when the rolling resistance felt a bit more . 40-622 size

now on a 47-406 wheel, not with touring load aboard, the TR tubes feel fine for weeks.

I brought some larger and thicker tubes. I hopes this will help with the flats too. Thanks for all your advice, TM.

Larger tubes are the way to go, up the point where the tube is so large that it becomes difficult to install without pinching it or creasing it.

I've found that I can often ride the bike home after getting a small puncture using bigger tubes, before the pressure gets too low.

The cyclocross guys I race against favor using standard "20-28mm" road tubes in their 28-35mm CX tires, for lightest weight. This is because of the repeated accelerations favoring lighter weight.

The latest technology for tubes is a thin-walled tube that is dimpled, such that as the tube is inflated and presses flat against the inside of the tire, the wall of the tube becomes compressed in all 3 dimensions. This leads to a degree of self-sealing after a puncture. Brilliant!

If you care what pressure you are riding, you have to refill before every ride all other factors notwithstanding. Bike tires and tubes are not like auto tires that hold pressure fairly constant for months. They are much thinner and smaller volume and lose a significant fraction of their air relatively rapidly. Sure you can over fill and "ride the pressure down" non-specificially over several days, but why not have the right pressure all the time? You are less likely to overlook a slow leak than can catastrophically fail on a ride if you are checking the pressure before every ride. On my 23mm road tires with very thin tubes, I have to use about four strokes of the pump if I skip riding a day, two if I ride every day. Is that such a huge inconvenience? Why would one spend big bucks on a fine bike and then ride on hard tires one day and soft tires next time?

Robert