Commuting - Tire Pressure

Why is it that bicycle tires seem to need air once a week. I have two bikes, ride each of them at least three times a week, and every weekend when I check the tire pressure they need about 10-20psi? I know if they sit for months they need air but is this typical (every weekend) or just me? I have a motorcycle that can go 6 months without an psi adjustment. Don't get it.
Kevin

Probably the higher pressure of bike tires mean they lose air more quickly. I don't really find it a hardship to pump up my tires once a week; I need to check them that often, anyway.

Better to need air once a wek then to need gasoline every other day.

Bicycle tubes are not 100% air tight and will allow air to slowly permeate through them. The higher the pressure, the faster the air loss.

i check my tires everyday before my ride. it's a habit.

I've had this discussion a few times. The question of why bicycle tires lose a larger proportion of their pressure in a given interval is not just their higher pressure.

Velocity of air between two regions is proportional to the ratio of pressures between the two regions, and is also a function of the permeability- in this case, the total area of all "holes" in the tube.
The effect of losing a given volume of air, say, a liter, on pressure inside a bicycle tire, is much greater than that on a motorcycle or auto tire due to the bicycle tire's much smaller volume when full.
Even with the higher pressures, bike tires will maybe take 1/6 to 1/20 of the moles of air than a motorcycle tire. Since pressure inside a vessel = (gas constant)*(moles of air)*(temp in Kelvins)/volume, one can see why one has to pump up a bike tire, say, 12 to 40 times as frequently as a motorcycle tire.

If I'm abusing PV=nRT, let me know.

Why is it that bicycle tires seem to need air once a week.
Kevin

I pump up my tires every time I take a ride over a mile or two. When I had a longer commute, I would inflate them every day.

It's enough more fun to ride with fully pressurized tires that it's easily worth it.

How often a tire needs air is somewhat related to the size of the tube and the thickness of the tube. On a road bike with 23mm tires and ultra-light tubes, you could drop from 95 PSI down to 80 PSI in a week, and 80 PSI is too low for a heavy rider on a 23mm tire...the bike will feel heavy and slow and you could risk a "pinch" flat.

On a mountain bike, with mega-fat tires and beefy inner tubes, you might drop from 60 PSI to 50 PSI in a week, but on a fat tire 50 PSI is a very usable pressure - most riders would not notice the difference.

So, a road bike really ought to be checked three or four times a week, while once a week would be enough with ultra-wide mountain bike tires.

A friend of my was telling me the tires on her van "never" need air. When I checked, they were at about 24 PSI, not the 34 PSI they are supposed to be at. The loss of pressure is so gradual that people get used to the look and feel of an underinflated tire and begin to think "low" is normal.

I find my tires need pumping after 2 or 3 days. To save myself the bother of remembering when I last pumped the tires (maybe alzheimer's starting), I always do them before I leave home. With a decent floor pump this only takes a few seconds and no effort.

Thanx for the replies, it all makes sense. Sounds like a nessecary nusance.
Kevin

I've had this discussion a few times. The question of why bicycle tires lose a larger proportion of their pressure in a given interval is not just their higher pressure.

Velocity of air between two regions is proportional to the ratio of pressures between the two regions, and is also a function of the permeability- in this case, the total area of all "holes" in the tube.
The effect of losing a given volume of air, say, a liter, on pressure inside a bicycle tire, is much greater than that on a motorcycle or auto tire due to the bicycle tire's much smaller volume when full.
Even with the higher pressures, bike tires will maybe take a 1/6 to 1/20 a of the moles of air than a motorcycle tire. Since pressure inside a vessel = (gas constant)*(moles of air)*(temp in Kelvins)/volume, one can see why one has to pump up a bike tire, say, 12 to 40 times as frequently as a motorcycle tire.

If I'm abusing PV=nRT, let me know.

Good thinking. Assuming that tires are a torus, the ratio of volume to surface area of my 1.25X27 bike tires is .31 and that ratio for my car tires is 1.5 so bike tires should need pumping 5 times more often than cars. The reality is greater than 5 so there must be more going on. I think that the thicker rubber is less permeable than the thin tube.

opps, how often do you check tire pressure? Haven't done this yet.

Good thinking. Assuming that tires are a torus, the ratio of volume to surface area of my 1.25X27 bike tires is .31 and that ratio for my car tires is 1.5 so bike tires should need pumping 5 times more often than cars. The reality is greater than 5 so there must be more going on. I think that the thicker rubber is less permeable than the thin tube.


Thanks for your comment and for the fact that you grok my point as well as extend it.
We need more engineering savvy on these forums.

opps, how often do you check tire pressure? Haven't done this yet.

I should check mine at least twice a week. To be perfectly honest, I should check every time I ride. I'm lazy. :(