Bicycle Mechanics - Inconsistent tire pressure

My road tires seem like they can't hold their pressure. I will pump them up to about 110psi and then two days later the pressure is around 80psi. It seems like I have to pump it up before every ride. Is this normal?

Yep

High pressure, low volume, it doesn't take much require refil. It's really not that hard, just make it a habit to check before each ride.

30 psi loss in two days is outside normal leakdown rates in my book. Are you sure you're really losing this much? On low-volume road tires, it's pretty hard to check a tire without losing 5-10 psi.

I lose about 20 psi over two weeks or so.

- Mark

Check that the valve cores are screwed in tight. Recently I've come across a bunch of tubes in which were leaking air from the valves. A spit test should tell you if they are leaking. I fixed mine with some blue LokTite on the threads of the valve core and none of them have leaked since.

How are you determining the tire pressure? As mentioned, high pressure/low volume of bike tires can lose significant pressure just from using the pressure gauge.

It's the "Heisenberg Uncertainty Principle" applied to bikes; the more you look, the less you know.

Yes. It depends on the tubes. Lighter tubes seem to lose pressure more quickly.

Butyl hold air much better than latex. That is a normal to slow leak rate for latex
tubes. Butyl vary a lot, new ones on our tandem lose less than a #/day, others
in the stable with more mileage can lose 3-5#/day. You may have a micro pinhole.
Doesn't take much of a hole to lose at 10-15#/day. Or a leaky valve.

I typically lose pressure from 110 to 80 psi in about 4 days (both tires). This is because of the porous nature bike tubes and the high pressure.

To check the pressure I use a floor pump which as a gauge included. The valve cores are always screwed in tight. I'll keep an eye on it some more but what is a normal amount of lose for 2 days?

I always wondered how much air pressure u lose just by attaching the pump to your valve.

I guess you could find out by pumping up ur tire, noting the pressure, removing the pump, then reattaching it to see what the pressure is then.

I think the leakdown rate varies a lot, but 15% a day is excessive and not normal, at least not with the common tubes and tires I've been using for decades. I did a little math based on my experience where a tire with 110 psi will be down into the 80's in a week or so and it comes out something like 3% per day.

Again, you lose a lot of air just checking with gauge, or hooking up a pump, so unless you do some experiments to correct for this, I think you always tend to over-estimate the loss. A typical scenario is that a person gets ready to go for a ride and the bike has been sitting a few days. They check the tires by hand and they feel slightly soft (they're down maybe 5-10 psi), so they hook up the floor pump and in the process of doing so, they lose more air and the gauge on the pump reads 80. That doesn't mean they lost 30 psi.

But if riding the bike with the tires down that 5-10 psi matters to you, then you do probably need to fill them every few days which may mean every ride.

Personally, I've found that the tradeoff between ride quality and rolling resistance is about an even one for me, so I don't mind if the tires are down 10 psi on a given ride - I just pedal slightly harder and enjoy the better ride.

- Mark

Would be normal in my book

Normal. Don't expect the laws of chemistry to change for you. If topping off tire pressure is too much, then consider something other then cycling.

I think it's close to normal. I ride at about 118 in the rear tire and 110 in the front and my wife rides at 5 pounds less than that due to her lighter weight. We'll lose 10 pounds in the first day, then 5 the second, and gradually lose less and less with more time. I always top off our tires immediately before each and every ride. It's easy to damage a presta valve by tightening it too much. All you need to do is snug the nut down to keep the dirt out. The pressure inside the tire should keep the valve closed.

Al

You shouldn't lose any pressure just by putting the pump head on a Presta valve, unless you're pushing the valve down or bending it sideways as you're doing it (poor technique). That's the whole purpose of Prestas. The valve doesn't actually open until you put some air pressure on it.

Normal. Don't expect the laws of chemistry to change for you. If topping off tire pressure is too much, then consider something other then cycling.

I have no problems filling up my tires before each ride. I just wanted to see if it was normal or not.

My road tires seem like they can't hold their pressure. I will pump them up to about 110psi and then two days later the pressure is around 80psi. It seems like I have to pump it up before every ride. Is this normal?

I find the same. It's much worse if you pump with a CO2 cartridge because CO2 permeates
butyl rubber much faster than N2 which is the main component of air. If you don't want to keep inflating your tires, use pure nitrogen as some car tire fitters do.