Bicycle Mechanics - Air Pumps: A question on airing the tires

I own a trek 1000 road bike. Tire size is 700X25c Bontrager. My LBS told me to fill up my tires after every ride as they lose air quicker than wider tires. However, I own a standard air "pump" (the kind you pump with the handle). After my ride(s), I try to pump a little air to fill the tires up but it seems like I can't put any more air in the tire. Do I have a wrong type of air pump? or I am I doing something wrong?

How much pressure are we talking about?

Not the pump, but the head may be wrong. At 700x25, your bike probably has Presta valves. Your pump may be Presta or Schrader (Schrader is the kind of valve you see on car tires). Or it may be interchangeable between the two. Even if your pump is a Presta pump, you also have to remember to loosen the valve stem nut on the valve, to allow the pump to open the valve each time you push the plunger.

Here's a description of Presta and Scrader valves (http://www.sheldonbrown.com/gloss_p.html#presta) (with pictures). To determine which head your pump has, look at the center of the pump head. If there's a metal or plastic "pin" in the center, it's probably a Schrader head (the pin pushes the valve open on a Schrader valve). If not, it's probably a Presta head.

HTH

I'd fill up the tires before a ride, not after. You want them at optimum operating pressure before you ride, not after you get home.

700x25 tires will lose air reasonably quickly - I usually top mine up every second day before I ride. We're talking about a very small volume of air held at a high pressure. You don't need to lose very much air to have a significant drop in pressure, which is usually due to a little bit of seepage through the tubes. Since mountain bike tires hold more air at a lower pressure, they need filling less often.

How much pressure are we talking about?


Well the LBS told me that I wanted to keep my tires at about 100. When I brought my bike in for maint. he checked the air in my tires and he said my air pressure was about 70, so he pumped it up to 100.

Not the pump, but the head may be wrong. At 700x25, your bike probably has Presta valves. Your pump may be Presta or Schrader (Schrader is the kind of valve you see on car tires). Or it may be interchangeable between the two. Even if your pump is a Presta pump, you also have to remember to loosen the valve stem nut on the valve, to allow the pump to open the valve each time you push the plunger.

Here's a description of Presta and Scrader valves (http://www.sheldonbrown.com/gloss_p.html#presta) (with pictures). To determine which head your pump has, look at the center of the pump head. If there's a metal or plastic "pin" in the center, it's probably a Schrader head (the pin pushes the valve open on a Schrader valve). If not, it's probably a Presta head.

HTH


That helped a lot, thanks. Yes I have Presta valves on my tires, so that means I need a Presta valve pump correct??

If your pump does not have a Presta head, you can buy an adapter cheap. I keep one screwed on a tire so I can use a gas station pump.

You should pump them up before every ride. After seems kind of strange to me.

Is your pump made for 100psi +? My Planetbike pump can BARELY do 110... and certainly can't go beyond that. So be sure you have a decent quality pump.

If your pump does not have a Presta head, you can buy an adapter cheap. I keep one screwed on a tire so I can use a gas station pump.

at my LBS the adapters are .25 each. since they are small and easy to lose, you might want to pick up a bunch of them.

It's worth buying a floor pump. They are ideal for topping up tyres, in a few seconds. I can't imagine trying to inflate a tyre fom 100psi to 120psi with a mini-pump or any hand pump for that matter. Floor pumps are worth the money.

CHEERS.

Mark

It's worth buying a floor pump. They are ideal for topping up tyres, in a few seconds. I can't imagine trying to inflate a tyre fom 100psi to 120psi with a mini-pump or any hand pump for that matter. Floor pumps are worth the money.

Amen, brother! On the road, I use a full-length frame-fit Zefal HP-X or Blackburn. Every few days should be about the right topping-up interval.

I own a trek 1000 road bike. Tire size is 700X25c Bontrager. My LBS told me to fill up my tires after every ride as they lose air quicker than wider tires. However, I own a standard air "pump" (the kind you pump with the handle). After my ride(s), I try to pump a little air to fill the tires up but it seems like I can't put any more air in the tire. Do I have a wrong type of air pump? or I am I doing something wrong?

You need to "burp" your presta valves before you inflate them. After losening the little nut on top of the valve, just push it down with your finger and let out a tiny bit of air. That breaks the seal on that poppet valve and makes it open more easily as you reinflate it.

Like the others, it sounds crazy to me to top up your tires AFTER a ride. I top mine off just prior to riding.

Your tires probably have even more PSI in them after a ride. Sort of like your car having a higher PSI reccomendation if ou have been driving.

You loose the PSI quickly to slowly over time. Meaning, you loose more in the first 12 hours than you do in the next 12, so you want to check before you ride.

Burping the prestas is key. I've recently been letting a bunch of PSI out of my tires between rides (120 -> 90). That way I quickly remember to fill them and the burbing is easier. Nothing worse than a pinch flat... keep air in them tires!

Your tires probably have even more PSI in them after a ride. Sort of like your car having a higher PSI reccomendation if ou have been driving.

i am not sure if that is true... car tires generally flex a lot more and they also spin a lot faster than do bike tires :)

i am not sure if that is true... car tires generally flex a lot more and they also spin a lot faster than do bike tires :)

It is a heat thing, not a flex thing. Simple PV=nRT -> P=(nRT)/V. With the tire, everything is constant in regards to pressure {(nR)/V = constant** when riding due to the high PSIs, so the only factor is temperature. There is friction between the tire and the road. That is the biggest cause of heat/temperature (watch race car dirvers swerve back and forth). Your weight and flex aren't a big deal at those relative PSI (P) and volume (V). Yes, the T delta between a car and a bike is much greater for a car. The V is also much greater, thus the temperature have a lower linear effect on the P.

In the end, is it at all measurable? Does it matter? Don't know or care. Kind of fun to think basic physics for a minute. Now back to my regularly scheduled job.

I think the morale is still to inflate before you ride so as not to pinch flat and enjoy.

It is a heat thing, not a flex thing. Simple PV=nRT -> P=(nRT)/V. With the tire, everything is constant in regards to pressure {(nR)/V = constant** when riding due to the high PSIs, so the only factor is temperature. There is friction between the tire and the road. That is the biggest cause of heat/temperature (watch race car dirvers swerve back and forth). Your weight and flex aren't a big deal at those relative PSI (P) and volume (V). Yes, the T delta between a car and a bike is much greater for a car. The V is also much greater, thus the temperature have a lower linear effect on the P.


I actually had to calculate the temperature of air inside an inflating bike tire for a 1989 mid-term in Physics 3201 Thermodynamics. Thanks for reminding me. :)

bkr

If you get a floor pump with a small diameter barrel, it will be much easier pumping at the high pressures, and get one with a built-in guage. The tire will hold the pressure OK for several days, but it is easier to get into the habit of pumping before every ride than trying to remember if it was 3 or 4 days ago that you last pumped them. You wont have to make more than 5 strokes of the pump to get up to pressure.

hmmm ok. So those of you who do use a built in pressure gauge does 100 psi sound right to you or too much??

hmmm ok. So those of you who do use a built in pressure gauge does 100 psi sound right to you or too much??

Look at the maximum on the tire. Mine are 120, so I pump to about 115 on my gauge.

Here is my rule of thumb for the road:
Look at the maximum on the tire, go a couple under. If it is going to be really hot, a couple more under. I don't go more than 8-10 under since I seem to suffer pinch flats at that point.

On the trail, I go a lot less... more volume, so less chance of a pinch flat, and the lower pressure equals more comfort.