Hell And High Water said:Seems I'm not smart enough to pump up a tire...
The little stem/rod on my tire valves keeps getting bent. I have a
decent Bontrager pump, but it has a nozzle that fits both presta and
schrader valves.
I can't seem to get it to seat correctly. When I first try to attach it
to the tire all the air rushes out, then I close it down, and it never
seems to be seated correctly. Sometimes no air is getting into the
tire, other times there's a delay, where the pressure goes way up, then
backs down to the correct amount.
Is this the bent stem? The two way pump nozzle or am I just dumb?
gracias
-Bob
Dear Bob,
If the tube is already inflated, a presta valve that's been unscrewed
will stays shut until you pump enough pressure to force it open.
That causes the delay when the pressure goes way up while you pump.
For example, with a sticky presta O-ring you might see your pump gauge
rise to 100 psi with a few stokes and then abruptly drop to 90
psi--the tire had 90 psi, but it took an extra 10 psi to push the
presta valve open.
Schrader valves don't do this because a Schrader pump has a doohickey
that pushes the valve open against a tiny spring as soon as you put
the chuck on the valve--no air pressure is needed.
A dual-head bicycle pump pushes air through whichever hole has some
back pressure, using a clever little ball arrangement.
But if neither hole in a dual-head chuck has back pressure because one
hole is open and the other hole is connected to a limp inner tube,
then pure stinking luck decides which hole gets the air that you pump.
So the odds are 50-50 that air will whoosh uselessly out the open hole
on a dual-head pump if you attach it to an empty inner tube.
The trick is to push the inner tube against the base of the valve,
which creates enough back pressure to switch the pump where you want.
If you're inflating a bare tube, it's easy to push the inner tube
against the base of the valve with a finger. If you're inflating a
tire, just push a little harder on the same place on the tire.
If that doesn't clear things up, you could have a defective pump.
The solution would be to stop by the local bike shop and to ask them
to run through it with you.
They're human, so they like being asked if they're experts. (This is
the basis of RBT.) Even if the LBS doesn't sell you something, they'll
be just as pleased to have someone walk through the door as anyone
else in retail sales.
Cheers,
Carl Fogel
