bikeformoney

Hi I bought a bike pump for my tires that use presta valves. (tires say "max 135 psi") It cost about 30$ (was 40 retail) after 80 psi, it is difficult to pump air into the tubes (even though I could keep going). Even if the tires feel only halfway flat, and I first hook the pump in, the gauge only reads about 5 psi (even though i'm guessing it has 55 psi) Is this common with floor pumps with pressure gauges? should I buy a tire pressure reader? maybe the presta adapter on the pump is cheap and I should buy a schrader adapter? I strongly suspect the gauge is not accurate.

1 more question: how much will a upgraded rear deraiuler help with smooth shifting, (I am getting some roughness and complaints when I shift into the very top gear) I know the one I have is fairly cheap (this is a road bike) or will the gears wear in after a while (bike is three months old) I clean the moving parts with dry degreaser and lube it with dry lube.

RonH

What brand/model pump did you get? Sounds like you need to return it.

Have you taken the bike in for adjustment after the break-in period?

Drew Eckhardt 

Presta pump heads generally rely on air pressure to open the presta valve so you won't get an accurate reading until you're adding air to the tire (it'll start out low, and it'll over-shoot if the valve is sticky and not opening).

It won't (unless it's really cheap without a floating top pulley). The limit screw might not be allowing enough movement or the cable tension could be a little off (I play with it a little bit while riding after swapping cables until I get the same shift quality going to bigger and smaller cogs).

MudPie

Actually, when you connect the pump to the presta valve it should read zero. The valve in a presta is simple - bascially slides depending on which side has more pressure. There is not spring in a presta valve. When the internal pressure in the innertube is greater than the pressure on the outside of the valve, the valve shuts so air cannot escape. Conversely, when you connect the pump and start pumping, the valve opens when the pump pressure is greater than the pressure in the innertube and air is allowed into the innertube. When you remove the pump head, the valve immediately shuts because the pressure inside the innertube is greater than the ambient air.

fietsbob

Floor pump, or a on the bike pump? if the latter you need to put more work into the handle.

bikeformoney

I could get more from it, but since the gauge seems to only know how much air I put into the tire, instead of the total combined air in the tire, I don't want to over do it because when it says "75 psi" and the tire feels hard and the handle is more difficult, just don't want to puncture the tube with too much air, not knowing the actual air pressure I am using

MudPie

What make and model pump do you have.

Assuming it's of decent quality, I would assume the gage is accurate. At 80 psi, a tire is "hard" (which is subjective). Note, to go from 80 to 100 psi, it may only take one piston stroke.

Perhaps one way you can test your gage (assuming you have a schrader adapter) is to check the tire pressure in a car tire, then connect your pump to the tire and see if it reads the same.

BCRider

You're over thinking the issue.

Assuming the valve is opening properley the pump will never generate more pressure than what is in the tire. The pressure you see the needle stabilize at following a pump stroke is what is in the pump and the tire. They are at the same pressure.

And the pressures do not "add up". What you see at the end of a pump stroke that TRANSFERS AIR to the tube is the same as what is in the tube. Note that the first pump stroke or even second one may not build up enough pressure in the connecting line to actually transfer air into the tube. Until you hear the honking sound of air flowing through the valve the reading on the pressure guage means zilch.

75PSI in a tire is darn hard to a hand trying to guage pressure with a pinching action. From around 60 to 70psi the tire gets so hard that a pinch test is no longer meaningful in any way whatsoever. At that point you need a guage to measure the pressure.

And finally I can't imagine a floor pump that won't go well past 80 psi unless it's a skinny 10 year old pushing down on the handle and at 80 the back pressure is lifting him off the ground. If it's leaking or something else then it is faulty.