Track pump gauge reading : Drops when I stop pumping?

[RSV_Ecosse]

Hi, quick question on the gauge reading on my track pump.

I pump the tyres up to @100psi ( 700x23 Schwalbe Stelvios ) which is fine, but once I see the gauge needle get to the 100psi mark and I stop pumping the needle drops very quickly. There's no air escaping anywhere so why would it do this?.

It's weird, 'cos I then can't really work out how much pressure I've got in the tyres?. As there is no air escaping it means that if I start pumping again until I reach 100psi then surely there's far too much air in them, if that makes any sense?. They feel absolutely rock hard and the ride is generally quite harsh.

 

[Paul_L]

it sounds like you haven't got a good seal on the valve. If it's like my Blackburn pump - and i'm sure they're all much of a muchness, you have to push the pump head right over the valve pin, and then lock it. If the guage pressure is dropping there must be air leaking from somewhere.

 

[RSV_Ecosse]

Cheers, yeah - you were right. I stopped pumping and the gauge started dropping as usual but I reached down and moved the gauge so it was square and tight against the valve ( the fit is a bit err, "floppy" ) and immediately the pressure reading went back up to just under 100psi where it should be, and stayed there.

*EDIT* And mine is a crappy Tesco own brand one but it does what it says on the tin ( with a bit of fiddling! ).

 

[youngoldbloke]

Surely the 100psi in the tyre will close the valve instantly you stop pumping, and then the gauge measures the pressure in the pump itself, which loses pressure quickly as soon as you stop? The gauge will only measure the tyre pressure when the valve is opened, as you push air into the tyre.

edit - just saw your last post - you've sorted it by making the pump more airtight, but I think the above still holds - the more accurate reading is on the push stroke of the pump?

 

[Paul_L]

Surely the 100psi in the tyre will close the valve instantly you stop pumping, and then the gauge measures the pressure in the pump itself, which loses pressure quickly as soon as you stop? The gauge will only measure the tyre pressure when the valve is opened, as you push air into the tyre.

edit - just saw your last post - you've sorted it by making the pump more airtight, but I think the above still holds - the more accurate reading is on the push stroke of the pump?

Mmm, you've got me thinking there.

With the bike valve open and the pump fully sealed the pressure in the tube will be the same as the pressure in the pump? On the down stroke of the pump you're basically pushing atmospheric pressure air into the bike tube and therefore the pressure increases. On the up stroke of the pump you allow atmosphere into the pump body. The guage pressure should slightly decrease owing to the intake of atmospheric air. On the pump downstroke the pressure will increase again.

To my mind you should be able to stop pumping and the pressure guage will read the same (apart from a slight drop to allow for the intake of atmospheric pressure).

 

[RSV_Ecosse]

Now you guys have totally confused me.

 

[youngoldbloke]

- and myself - I must pay attention next time I inflate
I expect someone out there knows the correct answer.

 

[Wigsie]

I think Paul is right... With mine The pressure guage increases on the down stroke then drops slightly to settle on the 'true' reading as the pressure balances.

 

[youngoldbloke]

- must be because you need initial higher pressure in the pump to open the valve and admit air into the innertube, once admitted - bottom of the stroke - the gauge drops back slightly - makes sense?

 

[Headgardener]

Am I right in thinking that when the pump head is fitted onto the valve something inside the head pushes the tube valve open. So that the pressure in the tube and the pressure in the head and hose are equal. Also that on better pumps there is a non return valve somewhere between the head and pump body to stop it leaking out again?

 

[Banjo]

I pump my hybrid tyres to 70 front and 80 rear. No leaks as far as I know but within a few days they both read about 60 psi. Is this due to the small amount of air lost when connecting/disconnecting the pump or must there be a leak somewhere?

 

[mickle]

I pump my hybrid tyres to 70 front and 80 rear. No leaks as far as I know but within a few days they both read about 60 psi. Is this due to the small amount of air lost when connecting/disconnecting the pump or must there be a leak somewhere?

Porous tubes or poor valve seals.

 

[Sheepy1209]

To the OP - is this Presta or Schrader (car type)?
When I first got Presta valves a few weeks ago I had a similar problem because I didn't understand how they work.

Sorry if I'm telling you what you already know (it's mostly new to me) - on the Schrader the pump head pushes down on the top of the valve which opens it against a spring.

On the Presta you have to loosen the collar (I'm assuming you've done that!), then apply excess pressure until the valve pops open (you'll hear a 'ping') - the pump will then read the pressure inside the tube and you're ready to begin pumping.

[edit: as per mickle below, give the stem a quick press to break the seal - I do that but forgot to mention it!]

Even if this wasn't your problem somebody else as dim as me might read this post and see the light

 

[mickle]

On the Presta you have to loosen the collar then apply excess pressure until the valve pops open .......

Push the valve stem in with your finger to break the seal before attaching the pump head, it's easier on the pump.

 

[RSV_Ecosse]

Yeah Sheepy, Presta valves and yeah - I knew about the "press the top of the valve to release a bit of pressure before connecting the pump head". But I was the same as you, I didn't realise this when I first got the bike either until I watched a Youtube vid explaining the difference between Presta and Schrader valves.