Using cycle pump on car tyres

[Rob Morley]

In article <[email protected]>, Pete Biggs
[email protected] says...
> Rob Morley wrote:
> > In article <[email protected]>, Pete Biggs
> > [email protected] says...
> >> Nick wrote:
> >>
> >>> I firmly believe that the length doesn't matter with regard to
> >>> pressure. I'm told that my 46cm is perfectly adequate.
> >>>
> >>> Could you propose a theory as to why length makes a difference?
> >>
> >> The longer (or narrower) the barrel, the more the air is compressed.
> >>
> >> Short pumps have to be narrow to get high pressure. Fat pumps have
> >> to be long.
> >>
> > You are confused. Do you want to go away and think about it, or
> > shall I try to explain it to you?
>
> Feel free to correct me if I'm wrong -- any time -- 3.30 am if you like!
>
Gosh, is that the time? I'd better get some beauty sleep.

 

[Nick]

Pete Biggs wrote:
> Nick wrote:

>>>
>>> Silca Pista barrel is 32mm (externally), 62cm long, 150 psi no
>>> problem for 11.5 stone.
>> Assuming an internal diameter of 30mm 11.5 stone dead weight down
>> force compares to about 147 psi. I dare say the with a bit of added
>> acceleration your entire body weight would be enough to get to 150
>> psi.
>
> I mean it's not difficult to inflate a 700x23 tyre to 150 psi with this pump
> for an 11.5 stone man (with modest muscles).
>

Forgetting the physics argument, is the Silca Pista a good pump, I could
do with a better one.

I see they have one at Parker International for 29.95, worth buying?

I did get a Zefal HPX after you recommended them and have been very
happy with it.

 

[Pete Biggs]

Nick wrote:

> Forgetting the physics argument, is the Silca Pista a good pump, I
> could do with a better one.
>
> I see they have one at Parker International for 29.95, worth buying?

It's a very effective and reliable old fashioned pump, except for the
head -- a simple push-on job, which tends to leak or slip off the valve. I
replaced mine with a Topeak SmartHead (complete with replacement hose).
This pushes up the total price so much that you might find better value for
money nowdays from another pump that comes with a better head.

~PB

 

[Mark]

> Forgetting the physics argument, is the Silca Pista a good pump, I
> could do with a better one.
>
> I see they have one at Parker International for 29.95, worth buying?

This track pump is the best value I've seen. It goes to 140psi with ease,
and will go higher (I've just had no need to). When it eventually goes
it'll be the pump head (though this is after 9 months of workshop use, so a
few lifetimes for us . You can buy the bits for 95 pence.

<www.edinburghbicycle.com/ebwPNLqrymode.a4p?f%5FProductID=4093>

If you want something that'll last for ever, the Joe Blow has a great
reputation amongst mechanics for reliability.

 

[Rob Morley]

In article <[email protected]>, Pete Biggs
[email protected] says...
> Rob Morley wrote:
> > In article <[email protected]>, Pete Biggs
> > [email protected] says...
> >> Nick wrote:
> >>
> >>> I firmly believe that the length doesn't matter with regard to
> >>> pressure. I'm told that my 46cm is perfectly adequate.
> >>>
> >>> Could you propose a theory as to why length makes a difference?
> >>
> >> The longer (or narrower) the barrel, the more the air is compressed.
> >>
> >> Short pumps have to be narrow to get high pressure. Fat pumps have
> >> to be long.
> >>
> > You are confused. Do you want to go away and think about it, or
> > shall I try to explain it to you?
>
> Feel free to correct me if I'm wrong -- any time -- 3.30 am if you like!
>
Pressure is a function of the force applied and the area to which it is
applied - length of stroke doesn't come into it (but see [1]). A
smaller bore will require less force than a larger bore to produce a
given pressure. There is a limit to the force your arm can apply to the
pump, so the smaller the bore the higher the pressure you can achieve.
Unfortunately although a small bore means you can achieve higher
pressure with less force it also means that each stroke delivers less
air. The swept volume increases with the bore and the stroke. When
designing a bike pump you have to compromise between the pressure that
can be achieved, the effort required to do it and the compactness of the
pump. You also have to consider ergonomic factors - how easily you can
grip a particular size of pump, and the range of movement of your
pumping arm.

[1] There is also the matter of wasted effort - on each down stroke a
small amount of the air that has been compressed doesn't get delivered
to the tyre, but stays in the bottom of the pump and expands again on
the up stroke - this is significant in old-style pumps that use a
flexible connecting hose as the entire volume of the hose is wasted on
each stroke, but with frame-pump type connectors it's pretty small and
track pumps have a non-return valve so you only waste the volume of the
hose once per inflation rather than once per pump stroke. The wasted
air volume can also limit the maximum achievable pressure (where the
force applied isn't the limiting factor) - e.g. if the volume is 10% of
the swept volume you'll have a 11:1 compression ratio, and the pump
won't be able to deliver more than 10 bar. For this reason a long pump
can outperform a short pump of otherwise identical contruction and
dimensions.

 

[Pete Biggs]

Rob Morley wrote:

> Pressure is a function of the force applied and the area to which it
> is applied - length of stroke doesn't come into it (but see [1]). A
> smaller bore will require less force than a larger bore to produce a
> given pressure. There is a limit to the force your arm can apply to
> the pump, so the smaller the bore the higher the pressure you can
> achieve. Unfortunately although a small bore means you can achieve
> higher pressure with less force it also means that each stroke
> delivers less air. The swept volume increases with the bore and the
> stroke. When designing a bike pump you have to compromise between
> the pressure that can be achieved, the effort required to do it and
> the compactness of the pump. You also have to consider ergonomic
> factors - how easily you can grip a particular size of pump, and the
> range of movement of your pumping arm.
>
> [1] There is also the matter of wasted effort - on each down stroke a
> small amount of the air that has been compressed doesn't get delivered
> to the tyre, but stays in the bottom of the pump and expands again on
> the up stroke - this is significant in old-style pumps that use a
> flexible connecting hose as the entire volume of the hose is wasted on
> each stroke, but with frame-pump type connectors it's pretty small and
> track pumps have a non-return valve so you only waste the volume of
> the hose once per inflation rather than once per pump stroke. The
> wasted air volume can also limit the maximum achievable pressure
> (where the force applied isn't the limiting factor) - e.g. if the
> volume is 10% of the swept volume you'll have a 11:1 compression
> ratio, and the pump won't be able to deliver more than 10 bar. For
> this reason a long pump can outperform a short pump of otherwise
> identical contruction and dimensions.

A certain amount of length (in relation to bore diameter) is needed for the
air to be suffiiciently compressed for the desired pressure. But thinking
again, I realise virtually no pumps are too short for this.

I think the Silca Super Pista must have a smaller bore diameter than some of
poorer track pumps (making high pressure easier), but it's nice and long as
well to minimise the number of strokes required.

Thanks,

~PB

 

[Rob Morley]

In article <[email protected]>, Pete Biggs
[email protected] says...
>
> A certain amount of length (in relation to bore diameter) is needed for the
> air to be suffiiciently compressed for the desired pressure.

It's not directly related to the bore:stroke ratio, it's about the
compression ratio. The way that pumps are typically contructed leaves
some wasted space around the piston sealing arrangement, either beside
the piston or below the washer:

| __| |__ | | | | |
|| || | __| |__ |
|O O| ||________||
||________|| |/ \|

Eliminate that (and any other wasted space) and you could get high
pressure from a very short stroke. The problem is you'd need high-tech
materials and high-precision engineering to make it work, and this is
bike pumps we're talking about.

 

[[email protected]]

In message <[email protected]>
Rob Morley <[email protected]> wrote:

>> A certain amount of length (in relation to bore diameter) is needed for the
>> air to be suffiiciently compressed for the desired pressure.

> It's not directly related to the bore:stroke ratio, it's about the
> compression ratio. The way that pumps are typically contructed leaves
> some wasted space around the piston sealing arrangement, either beside
> the piston or below the washer:

[snip]


Forgive my failed "O" level physics, but isn't piston suface area
incontact with the comressed gas a key element, as well as stroke and
bore? So if you have a cone shaped piston...

(And I have one of those Hi-tech ribbon drive pumps - great pump,
great guage, very noisy, and useless for car tyres.)


--
Charles
Brompton P-typeT6 in Motspur Park

 

[Tony Raven]

[email protected]m wrote:
>
> Forgive my failed "O" level physics, but isn't piston suface area
> incontact with the comressed gas a key element, as well as stroke and
> bore? So if you have a cone shaped piston...
>

No, its only the piston area projected onto a plane normal to the
movement of the piston which is the same whether its flat, conical,
spherical or whateverical. A cone shaped piston will just bottom out
earlier preventing you reaching higher pressures so might as well make
it flat.

Tony

 

[Simon Brooke]

in message <[email protected]>, Tony Raven
('[email protected]') wrote:

> [email protected]m wrote:
>>
>> Forgive my failed "O" level physics, but isn't piston suface area
>> incontact with the comressed gas a key element, as well as stroke and
>> bore? So if you have a cone shaped piston...
>>
>
> No, its only the piston area projected onto a plane normal to the
> movement of the piston which is the same whether its flat, conical,
> spherical or whateverical. A cone shaped piston will just bottom out
> earlier preventing you reaching higher pressures so might as well make
> it flat.

Unless the bottom of the cylinder has a matching conical depression, of
course... but even then it's still only equivalent to a flat ended piston
in a simple cylinder of the same diameter.

--
[email protected] (Simon Brooke) http://www.jasmine.org.uk/~simon/

;; I put the 'sexy' in 'dyslexia'

 

[Rob Morley]

In article <[email protected]>,
[email protected]m says...

> Forgive my failed "O" level physics, but isn't piston suface area
> incontact with the comressed gas a key element, as well as stroke and
> bore? So if you have a cone shaped piston...
>
That's why you failed and I got an 'A'.

 

[Nick]

Simon Brooke wrote:
> in message <[email protected]>, Tony Raven
> ('[email protected]') wrote:
>
>> [email protected]m wrote:
>>> Forgive my failed "O" level physics, but isn't piston suface area
>>> incontact with the comressed gas a key element, as well as stroke and
>>> bore? So if you have a cone shaped piston...
>>>
>> No, its only the piston area projected onto a plane normal to the
>> movement of the piston which is the same whether its flat, conical,
>> spherical or whateverical. A cone shaped piston will just bottom out
>> earlier preventing you reaching higher pressures so might as well make
>> it flat.
>
> Unless the bottom of the cylinder has a matching conical depression, of
> course... but even then it's still only equivalent to a flat ended piston
> in a simple cylinder of the same diameter.
>

Come on guys this is the easy part of the thread.

How about one of you physics types giving an answer to the adiabatic
heating of the air in the foot pump at 2 bar vs the bike pump at 10 bar.

I had a bash and got something like 100 K rise for the foot pump and a
450 K rise for the bike pump. But I'm really not sure what I'm doing and
the bike pump rise seems too high even given that most of the heat won't
be transferred to the valve where I can feel it.

 

[Tony Raven]

Nick wrote:
>
> Come on guys this is the easy part of the thread.
>
> How about one of you physics types giving an answer to the adiabatic
> heating of the air in the foot pump at 2 bar vs the bike pump at 10 bar.
>
> I had a bash and got something like 100 K rise for the foot pump and a
> 450 K rise for the bike pump. But I'm really not sure what I'm doing and
> the bike pump rise seems too high even given that most of the heat won't
> be transferred to the valve where I can feel it.

<Dredges memory banks>

IIRC, the formula you need is P^^0.4/T^^1.4 = constant for air (5
degrees of freedom)

So if P increase x2, T increases ~22% or ~65K and if P increases 10x, T
increases 93% or ~280K. Of course the compression is not adiabatic so
the temperature rise will be a lot less than that.

BICWBW

Tony

 

[Nick]

Tony Raven wrote:
> Nick wrote:
>>
>> Come on guys this is the easy part of the thread.
>>
>> How about one of you physics types giving an answer to the adiabatic
>> heating of the air in the foot pump at 2 bar vs the bike pump at 10 bar.
>>
>> I had a bash and got something like 100 K rise for the foot pump and a
>> 450 K rise for the bike pump. But I'm really not sure what I'm doing
>> and the bike pump rise seems too high even given that most of the heat
>> won't be transferred to the valve where I can feel it.
>
> <Dredges memory banks>
>
> IIRC, the formula you need is P^^0.4/T^^1.4 = constant for air (5
> degrees of freedom)
>
> So if P increase x2, T increases ~22% or ~65K and if P increases 10x, T
> increases 93% or ~280K. Of course the compression is not adiabatic so
> the temperature rise will be a lot less than that.
>
> BICWBW
>

Ah yes thanks Tony, I concur. A much more plausible answer.

Originally I was only using the exponent on pressure not temperature.

 

[Martin Dann]

Nick wrote:

> Come on guys this is the easy part of the thread.
>
> How about one of you physics types giving an answer to the adiabatic
> heating of the air in the foot pump at 2 bar vs the bike pump at 10 bar.
>
> I had a bash and got something like 100 K rise for the foot pump and a
> 450 K rise for the bike pump. But I'm really not sure what I'm doing and
> the bike pump rise seems too high even given that most of the heat won't
> be transferred to the valve where I can feel it.

The air will travel through the valve fast, and much of
this heat will be lost in the pump, of will go into the wheel.

A typical tyre [1] will have approx 0.6mol (17g) of air in
it.
In contrast a Mavic XC717 has a mass of 420 grams, 25
times more. If the heat capacities were the same[2], then
a 260K rise in the air temperature would lead at most to a
10K rise in the temperature of the wheel rim.


[1] 26in*1.5 at 5bar
[2] which they won't be.

 

[Nick]

Martin Dann wrote:
> Nick wrote:
>
>> Come on guys this is the easy part of the thread.
>>
>> How about one of you physics types giving an answer to the adiabatic
>> heating of the air in the foot pump at 2 bar vs the bike pump at 10 bar.
>>
>> I had a bash and got something like 100 K rise for the foot pump and a
>> 450 K rise for the bike pump. But I'm really not sure what I'm doing
>> and the bike pump rise seems too high even given that most of the heat
>> won't be transferred to the valve where I can feel it.
>
> The air will travel through the valve fast, and much of this heat will
> be lost in the pump, of will go into the wheel.
>
> A typical tyre [1] will have approx 0.6mol (17g) of air in it.
> In contrast a Mavic XC717 has a mass of 420 grams, 25 times more. If the
> heat capacities were the same[2], then a 260K rise in the air
> temperature would lead at most to a 10K rise in the temperature of the
> wheel rim.
>
>
> [1] 26in*1.5 at 5bar
> [2] which they won't be.

I've never felt a rim get hot. But the valve and bottom of the pump
certainly warm up.

 

[CoyoteBoy]

On 10 Aug, 19:58, Nick <[email protected]> wrote:
> CoyoteBoy wrote:
> >> I have a cheap track pump from argos and it doesn't seem to take any
> >> longer than a foot pump to pump my car tyre from flat.
>
> > Track pumps have fairly large volume, much like a car pump.
>
> >> I think the diameter of the barrel is too big for proper pressures on a
> >> bike.
>
> > ?? Not sure i understand how you mean?
>
> Large diameter means it is hard to exert enough force on the pump handle
> to get a high pressure.
>
> >> Also I don't see why you would have heat problems as car tyres are low
> >> pressure 2 bar compared to the 8+ bar on a road bike.
>
> > Because due to a larger volume having to pass through the hose, the
> > area remains heated for longer. I dont think it will be a "problem" as
> > such, but pumping a car tyre to 2bar would produce more heat in total
> > than pumping a bike tyre to 2 bar.
>
> I'm not a physicist and don't quite know how to apply the ideal gas law
> etc. But wouldn't compressing a gas 8 times produce much higher
> temperatures than compressing it 2 times. AIUI the heat was constant but
> just compressed into a smaller volume.

But theres a difference between heat and temperature, much like waving
a match under your finger - if you do it in short quick bursts the
temperature may be very high but the heat imparted to your finger is
low (like pumping a bike tyre, even to high pressure), but rest your
finger on a radiator at 60C and you'll burn it (akin to pumping a car
tyre up, even to low pressure). Obviously its all in relative and the
size/material etc of the hoses and pumps makes a large difference. But
in general, when heat is transferred by conduction as in this case,
the longer the heat source is in contact, the hotter the affected item
will get until it reaches equilibrium (which is often longer than the
time taken to pump up a bike tyre)