In article <
[email protected]>,
<
[email protected]> wrote:
>> Jobst, I'm curious what the stroke length is on your pump. I tried
>> a back-of-the-envelope calculation for volume a while ago, and
>> couldn't arrive at 10 strokes.
>
>The length is optional and in the drawing is it "L" because it was
>designed so that the long parts are all the same length, a length of
>your choice depending on body height. Most of the pumps built were
>24" and four were 25" and one 22". Mine is a long 1-5/8" OD one. The
>large diameter pumps were too much pull for some riders, so most were
>1.5" OD. All the 25" long pumps were 1-5/8" OD.
>
>In the cross sectional area noted in the lower left with the
>compression ratio and upward pull, you can calculate the effect of the
>larger diameter and stroke.
Stroke doesn't enter what's calculated on the diagrams I've
seen, only areas; there being calculations of force, but not volume.
Actually, I was a little puzzled by the calculations on the
left. The force you give at the end is (CR-1) * 1atm * A, where A is
the area of the downstroke chamber. This forces you give are the
initial push on the _downstroke_, after the low-pressure chamber has
fallen out of the picture.
In pulling upwards, the force on the low pressure cup is CR
times larger than this, for a net pull of (CR-1) times your final
numbers. For your 1-5/8" pump, this is a pull of 36 lbs (rather than
16.4 in the figure). I can appreciate how this would be too much for
some people. 20 lbs for the 1.5" pump sounds much more reasonable.
This high pull force is in part a product of how small your
output chamber is - ease of pushing coming at the expense of
difficulty pulling, for a given delivered volume per unit stroke. I'm
very comfortable with pushing Silca's standard floor pump diameter for
the pressures I need, and don't anticipate a need for 300PSI, so I
intend to use that diameter for my downstroke, and keeping my CR lower
at around 2 to 2.2. I think the Zefal Double-Shot HP is very similar
in dimensions, but its hose comes out the handle intead of the foot.
>> For the dimensions in the drawings for your 1.5" pump, the first
>> stage has an area equivalent to a 33.2mm ID, or 1.4x the area of
>> Silca's pumps. For the same stroke as a Super Pista, this would
>> take 16 strokes to fill my tires, and with 28mm tires needing even
>> more. The 1.625" pump works out to 36.55mm, or ~13 strokes.
>I only recall that a Clement criterium took 10 strokes to 100 psi.
>Even with my 25mm cross section tires it's not much more that that to
>100. Maybe 14 strokes.
Ah, I think this is the source of my confusion - I was under the
impression that your 10 strokes to 100psi was for a 28mm tire. What I'm
finding for the Clement criterium is that it's a 21mm tire, in which
case all the numbers make sense.
>As you can see, the dead space is the screw driver slot in the top
>screw, the 0.1" diameter 0.6"long air passage to the check valve and a
>small annular space between leather and top screw. This was considered
>carefully and is why the check valve is located where it is.
I think this is a little better than Silca's dead space, it
having relatively more volume between the flats of the hex nut and the
leather, and also more overhead from the extra height of the nylock
protrusion. But even with this volume, it's not bad at all. Pumping to
100psi, I just start to see the effects of this with the last 20psi
taking a stroke more than the 20 preceding it.
-Luns
