SpudFiles

So I have taken interest to pogo sticks recently, I live in finland so getting a pogo isn't very easy. I was first planning to build a spring-operating one, but then I read about these cool Vurtego sticks, and cwazy1 here built one himself. I read the topic thoroughly, but some of the stuff is still a little unclear for me. I (SHOULD) know what compression ratio means (for example 1:4 ratio means if you have a precompression of 50 psi, it will multiply to 200 when you jump?), precompression is the pressure pumped to the pogo before its ready to be used. I've done some research, but haven't found much info. I've got the compression cylinder already and it is very large in diameter and wall thickness. Worst thing is that I don't know where to get started exactly..

http://en.wikipedia.org/wiki/Compression_ratio

This is for internal combustion engines but you could apply the same principle to a pressurised chamber.

So is a higher compression ratio better in this pogo (if not in cars)? when I think of it like the precompression is 25 psi and the ratio is 1:10, then I'll get a very high pressure when i jump, 250 psi, and doesn't that also mean I'll jump higher?

You have to think of a spring analogy. If you have a small movement and high pre-compression, it's the equivalent of a thick stiff spring. If you have bigger movement and less pre-compression, it's the equivalent of a thinner longer spring.

Ok I'll make the piston long, so less precompression, here are the dimensions of the compression chamber: length 790 mm, diameter 92,5 mm, and the wall thickness is an amazing 7,5 MM!

So how are the precompression and stroke calculated? (I don't have a piston yet for the piston diameter)

but you must also remember, how is my weight gonna affect the performance? in other words you will want you weight to balance at about 5/8 of the way down the piston. also do not do not forget a buper otherwise testing can mess up your pogostick.

Oh finally, an answer, so how are the calculations done? Still don't know how....I do know that the stick will need a shock of sorts like in the vurtego, there is a shock donut, and some other facts

Lets do some math using simplified physics which disregards thermodynamics...

79.0 cm long, 9.25 cm diameter.

This gives us a piston area of 4,105 square inches.

At 10 psi, there is a force of 41 lbs (18.6 kg) on the piston.

If the piston moves half way, this force will double - 37.2kg

Move it to 75% of its travel and you now have 0.25 times the original volume. This means the force is now 4 times greater than the original, 74.4kg. This means that if you weigh say 60kg, by simply standing on the thing it will not reach the limit of its travel.

Of course jumping on it greatly increases the force you can subject it to.

I would look at the specs of a successful commercial model and make something similar.

I'm not sure a bumper is necessary, if the piston head actually touches the base of the cylinder then you have probably generated enough pressure to blow it apart...

Those calculations should really be done with absolute pressure.

Atmospheric pressure is about 15 psi. So a pre-compression of 10 psig (g for gauge) is actually 10 psi above that for 25 psia (a for absolute).

So a force on the piston is actually about 102.5 lbf... but that's counter-acted by a 61.5 lbf from the atmosphere on the other end of the piston, making it appear to be 41 lbf.

Move the piston halfway though (even disregarding the thermodynamics), and that internal pressure has doubled to 50 psia and a force of 205 lbf... but atmospheric pressure hasn't changed, so only exerts its same 61.5 lbf... so that's a total force of 143.5 lbf.

75% of the way would be 100 psia and a total force of 348.5 lbf. Calculated with the thermodynamics (gasses heat as they're compressed) and the raised temperature increases the pressure to 174 psia. 652 lbf.

As far as the bumper... well, with the thermodynamics, if you can compress it 90% of the way, the pressure is up to 628 psia, and the force is 2513 lbf - enough to lift a small car.

As Jack says, a bumper won't have an effect in normal use - the gasses will compress to very high pressures and stop the piston coming close to the bumper. That's not to say that one wouldn't be useful though, as it could be a very useful redundancy if the cylinder were to lose pressure...

Ok should have told this a looong time ago, but now when it's winter this project is suspended, in summer i'll get back to this, and merry christmas

Be careful with this because the russians did this with combustion cannon type shoes, and they were able to jump like 20 feet, however if your jump timing was of, then the user would break his legs.

The two cases aren't actually that similar. A pneumatic pogo stick is just using a pneumatic spring and, unlike the boots, doesn't actually inject any extra energy into the system.

In any case, last I'd heard, the risk with those boots was more about losing your balance than breaking your legs.

Interesting thread. I wonder if there is market for a propane or gas powered pogo stick

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You want to look up the "hot rod"