how much weight does seatpost psi add to a bike?- Mtbr.com

Thread: how much weight does seatpost psi add to a bike?

1. how much weight does seatpost psi add to a bike?

Tire psi adds weight to a bike. What kind of weight does dropper post psi add to a bike? For example, I have a OUC dropper that was pumped to 175psi and then I pumped it to 300psi. How much weight do you think this added to the bike?

2. Three

3. This is why I use Hydrogen

4. Originally Posted by titus
Tire psi adds weight to a bike. What kind of weight does dropper post psi add to a bike? For example, I have a OUC dropper that was pumped to 175psi and then I pumped it to 300psi. How much weight do you think this added to the bike?
Some light reading to get you started. I would calculate it for you, but I'm in the middle of figuring out how many angels can dance on the head of a pin. G'luck!

There is a well-known pV=mMRT universal gas formula. Here:

p is the pressure
V is the volume
m is the mass
M is the molar mass of the gas
R is the universal gas constant (8.314)
T is the absolute temperature (in Kelvin)
Although your question literally interpreted asks for the weight of the gas, from the context it is clearly visible that you are asking for its mass. These are different things:

weight means the force, with what a body would pull its suspension, or pushes the things below it. It is measured in Newtons (N).
mass is... well, the quantity of the matter. It is in kgs or in pounds. 1 pound is around 0.45 kg.
Now we know the V, it is clear. Convert the cubic feets to cubic meters. We also know the pressure, but don't forget to recalculate the psis to Pas (Pascals).

The molar mass is a little bit more problematic. Every gas has a molar mass (M), it essentially means the mass of its 6∗1023 molecules in grams. (1kg=1000g). It depends only on the gas, and it is rougly the same if you calculate all of the protons and the neutrons in its atoms.

For example, in methane, it is 16 (1 carbon atom, with atomic mass 12, and 4 hydrogene atoms, with atomic mass 1). I have a strong impression that you are experimenting with methane :-). It means, that the mass of 1mol of methane is 16g.

Now you want to know m. So, a little bit of reordering happens on our equation:

m=pVMRT
You know p, you know V, I now said R. I suspect T is room temperature, which is rougly 300K.

The only what we don't know is M, but if you say which type of gas is it, I will say it to you on the spot.

From that point, you need only unit conversions and elemental unit multiplications-divisions.

5. Its about 12 times what a seat post full of air weighs at zero psi, which is basically nothing.

A cubic foot of air weighs 36 grams. You could probably fit 20 post cartridges in a cubic foot, so we'll call one 0.9g. By 12 times, its about 11g.

Theres some obvious rounding and guessing errors, so call it 10-20g.

6. I think we've found a new level of weight weenie-ism here. Bravo.

7. BTW, you could just weigh the post at 0 and then at full pressure to eliminate any guess work.

8. Originally Posted by Jayem
Three
Three what ?

9. This is why I heat up my air before I fill up my tires, suspension or dropper.

10. I fill mine with superheated steam 421degF. very low weight per unit of rise

11. Man, using heated air to fill your seat post gives me an idea for a new product to help lighten your bike. Rather than compress/decompress air in a dropper to raise and lower it, why not just release the air to lower it (loosing some weight), and have a tube connected to your rear end through the seat and fart to refill it when you need to raise the post? Think about it. You loose the weight of the compressed air, and when you refill it with flatulence, you don't actually gain any air weight since you're just transferring it from you to the seat post!

Go ahead, tell me this is a bad idea. I dare you!

12. well, for me when I fart, angels come, pack them into balloons, and take them up to heaven. so in my case I will opt out of any research into that idea ^

13. This is why mountain biking on the moon will never be a big thing. Air is heavier there.

14. Originally Posted by Jayem
Three
Wrong. 42. Always 42.

15. Originally Posted by scatterbrained
I think we've found a new level of weight weenie-ism here. Bravo.
You've never been riding uphill and thought, "man if only the air in my dropper was lighter..." ?

16. Let's do some maths assuming some variables.

Air weight. 1.2 kg/m3 @ 20 deg C at 1barg (14.7 psi atmospheric pressure)

Rear shock volume. 200cc/0.0002m3 (vague approximation)
Shock air weight @ 14.7 pressure. 1.2kg/m3 x 0.0002 = 0.00024 kg, 0.24grams

Shock air weight at 275psi (18.7 barg 18.7 atmospheres) x 0.24 grams = 4.488 grams
Shock air weight at 300psi (20.4bar/atmospheres) x 0.24 = 4.8979591

Weight difference between 275psi and 300 psi at 20 Deg C= 0.4099591 grams.

17. Yeah, but what about moisture content?

18. Originally Posted by fredcook
Man, using heated air to fill your seat post gives me an idea for a new product to help lighten your bike. Rather than compress/decompress air in a dropper to raise and lower it, why not just release the air to lower it (loosing some weight), and have a tube connected to your rear end through the seat and fart to refill it when you need to raise the post? Think about it. You loose the weight of the compressed air, and when you refill it with flatulence, you don't actually gain any air weight since you're just transferring it from you to the seat post!

Go ahead, tell me this is a bad idea. I dare you!
Originally Posted by WHALENARD
Yeah, but what about moisture content?
You guys are gross.

Don't forget to factor in altitude.

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