CO2 in tyres

[compo]

When I was a kid I recall in science making carbon dioxide. The gas was stored in a jar. To prove we had made CO2 we "poured" the gas over a candle and extinguished the flame. This also proved that CO2 is heavier than air. This has me wondering why people spend hundreds of pounds on ultra lightweight wheels and tyres then fill them up with a gas that is heavier than normal atmospheric air.

This is not an April fool, and is totally light hearted, but is there a measureable difference in weight between air and CO2? I know it wouldn't make a scrap of difference either way. As I stated a while back my mind works in peculiar ways when cycling!

 

[Profpointy]

air weighs around 1.3 grammes per litre. C02 will be around 2 if my mental arithmetic serves me.
I can't be bothered working out the volume of a tyre, but say it's a litre - .8 grammes extra - mmm

There used to be a myth that they used helium in racing tyres to save weight - somewhere between 0.1 an 0.2 grammes per litre (again from memory mental arithmetic) ... but helium is very good at leaking out of things, so likely they'd have been cycling on flatish tyres

 

[MikeG]

Maths time

Assuming the cross sectional diameter of a tyre is 25mm, and that it is circular, then the cross sectional area (pi r squared) is 9.8 cm sq. Assuming the midline of the tyre forms a circle 712mm in diameter, then the length (circumference) is 223.6cm. Multiply those together and you get a volume per tyre of 2194CC's, and thus a total of 4388 cc's for a pair of tyres.

Tyres are normally pumped to around 100psi, which is about 7 atmospheres, so multiply the volume by 7, and the total amount of gas in the tyres is 30,700cc, or .0307 cubic metres.

Air weighs 1.2 kg/ cubic m, so 0.307 x 1200 grammes = 36.8 grammes.

Carbon dioxide weighs 1.977 kg/ cubic metre, so 0.307 x 1977 grammes = 60.7 grammes.

60.7 - 36.8 = approx 24 grammes.

Therefore, filling tyres with carbon dioxide will increase each tyres weight by about 12 grammes, or about half an ounce.

 

[Profpointy]

Maths time

Assuming the cross sectional diameter of a tyre is 25mm, and that it is circular, then the cross sectional area (pi r squared) is 9.8 cm sq. Assuming the midline of the tyre forms a circle 712mm in diameter, then the length (circumference) is 223.6cm. Multiply those together and you get a volume per tyre of 2194CC's, and thus a total of 4388 cc's for a pair of tyres.

Tyres are normally pumped to around 100psi, which is about 7 atmospheres, so multiply the volume by 7, and the total amount of gas in the tyres is 30,700cc, or .0307 cubic metres.

Air weighs 1.2 kg/ cubic m, so 0.307 x 1200 grammes = 36.8 grammes.

Carbon dioxide weighs 1.977 kg/ cubic metre, so 0.307 x 1977 grammes = 60.7 grammes.

60.7 - 36.8 = approx 24 grammes.

Therefore, filling tyres with carbon dioxide will increase each tyres weight by about 12 grammes, or about half an ounce.

duh - I forget the compression thing oh dear !

 

[MikeG]

Cross section 25mm gives 9.8 cm2?
1.25 x 1.25 x 3.14

Ooops. Don't know what I did there.

Correction factor applied to the calc.......divide the answer by exactly 2

In other words, the volume is halved, and the weight of gas is halved.....and thus the total saving in weight through using air rather than CO2 is 6 grammes per tyre, or 12 grammes per bike.

Sorry.

 

[compo]

OK, so tyres filled with CO2 are slightly heavier than when filled with air, though by a miniscule amount.
Thanks all.

 

[Archie_tect]

...and the drag resistance within the tyre of the gas on the inner tube reducing the turning force.