the weight of air in our tyres

Discuss light weight issues concerning road bikes & parts.
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Delorre
Posts: 866
Joined: Sat May 24, 2014 12:09 pm

by Delorre

I couldn't believe air does add weight to the bike, so tested it out myself, and :shock: Bike with both real 25mm tyres at around 90psi vs complete flat tyres made a whopping difference of 20gr! Not that you could win something in real usage, but it's a trick if you are just a few gr above 6kg and really want a scale pic at sub 6kg f.ex :mrgreen:

by Weenie


stormur
Posts: 1156
Joined: Tue Aug 12, 2014 3:50 pm
Location: FIN

by stormur

pump with Helium ;)
Go to Heaven for the climate, Hell for the company.
Mark Twain


I can be wrong, and have plenty of examples for that ;)

DaReef
Posts: 158
Joined: Mon Dec 19, 2016 2:36 am

by DaReef

The "weight" of air is highly variable.

KCookie
Posts: 848
Joined: Thu Feb 12, 2015 9:40 am
Location: Pom living in Australia

by KCookie

I put new tyres on the other day and did the same out of curiosity, air added 10gr.

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MJB
Posts: 168
Joined: Thu Jan 19, 2012 11:17 am
Location: bottom edge of Australia

by MJB

Low tyre pressures = WW trick! :P

hambini
Posts: 114
Joined: Tue Nov 15, 2016 8:13 am

by hambini

Also to note is if you are at altitude and pump your tyres up to the same pressure as at sea level. The bike will "weigh" more at sea level than at altitude.

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stockae92
Posts: 255
Joined: Fri Sep 25, 2015 5:13 pm

by stockae92

This is true WW :D

itguy
Posts: 129
Joined: Tue Jan 20, 2015 3:43 pm

by itguy

So if you aren't too fat, you can run lower psi, more weight savings

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TeslaOsiris
Posts: 20
Joined: Thu Dec 29, 2016 5:16 pm
Location: Bentonville, Arkansas USA

by TeslaOsiris

It isn't so much a change in weight as it is a change in density... You are adding more air molecules into the relatively same volume of space.

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TheKaiser
Posts: 434
Joined: Thu Sep 05, 2013 3:29 pm

by TheKaiser

TeslaOsiris wrote:It isn't so much a change in weight as it is a change in density... You are adding more air molecules into the relatively same volume of space.


That is a good technical correction and reminder of the physics at work.

964Cup
Posts: 187
Joined: Tue Dec 18, 2012 1:31 am

by 964Cup

TheKaiser wrote:
TeslaOsiris wrote:It isn't so much a change in weight as it is a change in density... You are adding more air molecules into the relatively same volume of space.


That is a good technical correction and reminder of the physics at work.

No, it's tautologous gibberish. By definition a change in density for the same volume is a change in weight, assuming constant gravity.

TeslaOsiris
Posts: 20
Joined: Thu Dec 29, 2016 5:16 pm
Location: Bentonville, Arkansas USA

by TeslaOsiris

964Cup wrote:
TheKaiser wrote:
TeslaOsiris wrote:It isn't so much a change in weight as it is a change in density... You are adding more air molecules into the relatively same volume of space.


That is a good technical correction and reminder of the physics at work.

No, it's tautologous gibberish. By definition a change in density for the same volume is a change in weight, assuming constant gravity.

I think the word you are looking for is "mass", not weight ;) Density is a ratio of mass:volume... Weight is a measurement of gravitational attraction. But what does my tautologous gibberish know? (That's not even a tautology is, FYI)

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wingguy
Posts: 3507
Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

TeslaOsiris wrote:I think the word you are looking for is "mass", not weight ;) Density is a ratio of mass:volume... Weight is a measurement of gravitational attraction. But what does my tautologous gibberish know?

Apparently your tautological gibberish doesn't know that we all live on the surface of the earth, therefore when you increase the mass of a bike you increase the weight by exactly the same degree, always. The word 'weight' is equally as valid as the word mass. If there's anything worse than being that pedantic it's being that pedantic and wrong :roll:

This applies equally to your original point. Air has weight. Adding more air to tyres adds more weight. This statement is no less valid because the density of the air in the tyres has also increased. It's not a situation where one or the other is true. They are both equally true at the same time.

964Cup
Posts: 187
Joined: Tue Dec 18, 2012 1:31 am

by 964Cup

TeslaOsiris wrote:
964Cup wrote:
TheKaiser wrote:
TeslaOsiris wrote:It isn't so much a change in weight as it is a change in density... You are adding more air molecules into the relatively same volume of space.


That is a good technical correction and reminder of the physics at work.

No, it's tautologous gibberish. By definition a change in density for the same volume is a change in weight, assuming constant gravity.

I think the word you are looking for is "mass", not weight ;) Density is a ratio of mass:volume... Weight is a measurement of gravitational attraction. But what does my tautologous gibberish know? (That's not even a tautology is, FYI)


I wonder why I said "assuming constant gravity" in my original post? And it is a tautology, because you are saying the same thing twice. Adding more molecules into the relatively same volume of space [sic] is indeed a change in density which is precisely as much a change in weight. They are (for a constant volume in constant gravity) exactly the same thing. (Even more amusingly, and tautologically, our common measures of mass are also measures of weight at Earth-normal gravity.)

[/pedant mode]

by Weenie


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