Pressure vs Temperature

[quixoticgeek]

I pump my tyres up in the nice warm of my flat, but when I get outside and start riding, they are always a bit squishier than I'd like.

Is there an easy way to calculate the pressure they will be outside at 5°C, if pumped up at 20°C ?

I aim for 5bar at 20°C.

J

[Feanor]

A a first approximation, I'd use the Combined Gas Law:

P₁V₁/T₁ = P₂V₂/T₂

Assuming a constant volume, we can ignore the V terms and just say:

P₁/T₁ = P₂/V₂

(Temperatures are Absolute)

[quixoticgeek]

A a first approximation, I'd use the Combined Gas Law:

P₁V₁/T₁ = P₂V₂/T₂

Assuming a constant volume, we can ignore the V terms and just say:

P₁/T₁ = P₂/V₂

(Temperatures are Absolute)

By absolute, do you mean in Kelvin?

If so, that would be

(5/293)*278 = 4.74. If it's 5°C out.

So not a massive drop. Certainly not enough to be noticeable when riding.

Thanks!

J

[Feanor]

By absolute, do you mean in Kelvin?

In SI units, P is in pascals, V is cubic meters and T in Kelvin.

We can ignore V in this case.
Since pressure in PSI is a straight linear multiplication from pascals, I'd reckon you can work with PSI on both sides of the equation.  It's just the constant value of PV/T will be different.

If so, that would be

(5/293)*278 = 4.74. If it's 5°C out.

So not a massive drop. Certainly not enough to be noticeable when riding.

I'm not going to mark your homework, but indeed in terms of absolute temperature the difference between 20c and 5C is pretty small so the corresponding pressure drop will be small.

[Kim]

And presumably the tyre compound becomes less flexible as the temperature drops, which will to some extent cancel out the effect of the drop in pressure...

[ElyDave]

You need pressure in absolute as well, I.e. 5 bar guage is 6 bar abs

[quixoticgeek]

You need pressure in absolute as well, I.e. 5 bar guage is 6 bar abs

So using pascals, and kelvin, and absolute, rather than gauge, I get:

(600000/298)*273   (using zero Celsius here)

= 549664.4295

which ignoring unnecessary precision from my irc bot.

Is a drop of 50.3 Kilo pascal or half a bar.

J

[BFC]

The air put in the tyre will be significantly warmer than ambient temperature whilst pumping up, amount depends on the pump used and the effort put in to some extent - unless using a CO2 inflater, that tends to be cooler.

The effect is more significant with a total inflation than a top up. The temperature of the pump should give a clue as to how much by, larger/heavier pumps are affected less.

CO2 is also slightly soluble in butyl, so that component of the air in the tyre will reduce over time as the CO2 migrates to the lower pressure.

Also water content of air is prone to a massive volume change when it transitions from warm vapour to cooler liquid.

The science is complex - most demonstrations/experiments go to great lengths to minimise/eliminate one or more of the variables.

In the real world you need to guess how much extra (or less) to put in the tyres to compensate for the method of inflation and the expected changes of environment.

[rogerzilla]

Going the other way, the heat of constant braking will certainly cause sufficient pressure rise to pop off a tyre that's near its maximum to start with (experimentally, some Schwalbes rated for 105psi will blow off at 115psi).

The rim can easily reach 50 deg C (i.e. hand hot, when you can't keep your finger on it) which is about a 10% rise in pressure, should it have time to soak through to the air in the tube. 

[salar55]

Going the other way, the heat of constant braking will certainly cause sufficient pressure rise to pop off a tyre that's near its maximum to start with (experimentally, some Schwalbes rated for 105psi will blow off at 115psi).

The rim can easily reach 50 deg C (i.e. hand hot, when you can't keep your finger on it) which is about a 10% rise in pressure, should it have time to soak through to the air in the tube.
[/quote

One Etape du tour was going to be a hot one, we had advice from Greg LeMond telling everyone to drop a few psi from the tyres. Glad he was next to us in one of the starting pens, would have been interesting if a tyre blew braking hard before going into a bend.

[quixoticgeek]

Is this as much of an issue if using disk brakes?

J

[Feanor]

Is this as much of an issue if using disk brakes?

J

No.

[salar55]

Is this as much of an issue if using disk brakes?

J

No.

Not a big issue with disk brakes but you are still putting stress into the tyre contact with the tarmac. Just need to look at all the car/lorry tyres that are destroyed lying at roadsides. More likely to scuff the tread off now we have powerful brakes that work in all weathers.

[rafletcher]

The other factor is that outside you're sat on the bike and progressing, and those loads will affect the "feel" of the tyres compared to inside. And volume can be significant if using, say 35mm tyres compared to 23mm tyres. 

[bobb]

As much fun as the number crunching is, if 5 bar inside is a bit too squishy outside, why not just pump up to just over 5 bar inside and everything should be good, right?!

[quixoticgeek]

As much fun as the number crunching is, if 5 bar inside is a bit too squishy outside, why not just pump up to just over 5 bar inside and everything should be good, right?!

Yes, except by applying the Maths™ I was trying to work out what pressure I should pump to inside, to be within bounds outside.

J

[hubner]

As much fun as the number crunching is, if 5 bar inside is a bit too squishy outside, why not just pump up to just over 5 bar inside and everything should be good, right?!

Exactly.

Or pump the tyres outside, and also get warmed slightly.

[quixoticgeek]

Exactly.

Or pump the tyres outside, and also get warmed slightly.

Take bike and pump outside, pump up tire, bring bike and pump back inside, up 7 floors, put pump away, take bike back out, go for a ride.

Sod that.

J

[hubner]

I use the pump I carry, either on the bike or in my bag.

[quixoticgeek]

I use the pump I carry, either on the bike or in my bag.

That's a lot of faff compared to about 5 or so puffs from my track pump. Feels a lot more complex than knowing "if it's about 5°C outside, pump tyres up an extra 0.5 bar"

J

[hubner]

I find using my frame pump is less faff than using a track pump but that's just me.

[quixoticgeek]

I find using my frame pump is less faff than using a track pump but that's just me.

I have never ever found a frame pump pleasant to use. Even the pump I carry on my bike is a mini track pump (topeak turbo morph g).

J

[BFC]

I use the pump I carry, either on the bike or in my bag.

That's a lot of faff compared to about 5 or so puffs from my track pump. Feels a lot more complex than knowing "if it's about 5°C outside, pump tyres up an extra 0.5 bar"

J

Do major fills then leave for "a while" for pressure/temp to stabilise before a gentle top up to an estimated target pressure based on the simple maths/science. The bigger or more vigorous the top up the greater the variables become.
Avoid putting high humidity air into the tyres if possible - don't do a total fill in the utility room with the fresh washing passing on it's moisture.

[Paul]

Exactly.

Or pump the tyres outside, and also get warmed slightly.

Take bike and pump outside, pump up tire, bring bike and pump back inside, up 7 floors, put pump away, take bike back out, go for a ride.

Sod that.

J

Aha! Another variable!

[Kim]