Ideal temperature to adjust tyre pressures

[Rich]

Good explanation

 

Yes there will be differences in the pressures when driving but I like to make sure I have them all the same when inflating from cold.

 

What if I had dropped the pressures on the warmer side so I got 31psi but left them on the cooler side. That would probably upset the handling as well as risk having uneven wear on that side, I bet alot of people do it without realising.

[NickT]

You're assuming an increase in pressie with temp on a fixed volume. The tyre is flexible and expands due to the expansion of air so this is an extra variable you need to consider in Boyles Law

[Sagitar]

 The tyre is flexible and expands due to the expansion of air

Yep - already said that . . . .

[Tony]

Another point to note is the actual tyre contact patch and radiated heat..... 

[Rich]

You're assuming an increase in pressie with temp on a fixed volume. The tyre is flexible and expands due to the expansion of air so this is an extra variable you need to consider in Boyles Law

 

No I know it's not a fixed volume but the tyre pressure in a cooler ambient is a good starting point for checking them IMO. If I was to check them on a hot summers day when they've been basking in the sun they would be quite high and if I adjusted them them accordingly the difference between each tyre may be alot greater.

 

What I've never got is when people check tyre pressures at petrol stations when they've been driving the car. If they've done a few miles the air would have expanded so if they drop them they could potentially be under-inflating the tyre when it's cooler.

 

There are alot of variables though but I'll stick to checking them when the car is in the shade.

[chrisgixer]

If using Nitrogen, as I have in the front, having picked up two cheap sc3's from costco...

 

Is there a standard increase applied over and above the standard "air" pressure?

 

Because, if using air, the tyre pressure increases as temp rises with use. So, if for track use for instance, (on bikes ime) you'd start with a much lower psi to achieve a similar operating temp pressure. Say 26 or 27 psi for a target operating temp of 36psi I think it was. Sorry I don't have a memory for numbers.

 

But if I had Nitrogen instead, the psi wouldn't increase, if advertising is to be believed, (must check my pressures more regularly or I'd know this) so with Nitrogen I'd have to start with a much higher psi nearer the target operating temp/pressure.

But I've not seen mention of this from tyre companies offering Nitrogen. So what am I missing..?

 

I'd expect to be told to add a couple of psi for Nitrogen while maintaining pressures for road use, and, say, add 5 or 6 psi for track use for example, over and above a cold pressure if using air. Because Nitrogen doesn't increase psi with temperature. Right?

 

I realise I don't have facility to add Nitrogen if topping up pressures for a slight leak or for heavy loads, so its a bit of a moot point, but if going to the lengths of using Nitrogen, I'd want to be accurate about it. Tbh.

 

 

Having said all that, there does appear to be other reasons for using Nitrogen. Reading this, I really must drain my garage compressor of water.

http://www.popularmechanics.com/cars/how-to/repair-questions/4302788

[liner33]

Its the water vapour that's the biggest factory rather than the oxygen , drag racers found dry air was much more stable than compressed air so you find many drag racers using a divers bottle to inflate their tyres, but their pressures need to be accurate to 1/4 of a psi , one of the guys i know runs his rear tyre at 5.25psi even a minor fluctuation is a major percentage increase at that kind of pressure.

 

But I still think there is too much over thinking going on here , for a normal road car and a normal road tyre a weekly check is fine , preferably in the morning before driving

[Sagitar]

 Because Nitrogen doesn't increase psi with temperature. Right?

 

No, for the umpteenth time, wrong.

 

Gay-Lussac's law says: At constant volume, the pressure exhibited by a sample of gas is proportional to the temperature of the gas on the absolute scale.

 

It doesn't matter which gas, if you raise the temperature, you also raise the pressure.

 

The temperature of a gas is a measure of the velocity with which its molecules dash about. Raise the temperature and they go faster.

 

The pressure of a gas is a measure of the energy with which the gas molecules impact on the container which restrains the gas. If they are moving faster they hit the container harder i.e. there is more pressure.

 

As Gay-Lussac says, at constant volume, you cannot have more temperature without more pressure.

[parthiban]

Wonder where the myth of nitrogen = consistent pressure came from?

 

So essentially it appears there are 3 benefits to using nitrogen in tyres:

 

1. It's inert

2. It's dry

3. Doesn't escape from tyres as easily

 

Number 3 is probably the most relevant to road use, particularly for those who don't check their tyre pressures enough (like me!), so seems like something worth doing if the cost is minimal, otherwise if you keep an eye on your pressures regularly you may as well stick with regular air

[Tony]

Where does the extra mass come from? I can understand excited molecules would effect the membrane but not the mass so in the absence of expansion due to water heat soak where does the actual internal pressure evolve from.  

[Sagitar]

Where does the extra mass come from? I can understand excited molecules would effect the membrane but not the mass so in the absence of expansion due to water heat soak where does the actual internal pressure evolve from.  

 

Tony - I'm not sure whether your question was addressed to me.  If it was then I don't really understand it.

The process that I described involves only temperature and pressure - there is no change of mass.

The volume of gas in the tyre is constant so no change of mass there. The mass of individual molecules of gas does not change so no change of mass there.

The change in pressure arises purely from the change of energy state created by the change of temperature.

The gas system inside the tyre is seeking always to achieve a state of equilibrium.

The molecules fly about freely at a velocity which is dictated by the temperature (it's a bit more complicated than that, because molecules have rotational and vibration energy as well as linear kinetic energy, but it's near enough for our analogy). When they hit the restraining vessel (the tyre or the wheel) there is some conservation of momentum, but there is also some loss of energy which causes heat to pass from the gas to the vessel so the gas temperature tends to fall until a state of equilibrium is achieved and the heat flowing into the gas (from the working of the tyre carcass, the external ambient temperature etc) is balanced by the heat flowing out from the gas as its molecules impact the restraining vessel.

[Tony]

I do love your descriptions, they are almost visual. In your opinion what's your take on Nitrogen for domestic car tyres, worth it or not? We know in aircraft it's mandatory and the likes of F1 use it albeit a more corrosive form but your very well educated on the topic and not bias so i would like to know.

[liner33]

You are presuming the reasons for filling and aircraft tyre with nitrogen are the same as filling a car tyre with it , they are not

 

I would also suggest that a F1 tyre bears little relation to road car tyres both in construction and operating environment

[Sagitar]

what's your take on Nitrogen for domestic car tyres, worth it or not?

If someone offered to fill or top-up my tyres with nitrogen I wouldn't say no, but I wouldn't pay for it, neither would I go out of my way to get it. Bluntly, for domestic motoring, it's an irrelevance.

 

But you and I both know that motoring markets are not driven only by proven science. The "go-faster-stripes" market is sizeable and for me, nitrogen in domestic vehicles has become part of it.

 

I don't blame any organisation for offering a nitrogen fill for domestic vehicles when there is a demand for it. It does no harm and its availability may give a useful commercial edge. I do however find quite ridiculous the claims for improved performance that are made by some suppliers.

 

There is plenty of hard data that debunks the improved performance claims and you might like to start with the Skeptics Stack Exchange site.

[Tony]

Fair points given..... As said the true benefits may be marginal but even if there's a 1% gain then it's a plus. I must say that we don't charge for it and never have so for the customers there's another plus.

[parthiban]

As I mentioned earlier it looks like the only real benefit is that it takes longer to leave your tyres, so there is a benefit for the general public who don't check their tyres often.

 

Still wondering where the myth came from that pure nitrogen maintains more consistent pressures - it seems like even Tony was under that impression?

[Tony]

True racing wheels have two valves so that the nitrogen is 100% rather than the % we apply +1 atmosphere of existing moist air. It's all very subjective to me when you factor in the likes of aircraft, F1 and even domestic cars like the Nissan GTR/ Vayron have to use nitrogen in various forms.  

[parthiban]

True racing wheels have two valves so that the nitrogen is 100% rather than the % we apply +1 atmosphere of existing moist air. It's all very subjective to me when you factor in the likes of aircraft, F1 and even domestic cars like the Nissan GTR/ Vayron have to use nitrogen in various forms.  

 

Those industries appear to use nitrogen because it is inert rather than for consistent pressure.

[Tony]

Isn't that the same thing?

[parthiban]

Isn't that the same thing?

 

No, inert simply means the nitrogen won't react with other things (whatever that might be). Not that its pressure won't increase with temp.

[Sagitar]

As I mentioned earlier it looks like the only real benefit is that it takes longer to leave your tyres, so there is a benefit for the general public who don't check their tyres often.

 

Still wondering where the myth came from that pure nitrogen maintains more consistent pressures - it seems like even Tony was under that impression?

 

The lower migration rate claimed is based upon the fact that nitrogen molecules are bigger than oxygen molecules. The difference in the rate is however wildly exaggerated by nitrogen proponents. It's difficult to find much hard evidence of the actual rate, but I've seen the results of a few experiments, some more scientific than others, that indicate very small differences.

 

The tests carried out by Consumer Reports in the U.S. are probably the best controlled and the most quoted. They tested 60 car tyres, 30 filled with air and 30 filled with 95% Nitrogen. (It's difficult to get 100% Nitrogen - generators leave some oxygen behind and filling a tyre through a single valve adds to the problem.) They stored all the tyres under controlled conditions for just over 12 months.  The air filled tyres lost an average of 3.5PSI and the Nitrogen filled tyres lost an average of only 2.2PSI. This difference is 1.3PSI over a year and I cannot see any way that such a difference could justify a lower frequency of checking tyre pressures.

 

I suspect that the myth of more consistent pressure starts from comparison with tyres heavily contaminated with water vapour and is fostered by the extensive marketing effort that has been made to sell nitrogen in the U.S.

 

Tiff Needell did a piece for Fifth Gear that actually showed the pressure in nitrogen filled tyres increasing by more than the pressure in air filled tyres, but I think he put that down to the nitrogen fill being contaminated by water vapour. I suspect the video must be around somewhere. I'll have a look for it.

[Sagitar]

Here we go,

[liner33]

I think its largely having something else to sell the public

 

I can see the logic for the aircraft industry and I can see the logic for F1 or any motorsport where control and consistency are so important, however I am still not convinced for road users

 

I also dont like the idea that nitrogen filled tyres dont need to be checked as often which could be the "sell" here , frequent tyre pressure checks and inspections are vitally important

[Tony]

I feel disappointed

[parthiban]

Hmmm, so then the argument about consistency goes out of the window as well.

 

So therefore the only benefit of nitrogen is that it is inert, which has no real relevance to road use making it a bit pointless then!