The tire pressure number that is usually listed on the inside frame of the drivers side door is that pressure for hot or cold tires?
Hot or cold?
- Thread starter Camprunner
- Start date
- Status
Tire pressure is always set/adjusted/recorded/recommended COLD.
In accordance with the owner's manual. What does that say?
its always cold pressure.. as is the max inflation number on the tire sidewall.
FWIW usually hot pressure for normal driving is 2-4psi depending on vehicle/tires/conditions.
my jeep requires 36psi cold my tires start out at 35-36psi and will be 38-40psi after 30min or so.
FWIW usually hot pressure for normal driving is 2-4psi depending on vehicle/tires/conditions.
my jeep requires 36psi cold my tires start out at 35-36psi and will be 38-40psi after 30min or so.
Originally Posted By: kschachn
In accordance with the owner's manual. What does that say?
It is always when cold.
In accordance with the owner's manual. What does that say?
It is always when cold.
cold
What is cold, and what is hot? Shouldn't it be ambient? Tires neither hotter nor colder than ambient.
I don't know if I've had a tire that felt hot. Maybe slightly warmer than the pavement. Makes sense though that it would get warmer than ambient. I should measure my tire pressure before I leave work, then after I get off the highway. Now I'm curious just how much it changes.
I don't know if I've had a tire that felt hot. Maybe slightly warmer than the pavement. Makes sense though that it would get warmer than ambient. I should measure my tire pressure before I leave work, then after I get off the highway. Now I'm curious just how much it changes.
I set over night cold before driving. Ed
Originally Posted By: supton
What is cold, and what is hot? Shouldn't it be ambient? Tires neither hotter nor colder than ambient.
I don't know if I've had a tire that felt hot. Maybe slightly warmer than the pavement. Makes sense though that it would get warmer than ambient. I should measure my tire pressure before I leave work, then after I get off the highway. Now I'm curious just how much it changes.
I grew up in New Mexico, you can really feel hot tires there.
I usually want tire pressure to increase by about 10% running from cold to hot.
BSW
What is cold, and what is hot? Shouldn't it be ambient? Tires neither hotter nor colder than ambient.
I don't know if I've had a tire that felt hot. Maybe slightly warmer than the pavement. Makes sense though that it would get warmer than ambient. I should measure my tire pressure before I leave work, then after I get off the highway. Now I'm curious just how much it changes.
I grew up in New Mexico, you can really feel hot tires there.
I usually want tire pressure to increase by about 10% running from cold to hot.
BSW
Ambient Temperature
So I measured. Granted it was a cheapo pencil gauge, but I used the same one. Ambient was about 85F and quite sunny. Tires are RT43's in 195/70R14. It was 6 minutes slow speed driving, 31 minutes of highway (70-80mph, mostly 75) and one minute to my stop. First is after sitting all day, second is psi after driving.
LF 38 39
LR 34 35
RR 35 36
RF 37 39
I'm not sure how my tires got those pressures, I must not have adjusted pressure since spring--not sure how the two got are different. But anyhow, 1-2psi elevation, that does count for something. Usual rule of thumb is what, 1psi for every 10F temperature change?
I suppose if one wanted, if they had an IR thermometer they could check at hot and adjust based on tire temperature.
LF 38 39
LR 34 35
RR 35 36
RF 37 39
I'm not sure how my tires got those pressures, I must not have adjusted pressure since spring--not sure how the two got are different. But anyhow, 1-2psi elevation, that does count for something. Usual rule of thumb is what, 1psi for every 10F temperature change?
I suppose if one wanted, if they had an IR thermometer they could check at hot and adjust based on tire temperature.
Originally Posted By: supton
So I measured. Granted it was a cheapo pencil gauge, but I used the same one. Ambient was about 85F and quite sunny. Tires are RT43's in 195/70R14. It was 6 minutes slow speed driving, 31 minutes of highway (70-80mph, mostly 75) and one minute to my stop. First is after sitting all day, second is psi after driving.
LF 38 39
LR 34 35
RR 35 36
RF 37 39
I'm not sure how my tires got those pressures, I must not have adjusted pressure since spring--not sure how the two got are different. But anyhow, 1-2psi elevation, that does count for something. Usual rule of thumb is what, 1psi for every 10F temperature change?
I suppose if one wanted, if they had an IR thermometer they could check at hot and adjust based on tire temperature.
That is actually how we do it at the track. I bleed down my tires to 40 PSI hot, but that is a temp that no street car will ever see (over 200F) Then we let them sit overnight, and check the pressures in the morning, that gives us our cold tire pressures. Cold pressures will be different at every track, and are normally different pressures for every tire.
At a track like Mosport, starting pressures on my Civic were:
LF 32
RF 33
LR 34
RR 35
So I measured. Granted it was a cheapo pencil gauge, but I used the same one. Ambient was about 85F and quite sunny. Tires are RT43's in 195/70R14. It was 6 minutes slow speed driving, 31 minutes of highway (70-80mph, mostly 75) and one minute to my stop. First is after sitting all day, second is psi after driving.
LF 38 39
LR 34 35
RR 35 36
RF 37 39
I'm not sure how my tires got those pressures, I must not have adjusted pressure since spring--not sure how the two got are different. But anyhow, 1-2psi elevation, that does count for something. Usual rule of thumb is what, 1psi for every 10F temperature change?
I suppose if one wanted, if they had an IR thermometer they could check at hot and adjust based on tire temperature.
That is actually how we do it at the track. I bleed down my tires to 40 PSI hot, but that is a temp that no street car will ever see (over 200F) Then we let them sit overnight, and check the pressures in the morning, that gives us our cold tire pressures. Cold pressures will be different at every track, and are normally different pressures for every tire.
At a track like Mosport, starting pressures on my Civic were:
LF 32
RF 33
LR 34
RR 35
Last edited:
Originally Posted By: supton
...... But anyhow, 1-2psi elevation, that does count for something. .....
Yes it does. The rule of thumb is that you don't want more than a 10% pressure buildup. Below that level, the internal tire temps are within the design limitations of the rubber. Above that level, the temperatures are too high for the rubber to sustain integrity over the life of the tire. (Note: Rubber deteriorates over time regardless of temperature, but elevated temperatures accelerate the process. See Arrhenius's rule)
If you get more than 15%, STOP!! You have to do something IMMEDIATELY!!! - usually increasing the inflation pressure (or reducing the load), but driving more slowly also works.
...... But anyhow, 1-2psi elevation, that does count for something. .....
Yes it does. The rule of thumb is that you don't want more than a 10% pressure buildup. Below that level, the internal tire temps are within the design limitations of the rubber. Above that level, the temperatures are too high for the rubber to sustain integrity over the life of the tire. (Note: Rubber deteriorates over time regardless of temperature, but elevated temperatures accelerate the process. See Arrhenius's rule)
If you get more than 15%, STOP!! You have to do something IMMEDIATELY!!! - usually increasing the inflation pressure (or reducing the load), but driving more slowly also works.
It is also very important to make sure you are using DRY air to inflate tires.
Any moisture in the tire and the tire pressure will increase a LOT faster as the water in the tire boils (when above 100 C).
Using a small portable compressor will put moisture in the tire.
Any moisture in the tire and the tire pressure will increase a LOT faster as the water in the tire boils (when above 100 C).
Using a small portable compressor will put moisture in the tire.
ambient cold shifts also. That's why you check your tire pressure regularly, to take into account the season changes
Originally Posted By: mightymousetech
It is also very important to make sure you are using DRY air to inflate tires.
Any moisture in the tire and the tire pressure will increase a LOT faster as the water in the tire boils (when above 100 C).
Using a small portable compressor will put moisture in the tire.
Ah ..... Mmmmmm ..... Not exactly.
If the compressor is putting liquid water in the tire (or there is liquid water already in the tire), then that affects the pressure buildup - and it's not 100C either. Water at elevated pressures boils at a higher temp. In fact at the pressures used in tires, the water will not boil at all! (at least under street conditions! Racing is another story!)
Plus, water vapor acts like an ideal gas - just like air!
- AND - water vapor will eventually equalize with the ambient air. So even if you start with liquid water in a tire, it will eventually evaporate and the tire will act like dry air - and dry air in a tire eventually gets the same amount of water vapor as the outside air.
It is also very important to make sure you are using DRY air to inflate tires.
Any moisture in the tire and the tire pressure will increase a LOT faster as the water in the tire boils (when above 100 C).
Using a small portable compressor will put moisture in the tire.
Ah ..... Mmmmmm ..... Not exactly.
If the compressor is putting liquid water in the tire (or there is liquid water already in the tire), then that affects the pressure buildup - and it's not 100C either. Water at elevated pressures boils at a higher temp. In fact at the pressures used in tires, the water will not boil at all! (at least under street conditions! Racing is another story!)
Plus, water vapor acts like an ideal gas - just like air!
- AND - water vapor will eventually equalize with the ambient air. So even if you start with liquid water in a tire, it will eventually evaporate and the tire will act like dry air - and dry air in a tire eventually gets the same amount of water vapor as the outside air.
I think this ref explains it well: Why Nitrogen is Better Than Air - No Water Vapor
It’s all about what’s missing.
The biggest advantages cited about nitrogen-filled tires have to do with how nitrogen-filled tires maintain a much more consistent pressure, even as the tire heats up, than air-filled tires.
As a BMW representative explained to me:
The main advantage of using Nitrogen in tires is less pressure growth as the tire heats up.
And the reason that nitrogen suffers from less pressure growth is primarily due to one thing:
Moisture in the air in tire cause more pressure rise as the tires heat up. What is worse is that the pressure increase is somewhat unpredictable since depends on the humidity of the air added to the tires.
Water. Humidity. Clamminess. The real hero here isn’t nitrogen, it’s the lack of water vapor. About 1 percent of regular air is water vapor at sea level, and when that air is compressed to go into your tire, that concentration can increase as well.
Water vapor is much more susceptible to changes in pressure from thermal changes. It expands when hot, contracts when cold, and changes the tire’s PSI as it does, which can cause problems for precision track and racing cars with handling tuned to very specific tire pressures.
Tire temperatures will increase while driving, due to friction and rubber deformation, and these temperature increases are independent of ambient air temperature. Tires can increase in temperature by around 50° over 30 minutes of highway driving (and much more for racing tires), and if you look at a chart of the vapor pressure of water at varying temperatures, you can see that a temperature increase like that can cause dramatically higher pressures.
Let’s say a tire starts at 60°F and goes up to, say 90°F; in that case, the water vapor pressure more than doubles. That’s a big deal.
Also, water can freeze, which is why nitrogen is specified for aircraft (and at least a couple spacecraft, like the Space Shuttle orbiter and that Air Force X-37B mini-shuttle).
So, really, the advantage of nitrogen is that it’s very dry, unlike air from a compressor, which may have a good bit of water vapor in it. It’s possible to dry out regular air as well, but it’s not terribly efficient and no easier or cheaper than extracting nitrogen, which, because of the processes used to isolate it, will always be free of water vapor.
Aside from keeping a more consistent pressure, there’s other advantages to removing water vapor from your tire: the water vapor can cause rust and corrosion inside your valve stem or to your wheel itself, so why deal with that [censored]? There’s also the idea that oxygen is more reactive with the rubber of the tire, and replacing that with a more inert gas like nitrogen will help the tire life as well.
HTH
It’s all about what’s missing.
The biggest advantages cited about nitrogen-filled tires have to do with how nitrogen-filled tires maintain a much more consistent pressure, even as the tire heats up, than air-filled tires.
As a BMW representative explained to me:
The main advantage of using Nitrogen in tires is less pressure growth as the tire heats up.
And the reason that nitrogen suffers from less pressure growth is primarily due to one thing:
Moisture in the air in tire cause more pressure rise as the tires heat up. What is worse is that the pressure increase is somewhat unpredictable since depends on the humidity of the air added to the tires.
Water. Humidity. Clamminess. The real hero here isn’t nitrogen, it’s the lack of water vapor. About 1 percent of regular air is water vapor at sea level, and when that air is compressed to go into your tire, that concentration can increase as well.
Water vapor is much more susceptible to changes in pressure from thermal changes. It expands when hot, contracts when cold, and changes the tire’s PSI as it does, which can cause problems for precision track and racing cars with handling tuned to very specific tire pressures.
Tire temperatures will increase while driving, due to friction and rubber deformation, and these temperature increases are independent of ambient air temperature. Tires can increase in temperature by around 50° over 30 minutes of highway driving (and much more for racing tires), and if you look at a chart of the vapor pressure of water at varying temperatures, you can see that a temperature increase like that can cause dramatically higher pressures.
Let’s say a tire starts at 60°F and goes up to, say 90°F; in that case, the water vapor pressure more than doubles. That’s a big deal.
Also, water can freeze, which is why nitrogen is specified for aircraft (and at least a couple spacecraft, like the Space Shuttle orbiter and that Air Force X-37B mini-shuttle).
So, really, the advantage of nitrogen is that it’s very dry, unlike air from a compressor, which may have a good bit of water vapor in it. It’s possible to dry out regular air as well, but it’s not terribly efficient and no easier or cheaper than extracting nitrogen, which, because of the processes used to isolate it, will always be free of water vapor.
Aside from keeping a more consistent pressure, there’s other advantages to removing water vapor from your tire: the water vapor can cause rust and corrosion inside your valve stem or to your wheel itself, so why deal with that [censored]? There’s also the idea that oxygen is more reactive with the rubber of the tire, and replacing that with a more inert gas like nitrogen will help the tire life as well.
HTH
Thank you
Thank you very much
gaijenny
Thank you very much
gaijenny
The nitrogen machines at the tyre places aren't pure nitrogen, and they aren't the purity of bottled nitrogen.
Drying air with a desiccant regenerative air drier is pathetically simple compared to the "making" nitrogen at the tyre shop.
If the air is 1% water vapour and you compress it, it's still 1% water vapour. Any drop out in the receive makes it less than 1%...not more concentrated.
Nitrogen and oxygen change pressure with temperature also.
http://www.msduncanchem.com/Reference_Tables/water_vapor_pressure_chart.htm
for pressure changes of water vapour...yes, 15.5C it's 1.76kPa, 35C 5.63
If' it's genuinely at 1%, the example given (e.g. straight from an air shark into the tyre with no reciever like you'd find at a garage), the effect of this inside a tyre at 250kPa is 0.01*(5.63-1.76) = 0.0076kPa
(0.0011 psi in 36psi)
Drying air with a desiccant regenerative air drier is pathetically simple compared to the "making" nitrogen at the tyre shop.
If the air is 1% water vapour and you compress it, it's still 1% water vapour. Any drop out in the receive makes it less than 1%...not more concentrated.
Nitrogen and oxygen change pressure with temperature also.
http://www.msduncanchem.com/Reference_Tables/water_vapor_pressure_chart.htm
for pressure changes of water vapour...yes, 15.5C it's 1.76kPa, 35C 5.63
If' it's genuinely at 1%, the example given (e.g. straight from an air shark into the tyre with no reciever like you'd find at a garage), the effect of this inside a tyre at 250kPa is 0.01*(5.63-1.76) = 0.0076kPa
(0.0011 psi in 36psi)
- Status
