If you've been for a run on a dual carriageway they should certainly be 'warm'.

Even a few. Miles around town they should be 'warm' as well.

Yeah but you never know how 'warm' they are. Far Too many variables to just say add 3psi.

The warmer they get, the more expansion takes place, and unless you're super clever with too much time on your hands, and can get the temperature of the tyre and air inside, then calculate how much its expanded over normal temperature, you cant really say.

I cant do that. So i just do mine cold

Not having a go, just saying (:

It's not an exact science, and outside of motorsports doesn't need to be.

You'll be able to tell by touching the tyre if they're warm, even on a really cold day they'll be warm.

Seem to recall reading somewhere years ago tyres will be somewhere between 45-55C.

Most manufacturers quote adding 3psi if you've been driving for 10min or more at 30mph+ average speed to ensure your cold temp is right.

Have a go after your next drive and check your tyre temps warm and you should see the 3psi increase

& for Winter / Cold Weather they warn you on setting and adjusting tyre pressures in a Garage / Building. 

ambient temperatures obviously.

Not only is it not Rocket Science, it is just Common Sense often.

 

http://michelin.co.uk/tyres/learn-share/care-guide/cold-weather-tips

Edited 19 April, 201511 yr by goneoffSKi

If you're going to do it though, you might aswel do it properly, which IMO is when its cold, the most accurate i can get it.

If you're going to do it though, you might aswel do it properly, which IMO is when its cold, the most accurate i can get it.

As I say, try it.

I've done it for years, and cold the tyres are spot on cold pressures. No issues with tyres at all.

Contact the Supplier you got them from, for them to contact their Supplier to have a Manufacturers Rep have the Tyres Inspected.

 

^^^^^ This.

 

Tyres come with a lifetime pro-rata warranty against manufacturers defects.   If the tyre is half worn you'll get a 50% discount off RRP (note this - not the street price) for a replacement tyre.   

 

Generally, if you only have 30% tread left there is no point making a claim due to the difference in RRP & street prices but it's worth asking.

On the "hot or cold" point, I once did a trip driving more assertively than usual. After about 12 miles I checked tyre heat manually, and the front tyres were noticeably warmer than the rears. On that basis I'd suggest that the only time you can be sure of getting all 4 at the same temperature is before you use the car for the first time that day, and then only if it's parked in the shade.

Okay, on the subject of tyre pressures, the fitter puts a new tyre on your wheel and then inflates it off the vehicle to your set pressure (for arguments sake let's say 2.5bar). Then fits it to your car and drops it back on the ground. What's the pressure in the tyre now with the added weight of the car pressing down on the tyre? Corner weight will be somewhere round about 400Kgs on a front wheel at a guess, so that extra weight compressing the tyre will affect the pressure.

 

So, do you check tyre pressures with no load on the wheel/tyre. or check the absolute pressure when the vehicle is loaded?

 

 

So, do you check tyre pressures with no load on the wheel/tyre. or check the absolute pressure when the vehicle is loaded?

 

 

although I haven't read this I'd find it implausible that a manufacturer would require you to jack up your car before inflating tyres.

 

So pressure will be as on the ground at ambient temperatures i.e. approx 20c.

That's reasonable to assume, yes. So perhaps tyre fitters should check pressures once on the car on the ground rather than on the fitting machine as they always do.

Okay, on the subject of tyre pressures, the fitter puts a new tyre on your wheel and then inflates it off the vehicle to your set pressure (for arguments sake let's say 2.5bar). Then fits it to your car and drops it back on the ground. What's the pressure in the tyre now with the added weight of the car pressing down on the tyre? Corner weight will be somewhere round about 400Kgs on a front wheel at a guess, so that extra weight compressing the tyre will affect the pressure.

 

So, do you check tyre pressures with no load on the wheel/tyre. or check the absolute pressure when the vehicle is loaded?

 

 

although I haven't read this I'd find it implausible that a manufacturer would require you to jack up your car before inflating tyres.

 

So pressure will be as on the ground at ambient temperatures i.e. approx 20c.

The question lurking here is "does the volume of the tyre change when load is placed on it?" The answer is "NO!"

The volume of air isn't relevant. In the same way, if you squeeze a balloon full of air, the pressure inside increases despite no increase in volume.

Boyle's Law.

Different from squeezing a Boil though, that is a right pussie mess.

 

 

 

Barometric Pressure,

thought of the day.

That's reasonable to assume, yes. So perhaps tyre fitters should check pressures once on the car on the ground rather than on the fitting machine as they always do.

I know Costco do, but most just put any old pressure in from experience

I find working beside tyre fitters that Most put the pressures high, have the beads in the rim, then let the pressure back down to much the same pressure depending on what type of tyre / vehicle, on a car this will Often be 35 psi.

So not just any old pressure, but not necessarily the correct pressures, but then a Customer can always ask what they are at,

or say what they want them at.

The volume of air isn't relevant. In the same way, if you squeeze a balloon full of air, the pressure inside increases despite no increase in volume.

 

Mate if you squeeze a balloon the pressure increases because of a decrease in volume.

 

Unless you are reducing temperature at the same time - Gay-Lussac's law I think.

 

Been a long time since I did chemistry.

The volume of air isn't relevant. In the same way, if you squeeze a balloon full of air, the pressure inside increases despite no increase in volume.

Yes it is; if the mass of air remains constant (it does), and the volume inside the tyre remains constant (it does, but the proof of this is complex: For now consider that a balloon doesn't have fabric or woven steel reinforcement) then the only way the pressure can rise is by increasing the temperature of the air.

By compressing the tyre, you're decreasing the internal volume of the tyre, so you'd increase the pressure of the air inside.

By compressing the tyre, you're decreasing the internal volume of the tyre, so you'd increase the pressure of the air inside.

 

It deforms but does the volume change?

 

I don't know the answer just posing the question

Just been puzzling this myself. So had a quick youtube of someone else who also had the same puzzling thought.

 

https://www.youtube.com/watch?v=wOHSJ7zUBIA

 

Seems it might or might not increase, depending on the accuracy of his gauge.

Just been puzzling this myself. So had a quick youtube of someone else who also had the same puzzling thought.

 

https://www.youtube.com/watch?v=wOHSJ7zUBIA

 

Seems it might or might not increase, depending on the accuracy of his gauge.

So effectively the answer is no, it doesnt increase.

Okay, on the subject of tyre pressures, the fitter puts a new tyre on your wheel and then inflates it off the vehicle to your set pressure (for arguments sake let's say 2.5bar). Then fits it to your car and drops it back on the ground. What's the pressure in the tyre now with the added weight of the car pressing down on the tyre? Corner weight will be somewhere round about 400Kgs on a front wheel at a guess, so that extra weight compressing the tyre will affect the pressure.

 

So, do you check tyre pressures with no load on the wheel/tyre. or check the absolute pressure when the vehicle is loaded?

The change in volume of the tyre flattening out under load is minimal in relation to the total volume of the tyre, therefore the change in air pressure is so minimal as to be insignificant compared to the pressure changes experienced when the tyre heats up.  For a bicycle tyre it's a change of 1/1,000,000 of the actual pressure

 

These aren't my maths but:

with a 23mm tyre you probably have about 20mm diameter of air inside. A 700c wheel is ISO 622, so has an inner radius of 311mm. So your tube forms a torus with major radius 321 and minor radius 10mm. A litre is a cubic decimetre (a cube 1/10th of a metre or 10 cm on a side) so it's easier to use dm for this. So, major radius = 3.21dm, minor = 0.1dm.

Now, at 100psi with a 200lb rider the flattened area is 2 square inches, or 1290 mm². So we want the volume of a toroidal section having that flat area. Conveniently the section is an ellipse, so the area is π (major radius)(minor radius) (and a circle is πr² because both radii are the same). As a first approximation let's say that the minor axis is half the width of the tyre (so 10mm) and see what happens:

Volume of a toroid = 2 π² minor² major = 2 π² 0.1² 3.21 = 0.63 litres

That seems a little on the short side, but it's vaguely plausible, so what's the volume? Unfortunately the volume of a toroidal section is a bit beyond my rusty calculus, so I'm going to cheat. A lot.

a = π R r => R = a / π r = 1290 / 10π = 41mm

First, what arc does this represent?

That's about 3.5° each side of the centre of the flat spot.

sine θ = 41/321 => θ = 0.0004 radians (about 7°)

How deep is it? Since sine X = x for small x, and we definitely have small x, if we go out 20.5mm from the centre of the flat spot we've got a right triangle 20.5mm on the long side. The short side is that times sine 3.5° as above:

As you can see that's a pretty flat shape, so even a crude pyramidal approximation is not going to matter too much (if it's out by a factor of 5 or 10 it's not really going to matter, it's still going to be "roughly zero" as a fraction of the total volume above).

depth = 20.5 * (20.5 / 321) = 1/321 = 0.003mm

          = 0.00003dm (since we're using dm for volume calculations)

Conveniently we can express the volume of a pyramid as a factor of the area of the base:

The area from above was 1290 mm², which we need to divide by 100² to get it in dm² for the volume calculations = 0.129 dm²

volume of a pyramid = 1/3 height * area of base

What fraction of the original volume is this?

volume lost = 1/3 * 0.00003 * 0.129 = 1.34e^-6 litres

See, even if that's out by a factor of ten, we're still looking at a volume change of 10^-5. So the pressure change will be about the same, a change of 1/1,000,000 the actual pressure.

fractional change = lost volume / total volume = 1.34e^-6 / 0.63 = 2.12e^-6

 

 

This thread has got far too technical. Lol

On the "hot or cold" point, I once did a trip driving more assertively than usual. After about 12 miles I checked tyre heat manually, and the front tyres were noticeably warmer than the rears. On that basis I'd suggest that the only time you can be sure of getting all 4 at the same temperature is before you use the car for the first time that day, and then only if it's parked in the shade.

Yeah. My bike tyres get toastey! Haha.

Thats a thought actually. I'll ask my mate how much his GTR tyres go up when its had a run, as you can watch each tyres pressure on the display, and i know he does keep an eye on that