Recommended Tyre Pressure (in Sri Lanka)

[Sifaan]

Hi!

(My first question on AL )

There are websites that give recommended tyre pressures for various vehicles.

Can these numbers be used as-is in Sri Lanka, or should they be adjusted for the climate here?

e.g. the Agent says they usually advise 24 (front) / 26 (rear) but the website suggested 34!

even for the previous car the recommended pressure we found on some websites were higher than what was recommended here (but I don't recall the numbers now)

Thanks

/Sifaan

[SeanD]

I understand tire pressure differences due to temperature change. However, if you need 33 psi while you're running the vehicle, shouldn't they be pumping 33psi? Simply because tire pressure is what we're particular at, not the volume of air inside.

No, because my pumping place is more than 1km away and by the time I've got there, the heat generated by the tire means that whatever reading is shown at the station is inaccurate. However, I usually check the cold pressure at home before I leave so I know exactly what it should be. To clarify:

I need 33. I check the pressure at home with my digital gauge, it reads 28.

After my 1.5km run to the petrol shed on hot surface roads, the digital gauge in the shed's pumping machine now reads 30. However, that is because the air inside has expanded due to heat, and is not the real pressure. If had had them inflate to 33 based on that reading, I would really be only achieving a cold pressure of about 30-31 when the tires have cooled (negates the purpose of filling them up in the first place, since I'm still underinflated when the tires are cold). So I add the difference between shed pressure (30) and my cold pressure (28) at home to the manufacturer recommended 33 psi (which is the rating for cold tires), and that = 35psi.

After I come home and the tires have cooled down, the pressure is now where it should be = 33psi.

Incidentally, my cheapo digital gauge and the digital gauges at petrol sheds in SL give the same reading. I don't think they need to be calibrated, unlike the old analog ones that some sheds still have which are probably wrong.

or else you want it to be at 33psi when you keep the vehicle parked for some reason ?

I want it to be 33psi when the tires are cold, because the manufacturers spec is for cold tires, otherwise it would make no sense as the pressure inside constantly fluctuates due to driving and road conditions. I've seen my tires at somewhere around 38psi after long trips sometimes, but that's due to the expanded hot air inside them. If I were to panick and deflate the tires down to 33 thinking that my tires are overinflated at that point, I'd have the rude shock of seeing my tires are actually around 28psi the next morning when they are cold.

Edited August 18, 2010 by SeanD

[Sifaan]

I've seen my tires at somewhere around 38psi after long trips sometimes, but that's due to the expanded hot air inside them. If I were to panick and deflate the tires down to 33 thinking that my tires are overinflated at that point, I'd have the rude shock of seeing my tires are actually around 28psi the next morning when they are cold.

I think I read somewhere that you should never reduce pressure by letting air out of a hot tyre. I don't remember the website giving a reason... any idea why you shouldn't do that?

[harshansenadhir]

No, because me pumping place is more than 1km away and by the time I've got there, the heat generated by the tire means that whatever reading is shown at the station is inaccurate. However, I usually check the cold pressure at home before I leave so I know exactly what it should be. To clarify:

I need 33. I check the pressure at home with my digital gauge, it reads 28.

After my nearly run to the petrol shed on hot surface roads, the digital gauge in the shed's pumping machine now reads 30. However, that is because the air inside has expanded due to heat, and is not the real pressure. If had had them inflate to 33 based on that reading, I would really be only achieving a cold pressure of about 30-31 when the tires have cooled (negates the purpose of filling them up in the first place, since I'm still underinflated when the tires are cold). So I add the difference between shed pressure (30) and my cold pressure (28) at home to the manufacturer recommended 33 psi (which is the rating for cold tires), and that = 35psi.

After I come home and the tires have cooled down, the pressure is now where it should be = 33psi.

Incidentally, my cheapo digital gauge and the digital gauges at petrol sheds in SL give the same reading. I don't think they need to be calibrated, unlike the old analog ones that some sheds still have which are probably wrong.

I want it to be 33psi when the tires are cold, because the manufacturers spec is for cold tires, otherwise it would make no sense as the pressure inside constantly fluctuates due to driving and road conditions. I've seen my tires at somewhere around 38psi after long trips sometimes, but that's due to the expanded hot air inside them. If I were to panick and deflate the tires down to 33 thinking that my tires are overinflated at that point, I'd have the rude shock of seeing my tires are actually around 28psi the next morning when they are cold.

Great, now me convinced to do the same thanks for the detailed explanation.

[SeanD]

I think I read somewhere that you should never reduce pressure by letting air out of a hot tyre. I don't remember the website giving a reason... any idea why you shouldn't do that?

Well aside from the fact that your tires will be severely underinflated when the tire has cooled down, I'm not sure why. Perhaps it's because hot tires under very high pressure are more likely to burst if the sidewall has flaws when the air is suddenly released? Not sure about that, but you will be left with some underinflated tires for sure.

[Sifaan]

I think I'll just make sure to measure once a week with the gauge I ordered

small update... after using the tyre pressure gauge for a while, one thing I can say for sure is that it is only with the guage that I can detect underinflation upto 10% or so (i.e. when the guage shows 30 when I should have 33.5) - visually there's no bulge at the bottom, tyre feels solid if you kick it, etc.

so, for people relying on visual inspections to detect tyre underinflation, chances are you're running a little low on pressure!

[Pericles]

In keeping with the green theme prevalent these days...

http://gajitz.com/reinventing-the-wheel-airless-tires-are-tough-green/

[arunap55]

My experience is 28 for Rear and 30 for front is the tyre pressures maintaining in most vehicles in SL and this figure is used by almost all air filling stations in SL.

[Sifaan]

My experience is 28 for Rear and 30 for front is the tyre pressures maintaining in most vehicles in SL and this figure is used by almost all air filling stations in SL.

hmm... silly of the manufacturers to bother quoting recommended pressures then

[heavyD]

My experience is 28 for Rear and 30 for front is the tyre pressures maintaining in most vehicles in SL and this figure is used by almost all air filling stations in SL.

Wow, what a silly statement. I do hope your 'experience' is limited to air pressure and filling stations...

[MrCat]

This thread is awesome. Bought a car day before and checked tire pressure just a while ago. Tires are 215/45R17 and when I checked after driving for a while it was reading 34Psi. Manufacture recommends 30Psi both front and rear so reduced to 30 while cursing the dealer for over inflating.

Forgot all about “cold tire pressure” until I read this thread. Will have to re-inflate them all first thing tomorrow morning when they are cold.

Edited December 15, 2010 by MrCat

[Watchman]

This thread is awesome. Bought a car day before and checked tire pressure just a while ago. Tires are 215/45R17 and when I checked after driving for a while it was reading 34Psi. Manufacture recommends 30Psi both front and rear so reduced to 30 while cursing the dealer for over inflating.

Forgot all about “cold tire pressure” until I read this thread. Will have to re-inflate them all first thing tomorrow morning when they are cold.

i take it it was it was a brand new or recon car? I think its the normal practice for the shippers to over inflate the tyres before loading them onto the ship. This is just in case there's a leak, it'll save the hassle of having to change tyres in the middle of the unloading process. I remember ours was around 38 when we received delivery.

Might be the same for normal car sale guys who have a whole bunch of cars on their lots/yards which have been sitting there for weeks/months.

[MrCat]

i take it it was it was a brand new or recon car? I think its the normal practice for the shippers to over inflate the tyres before loading them onto the ship. This is just in case there's a leak, it'll save the hassle of having to change tyres in the middle of the unloading process. I remember ours was around 38 when we received delivery.

Might be the same for normal car sale guys who have a whole bunch of cars on their lots/yards which have been sitting there for weeks/months.

This was a used car that I bought from the dealer mate. They`ve done the 30k service before selling so im sure they must have balanced and rotated the tires then..