Yesterday we we were down to 2 degrees and coincidentally I had to take my dad for a hospital appt.
On way to his, my BMW tyre monitoring system told me to check my pressures. So after I dropped him at hospital I got my compressor out and sure enough, where tyres should be 35/32 they were nearer 31/28.
Strange that all 4 were so far out.
So pumped them all up and reset the monitor.
Had to take him back this morning for follow up visit and it was 1 degree this morning.
I put on my tyre monitor thingy and my pressures were all away down again, down to 31/27.
But, on way home there, although outside is still at 3 degrees I have driven about 12 miles and the tyres were now almost touching 35/32 again!
I know they always say check pressures when cold but that is some difference, I didn’t expect to see that variance. But either way, I am guessing that it did mean that my tyres were all soft although going by this mornings pressures I should be pumping them all up again!
!
Going up North for a week tomorrow so might give them another check this afternoon at the petrol station when filling up with diesel.
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Tyre pressures change by 1 PSI for approx 4C change in temperature.
So a reduction from (say) 12C to 2C would account for a pressure change of 3/4 PSI.
Most handbooks say check tyre pressures when cold. Driving creates tyre movement, friction creates heat. A temperature increase of 20-25C after driving a few miles from cold is likely.
Because of the thermal mass of the tyre and wheel, the tyres do not immediately cool down - it may take 30 minutes.
I have always thought it more important for the tyre pressures across axles to be consistent even if the pressure varies by 1/2/3 PSI from what is recommended.
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The pressure change is by temperature is less than 1 per 4 deg..
To calculate you need degrees Kelvin and absolute pressure (e.g. 30psi is actually 30psi above atmospheric, so about 44.5psi altogether). Then the pressure change is proportionate to the temperature change.
It's about 0.15psi per degree C/K in the relevant range, so 0.6psi per 4 degrees C. If the pressures were set to 30 psi at 20C then at 0C I'd expect a reading of 27psi.
I can't explain why you've had to reset yours twice but if one adjustment was with warm tyres then it will have messed things up.
E&EO. I'm not a real physicist.
My TPM went off last week when the temperature first dropped. They were about 5psi low, probably a combination of the temperature change and the fact that I hadn't checked them for months.
Last edited by: Manatee on Thu 4 Nov 21 at 11:14
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>> I'm not a real physicist.
For a minute I thought Number Cruncher had returned ;)
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>> I know they always say check pressures when cold but that is some difference, I didn't expect to see that variance.
Isn't this one reason why they suggest inflating the tyres with nitrogen gas?
eg
www.continental-tyres.co.uk/car/all-about-tyres/tyre-care-maintenance/tyre-pressure/nitrogen-in-tyres
Some people however, might just class it as snake oil.
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...I use 78% Nitrogen in mine....
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>>Isn't this one reason why they suggest inflating the tyres with nitrogen gas?
Nitrogen, oxygen both follow the same gas laws, in my calculation I am assuming constant volume which will be some sort of approximation but probably not much.
I suppose you're thinking oxygen might leak out faster than nitrogen. I don't think the difference, if there is one, matters. Certainly not for any critical application where there are regular checks or TPM, and where 'ambient' air is 78% N to start with. Unless you flush the tyres when inflating with N then there will still be around 7% O in there anyway.
I think it's probably to do with using dry gas for inflation rather than ambient air compressed, with unknown water content..
Last edited by: Manatee on Thu 4 Nov 21 at 11:25
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Nitrogen is used on inflation of aircraft tyres, it's to reduce corrosion primarily and has a secondary of reduced Oxygen content in cases of fire hot brakes and/or if there's a rapid decompression of the tyre through the plug*.
Probably a marketing thing on cars to get an extra couple of quid out of customers, it won't do any harm, but it's of no real benefit on cars. No harm either, I wouldn't care if it was free, but wouldn't pay extra.
* The exact name of which escapes me right now.
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>>> * The exact name of which escapes me right now.
Schrader valve?
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>> >>> * The exact name of which escapes me right now.
>>
>> Schrader valve?
Aircraft have some sort of fusible plug in the wheel that allows relatively controlled* decompression of a tyre that has seriously overheated.
*Controlled as to what happens if the tyre explodes.
Explanation here:
www.airliners.net/forum/viewtopic.php?t=771699
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>> >> >>> * The exact name of which escapes me right now.
>> >>
>> >> Schrader valve?
>>
>> Aircraft have some sort of fusible plug in the wheel that allows relatively controlled* decompression
>> of a tyre that has seriously overheated.
Indeed, its for when the wheel is retracted in the wheel well, where a tyre explosion would be terminally serious,
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>> Indeed, its for when the wheel is retracted in the wheel well, where a tyre
>> explosion would be terminally serious,
Most reports I've seen refer to them melting under heavy braking - rejected t/o or overweight landings for example. However (qv Concorde at Paris) a burst with the wheels down is still bad news.
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Fusible plug, that's the word I was looking for.
Normally made partially from a wax material to operate at different temps, also colour coded to help differentiate between the different temperature versions.
Better to have a flat tyre during braking than one in bits behind you.
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I have tweaked by Beemer to display the tyre temp as well as pressure. I never reset the monitor, it just displays the new pressures.
Its fun to watch them rise on the dash when you pump them up.
And no, before you all start shouting, it does NOT work on speed sensors*. My mate with a Merc insisted it did, and is now 50 quid lighter after betting me - then sitting in the car watching the pressure go down as I let some out, and then up again when I pumped it up.
*on my car.
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The biggest differential between ambient and tyre temperature I ever managed in my last BMW was 23 degrees. In the Merc I haven’t got near that…different tyres and a more relaxed drive I guess. The temperature of the tyres rises by 10 degrees pretty quickly though, even when driving sedately - it’s a heavy car... But if the car’s capable of showing individual tyre pressures my understanding is it needs the sensors in the valves; the abs sensor version just measures relative differences in rolling radius doesn’t it, and bongs if the difference gets too big?
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Cold tyre pressures - Terry
