Why is it so hard to check your tyre pressures.
Discussion
playalistic said:
I have this, it's very good. And after borrowing mine, two of my neighbours now also have the same.
Raize said:
Can't be bothered. Didn't check them in 2 months, went to check them, they were still correct from last time after about 1500 miles.
Can I offer a precautionary tale for those who don't regularly check tyre pressures?I check mine religously. No idea if the tyre pressure gauge I use is accurate or calibrated, but it doesn't appear far wrong. I noticed that one tyre was losing 4-5psi over the space of a week or so, the rest maybe 1psi a month. If nothing else, it was a chore having to re-inflate it to the right pressure every time I went for a long drive. So when I got the car serviced I had them check the tyre was seated correctly.
Turns out there was a huge nail through the rear tyre, the end of which had been scoring away the inside of the tyre sidewall. I hate to think what might have happened if I'd left it, and the tyre failed at speed. So check your pressures, you never know what that slow leak might be!
Also, for anyone thinking it's hard to check tyre pressures - go speak to an Alpina owner for a different perspective.
RobM77 said:
doogz said:
RobM77 said:
Same here 
oh, and doogz: yes, I coverted from F to C
You did?oh, and doogz: yes, I coverted from F to C
How?
1C is 1.8F.
So a 20C difference is an 36F difference, yes?
So, how did you come to this 6PSI difference? As that's not how it reads to me.
Regardless of what your random graph says, pressure divided by absolute temperature, is a constant, assuming constant volume.
You can argue with physics if you want, but there's no point in arguing with me about it.
See Charles Law.
BTW 1C isn't 1.8F
Edited by Vipers on Thursday 8th August 16:32
Edited by Vipers on Thursday 8th August 16:34
Fair point, so a typical tyre pressure would be about 45 PSI... going from 27 to -3 Celsius would drop the pressure by 4.5 PSI instead of 3. So you'd go to 40.5; a drop from 30 to 25.5 relative pressure which your gauge reads.
Still not exactly life threatening through the likely temperature range you'd experience at ambient in this country, but slightly more significant.
Still not exactly life threatening through the likely temperature range you'd experience at ambient in this country, but slightly more significant.
Vipers said:
If you want to work in absolute temperature, you should also work in absolute pressure. If your working in bars, just add 1 bar, after the calculation, subtract 1 bar to get gauge pressure.
See Charles Law.
There's a discussion here on the polytropic expansion of a real gas(...lots of assumptions) but you're all ignoring the fact that this set the tyre pressure based on ambient temp is rubbish. You are aiming for a hot tyre pressure (that's why you'll see people running around with guages in pitstops for formulae without tyre warmers)See Charles Law.
Edited by Vipers on Thursday 8th August 16:32
The hot temp of the tyre really won't change much with ambient temp. If you really want to do it properly then set to correct cold pressure at the average yearly temperature in the country of origin of you car. Drive like a loon for 20 mins at least. Measure pressure. In the future drive like a loon for 20 mins before checking to this new pressure.
No I thought not, so check pressures when the tyres are anywhere between around 5-25 celcius as a deeply unscientific guess and have done with it. That's enough to keep you safe, if your car is so sensitive to pressures that you can feel the difference then find a hot pressure that feels right and check on that, it'd be far more consistent... but I don't want to meet you if you can be bothered with all that
Thankyou4calling said:
Front drivers side looked a bit low so drove to the filling station, airline out of order, station two, no airline, station three, success, BUT, im behind a bloke carrying out a 12000 mile service, he does tyres, radiator, washers, oil cleans screen then goes to vacuum the interior! I've driven seven miles so I'm sticking with it. Finally get my turn, it's 20p and I don't have change, so I have to buy a mars bar to get change. Remove all the valve caps, set the meter to 32 and off I go. Does the airline reach all the tyres? Barely, can I see if the pressures are down? Nope, can't see the readout so I listen for the bell which you can barely hear. Then my 2 minutes of air runs out before I've done all four tyres omitting the suspect front! Can't be bothered to get another 20p so replace all valve caps but, as is the way, one is missing (nice alloy caps too) I just need to get out of this place. Head home and I've driven 14 miles and its taken an hour. Is it just me who hates this job?
Left the 'Ring yesterday. Went to Ed's Tankstelle before the drive home to replace the air I'd removed over the course of the trackday. 4 air lines, 2 digital, all free to use. It wasn't that long ago that airlines were free in all UK petrol stations. Then suddenly a few started charging, now they all want about 50p to a £1 for a few cu cms of air. Thanks.
doogz said:
Vipers said:
BTW 1C isn't 1.8F
I'm not talking about temperature. I'm talking about temperature difference. 1.8006 IIRC.Edited by Vipers on Thursday 8th August 16:32
Edited by Vipers on Thursday 8th August 16:34
Do you disagree?
doogz said:
Vipers said:
BTW 1C isn't 1.8F
I'm not talking about temperature. I'm talking about temperature difference. 1.8006 IIRC.Edited by Vipers on Thursday 8th August 16:32
Edited by Vipers on Thursday 8th August 16:34
Do you disagree?
Am I on the right track.
doogz said:
Vipers said:
Had a rethink, do you mean an increase of 1C is an increase of 1F, this is true at around an increase of one degree At 140 C, but is miles out for an increase of 1C at 26C.
Am I on the right track.
No.Am I on the right track.
I don't know how to explain it any better. I already told you the answer was 1.8. You told me I was wrong, then that you didn't understand, and now that you think the answer is 1.
If the temperature increases by one degree celcius, it increases by 1.8 degrees fahrenheit. It doesn't matter whether the starting temperature was freezing point, boiling point, or anywhere inbetween.
Thanks for your patience.
doogz said:
Buckling. Every engineers least favourite mode of failure?
Laminate peeling?Engineer1 said:
If we are messing around with the ideal gas laws PV=nRt n and r should be constant meaning a change in pressure should also see a change in volume assuming it is solely due to temperature.
Hmm I got stuck here. On the one hand, the amount of air in the tyre doesn't change. So if the temperature varies, both volume and pressure will vary. I think for a fully-inflated tyre you can assume the volume doesn't change, so you end up with pressure varying with temperature. I start to struggle when considering changes in p, V and t when inflating a flat tyre (where I imagine both volume and pressure increase isentropically up until the point V can no longer change).steve singh said:
So guys, with a North Westerly wind and being 500ft above sea level - am I ok to drive my car home from work or will the tyres explode?
Depends if you inflated the tyres with air from the same altitude at the same temperature. Also make sure there are no hills or valleys as we all know temperature changes with altitude and no one in history has survived changes in ambient temperature without ther tyre falling off and hitting a cameraman in the face.obob said:
steve singh said:
So guys, with a North Westerly wind and being 500ft above sea level - am I ok to drive my car home from work or will the tyres explode?
Depends if you inflated the tyres with air from the same altitude at the same temperature. Also make sure there are no hills or valleys as we all know temperature changes with altitude and no one in history has survived changes in ambient temperature without ther tyre falling off and hitting a cameraman in the face.
Being curious about this 1.8 business, finally figured it out.
Take a number in centigrade, say 40 degrees, this equals 104 Fahrenheit.
Then convert both figures to absolute values, you get 313.15 Rankine and 563.67 Kelvin.
Divide degrees in Kelvin by Rankine and we get 1.8
Boy that hurt the little grey cells.
Take a number in centigrade, say 40 degrees, this equals 104 Fahrenheit.
Then convert both figures to absolute values, you get 313.15 Rankine and 563.67 Kelvin.
Divide degrees in Kelvin by Rankine and we get 1.8
Boy that hurt the little grey cells.
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