I know the accuracy of these varies from device to device (and I have not felt confident to trust any of them after trying 4 different types of which the variance was over 50% - sticking with a simple metal 'pen' like I used years ago), but irrespective of this, does it make any difference if the car is on the flat or on a slope when you check pressures?
Common sense says that on a slope it changes the weight on either the front or back of the car respectively, but does that actually make much odds to the tyre pressures?
I park my car on the drive which is on a reasonable slope, with the nose of the car raised.
Should I move it onto the flat to check the pressures, or does it make no odds in the real world?
Last edited by: VxFan on Tue 30 Sep 14 at 13:59
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>> does it make any difference if the car is on the flat or on a slope when you check pressures?
No.
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That is the least of your problems, have you factored in ambient temperature, tyre temperature, if one side of the car is in direct sunlight and the other in the shade, fuel tank contents, boot load, expected speed, number of passengers, standard or economy pressures, etc? :)
Last edited by: Old Navy on Tue 23 Sep 14 at 12:50
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OK, OK! :) Point taken! :)
It was one of those funny thoughts I had as I was looking out of the kitchen window at the car....
Interestingly, if I had relied on the 'Michelin' foot pump (cheap and nasty looking 'made in China' thing that was anything but cheap) then I would be driving on either shredded or inflated-to-bursting-point tyres (can't remember which way round it was - it was a year or so ago....).
The digital gauge I had was still some way out too, and in the end, using a few forecourt gauges (when tyres still pretty cold) and the cheap metal 'pen' thing, it showed the 'pen' to be closest to the average.
Recently bought a high-quality hand-operated (well, stand on the foot rests and pump up and down with your hands) pump from local bike shop. That looks to be very good.
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I have a cheap (<£5) digital gauge which came from one of the food sheds, Tesco I think, I have also seen it Draper branded. It matches the pressures displayed on the digital display of my small 12v pump.
Last edited by: Old Navy on Tue 23 Sep 14 at 16:11
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The cousins have a couple of old tractors, so for convenience they have a compressed air container, a forged steel sphere with gauge attached, able to hold 200 atmospheres. That can be handy at times.
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I have a couple of the bulb type that came from one, other or both of our respective fathers. Both devices are likely to be older than Mrs B and I.
I reckon they're as accurate as anything new and better than some.
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>...............able to hold 200 atmospheres. That can be handy at times.
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You would have to be very careful using that on a tyre.
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>> You would have to be very careful using that on a tyre.
I think you can set it to the pressure you want, and when it gets there it pings or something.
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200 atm=3,000psi almost.I don't think I'd want that anywhere near me.
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>> 200 atm=3,000psi almost.I don't think I'd want that anywhere near me.
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>and when it gets there it pings or something.<
I dont think I want to be standing next to it when it goes ping!
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I dunno, perhaps it's only 100 atmospheres. I'll ask this evening. It does hold a very high pressure. I don't think an ordinary forecourt pump can go high enough to really charge the thing.
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The local trading standards told me as they tested a number of tyre gauges for me that on the whole, the simple pencil type gauge has the best reputation for accuracy.
Certainly the racing gauge they tested for me was way out and I returned it to the supplier who to their credit replaced it straight off and paid for two lots more P&P. Don't know what the experience is on modern digital gauges but I suspect they are another thing which purports to great accuracy but in reality is not very good. I certainly don't trust cheap digital voltmeters.
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A good tyre gauge is only as good as the person using it!
Bought my X-trail in 2007 and a day later thought the tyres looked a bit flat - tested the oressures. Instead of 30 PSI it was 20psi all around.....culprit was the sticker on the door frame -highlighted was the pressure 2.0 bar which was read as 20 psi.
Roll on 5 years - CRV from same outlet and checked them immediately - off the scale >55psi. Obviously the factory inflates them not knowing how long they will sit in a boat / compound.
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My first 'company car' was a Hillman Imp. The tyre pressures on that were 15psi at the front and 45psi at the back. One day after it had been serviced, I couldn't work out why I couldn't drive in a straight line. The had swapped the wheels front to rear, but hadn't adjusted the tyre pressures!
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I have found the tyre pressure monitors on my current car to be interesting. I have to assume they are accurate I suppose.
Pressures change so much with temperature of course. And I see this in winter/summer and obviously when they are warmed up from driving.
Only time it has told me they are underflated (all 4) was just after an MOT. Hmm.
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>> Pressures change so much with temperature of course
Not if the tyres are inflated with nitrogen gas apparently.
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Does Nitrogen have an exemption from Boyle's law then?
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Most forecourt pumps would max out at 100-125 psi-that's approx. 7-8 atm(bar).
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>> Does Nitrogen have an exemption from Boyle's law then?
I dis say "apparently"
That's what everyone who fills tyres with it claims anyway.
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I can't afford pure nitrogen. I use 80% nitrogen and a blend of various other gases.
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Ye'd think, well I would anyway, that it couldn't be pure nitrogen, surely even an uninflated tyre has some "normal" air in it before you start blowing it up. Got to be a mixture of what was naturally occurring in the tyre garage's atmosphere ( don't think too hard about that if you're eating ) and whatever is pumped in.
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>> I dunno, perhaps it's only 100 atmospheres. I'll ask this evening. It does hold a very high pressure.
I remember the thing, and it did hold an impressive number of atmospheres. But I could easily have forgotten the correct number.
Anyway the cousin in question, disappointingly, says he can't remember the pressure. He now has a small electric pump that you plug into your car's lighter socket thing. Not as good as the sphere when you have a real flat though. But it seems to work in the end. You have to run the car engine when it's a long job, unless you want bother tomorrow morning.
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Or when it goes bang!
Guy Goes Flying From Exploding Tire Accident at S…: youtu.be/ZuGZxmIBkaU
Tire Safety Video.mov: youtu.be/dKlJJqHFfoQ
A good reason for not driving alongside a HGV at speed.
Last edited by: Old Navy on Wed 24 Sep 14 at 08:50
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>> A good reason for not driving alongside a HGV at speed.
After I passed my driving test, I had one motorway lesson. One thing I was told was don't stay alongside an HGV for long. Put your foot down even if it means speeding a bit. But now I think it's common sense not to be alongside!
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>>But now I think it's common sense not to be alongside!
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Maybe I should have said "Another good reason.......". :)
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If you're alongside he can't see you in his mirrors-drop back or overtake!
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"does it make any difference if the car is on the flat or on a slope when you check pressures"
Strictly speaking, I think it must do. Vector graphics and all that.
Car on level, front (engine weight etc) acts down vertically through wheel trying to flatten tyre.
Car approximately vertical and tyre is not pressing on concrete. (yes I know, what holds it up, but this is an academic exercise and the easiest way to show it must change is to look at an extreme change of angle)
Rotate car back towards horizontal and pressure on the tyre increases steadily as is goes back towards the horizontal. At 45 degrees the tyre force on the ground is expected to be roughly about 70% of what it was at horizontal, although that could be affected somewhat by the distributed weight of the car.
What contribution to the tyre pressure the car weight makes could easily be measured. Just measure the pressure and then jack the wheel clear of the ground. As a typical pressure measurement lets a little air out each time you measure it, you would need a gauge fixed on so as not to leak significantly while you jacked the wheel up.
Be interested in what you find or measure.
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>> Be interested in what you find or measure.
So will I. But I won't believe anyone who claims to measure a discernible difference. There isn't one.
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Don't confuse pressure with force. Apply more force and the contact patch will increase in size, so pressure will not be proportional to the increased load.
Last edited by: Manatee on Thu 25 Sep 14 at 23:02
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If the volume of the tyre is reduced by the flattening, pressure must increase. So it comes down to what flattening does.
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>> If the volume of the tyre is reduced by the flattening, pressure must increase
The relevant word there is "if". Doesn't the rest of the tyre effectively bulge a little so compensating for the reduced volume in the flattened area? Bit likes squeezing a balloon, the overall volume remains the same and there is therefore no increase in pressure.
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I've tended to go for consensus amongst tyre pressure gauges.... Having acquired a few 'pen' gauges over the years and a cheap digital one when I got to Aus. I find that an old gauge (possibly my Dad's and 40yo) matches the digital and the readout on my electric pump whereas the other pen gauges read 'high'. So, I trust the digital & old pen gauge.
The other comments on temperature are so relevant though, I always wonder at people inflating their tyres at the garage forecourt where the tyre must be warm surely ? Ambient temperature is quite a problem here too - as it ranges between 1C and 47C albeit over (say) a 3 month period.
I was told that using nitrogen to fill tyres meant less 'leakage' of the gas, molecular size and permeable membranes and such like ?
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Some sensible information on the subject of nitrogen from the AA.
www.theaa.com/motoring_advice/safety/filling-tyres-with-nitrogen.html
Last edited by: CGNorwich on Tue 30 Sep 14 at 08:11
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I'm prepared to be surprised but how would O2 molecules be smaller than N2, when N has atomic number 7 and O 8?
Last edited by: WillDeBeest on Tue 30 Sep 14 at 13:06
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Yes they are apparently. Here's a brief explanation. I'll test you on it later. :-)
www.getnitrogen.org/pdf/graham.pdf
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That was really interesting, and you can see from that how both the AA's position of "don't bother cos it will still need the pressure checking anyway" can be squared with Murphy's answer of "yes, but the tyre is effectively kept in better condition for three to four times longer".
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Good find, CGN. Consider me surprised - but a little better informed.
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" Bit like squeezing a balloon, the overall volume remains the same and there is therefore no increase in pressure."
WOW! You can squeeze a balloon and the pressure does not change. Really!
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Wish-full thinking methinks.
Squeezing a balloon stretches the rubber. Which then exerts more force. So the air is more forcibly retained. Thus it will not expand to equal the original volume, but to somewhat less.
In effect it is similar to using a stronger balloon and no squeezing.
I rest my case.
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If you have to pay to use garage air line,then pressure they are controlled by the weights and measures people
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Type pressures - Armel Coussine
