I've just discovered that I have been running my A1 with tyre pressures at 2psi below manufacturers recommendation. The garage put a little sticker inside the windscreen for my convenience but unfortunately they quoted the recommended pressures for 16 inch wheels and not the 17s that my model has... Presumably I have shortened the life of my tyres?
On an unrelated note, why should pressures be so much higher for speed or load? OK, load I can understand, but why does high speed necessitate higher pressures? Surely the pressure must increase naturally with the additional heat as the tyres spin faster?
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Tyre pressure is a peculiar concept on which to base the desirable degree of inflation, because as you say it varies so much with temperature.
It would appear that the quoted pressures are those for a car that has been standing for many hours, out of the sun, so that the tyres are at shade air temperature. But as soon as you you start driving the temperatures rise, either from friction or hot sun or both.
So logically if you are setting out on a long journey involving the baking sun on one side continuously, you should set the pressures differently for each side.
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I run my tyres at half way between standard and loaded pressures. I check the pressures when the car has been in the garage overnight. I have never had a problem apart from a couple of punctures in the last ten years or so.
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This is a very relevant query in light of my recent purchase of just over 2 weeks ago when I bought a new Golf MK7 Match Edition 1.4TSI BMT and the sticker on the B pillar says, amongst other things, that a light or normal pressure (for me, anyway) is 35psi, which gives a rather unpleasant hard ride and the full load pressures were higher but cannot remember by how much without looking again.
But what did interest me, was a recommendation of 30psi for 'Comfort Tyres' but no one was quite sure to what this referred. I assume that it means that for more comfort and possibly higher fuel consumption I can lower them to that figure. Anyway, at the moment, I have compromised with a figure in between 35 and 30 i.e. about 32psi.
As people will say, there are so many variables of tyre pressure accuracy and reproducibility and the in what temperature and conditions one's car has been standing in overnight.
I only ever check my pressures early morning before the sun gets to work on the car but of course that could be on a Winter's or Summer's morning when the differences in temperature could easily be 10C. It's all a bit smoke and mirrors, if you ask me.
My two previous Golfs ran the recommended pressures of about 30psi.
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>> the sticker on the B pillar says, amongst other things, that a light or normal
>> pressure (for me, anyway) is 35psi, which gives a rather unpleasant hard ride and the
>> full load pressures were higher but cannot remember by how much without looking again.
>> But what did interest me, was a recommendation of 30psi for 'Comfort Tyres' but no
>> one was quite sure to what this referred. I assume that it means that for
>> more comfort and possibly higher fuel consumption I can lower them to that figure.
I think it's something about Eco tyres having thinner sidewalls so generate more heat as they flex more - they need more pressure to reduce this effect.
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...for the cars, I also run at part-way between standard and loaded pressures.
TBH, I wouldn't be too worried about a potential over-pressure of 2psi.
The motorhome motorcaravan campervan, however, is a different beast. For such vehicles fitted with "Camper" tyres it has become the norm for the pillar sticker to show only a single pressure - the maximum the tyre can take - recommending very high front and rear pressures (on my 225/75R16 79.5psi all round).
The motorhome motorcaravan campervan forums are full of discussion about the ride that arises from these extreme pressures - it can be filling-threatening!
In the past, I have had direct advice from Michelin recommending much lower pressures, but they have become rather more circumspect about recommendations for the rears (they will still give lower pressures fro the front).
Continental, however (and it is indeed Continental's I have fitted) will still recommend pressures appropriate to actual axle loads, and these are always considerably lower (it is also possible to download their data tables from the web, so you can make your own decisions).
So, following a visit to the local weighbridge fully-loaded to determine real axle-loadings, and then adding a not insubstantial margin for variation, I now run at 60F/65R instead of the "stickered" 79.5 all round.
The ride is somewhat more "compliant".
The odd thing is that, if I were to take the (not insubstantial) maximum legal axle loads as the guideline, then the appropriate pressures would still be somewhat below the 79.5psi recommendation (and in that state, the vehicle would be well over it's legal GVW).
It is very difficult to get any info from the tyre-manufactures as to why such pressures are recommended, but the conjecture is that the "Camper" tyre, which is additionally reinforced, is designed to cope with the overloading which is rumoured to be rife on vehicles to which it is fitted, and is specified with pressures accordingly.
Luckily, in fully-loaded, extended holiday condition, my 'van is just on it's plated maximum weight (though in fact, it's design weight is some 750kg higher!), and well below either of its maximum axle loads.
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>> I've just discovered that I have been running my A1 with tyre pressures at 2psi
>> below manufacturers recommendation. The garage put a little sticker inside the windscreen for my convenience
>> but unfortunately they quoted the recommended pressures for 16 inch wheels and not the 17s
>> that my model has... Presumably I have shortened the life of my tyres?
>>
Often the same tyre pressures are indicated for a car whether it's on 16, 17 or 18" tyres as the foot print stays the same. 2PSI is neither here nor there - most gauges probably aren't that accurate.
>> On an unrelated note, why should pressures be so much higher for speed or load?
>> OK, load I can understand, but why does high speed necessitate higher pressures? Surely the
>> pressure must increase naturally with the additional heat as the tyres spin faster?
>>
There are charts for pressure vs load but load and speed increase the flexing of the tyre and higher pressure reduces that.
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There were pictures in older motoring books showing the effects of under or over inflation.
Under caused tyre wear in the centre of the tread, over caused wear on the edges, because of the resulting curvature of the tread footprint.
That makes sense if you see the tyre as a balloon - obviously at a certain pressure compared with load you get a perfectly flat footprint. But presumably there is more to tyre construction than that - the tread must surely be pre-braced in some way. Does tyre pressure now affect the footprint, or was that only for old cross-ply tyres?
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I remember getting a puncture some years back and the tyre man saying he couldn't repair the tyre as the walls were weakened by having driven too far on low pressure. Annoying as there was plenty of tread left.
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>> Does tyre pressure now affect the footprint, or was that only for old cross-ply
>> tyres?
>>
It's one of those hideously complicated things but if you exaggerate it by letting the tyre go flat then it'll clearly have a bigger contact patch than when at normal pressure.
The reason pressures normally stay the same for different size tyres (within reason) on the same car is as you go up in diameter the tyre generally goes wider. So the contact patch gets wider and therefore reduces front to back.
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The Renault has a funky modern tyre pressure monitoring system. On day one, straight from the dealer a system message in the dash said "Learning tyre pressure".
What conclusions do you draw from that?
So I left it to learn. On day two it said the same. After a week it said the same. Some googling and a reset later, by pressing a few buttons, and it cleared, then came back with "Learning tyre pressure".
Took me ages to discover that "Learning" is clearly the French using Google translate. They mean "Monitoring". How very confusing. Can't wait to see the message if a tyre gets a puncture.
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...the mention of TPMS brings in yet another facet of the use of "Camper" tyres and the (virtually unusable) high quoted pressures for them.
Most TPMS systems (and I believe they are now mandatory just about across the board) allow the user to "program" desired values in some way - in order to cope with the different pressures specified by varying load/speed parameters.
So, the Fiat Ducato, on which most motorhomes are built, is delivered with inordinately high tyre pressures and the TPMS set to monitor those values.
Most owners want to drop the pressures significantly (to preserve their fillings) and thus need to reprogram the monitoring to accept those values, but no! - the Fiat system is a fixed value (though I suspect you might be able to vary the values via the OBD socket with the correct computer and software).
So, you either lose your fillings, or keep them and live with a constant warning. Luckily, my 'van just predates the standard fitting.
(I understand the BMW I am about to take delivery of allows you to tell the TPMS to accept "the current values" at any time, and it then learns and uses these as the benchmark, until/unless they are reset).
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My Honda (on order) uses wheel rotation rates to detect a deflating tyre. A reset and it monitors the current pressures.
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Whatever happened to kicking tyres on a Sunday morning?
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>> Whatever happened to kicking tyres on a Sunday morning?
>>
I thought that was for time-wasters who replied to a for-sale advert but only came to talk and nit-pick with no intention of buying?
I have noticed though that I can pretty well judge the pressure of my tyres just by looking.
There is a slow leak in one - it goes down by about 2 psi per week, annoying but not worth doing anything about. It's meant to be 27, but if it's gone down to 25 there is a perceptible bulge in the sidewall in the fore/aft direction from contact with the ground. When pumped back up to 27 the bulge flattens out. So I generally pump it up to 29 and then kick it on Sunday morning to see if it has gone down enough to be worth pumping up again.
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>> My Honda (on order) uses wheel rotation rates to detect a deflating tyre. A reset
>> and it monitors the current pressures.
>>
Mine uses ABS /wheel rotation as well and so doesn't have to rely on those vulnerable and expensive sensors at the baseof each tyre's Schrader valve.
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Tyre pressure questions - Cliff Pope
