Why is it so hard to check your tyre pressures.

[RobM77]

doogz said:

RobM77 said:

If you're driving the car to a garage just before you check the pressures it makes the whole operation pointless. The act of driving your car will create heat in the tyres and therefore heat the air in them, raising the pressure. This means a) the cold pressures given in the handbook won't apply and b) the heating will be uneven depending on the number of left and right bends, acceleration and braking. Ergo what you'll end up doing is having all of the pressures too low and they'll all be different - utterly pointless.

Get yourself a tyre pressure gauge and a pump and do it yourself when the car is sat cold and out of the sun; then you can enjoy the handling, economy and safety that was designed into your car in the first place. It'll take five minutes and it'll be done properly.

By a poofteenth (That's a technical term) unless he's driving hard or fast. It'll likely be less than the temperature difference between a chilly winters day and a hot summers day in this country, and I've never seen anyone whip out a calculator to calculate pressure differences due to different ambient temperature.

Nor have I ever seen a car manual that states an ambient temperature for when the tyres are cold.

Ambient temperature is surely irrelevant if the pressures are set accordingly for that temperature?

If, for example, your tyres are sat at 35psi cold in the shade on a 20 degrees C summer's day, then the act of driving them could raise their pressures to, for example, 45psi. Do that in winter, then they'll still go up from 35 to about 45. Sure, they'll be different thermal insulative losses through the tyre wall etc, but the basic concept of a cold tyre pressure is that you maintain it throughout the year so that the hot pressures you're driving on are just a set amount above it.

[RobM77]

martinnitram said:

At what ambient temperature would you recommend taking tyre pressures?
Would you make an allowance for a hot summers day, there is no such thing as cold tyre's really is there.
I think if you had just done ten laps of Donnington you perhaps would have tyre's at a higher pressure, nipping to the garage and waiting half an hour to pump them up wouldn't have life threatening consequences on pressure.
IMO

As I explained above, you take the pressure at the temperature you're going to be driving at. This is one of the reasons why you need to check and set your tyre pressures regularly - to keep pace with seasonal temperature changes. If your pressure is recommended as 35 degrees cold, that applies at 5 degrees C in January just as much as it does at 25 degrees C in July. Obviously if you set them at 35psi in July they'll be a lot less than that in January, but the idea is that you check and set them once a week (for example) and re-adjust the pressures to read 35psi - in effect you'll add air as the months get colder towards winter and then bleed it out as the air warms again.

Regarding your second point, no, it'll make a noticeable difference. Most people will feel a few psi difference in their tyres in a good sports car like an Elise - that'll be at least the amount you'll raise the pressures by when driving to the garage, not to mention the fact that the pressures won't all go up the same amount. Driving somewhere to set pressures if you've only got a cold reading to go by is just stupid.

Edited by RobM77 on Wednesday 7th August 18:09

[RobM77]

doogz said:

RobM77 said:

Ambient temperature is surely irrelevant if the pressures are set accordingly for that temperature?

If, for example, your tyres are sat at 35psi cold in the shade on a 20 degrees C summer's day, then the act of driving them could raise their pressures to, for example, 45psi. Do that in winter, then they'll still go up from 35 to about 45. Sure, they'll be different thermal insulative losses through the tyre wall etc, but the basic concept of a cold tyre pressure is that you maintain it throughout the year so that the hot pressures you're driving on are just a set amount above it.

Ok, so what about when I set my tyre pressures when I get home tonight. Then tomorrow when I drive the car next, it's already 15 degrees celcius warmer? Or vice versa? If I set my tyre pressures during the day, Do I have to inflate them some more when I want to drive at night?

You see my point?

The differences in pressure are pretty miniscule. A tyre @ 30PSI at freezing, will be at 32PSI when 20 degrees warmer.

I see your point/question and the answer is that you set the pressures at roughly the temperature you're going to be driving at.

Your point about the pressure vs temperature is wrong; it would rise to around 36Psi. See this chart:

Edited by RobM77 on Wednesday 7th August 18:15

[martinnitram]

RobM77 said:

martinnitram said:

At what ambient temperature would you recommend taking tyre pressures?
Would you make an allowance for a hot summers day, there is no such thing as cold tyre's really is there.
I think if you had just done ten laps of Donnington you perhaps would have tyre's at a higher pressure, nipping to the garage and waiting half an hour to pump them up wouldn't have life threatening consequences on pressure.
IMO

As I explained above, you take the pressure at the temperature you're going to be driving at. This is one of the reasons why you need to check and set your tyre pressures regularly - to keep pace with seasonal temperature changes. If your pressure is recommended as 35 degrees cold, that applies at 5 degrees C in January just as much as it does at 25 degrees C in July.

I think if you had just done ten laps of Donnington you perhaps would have tyre's at a higher pressure, nipping to the garage and waiting half an hour to pump them up wouldn't have life threatening consequences on pressure.
IMO

[lbc]

martinnitram said:

At what ambient temperature would you recommend taking tyre pressures?
Would you make an allowance for a hot summers day, there is no such thing as cold tyre's really is there.

On a hot day, wait until late evening to check the pressures.

As long as all tyres are near enough the same pressure per axle, and between 30-35psi you will be fine.

[braddo]

Bluebarge said:

Get a foot pump with a pressure gauge - costs about £7-8.

I bought a Michelin one from Halfords (so more expensive, maybe £20) and the gauge is accurate such that the digital pressure gauge I bought is redundant.

Using a foot pump takes more time obviously, but for me it's still better than going to a service station and doing the lucky dip of whether their machine is working.

[RobM77]

doogz said:

Oh right.

I don't think you did see the point then. As it was that ambient temperature isn't a fixed quantity, it's variable.

Yes, so as I've already explained, you set the pressures at a typical temperature for the week ahead. Currently where I live it's about 15 at night and 20 during the day, so on a Sunday morning after breakfast it's about 17 - just right for setting the pressures.

[RobM77]

martinnitram said:

I think if you had just done ten laps of Donnington you perhaps would have tyre's at a higher pressure, nipping to the garage and waiting half an hour to pump them up wouldn't have life threatening consequences on pressure.
IMO

It will change them a small amount and it'll also take a lot longer than just using a pump and gauge at home, which as already explained can be bought very cheaply. Driving to a petrol station to check something so simple when you're also invalidating the readings by driving the car is just utterly stupid.

[Ullevi]

RobM77 said:

It will change them a small amount and it'll also take a lot longer than just using a pump and gauge at home, which as already explained can be bought very cheaply. Driving to a petrol station to check something so simple when you're also invalidating the readings by driving the car is just utterly stupid.

Add the fact that the petrol station equipment is probably ancient and gets dropped onto the ground from a height by Bazza and his mates, and driven over by Dotty Doris and her crew, making its accuracy debatable.

I'd rather invest in a decent gauge and a foot pump, and measure my tyre pressures cold.

[kambites]

doogz said:

Secondly, your graph is in fahrenheit, you've considered that, yes? Assuming volume remains constant, pressure divided by temperature is a constant.

So if you set your tyre pressure to 30 PSI at 27 Celsius (300 Kelvin) and then the temperature dropped to to 17 Celsius (290 Kelvin), your pressure would drop to 29 PSI. At -3 Celsius they'd be 27PSI.

The difference between a particularly hot mid-summer day and a particularly cold mid-winter one is only 3PSI. Certainly enough to feel in a sports car, but not really a safety concern.

[martinnitram]

RobM77 said:

martinnitram said:

I think if you had just done ten laps of Donnington you perhaps would have tyre's at a higher pressure, nipping to the garage and waiting half an hour to pump them up wouldn't have life threatening consequences on pressure.
IMO

It will change them a small amount and it'll also take a lot longer than just using a pump and gauge at home, which as already explained can be bought very cheaply. Driving to a petrol station to check something so simple when you're also invalidating the readings by driving the car is just utterly stupid.

You are obviously very serious when it comes to tyre pressures,
But i wouldn't call a chap at the petrol station checking his tyres as utterly stupid, i wouldnt be so rude,
but, nipping to the garage and waiting half an hour to pump them up wouldn't have life threatening consequences on pressure.
Would it.

[Raize]

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.

[Thankyou4calling]

martinnitram said:

You are obviously very serious when it comes to tyre pressures,
But i wouldn't call a chap at the petrol station checking his tyres as utterly stupid, i wouldnt be so rude,
but, nipping to the garage and waiting half an hour to pump them up wouldn't have life threatening consequences on pressure.
Would it.

I thought that was rude too. Blimey, I was only releasing a bit of frustration at the hassle ( a bit self inflicted) of checking the pressures. I don't think temperature is really an issue on a regular car used for nipping around in. I thought there would be more peeps adding to what a general palaver it an be.

[kambites]

I must admit it had never really occurred to me that anyone would ever use those garage things except in an emergency. It's just so much easier to use your own three quid pressure gauge.

I doubt the garage's gauges are accurate enough for temperature to make any different anyway.

[Vipers]

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.

Dear oh dear, had to read the post twice, didn't believe what I had read first time.

Good luck.

[entropy]

Go to Sainsburys. Its free to use the air hose.

[clockworks]

As said above, Sainsbury's don't charge for air. Means it's always busy though.
Don't know if all Sainsbury's air/water machines are the same, but ours gets broken about once a month. Airline retract drum stops retracting, so people leave it lying across the forecourt. Some tt drives over it, and breaks the nozzle. Out of action for a week.

[braddo]

kambites said:

I must admit it had never really occurred to me that anyone would ever use those garage things except in an emergency. It's just so much easier to use your own three quid pressure gauge.

I doubt the garage's gauges are accurate enough for temperature to make any different anyway.

I would say that's PHer/car bore comment. I think the vast majority of motorists would not consider it normal to have their own tyre pressure gauge or pump, and that the local garage is the obvious place to 'pump up the tyres'.

Plenty of people will never even think about tyre pressures unless they see a tyre is half flat; other people who have a basic awareness that they should check the tyre pressures from time to time will know that they can pop to the local garage to pump up their tyres (and proceed to have an experience like the OP's ).

[kambites]

braddo said:

I would say that's PHer/car bore comment. I think the vast majority of motorists would not consider it normal to have their own tyre pressure gauge or pump, and that the local garage is the obvious place to 'pump up the tyres'.

Really? My parents have absolutely no interest in cars but still have their own foot pump with a gauge on it (which gets used for the car, bikes, wheel barrow and anything else that has pneumatic tyres on it). I doubt they've ever even noticed that petrol stations have air lines to be honest.

I don't know if they ever actually check their tyre pressures unless they look flat (almost certainly not), but they certainly have the equipment to do so if required.



I suppose I'd expected anyone who knew enough about cars to know they were meant to check tyre pressures more often than the MoT (which is probably a small minority of drivers) would have their own pump and gauge just for the convenience of it.

Edited by kambites on Thursday 8th August 09:08

[steve singh]

kambites said:

Have you thought of just buying a tyre pressure gauge? You can get one which will be at least as accurate as the petrol station ones for about a fiver.

If you can show me one I'll buy it.

Had tons of ones to use at home and they are rubbish.

Now thinking of buying an air compressor and have done with it !