Air Pressure In Tyres

[cfc1]

This is why it's advisable to check your tyre presures once per week.:) it's also one of the reasons tyres crack.. they can expand and contract due to different "ambient air temperature". Suspension bridges also do this.. which is why there is gaps between each section (oh! no!.. engineering )

i know exactly what you mean, lower profile tyres bulge more, but if you pump them up then the bulge gets smaller. which is why i disagree with you saying that "bulge has sod all to do with pressure."

If you pump both of them up at the same pressure.. the bulge on both gets smaller.. but the bulge on the lower profile tyre is still larger than the standard profile tyre.

This is quite good innit m8

[reece]

Plus I don't think the advice that Matthew McNally gave regarding pumping tyres up was good advice. 

as a crude tip, pump the tyres up on a flat surface and weight for the front tyres to bulge slightly at the bottom and that should be perfect! back tyres should have zero or very little bulge.

EXAMPLE

I run my tyres @ 35 front / 32 rear.

I measure them stone cold, and the rears are 30.5.

I make a note I need to add 1.5.

I drive to the garage, and measure again. They are 33.

So I fill up to 34.5.

Now let me see which one I will choose!

A crude tip or a sensible safe tip?

Choices choices...

:D

[nrgizerbunny]

cfc1, i believe you just agreed with me! hehe. I do apologize if i sounded patronizing it was certainly not my intention.

in a world of choices:

recommended by goodyear and dunlop and endorsed by the society of automotive engineers or Matthew McNally?

[Matthew_McNally]

Plus I don't think the advice that Matthew McNally gave regarding pumping tyres up was good advice. 

←

what better advice are you offering?

your own description of your advice was a crude tip.

I described a sensible method of ensuring that your tyres are checked regualrly, and to fairly a consistent pressure.

Yes - the ambient temperature will make a slight difference, but not that much.

recommended by goodyear and dunlop and endorsed by the society of automotive engineers or Matthew McNally?

←

so - you say Goodyear and Dunlop say not to measure your pressures cold, not to do so regularly, and not to attempt to keep your tyres at the pressure recommend by the manufacturer.

Instead - we should just fill 'em to a slight bulge? No matter what size and profile the tyres?

I'm not trying to set myself up as some sort of expert - but the advice I posted is sound.

If you are going to disagree, please provide a better option

[Al J]

Almost scared to venture back into this thread but surely the only sensible way is that you take the trouble to find out the correct tyre pressures and set them to that?

Which is what starletsr was asking in the first place.

I understand pv=Rt and all that but you can't use that for day to day setting up of tyre pressures. There are far too many other parameters to take into account. You'd go out for the day in the hot Summer sun and have to pump up the tyres a bit for your return trip at night.

Find the correct pressures, pump them up to that and check regularly, I say. The recommended pressures take into account expansion/contraction due to differing conditions.

[starletsr]

wow didnt know that this thread would start off like this lol.

Some people told me "28-30psi in the front and 28 in the rear, if tyres are too hard Starlets or Glanza tends to hop at the back during corning....

also it depends on what ou want comfort or economy....play around till you get what you like. hope that helps"

"I ran my GT on 195/50/15 and used 30psi front and 28psi rear.

Seemed to take the rough roads better but could have be my imagination"

But thanks for the helpful information on here

[nrgizerbunny]

what better advice are you offering?

your own description of your advice was a crude tip.

I never said that the bulge method was a substitute for getting the recommended tyre pressures from the manufacturer - if they were unavailable then what else can you do. the best reply has been to phone up the dealer - what are you supposed to do until then? :(

I described a sensible method of ensuring that your tyres are checked regualrly, and to fairly a consistent pressure.

yes i totally agree with most of what you said :D , but the bit about adding on 1.5psi is not good IMO :( because you'll end up with too much or too little air pressure in your tyres, a better way would be to pump them up when they are stone cold (first thing in the morning) as opposed to driving to the garage forecourt and adding the difference because that does 2 things, it warms your tyres up and also brings into question another gauge which may or may not be more accurate than your one at home. I suppose you could take your accurate gauge with you

Yes - the ambient temperature will make a slight difference, but not that much.

I'm not sure how much you mean by slight and not much, but bear in mind that the tyre pressure you measure is a gauge pressure, gauge pressure is the difference between absolute and atmospheric pressure. when the ambient is hot, the atmospheric is low and vice versa.

what i am really trying to say is the measured pressure in a car tyre is the difference between tyre pressure and atmospheric. If the atmospheric changes (winter/summer) and the tyre pressure changes (hot/cold) you are going to get compounded effects.

so - you say Goodyear and Dunlop say not to measure your pressures cold, not to do so regularly, and not to attempt to keep your tyres at the pressure recommend by the manufacturer.

Instead - we should just fill 'em to a slight bulge? No matter what size and profile the tyres?

I'm not trying to set myself up as some sort of expert - but the advice I posted is sound.

If you are going to disagree, please provide a better option 

←

No I don't disagree with any of that :D , i disagreed with the 1.5psi fiddle as the assumption is that air in tyres varies linearly with temperature . Goodyear dunlop and even when i got to uni today and had a look int he Michelin handbook says the following:

1. Tyre pressures should be checked and adjusted at ambient conditions

2. During sharp climatic changes, tyre pressure should be checked/adjusted

3. Tyre pressure should be in accordance with the tyre manufacturer not the Vehicle manufacturer (although these two are usually the same)

4. Tyres should be checked for pressure/damage/wear every week

5. Spare tyre should be checked every month

8. In the event of tyre pressures being unavailable or non standard tyres being fitted, Michelin recommends setting the tyre pressure at a level where the portion of the tyre in contact with the road shows some slight deformity as shown in figure (in book). The deformation can vary between 10 and 30% and typical values have been listed in table (in book).

*Michelin suggest the car owner consult their tyre manufacturer asap as incorrect tyre pressures may lead to tyre failure and personal injury.

I don't proclaim to be an expert, but I look after the front suspension on our uni formula student car and part of my job is fiddling with the tyre pressures and I am only going by what i've read in books. CFC1 is a tyre fitter so i am sure his info is good and accurate but if you actually read what I have said, apart from the bulging and the 1-2psi fiddle factors it pretty much concurrs with what he said. - although i didn't go so far as to say everything was the biggest pile of rot in 15 years!! LOL :D

Avon the local tyre manufacturer in this area supplies tyres to our formula student team. I certainly wouldn't recommend their tyres (I think they are dreadful ) but they also use the bulge method (they have a chart too!) when no data is unavilalbe (eg. if you put 20" wheels on a starlet - the dealer isn't going to know , and i doubt that kwik fit would know either. They'd have to ring up the tyre manufacturer and find out)

[reece]

I'm bored of this topic now...

nrgizerbunny, you can kick your tyres and check your bulge as aften as you like, at least I know I've checked my tyres correctly

[nrgizerbunny]

I'm bored of this topic now...

me too. but thats because my hands hurt from typing :D

[cfc1]

I think people will have the idea now

This could go on forever.. way too many variables to be 100% accurate ALL the time. (F1 cars have on board (or in pit) pressure guages.. and they get corrected lap by lap corner by corner).

I think in short the best thing to do is get hold of a digital tyre pressure guage to check your pressures, get one of them compressors that plug into the cigarette lighter (great wee things them.. if only i could get mine back from the kids.. footballs, bikes, airbeds, swimming pools etc :().. and check your tyres on a weekly basis.

It is very important that your tyres are kept in good condition.. at 70mph.. they are the only thing touching the road.. unless your exhaust is hanging off :P

[cfc1]

Which reminds me...

Exhausts.. the best way to........ :P :P :P :P