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Pressures for larger tyres


glaggs
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The theory with tyres is, it is the volume of air that is critical not the pressure. Therefore if you fit larger tyres inflated with the same volume of air the resulting pressure will be lower. Couldn't find any sites that could give a conversion matrix so had to resort to some basic GCSE physics. According to my calculations the Jimny with 31x10.5 -15 tyres fitted should now be running 13.6psi ! Currently they have 20 psi in, and the oe fittment 205/70-15s recommended pressure was 24 psi. With 20 psi in they hardly deform at all at the bottom, so perhaps the calculation is right! Anyone got any links for easy calc. sites?

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Never heard that theory and I don't know if it is correct anyway.

I would say that the critical bit is the pressure required to support a given load, more pressure produces a smaller contact patch with a given load, or increasing the load squashes the tyre out so you need to increase the pressure to restore its shape. I don't know how manufacturers arrive at published pressures though.

I used to use about 20psi in 33x12.50R15s on my Defender 90, which "looked" about right and gave good flotation in soft ground. I shall probably use about 22 front 25 rear in the 110 due to the extra weight, and see how that looks.

Edited to add the above is with road speeds rarely exceeding 50mph and may well be unsafe if you plan on bombing up the outside lane of a motorway all day!

Edited by BogMonster
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OK

How to work out what tyre pressures you need :

1. Find a Nice quiet straight road.

2. Park up

3. On the fronts 1st -

4. With some of the yellow Chalk from your friendly Tyre shop draw a straight line from inside to outside of the thread, right across

5. also place a mark that identifies where this is on the tyre wall outside

6. Do the same and in the same position on the other front tyre

7. get in truck and drive DEAD STRAIGHT for just a few yards.

8. Park without moving steering - stop so you can see the marker on the wall means you can see the tyre marking too

9. Get out and look at tyre

a - Chalk gone in the middle - pressures too high - lower remark retest

b- Chalk gone on outer - pressures too low, inflate remark and redo

c- If on the 1st view there is no chalk you have driven too far, just a few yards does it !

Eventually you will find 95% of the chalk gone in a few yards - write down pressures - thats what your truck should have in it

Then repeat on rear

I then add a tad extra like 2 psi for "Load" and job done

:)

HTH

Nige

6. Get out and look

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wot he says ^^^^^ though you can get a good starting point by using the manufacturers max load/pressure info to determine a likely pressure that will result in the correct footprint/tyre profile for the specific application.

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OK

How to work out what tyre pressures you need :

1. Find a Nice quiet straight road.

2. Park up

3. On the fronts 1st -

4. With some of the yellow Chalk from your friendly Tyre shop draw a straight line from inside to outside of the thread, right across

5. also place a mark that identifies where this is on the tyre wall outside

6. Do the same and in the same position on the other front tyre

7. get in truck and drive DEAD STRAIGHT for just a few yards.

8. Park without moving steering - stop so you can see the marker on the wall means you can see the tyre marking too

9. Get out and look at tyre

a - Chalk gone in the middle - pressures too high - lower remark retest

b- Chalk gone on outer - pressures too low, inflate remark and redo

c- If on the 1st view there is no chalk you have driven too far, just a few yards does it !

Eventually you will find 95% of the chalk gone in a few yards - write down pressures - thats what your truck should have in it

Then repeat on rear

I then add a tad extra like 2 psi for "Load" and job done

:)

HTH

Nige

6. Get out and look

has it got to be yellow? :ph34r::lol:

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Look at the load rating together with max PSI for that load stamped onto the tyre wall. Measure or guesstimate the load on each corner and then calculate the correct (ish) pressure from the max load figures ………… I run my superswampers at 15 psi on the front and 17 psi on the rear …………then do the HFH test with the chalk ………

:)

Ian

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OK I know the suck it and see method of calculating pressure but was looking for a more 'scientific' method of working out pressure for road use when fitting over size tyres. Yes pressure is important for a given size of tyre carrying a given load. Pressures are quoted because they are the easy way to check if a tyre has the right volume of air. Check your Discovery (or what ever ) manual and different tyre sizes are shown with different recommended pressures. Add weight and the book recomends adding more air volume to support the weight, but again the easy way to check this is by checking pressure. On racing tyres the temp of the tyrte can be tuned by adjusting the volume of air doing the work. More volume = higher pressure = trye runs cooler but looses grip. Less volume = lower pressure = hotter tyre and more grip. This is simplifying things a bit but the theory is soun. I was looking for a quick calc matrix which might help with tyre size/pressure calcs?

Boy racers fitting bigger rims and ultra low profile tyres have to run higher pressure to maintain the volume of air in the tyre and thus keep tyre performance at its optimum. The reverse must apply for bigger tyres to be run safely on the road?

http://answers.google.com/answers/threadview?id=233041

This link may help, but its back to GCSE physics time!

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OK I know the suck it and see method of calculating pressure but was looking for a more 'scientific' method of working out pressure for road use when fitting over size tyres. Yes pressure is important for a given size of tyre carrying a given load. Pressures are quoted because they are the easy way to check if a tyre has the right volume of air. Check your Discovery (or what ever ) manual and different tyre sizes are shown with different recommended pressures. Add weight and the book recomends adding more air volume to support the weight, but again the easy way to check this is by checking pressure. On racing tyres the temp of the tyrte can be tuned by adjusting the volume of air doing the work. More volume = higher pressure = trye runs cooler but looses grip. Less volume = lower pressure = hotter tyre and more grip. This is simplifying things a bit but the theory is soun. I was looking for a quick calc matrix which might help with tyre size/pressure calcs?

This link may help, but its back to GCSE physics time!

I wouldn't trust GCSE anything as far as I could spit a rat. Dumbed down way lower than older fashioned CSEs and mostly worthless.

Assuming your tyre at rest defines a fixed and constant volume then 'increasing the volume of air' in the tyre makes no sense at all. You can increase the density of the air contained within the volume but that is just a fancy way of saying increasing the pressure. Pressure is rightly measured in units of Force over units of area and that should clue you into what it's important in tyres. The pressure is constant throughout the defined volume and acts universally against the inner walls of that volume, notably in this case the tyre sidewalls. It is those same tyre sidewalls that you are seeking to prevent from deforming which would allow the tyre to run out of the wheel well and cause mayhem or spontaneous death. A correctly pressured tyre will by design offer the maximum tread pattern to the road surface and all will be well in your world.

So, by passing more volume of air through your pump you increase the the density of air in the tyre which as it's a fixed volume causes the pressure to rise which increases the force applied to the sidewalls.

Tyre manufacturers are loathe to recommend pressures for cars from manufacturers who haven't paid them to do so. Car manufacturers recommend tyre pressure by the simple expedient of sticking it on a test track and seeing how it handles.

To sum up, go get some chalk :rolleyes:

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Surely unless it's gone dead flat, the volume of a tyre is reasonably constant?

Mind you, I only have the two Science GCSE's :unsure:

You'd be surprised. There are whole fleets of mathematicians carving careers out of answering questions like that. Take a toilet roll inner and look down it. It's round which enables it's cross-sectional area to be maximally efficient with respect to it's border (circumference in this special case 'cos it's round). Now squish it flat and you'll see its area reduced to near zero whilst its border stays constant - it is now minimally efficient wrt its cross-sectional area. The various states in between are where clever maths comes in and I leave :blink:

For a tyre though, yes, unless it is squished flat by being, err, flat, then it's internal volume is constant within reasonable bounds. Really all the air is doing is keeping the sidewalls from buckling in and so maintaining the tyre's profile for the given load. The amount of force required to be applied to the sidewall to do that is a function of the deforming force from the weigh of the vehicle and the rigidity of the tyre which is usually related to its construction type and size. If you have a bigger tyre and hence a bigger sidewall, assuming that it is equally resistive to deformation (which it won't be) you'd need a larger force which might well be best supplied by using the same pressure as the smaller tyre - pressure being force/area. Trouble is that larger tyres often have stronger sidewalls requiring less force to maintain profile and so requiring less pressure.

When I was a college we had a lecturer in engineering physics who would delight in setting mind numbing circular problems like this. In his honour we created the SI unit of confusion, the Boston. This is getting to at least 2 Bostons and rising.

Chalk really is easier.

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Trouble is that larger tyres often have stronger sidewalls requiring less force to maintain profile and so requiring less pressure.

I just read the whole thread waiting for that comment! Sidewall strength and tyre section do play a large part in what is right and no rule-of-thumb will always work when totally changing both.

Chalk really is easier.

Yes, but you won't always get wall to wall removal of the chalk - IMHO some tyres appear to be designed to have little or no contact at the outer edges... And I would also do a sanity-check after arriving at your ideal pressure...

When all's said and done, tyre pressures are always a compromise and the absolute best pressure for a given car/wheel/tyre-type/ tyre-section/surface/temperature/load/driver/wheel <breath in again> will always be a moot point.

TwoSheds.

p.s. The chalk trick doesn't work for motorcycle tyres :huh:

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Surely unless it's gone dead flat, the volume of a tyre is reasonably constant?

Mind you, I only have the two Science GCSE's

The internal volume of the tyre is constant, but as air is a gas it can therefore be compressed, this is why as you add more air the pressure increases. You are forcing more air volume into a fixed(for sake of argument) area. If the volume of air a tyre could hold was constant you wouldn't have to add air, there would be enough in there when it was fitted and you'd never have worry about punctures!. Max pressure is limited by the tyres strength of construction not by its internal volume. And sorry I didn't do GCSE's I did good old fashioned O levels. If your not convinced by the theory pleaes check the links.

I agree that tyre manufacturers finalise recommended pressures by testing, after applying an educated guess.

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Surely unless it's gone dead flat, the volume of a tyre is reasonably constant?

Mind you, I only have the two Science GCSE's

The internal volume of the tyre is constant, but as air is a gas it can therefore be compressed, this is why as you add more air the pressure increases. You are forcing more air volume into a fixed(for sake of argument) area. If the volume of air a tyre could hold was constant you wouldn't have to add air, there would be enough in there when it was fitted and you'd never have worry about punctures!. Max pressure is limited by the tyres strength of construction not by its internal volume. And sorry I didn't do GCSE's I did good old fashioned O levels. If your not convinced by the theory pleaes check the links.

I agree that tyre manufacturers finalise recommended pressures by testing, after applying an educated guess.

I think your confusing volume with pressure.

Volume is a finite measure of a space expressed in units on length (albeit cubed ones). As such it doesn't change.

The amount of volume that the air in your tyre might occupy at stp in a free environment is of no interest at all other than to gauge how long it might take you to pump up your tyres.

Sure, cramming more air in a tyre means that a greater volume of stp air passes through your pump but so what? If you had a puncture you could pump all day and measure huge volumes of air but your tyre would still be flat.

As the desired outcome of all that pumping is a force applied to the inner surface of the tyre wall and as that force is directly proportional to the pressure of the air contained within the fixed volume of the tyre, it makes sense to ensure that the tyre is correctly inflated by checking the pressure of that air. You could try checking its volume but you would find it equal to the internal volume of the tyre irrespective of the pressure you achieve.

Really, knowing the volume that the air in your tyre would occupy if released to stp is irrelevant, even more so if you start to use nitrogen or other inert gases to inflate your tyres. Different gases with different densities, have differing volumes per mass unit of comparison making any standardised 'volume pump throughput' spec useless. To a less extent the same is true for atmospheric air, varying as it does in ambient temperature & pressure - air occupies less free volume when it's cold so any throughput volume figure calculated for say, Saudi Arabia would be useless in Scotland.

"Och Aye Jimmy, you're looooki' a wee flat aboot the ground there."

"And what do yous expect Jock McSpangle? What with the price o' desert air being so high..I cannae joost stuff another cubic metre of our fair Highland air in, it's too damp an' heavy. See cannae take anymore. (capt'n)"

Poor Jock, if only there was a way to measure the actual force being applied by all that squished air...but wait, there is. :P

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I agree that a gas at a given pressure will occupie a given volume of space ie the inside of the tyre. By putting more gas into the given space it is compressed and thus presure increases, this allows a larger volume of gas than the tyres actual internal volume, again increasing pressure.

http://answers.google.com/answers/main?cmd...w&id=232922

http://answers.google.com/answers/threadview?id=233041

These links does help expain the science.

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