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Topic: Tyre Pressure

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saxo1

23/7/2014 at 5:42pm
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If the pressure varied with the load then all HGV vehicles would need to decrease or increase the pressure to remain within the law depending they were fully loaded or empty.
Saxo1

arc systems

23/7/2014 at 10:22pm
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Quote: Originally posted by trog100 on 23/7/2014

you are wrong with that one.. its is the air pressure that carries the load.. nothing else does.. he he

No I'm not, it's a fundamental fact and that's my whole point, the given psi pressure is simply a handy measurable result you can then use for a given load to replicate putting the required volume into tyres of the same size.

For your statement to be correct, an xx pressure would suit any size tyre if the load was the same, but that is obviously wrong
It's also why then for the same type construction tyre and as the tyre size increases, it can carry more load for the same given pressure...again simply because with a bigger tyre, you get more volume of air inside for that same pressure.

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trog100

24/7/2014 at 2:53pm
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my statement is perfectly correct.. its the air pressure inside the tyre that supports the load.. the tyre contains the air pressure.. it has to be strong enough to do this

different loads need different pressures for (what should be) obvious reasons.. the greater the load the great the pressure needed to support it..

as for saxo1s statement.. to be correct the lorry tyre pressure should be varied to suit the load.. they just mostly dont bother..

to use a couple of examples.. take my boat trailer.. with a boat on its heavy with no boat on its not so heavy.. should the tyre pressures be adjusted accordingly.. yep without a doubt they should..

take my 4 x 4 truck.. the tyres are rated at a silly 1600 kg at 36 psi.. this is over six tons for four of them.. my 4 x 4 truck weighs around two tons.. do my tyres need to be at 36 psi.. nope.. in fact for off road use they should be let down to around 12 psi.. on road use.. maybe 28 to 30 psi..

my boat trailer with no boat on.. hit a bump and it will leave the ground and bounce two feet in the air.. if i run the tyres at 20 psi instead of their max 90 psi (needed with the boat on) it dosnt bounce as high.. he he he

the greater the pressure in the tyre the greater the stress on the tyre.. the more likely the tyre is to fail..

blow them up as hard as you have to but no harder.. how hard you have to blow them up is entirely governed by how much weight they have to carry..

trog

saxo1

24/7/2014 at 3:26pm
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The only way you can increase the pressure is by increasing the volume of air or by decreasing the size of the container.
When you increase the load on the tyre the tyre doesn't get smaller the volume is constant as is the pressure.
The air in the tyre doesn't support the weight the sidewalls are the loadbearing part the air pressure is to stop the tyre deforming.
The reason it is recommended to increase the tyre pressure according to the load is to reduce the tyre deformation, which would lead to increased rolling road resistance and and an increase in the temperature of the tyre and increase fuel consumption.
Saxo1

trog100

24/7/2014 at 6:31pm
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this can go on for ever.. if too much weight is added or there is too little pressure in the tyre to support the weight.. the tyre will deform.. it will go flat at the bottom.. the container will get smaller and the pressure in the tyre will get higher than if no weight was being placed upon it..

under normal circumstances the pressure in the tyre matchers the weight it has to carry.. its a pressure that dosnt allow too much tyre deformation to take place..

a radial ply tyre sidewall will deform more than the more rigid cross ply type sidewall will..

the sidewall is flexible to allow it to flex and the tread remain flat on the road.. a radial ply tyre is probably not the best tyre to use on a caravan or trailer.. sidewall flexing isnt a good thing on a caravn..

keep the thread going and some useful information may come out of it.. :)

trog

Bob61

24/7/2014 at 7:53pm
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Quote: Originally posted by trog100 on 24/7/2014
this can go on for ever.. if too much weight is added or there is too little pressure in the tyre to support the weight.. the tyre will deform.. it will go flat at the bottom.. the container will get smaller and the pressure in the tyre will get higher than if no weight was being placed upon it..

trog

As I said in a post a long time ago, yes, weight on the tyre will cause it to go flat at the bottom but to compensate and receive the displaced air, the rest of the tyre expands, thus the air pressure within stays the same.

arc systems

26/7/2014 at 12:11am
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Quote: Originally posted by trog100 on 24/7/2014

different loads need different pressures for (what should be) obvious reasons.. the greater the load the great the pressure needed to support it..

If your right, then for road use why does my mates massive tractor with an equally massive disc harrow hanging off the back only require 20psi?
He would use 10psi in the fields except it takes ages to pump them back back up from 10psi to 20psi?

The tyres max pressure BTW is 25bar 367psi!

Of course I fully agree that different loads in different size/type tyres need different pressures, BUT, the volume of air that pressure represents does not change, the volume of air required is a constant for any given load, (technically that's at sea level)
So to say 'pressure' in a tyre supports a load,(or the side walls), is not correct, it is the volume of air and when that's correct the pressure is what it turns out to be.

so what are these 'obvious reason'?

trog100

26/7/2014 at 2:39pm
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okay i will explain.. PSI which i think stands for pounds per square inch..

the way it works.. for each square inch of load bearing area.. lets just use 20 PSI for ease..

with a piston an area of 10 square inches at 20 PSI would exert a force 10 x 20.. or 200 pounds.. up the pressure to lets say 50 PSI and the same piston would exert a force of 10 x 50 or 500 pounds.. a pressure of 100 PSI for the same piston would be 10 x 100 or a force of 1000 pounds..

the force can be a pushing force or simply a load carrying force..

the load carrying area of a tyre is its footprints or the bit in contact with the road or muddy field in the case of a farmer.. :)

the bigger and fatter the tyre the bigger the footprint and the greater the load carrying ability at any given pressure..

or to carry the same load big fat tyres dont need to be blown up as hard as small thin tyres..

trog

ps.. the obvious reasons.. a basic understanding of how things work.. which if this thread is anything to go by is sadly lacking.. ;)

phil76

26/7/2014 at 3:34pm
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I have to agree with trog he is correct in saying that the air in the tyre supports the load and increasing the air pressure allows a tyre to carry more weight or vice versa
The air in the tyre not only carries the vertical load but also a gives the strength to the sidewalls to allow them to resist horizontal forces when cornering
Incidentally has anyone tried ceramic beads to balance their tyres I have used them on my work van and they seem to work really well although I think there is some magic involved
Phil

trog100

26/7/2014 at 3:53pm
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Quote: i have to agree with trog he is correct in saying that the air in the tyre supports the load and increasing the air pressure allows a tyre to carry more weight or vice versa

the remarkable thing phil is so far you are the only one that agrees with what i have said.. the general consensus seems to be trying to do the opposite.. prove me wrong.. he he

trog

Bob61

26/7/2014 at 5:30pm
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I think the issue has become clouded a little. Adding more air to carry a greater load is one thing, but it has been suggested that adding a greater load increases the air pressure already inside the tyre. It has been proved from the earlier video that is not the case.

trog100

26/7/2014 at 5:46pm
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if you keep adding more and more load bob as the tyre deforms at the bottom and goes flat the air pushed out from the bottom of the tyre has to go somewhere.. the same air but less room for it to occupy.. the pressure must go up.. the tyre (its canvas) unlike a balloon will not stretch so the pressure just has to go up.. simple logic..

your might want to use a youtube video as proof this wont happen but it will and given enough weight the tyre will fail and go bang..

as you say several issues have been brought up and i am correct in every one of them.. he he..

trog

Bob61

26/7/2014 at 7:33pm
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Obviously a tyre isn't as flexible as a balloon but if the tyre wasn't flexible at all then you wouldn't be able to pump much air into it and the ride would be solid. When the tyre is squashed at the bottom by the weight of a vehicle the displaced air expands the rest of the tyre evenly and the internal air pressure remains the same.

Google Newtons Laws

trog100

27/7/2014 at 9:43am
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under normal use a tyre is selected to suit the purpose it will be used for..

a size and pressure is chosen so that there will not be that much deformation for road use vehicles..

for off road vehicles this often dosnt apply.. tyre deformation is desirable..

but assuming that in certain circumstances an air filled tyre will deform.. go flat at the bottom.. if it does the pressure inside will go up..

it come down to one simple thing.. does extra weight cause the internal area of the tyre to decrease or dosnt it.. if it does the pressure will go up.. if it dosnt it wont go up..

i am reasonably sure that adding enough extra weight will cause the tyres internal pressure containing area to decrease.. thus the pressure will go up..

i could be wrong.. but i dont think so.. he he..

tyres have air in them as opposed to being solid because an air filled tyre absorbs shock much better than a solid one.. assuming people dont blow them i up too hard.. i think most people do blow their tyres up too hard.. but that would be a whole new debate.. :)

trog

saftlad

27/7/2014 at 10:01am
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Right or wrong, I always check my tyre pressures once we're loaded and ready to travel.

Since the van doesn't move when it's unloaded, tyre pressures then would be a moot point. I don't make a habit of pulling my van around just for the hell of it lol

trog100

27/7/2014 at 10:30am
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Quote: I don't make a habit of pulling my van around just for the hell of it lol

i do its what i bought it for.. i call it touring.. he he

but all my comment isnt about what people do its about a basic understanding of how things work..

in the three months i have owned it i have done 1300 miles worth of "pulling it around".. :)

some of it at 60 mph on motorways some of it at 25 mph on bumpy country lanes.. the same tyre pressure isnt the best for both types of conditions.. only if you dont care about how things work.. he he

trog

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