[fordem]

gregdixon wrote:

It is not the foot print

It's the air inside the tyre pushing up on the top of the tyre is what carry's the load, the reason you air down is to get a larger foot print to reduce the ground contact pressure not increase your tyre's carrying capacity. When you air down your tyre load capacity is reduced and is why you have to go slow, it does not increase as your theory would suggest.

I will get some data to post up to show everyone how a tyre works just to clarify as it seems there are many different opinions on this.

Two points - first - I am not suggesting that airing the tyre down will increase it's load carrying capacity - I merely used that as an example to explain why the increased foot print allows the same load to be spread over a greater area, or allows a greater load to be carried in the same area - I believe you'll find that this equates quite nicely to the concept of pounds per square inch- with a given tire pressure in pounds per square inch, measure the footprint and multiply.

Second - you're suggesting that the air inside the tyre pushing UP on the TOP of the tyre that carries the load - interesting theory - I won't argue with it, but at the end of the day, don't forget that it is still the contact patch (or footprint) at the bottom of the tyre - so to speak - where the rubber meets the road - that has to support the load.

[matty87]

fordem wrote:

gregdixon wrote:

It is not the foot print

It's the air inside the tyre pushing up on the top of the tyre is what carry's the load, the reason you air down is to get a larger foot print to reduce the ground contact pressure not increase your tyre's carrying capacity. When you air down your tyre load capacity is reduced and is why you have to go slow, it does not increase as your theory would suggest.

I will get some data to post up to show everyone how a tyre works just to clarify as it seems there are many different opinions on this.

Two points - first - I am not suggesting that airing the tyre down will increase it's load carrying capacity - I merely used that as an example to explain why the increased foot print allows the same load to be spread over a greater area, or allows a greater load to be carried in the same area - I believe you'll find that this equates quite nicely to the concept of pounds per square inch- with a given tire pressure in pounds per square inch, measure the footprint and multiply.

Second - you're suggesting that the air inside the tyre pushing UP on the TOP of the tyre that carries the load - interesting theory - I won't argue with it, but at the end of the day, don't forget that it is still the contact patch (or footprint) at the bottom of the tyre - so to speak - where the rubber meets the road - that has to support the load.

in terms of pure physics the equation comes out to be force=pressure X area. if we go the opposite way and go small instead of big. have you ever ridden a road bike (bicycle) with say 30psi in the tyres? it is rather slow and painfull as the tyres are nearly flat these tyres are designed to run at 80 to 120 psi as the surface are of the tyre is so small it needs a far greater pressure to provide the force to counteract the 80 kg person riding the bike.

this theory also extends to pressure vessels as the larger the pressure vessel for a given pressure the higher the force inside said pressure vessel.

[TZAR]

fordem wrote:

you're suggesting that the air inside the tyre pushing UP on the TOP of the tyre that carries the load - interesting theory - I won't argue with it, but at the end of the day, don't forget that it is still the contact patch (or footprint) at the bottom of the tyre - so to speak - where the rubber meets the road - that has to support the load.

The tyre is just a pressure vessel. It is the air which carries the load not the tyre. The higher the ply rating the higher the pressure which can be put in the tyre to support more load

Have 0 psi in a tyre and it will be flat on the ground supputing nothing. Put 30 psi in it the car is supported on a the air inside the tyre

This is tyres in the most simpliest form. Don't think they are just little expensive black thingies which hold my car of the groaund and me drive over awesome stuff.

They are an intergral part of the suspension and braking system of the vehicle. Best thing is try different pressures yourself.

Now for us to give you the most accurate advice we need answers to these question

Tyes - LT225/75R16 load index 115R
Wheels- ????? standard aftermarket size offset etc
Driving style- hard, lunatic, grandma, Lunatic grandma, etc
Driving places- highway, city surbabia, dirt bitumen etc
Load carried in car- standard, winch bar, winch bar with winch etc

all this info can be obtained by looking at the car, but since we can't see it then we need it to give you the best answer.

Anyway I think most of the problem is the fact you have tyres which are designed to be carring 1215 kgs each on the front of your car at 30 psi.

[fordem]

@matty87 - that's exactly where I'm coming from - the smaller the surface area of the footprint or contact patch, the more pressure you need to support a given load, conversly, the same pressure, over a larger surface area (or footprint, or contact patch) is capable of supporting a greater load.

@TZAR - maybe you should re-read the thread and get a good idea of who posted what, I don't have any problems with my tyres or the pressures in them, and I'm pretty certain that none of my posts suggest that I do.

[alien]

someone say pressure vessel again... go on!

[gregdixon]

Here is some data showing how the load is supported by the tyre and how increases in pressure, casing strength and chamber volume increase the tyre load capacity.

Now if this is wrong you will have to write to the Bridgestone technical center in Tokyo and let them know because this is where the data came from

This first slide likens a tyre and rim to a bike wheel. As the basic principles work the same.


This shows where the load is supported by the air pressure in the chamber. As you can see the rim is more hanging in the air from the top of the tyre than getting pushed up from the bottom of the tyre.



This shows the increase in load capacity with increase of chamber volume with the same inflation pressure


This just shows the increase in load capacity with pressure


This is the allowable increase in pressure with an increase in casing strength allowing greater carrying capacity

[gregdixon]

VESSEL, VESSEL, VESSEL!!!

You asked Alien

[crazynic]

TZAR wrote:

[


Tires - LT225/75R16 load index 115R
Wheels- Stock enkei alloy 3 spoke 16"
Driving style- Prudent+skillfully piloted through impossible terrain on weekends.
Driving places- highway+weekend dirt,snow,sand,grass,gravel,rocky river beds,mud ect.
Load carried in car- standard+light loads on weekends.

all this info can be obtained by looking at the car, but since we can't see it then we need it to give you the best answer.

Anyway I think most of the problem is the fact you have tyres which are designed to be carring 1215 kgs each on the front of your car at 30 psi.

Are you suggesting the tires are overinflated?

[Bucket]

Generally speaking, we set the tyres on our services to 34/36