[crazynic]

Hey guys,

Just wondering how others decide the correct on-road inflation pressures when using non-standard tire sizes?

I got thinking about it today when checking the tire wear on my 225/75/16 Mickey Thompson ATZ 5 rib tires fitted to my 99 V6 grand vitara and noticed that while the tires are in general wearing very well and evenly they appear to be wearing slightly more on the outer edges as the center tread is very slightly deeper not by much and the tires are still pretty young.

I understand that generally more wear on the outer edges compared to the center indicates under inflation.

I have been running these tires at 30psi because the factory tire placard states that the factory tire size (235/60/16) should have a cold inflation of 26psi so i figured because i have increased the air volume inside the tire i would need to increase the pressure to support the same weight.

There are heaps more factors to as the new tires are of light truck construction so this will also have a bearing but i just wanted some feedback from others and their experiences.

[Jester]

i run my 30s at about 24psi myself for my vit

[steak_knife]

I used to run my Gv at 36psi..

[Built4thrashing]

Tyre pressure relates to air pressure not volume. stick with the factory ratings for the road. then down as low as ya need for the bush


B4t

[gvzookdriver]

steak_knife wrote:

I used to run my Gv at 36psi..

Wow 36 i run 28-30 like the factory plate says

[Joe]

Jester wrote:

i run my 30s at about 24psi myself for my vit

x2

[shakes]

There is some actual formula around that I think is run them as low as reasonably possible and after a few freeway k's re-check pressures, if your at 4psi difference between cold and warm you've got the pressure right. If it's less you've got the pressure too high, If it's more you need to add more air.

I could be wayy off the mark too

[gregdixon]

The larger the chamber volume means less pressure required to carry a given load. The larger surface area in the chamber works the same as increasing the pressure of the standard tyre.

Just adjust the pressures until you get good wear.

The formula mentioned works on the principle of Boyle Charles law, which is the relation of pressure increase in a chamber to heat. The air molecules expand when heated which increases the pressure inside a chamber. I use this at work as a guide on how hard a tyre is working on a specific haul route. I have more technical ways but this is the quickest.

[shabz]

Which tyres are wearing the outer edges - front or rear?

The fronts will wear slightly differently to the rear - the rears will wear fairly flat across the tyre, whereas the fronts will wear the edges more, this is due to the IFS setup on the front of the GV.

GVs front factory have slight positive camber in the front - meaning the top of the tyre tilts outward slightly, hence wearing the outer edge more. If you've got raised suspension then this angle is exaggerated.

FYI i run my pressures at 24psi on BFG AT tyres 225/76/16 - which i worked out using the 4psi rule.

Before you worry about pressure too much i'd get the wheel alignment checked.

[MacDaddy]

Built4thrashing wrote:

Tyre pressure relates to air pressure not volume. stick with the factory ratings for the road. then down as low as ya need for the bush
B4t

X 2
Always stick to factory settings as on the tyre placard... the only time i would increase pressure is when heavily loaded... if the tyres are wearing uneven, that suggests to me more like lack of tyre rotation, than tyre pressure if you only have 30psi in them... and has been said, the front tyres will wear more then rears...

[royce]

there are a million different ways pressure can make a tyre wear!!!!!

I would listen to those saying check your hot - cold pressure and work from there, and it sounds like gregdixon might know about these things, always pays to listen to those that know about these things over the 2 centers

[whincup]

just another 2 center what works for one persons car wont necessarily work for yours. all manner of things can affect it, such as how sagged or misaligned the suspension setup is, other mods made that increase/decrease the weight, or for example, quite a few sierra owners run lower pressures onroad to make it more comfortable. where 20psi might suit some, i run 28 up front and 25 on the rear because i dont ever have anything in the back. what you need to work out is what suits you best... and best way to do that is the 4psi rule.

just my 2 cents

[shep]

whincup wrote:

just another 2 center what works for one persons car wont necessarily work for yours. all manner of things can affect it, such as how sagged or misaligned the suspension setup is, other mods made that increase/decrease the weight, or for example, quite a few sierra owners run lower pressures onroad to make it more comfortable. where 20psi might suit some, i run 28 up front and 25 on the rear because i dont ever have anything in the back. what you need to work out is what suits you best... and best way to do that is the 4psi rule.

just my 2 cents

i guarantee you that your sierra will drive, go and stop better if you run the recommended
tyre pressures.

[crazynic]

Hey guys thanks for all the info.

The wheel alignment is within factory spec as it has been done recently and I rotate the tires every 5000km's (5 tire rotation) when i service it.

The wear on all 5 tires is very even and consistent i just noticed that the center of the tread is very slightly deeper giving the tread area a slight convex look.

The tires are pretty new with about 90% tread and I'm probably just being anal, I guess i will play with the pressures and monitor the wear to keep it as even as possible.

Just thought i would ask as i don't see how the factory tire pressure will necessarily work with a larger than stock tire.

[shakes]

total Pressure is total pressure, tire volume doesnt come into it at this point.

If the tire is crowning as you describe it probably means too much pressure

[whincup]

shep wrote:

i guarantee you that your sierra will drive, go and stop better if you run the recommended
tyre pressures.

...yes. but my point is what works for one setup doesnt necessarily mean it will work for someone elses.

[TheOtherLeft]

Are you running the standard rims? Like you I am running 225/75/16 tyres on standard rims. I get uneven wear on the front as well.

It's worth noting that factory rims are at the maximum width for 225mm wide tyres and actually pull the centre of the tyre (bead) slightly outwards. This causes the tread shoulder to be raised slightly relative to the centre. This has the same effect as underinflation and hence increased wear on the shoulder.

Of course being IFS increased shoulder wear occurs too.

Well that's my theory.

[gregdixon]

Quote:

tire volume doesnt come into it

I beg to differ. If tyre volume doesn't come into acount, how come a larger tyres carrying capacity is greater than a smaller one at the same air pressure?

[royce]

gregdixon wrote:

Quote:

tire volume doesnt come into it

I beg to differ. If tyre volume doesn't come into acount, how come a larger tyres carrying capacity is greater than a smaller one at the same air pressure?

tyre construction doesnt come into it too much? IE all things being equal a larger tyre will still carry more?

[crazynic]

I would imagine tire construction has a lot to do with a tires load carrying capability's.

As a tires load rating goes up so does the stiffness of its construction.

[gregdixon]

True a larger tyre will carry more as there is more surface area inside the tyre for the pounds square inch (psi) to push against. the more square inches inside the chamber the more capacity. ( does this make sence trying to make it easy to understand?)

The tyre ply rating / construction will allow an increase in tyre pressure due to it being stronger construction to contain the greater pressure. For example a 6 ply tyre @ 30 psi will carry the same as a 8 ply tyre @ 30 psi (same size). But you can increase the pressure of the 8 ply tyre as it has a higher max inflation pressure than the 6 ply due to the stronger construction allowing a greater load.This is only useful if you need to increase tyre load capacity, but on a car you aren't changing the tyre load.

[gregdixon]

This tyre next to me (59/80R63)can carry upward of 70 tonne whereas a regular truck tyre i.e 10.00R20 will only carry roughly 3 tonne. They both run 110 psi. It is only the larger tyre voulme that allows the load increase.

This is an extreme example but it highlights what i am saying.

I know it's a Michelin just don't tell my boss

[royce]

that makes sense to me

would that tyre fit if I fitted a bodylift

[alien]

speaking of tyres etc, i was talking about tyre testing on truck rims today when we were airing up... it wasnt testing - it was welding the rim... heres the vid: http://www.youtube.com/watch?v=pNZ4siYGkag

apparently theres tons of deaths each year because of numpties doing this...

[gregdixon]

Quote:

would that tyre fit if I fitted a bodylift

If you did manage to get it to fit you might have trouble turning 4x 6 tonne tyres. Might need some good rock crawler gears

Quote:

speaking of tyres etc, i was talking about tyre testing on truck rims today when we were airing up... it wasnt testing - it was welding the rim... heres the vid: www.youtube.com/watch?v=pNZ4siYGkag

apparently theres tons of deaths each year because of numpties doing this...

lightning is also a big cause of "pyrolisis" which is the technical term for this.

[fordem]

gregdixon wrote:

This tyre next to me (59/80R63)can carry upward of 70 tonne whereas a regular truck tyre i.e 10.00R20 will only carry roughly 3 tonne. They both run 110 psi. It is only the larger tyre voulme that allows the load increase.

This is an extreme example but it highlights what i am saying.

I know it's a Michelin just don't tell my boss

I don't think it's the volume as much as the the footprint - but you can't increase the footprint without increasing the volume - wider tyre = wider footprint, larger tyre = longer foot print - in both cases greater surface area, and let's face it 80% of that tyre's volume is not in contact with anything and not supporting any load.

Let's look at this from a slightly different angle - why do you "air down" a tyre before going off road - so that it balloons at the bottom (the part that's supporting the load) increasing the footprint and spreading the same load over a larger surface allows you to float over the top rather than sink in to it.

You do also need to factor in the construction of the tyre into the equation - I'm pretty certain that 59/80R63 tyre has many more plies than the 10.00R20 tire does.

[gregdixon]

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.

[royce]

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.

Please do, I am learning a lot that seems obvious but I hadnt twigged to yet

[shakes]

gregdixon wrote:

This tyre next to me (59/80R63)can carry upward of 70 tonne whereas a regular truck tyre i.e 10.00R20 will only carry roughly 3 tonne. They both run 110 psi. It is only the larger tyre voulme that allows the load increase.

This is an extreme example but it highlights what i am saying.

Soooo... The larger volume means there is more internal surface area for the pressure to "hold" the weight against?

I understand a bigger tire gives you bugger all increase in footprint, but providing they are of the same construction it will usually give you a bigger carrying capicty possibly at the price of speed restrictions?

[shakes]

royce 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.

Please do, I am learning a lot that seems obvious but I hadnt twigged to yet

x2